Article records
https://feeds.library.caltech.edu/people/Dimotakis-P-E/article.rss
A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenFri, 12 Apr 2024 23:29:44 +0000Local temperature measurements in supercritical counterflow in liquid helium II
https://resolver.caltech.edu/CaltechAUTHORS:DIMpof73
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Broadwell-J-E', 'name': {'family': 'Broadwell', 'given': 'James E.'}}]}
Year: 1973
DOI: 10.1063/1.1694214
In order to investigate the validity of appending the Gorter-Mellink friction term to the equations of motion of liquid helium, the temperature was measured along the axis of a channel carrying a supercritical heat current. A single thermomenter on a traversing assembly was used permitting local measurements both in the interior of the channel and in the jet formed in the free fluid. The temperature gradient in the interior of the channel is found to be in agreement with the Gorter-Mellink law up to the lambda point, but goes to zero within a channel radius, in the free jet. The Gorter-Mellink A (T ) was also measured up to the lambda point. A much stronger divergence is found as T λ is approached than was indicated by previous measurements.https://authors.library.caltech.edu/records/7p69a-w0a13Gorter-Mellink scale, and critical velocities in liquid—helium-II counterflow
https://resolver.caltech.edu/CaltechAUTHORS:DIMpra74
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1974
DOI: 10.1103/PhysRevA.10.1721
It is found that, in liquid—helium-II pure counterflow, the dimensionless number A=ρsAwl (where A is the Gorter-Mellink constant, w is the mean relative velocity between the two fluids, and l is a characteristic dimension of the geometry) scales the mutual friction between the two fluids. Critical relative velocities in cylindrical channels are shown to be correlated by the rule Ac≡ρsAwcπd-1.https://authors.library.caltech.edu/records/fftkq-1s415The mixing layer at high Reynolds number: large-structure dynamics and entrainment
https://resolver.caltech.edu/CaltechAUTHORS:20120731-074114467
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Brown-G-L', 'name': {'family': 'Brown', 'given': 'Garry L.'}}]}
Year: 1976
DOI: 10.1017/S0022112076002590
A turbulent mixing layer in a water channel was observed at Reynolds numbers up to 3 × 10^6. Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observation of the reaction of a base and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using a laser-Doppler velocimeter (LDV) revealed a fundamental periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing-layer dynamics at any point are coupled to the large structure further downstream, and some possible consequences regarding the effects of initial conditions and of the influence of apparatus geometry are discussed.https://authors.library.caltech.edu/records/yjnmg-af562Investigations of turbulence in a liquid helium II counterflow jet
https://resolver.caltech.edu/CaltechAUTHORS:DIMprb77
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Laguna-G-A', 'name': {'family': 'Laguna', 'given': 'Glenn A.'}}]}
Year: 1977
DOI: 10.1103/PhysRevB.15.5240
Using the phase information in a high-frequency second-sound beam, it was possible to investigate the temperature and velocity in a counterflow jet a few diameters downstream of the jet exit. At the heat fluxes investigated, the jet was characterized by turbulent velocity fluctuations. No temperature difference could be measured between the jet and the surrounding fluid, within the experimental accuracy (∼ 10^(-5) K). Velocity-sensitive phase measurements suggest that the normal-fluid jet entrains the surrounding superfluid to minimize any relative velocity and mutual friction between the two fluids. To our knowledge these observations constitute the first direct measurements of liquid-helium fluid velocities, as they enter the equations of motion.https://authors.library.caltech.edu/records/rd3wb-0az16Anatomy of a turbulent spot
https://resolver.caltech.edu/CaltechAUTHORS:CANpof77
Authors: {'items': [{'id': 'Cantwell-B', 'name': {'family': 'Cantwell', 'given': 'Brian'}}, {'id': 'Coles-D-E', 'name': {'family': 'Coles', 'given': 'Donald'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul'}}]}
Year: 1977
DOI: 10.1063/1.861752
Measurements have been made in the plane of symmetry of a turbulent spot to study the issue of entrainment. The mean flow is assumed to be similar in coordinates (x/t, y/t), and laser Doppler velocimeter data at several stations are used to determine the effective origin in x and t and to establish particle trajectories with respect to the stationary interface. Entrainment occurs over most of the rear interface and also close to the wall at the front of the spot. Flow visualization has also been used to obtain additional information about entrainment and sublayer structure. Pictures of the underside of spots, wedges, and boundary layers were taken through a glass wall, using a very heavy concentration of aluminum flakes, so that only motions occurring for y+ less than about 30 were visible. Sublayer streaks have essentially the same streaky, knotted appearance in all three types of flow. Optical correlations confirm the value λ+~100. An attempt has been made to estimate the strength of sublayer streaks as streamwise vortices, assuming that the streaks are a manifestation of local Taylor–Görtler instability. The results account for many of the phenomena actually observed in the sublayer.https://authors.library.caltech.edu/records/hgnce-dfw88Heat Release Effects on Shear-Layer Growth and Entrainment
https://resolver.caltech.edu/CaltechAUTHORS:20141020-151118820
Authors: {'items': [{'id': 'Hermanson-J-C', 'name': {'family': 'Hermanson', 'given': 'J. C.'}}, {'id': 'Mungal-M-G', 'name': {'family': 'Mungal', 'given': 'M. G.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1978
DOI: 10.2514/3.9666
The effects of heat release were studied in a planar, gaseous reacting mixing layer formed between two subsonic
freestreams; one containing hydrogen in an inert diluent, the other containing fluorine in an inert diluent.
Sufficiently high concentrations of hydrogen and fluorine reactants were employed to produce adiabatic flame
temperature rises of up to 940 K (adiabatic flame temperature of 1240 K absolute). Although the displacement
thickness of the layer for a zero streamwise pressure gradient showed an increase with increasing heat release. the actual thickness of the mixing layer at a given downstream location was not observed to increase and, in fact, was characterized by a slight thinning. The overall entrainment into the layer was seen to be substantially reduced by heat release. The large-wale vortical nature of the flow appeared to persist over all levels of heat release in this investigation. Imposition of a favorable pressure gradient, though resulting in additional thinning of the layer was observed to have no resolvable effect on the amount of chemical product formation and hence on the
mixing.https://authors.library.caltech.edu/records/52zaf-0x328Structure and entrainment in the plane of symmetry
of a turbulent spot
https://resolver.caltech.edu/CaltechAUTHORS:20120725-101139034
Authors: {'items': [{'id': 'Cantwell-B', 'name': {'family': 'Cantwell', 'given': 'Brian'}}, {'id': 'Coles-D-E', 'name': {'family': 'Coles', 'given': 'Donald'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul'}}]}
Year: 1978
DOI: 10.1017/S0022112078001809
Laser-Doppler velocity measurements in water are reported for the flow in the plane of symmetry of a turbulent spot. The unsteady mean flow, defined as an ensemble average, is fitted to a conical growth law by using data at three streamwise stations to determine the virtual origin in x and t. The two-dimensional unsteady stream function is expressed as ψ=U^2_∞tg(ξ,η) in conical similarity co-ordinates ζ = x/U_∞t and η = y/U_∞t. In these co-ordinates, the equations for the unsteady particle displacements reduce to an autonomous system. This system is integrated graphically to obtain particle trajectories in invariant form. Strong entrainment is found to occur along the outer part of the rear interface and also in front of the spot near the wall. The outer part of the forward interface is passive. In terms of particle trajectories in conical co-ordinates, the main vortex in the spot appears as a stable focus with celerity 0·77U_∞. A second stable focus with celerity 0·64U_∞ also appears near the wall at the rear of the spot.
Some results obtained by flow visualization with a dense, nearly opaque suspension of aluminium flakes are also reported. Photographs of the sublayer flow viewed through a glass wall show the expected longitudinal streaks. These are tentatively interpreted as longitudinal vortices caused by an instability of Taylor-Görtler type in the sublayer.https://authors.library.caltech.edu/records/wbwjs-krd71Two-Point LDV Measurements in a Plane Mixing Layer
https://resolver.caltech.edu/CaltechAUTHORS:20130911-101642275
Authors: {'items': [{'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'M. M.'}}, {'id': 'Catherasoo-C-J', 'name': {'family': 'Catherasoo', 'given': 'C. J.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Gharib-M', 'name': {'family': 'Gharib', 'given': 'M.'}}, {'id': 'Lang-D-B', 'name': {'family': 'Lang', 'given': 'D. B.'}}]}
Year: 1979
Investigations into the nature of the large structures in a two-dimensional shear layer were carried out using
laser Doppler velocimetry in the GALCIT free-surface water tunnel. By simultaneous measurements of velocity
at two points outside the turbulent region, above and below the shear layer, it was possible to measure the
strength (total circulation) and location of the vorticity center of the large structures. It was found that structures
not in the process of pairing convect downstream with the center of their cores close to the ray y/x along which
the mean velocity is given by U_m = l/2(U_1 + U_1 ). The determined value of the mean circulation is consistent with
the independent measurements of the mean spacing between the structures. Results indicate that if the large
structure vorticity distribution is elliptical, the inclination angle of its axis of symmetry with respect to the now
direction is small.https://authors.library.caltech.edu/records/s13xs-d4829Particle streak velocity field measurements in a two-dimensional mixing layer
https://resolver.caltech.edu/CaltechAUTHORS:20120718-100125180
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Debussy-F-D', 'name': {'family': 'Debussy', 'given': 'Francois D.'}}, {'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'Manoochehr M.'}}]}
Year: 1981
DOI: 10.1063/1.863481
Using digital image processing of particle streak photography, the streamwise and perpendicular components of the velocity field were investigated, in the mid‐span plane of a two‐dimensional mixing layer, with a 6:1 velocity ratio. The Reynolds number of the flow, based on the local vorticity thickness and the velocity difference across the layer, ranged from 1360 to 2520, in the plane of observation. The significant result of this experiment was that the region of vorticity bearing fluid is confined to a small fraction of the flow. A second finding, consistent with the small regions of concentrated vorticity, was the observation of instantaneous streamwise velocity reversal, in the laboratory frame, in small regions of the flow.https://authors.library.caltech.edu/records/2s8bb-64q06Structure and dynamics of round turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:20120712-110937678
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Miake-Lye-R-C', 'name': {'family': 'Miake-Lye', 'given': 'Richard C.'}}, {'id': 'Papantoniou-D-A', 'name': {'family': 'Papantoniou', 'given': 'Dimitris A.'}}]}
Year: 1983
DOI: 10.1063/1.864090
Laser‐induced fluorescence and particle streak velocity measurements were conducted to investigate the structure and dynamics of round turbulent jets. The results suggest that the far‐field region of the jet is dominated by large‐scale vortical structures, which appear to be axisymmetric or helical a large part of the time. Entrainment and mixing of the reservoir fluid with the jet fluid is found to be intimately connected with the kinematics of these structures. Unmixed reservoir fluid is found to reach and cross the jet axis.https://authors.library.caltech.edu/records/rqkrc-zpx64Turbulent mixing and combustion in a reacting shear layer
https://resolver.caltech.edu/CaltechAUTHORS:20160602-171748816
Authors: {'items': [{'id': 'Mungal-M-G', 'name': {'family': 'Mungal', 'given': 'M. G.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Broadwell-J-E', 'name': {'family': 'Broadwell', 'given': 'J. E.'}}]}
Year: 1984
DOI: 10.2514/3.8682
The temperature field is investigated in a gaseous mixing layer consisting of low-concentration hydrogen and fluorine. The results show the presence of large, hot structures separated by tongues of cool fluid that enter the layer from either side. The cores of the structures appear to be well mixed. the usual bell-shaped mean temperature profiles result from a duty cycle whereby a given point sees alternating hot and cool fluid, which results in the local mean. The adiabatic flame temperature is not achieved on average at any point across the layer. It is found that, in general, two different mean temperature profiles result from a given set of reactant compositions if the sides of the layer on which they are carried are reversed. These observations are not consistent with gradient diffusion concepts.https://authors.library.caltech.edu/records/qa9y8-bkr15Mixing and combustion with low heat release in a turbulent shear layer
https://resolver.caltech.edu/CaltechAUTHORS:20120709-071921964
Authors: {'items': [{'id': 'Mungal-M-G', 'name': {'family': 'Mungal', 'given': 'M. G.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1984
DOI: 10.1017/S002211208400238X
Turbulent mixing and combustion are investigated in a gaseous shear layer formed between two streams: one containing a low concentration of hydrogen in nitrogen and the other containing a low concentration of fluorine in nitrogen. The resulting temperature field is measured simultaneously at eight points across the width of the layer using fast-response cold-wire thermometry. The results show the presence of large, hot structures separated by tongues of cool fluid that enter the layer from either side. The usual bell-shaped mean-temperature profiles therefore result from a duty cycle whereby a fixed probe sees alternating hot and cool fluid, which results in the local mean. The adiabatic flame temperature is not achieved in the mean, at any location across the layer. For fixed velocities, it is found that, in general, two different mean-temperature profiles result from a given pair of reactant compositions if the sides of the layer on which they are carried are exchanged ('flipped'). This finding is a direct consequence of the asymmetric entrainment of fluid into the layer. Results are compared with the predictions of Konrad and discussed in the context of the Broadwell–Breidenthal model. By comparison with the liquid result of Breidenthal, the amount of product formed in the layer at high Reynolds number is found to be dependent upon the Schmidt number. Results for a helium–nitrogen layer are discussed briefly.https://authors.library.caltech.edu/records/vcwx1-6pj49A "flip" experiment in a chemically reacting turbulent mixing layer
https://resolver.caltech.edu/CaltechAUTHORS:20160602-172732465
Authors: {'items': [{'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'M. M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Broadwell-J-E', 'name': {'family': 'Broadwell', 'given': 'J. E.'}}]}
Year: 1985
DOI: 10.2514/3.9063
An experimental investigation of entrainment and mixing in a reacting, uniform density, liquid plane shear layer has been carried out using laser-induced fluorescence diagnostics. Results indicate that the reactants mix on a molecular level and react at a composition that is nearly uniform across the transverse extent of the layer. The composition of the mixed fluid is found to be asymmetric with an excess of high-speed fluid, suggesting that entrainment into the shear layer is also asymmetric. These results are at variance with predictions of conventional models of turbulent transport and mixing.https://authors.library.caltech.edu/records/dtp8n-8w135Reynolds number effects on mixing and combustion in a reacting shear layer
https://resolver.caltech.edu/CaltechAUTHORS:20160602-172201462
Authors: {'items': [{'id': 'Mungal-M-G', 'name': {'family': 'Mungal', 'given': 'M. G.'}}, {'id': 'Hermanson-J-C', 'name': {'family': 'Hermanson', 'given': 'J. C.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1985
DOI: 10.2514/3.9101
The temperature field is investigated in a gaseous, reacting mixing layer between two streams containing low concentrations of hydrogen and fluorine, over a wide range of Reynolds numbers. The results show the presence of large, hot structures that govern the flow dynamics at all speeds. The mean-temperature profile is found to decrease modestly with increasing speed, a result suggesting a weak Reynolds number dependence of the production rate. Tripping ht high-speed boundary layer is found to have a significant effect on the width of the mixing zone and the profile of the mean temperature rise.https://authors.library.caltech.edu/records/hs4wv-rdk80Laser-induced fluorescence measurements of mixed fluid concentration in a liquid plane shear layer
https://resolver.caltech.edu/CaltechAUTHORS:20160602-172456643
Authors: {'items': [{'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'M. M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1985
DOI: 10.2514/3.9154
The processes of entrainment and mixing are investigated in a nonreacting, uniform density, liquid mixing layer. Laser-induced fluorescence diagnostics and high-speed, real-time digital image acquisitions techniques are combined to measure the probability density function of the composition field. Results show that the vortical structures in the mixing layer initially roll up with a large excess of high-speed fluid in the cores. It is found that the mixed fluid composition, above the mixing transition, is essentially uniform across the entire transverse extent of the layer and is asymmetric with a bias in favor of the high-speed fluid. Preliminary observations indicate that the composition of the mixed fluid is more uniform across a liquid shear layer than that in the gaseous layer. The important effect of the resolution capability of the measurement apparatus on the results are discussed and comparisons with recent theoretical calculations are presented.https://authors.library.caltech.edu/records/52dhe-qqs16Implications of recent experimental results for modeling reactions in turbulent flows
https://resolver.caltech.edu/CaltechAUTHORS:20160602-173151038
Authors: {'items': [{'id': 'Broadwell-J-E', 'name': {'family': 'Broadwell', 'given': 'James E.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1986
DOI: 10.2514/3.9363
The general subject of this paper is turbulent mixing, chemical reactions, and combustion in fully developed shear layers and jets. More specifically, the purpose is to review the results of a series of experiments that seems to us to have important implications for understanding and modeling of these flows, with possible implications for dealing with turbulent transport in general. A comprehensive review of all important and useful recent experiments on the subject will not be undertaken; instead, attention is focused on some experiments that have, in our opinion, revealed new features of the mixing and combustion processes in turbulent shear flows.
The recognition of the importance of the large-scale organized structure (e.g., Ref. 1) in turbulent shear flows has given new directions to research in this field. While there is no doubt that the existence of the large structures has been recognized for some time (see, for example, Refs. 2 and 3), it was believed that their main effect on turbulent transport was the convolution at low wavenumbers of the turbulent/nonturbulent interface in the flow, which could be accounted for through the notion of intermittency. Much of the recent work, both theoretical and experimental, has had the objective of clarifying the fluid mechanics. For the purposes of the present discussion, our interest is in the implications for turbulent transport and diffusion of scalars, where the role of the large-scale organized motions is perhaps clearer. A complete theory would, of course, deal with the momentum and scalar equations simultaneously, but since it is likely to be some time before such a theory is available, it seems worthwhile to see what can be deduced from the evidence already at hand.https://authors.library.caltech.edu/records/81vff-hy763Mixing and chemical reactions in a turbulent liquid
mixing layer
https://resolver.caltech.edu/CaltechAUTHORS:20120627-085420456
Authors: {'items': [{'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'M. M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1986
DOI: 10.1017/S0022112086000812
An experimental investigation of entrainment and mixing in reacting and non-reacting turbulent mixing layers at large Schmidt number is presented. In non-reacting cases, a passive scalar is used to measure the probability density function (p.d.f.) of the composition field. Chemically reacting experiments employ a diffusion-limited acid–base reaction to directly measure the extent of molecular mixing. The measurements make use of laser-induced fluorescence diagnostics and high-speed, real-time digital image-acquisition techniques. Our results show that the vortical structures in the mixing layer initially roll-up with a large excess of fluid from the high-speed stream entrapped in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but its composition also changes. It is found that the range of compositions of the mixed fluid, above the mixing transition and also throughout the transition region, is essentially uniform across the entire transverse extent of the layer. Our measurements indicate that the probability of finding unmixed fluid in the centre of the layer, above the mixing transition, can be as high as 0.45. In addition, the mean concentration of mixed fluid across the layer is found to be approximately constant at a value corresponding to the entrainment ratio. Comparisons with gas-phase data show that the normalized amount of chemical product formed in the liquid layer, at high Reynolds number, is 50% less than the corresponding quantity measured in the gas-phase case. We therefore conclude that Schmidt number plays a role in turbulent mixing of high-Reynolds-number flows.https://authors.library.caltech.edu/records/4z34q-fcf57Two-dimensional shear-layer entrainment
https://resolver.caltech.edu/CaltechAUTHORS:20160602-172858956
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1986
DOI: 10.2514/3.9525
It is observed experimentally that a spatially growing shear layer entrains an unequal amount of fluid from
each of the freestreams, resulting in a mixed fluid composition that favors the high-speed fluid. A simple argument is proposed, based on the geometrical properties of the large-scale now structures of the subsonic, fully
developed, two-dimensional mixing layer, which yields the entrainment ratio and growth of the turbulent mixing
layer. The predictions depend on the velocity and density ratio across the layer and are in good agreement with
measurements to date.https://authors.library.caltech.edu/records/c33bw-tr154Measurements of entrainment and mixing in turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:20160602-173708153
Authors: {'items': [{'id': 'Dahm-W-J-A', 'name': {'family': 'Dahm', 'given': 'W. J. A.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1987
Entrainment and mixing were investigated in the self-similar far field of a steady, axisymmetric, momentum-driven, free turbulent jet fluid concentration field. Results show that the entrainment and mixing processes in the jet display a roughly periodic organization characterized by temporal and spatial scales approximately equal to the local large scales of the flow. It is found that instantaneous radial profiles of the jet fluid concentration do not resemble the mean profile, indicating that the mean profile is a poor representation of the mixed fluid states in the jet. The instantaneous profiles show that unmixed ambient fluid is transported deep into the jet, and that the mixed fluid composition can be fairly uniform within large regions. The probability of finding unmixed ambient fluid on the jet centerline is found to increase markedly at intervals typically separated by the local large-scale time of the flow. These results are interpreted in terms of a simple conceptual picture for the large-scale organization of entrainment and mixing in the far field of turbulent jets.https://authors.library.caltech.edu/records/56qcj-vws22Effects of heat release in a turbulent, reacting shear layer
https://resolver.caltech.edu/CaltechAUTHORS:20160602-174405864
Authors: {'items': [{'id': 'Hermanson-J-C', 'name': {'family': 'Hermanson', 'given': 'J. C.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1989
DOI: 10.1017/S0022112089000406
Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in
an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to
940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren
spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori
calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures
were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the
corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean
temperature rise profiles; normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A
small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat
release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The
additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.https://authors.library.caltech.edu/records/k1d25-fg173Effects of a downstream disturbance on the structure of a turbulent plane mixing layer
https://resolver.caltech.edu/CaltechAUTHORS:20141028-160303422
Authors: {'items': [{'id': 'Koochesfahani-M-M', 'name': {'family': 'Koochesfahani', 'given': 'M. M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1989
Using a two-dimensional airfoil, a disturbance was introduced into a plane mixing layer some distance
downstream of the splitter plate trailing edge. Results indicate that it is possible to induce very large changes in
the layer growth rate downstream of the disturbance location, while leaving the portion of the shear layer
between the splitter plate and the disturbance source essentially unaffected. Furthermore, the use of forcing for
modification of the mixing layer in the region upstream of the disturbance is demonstrated. It Is shown that two
different mechanisms are responsible for coupling such disturbances to the flow in the present forcing of
upstream and downstream regions.https://authors.library.caltech.edu/records/82kpr-gbd13An improved laser-Rayleigh scattering photodetection system
https://resolver.caltech.edu/CaltechAUTHORS:20160602-174109500
Authors: {'items': [{'id': 'Downling-D-R', 'name': {'family': 'Dowling', 'given': 'D. R.'}}, {'id': 'Lang-D-B', 'name': {'family': 'Lang', 'given': 'D. B.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1989
DOI: 10.1007/BF00187060
An improved photodetection system for high resolution laser-Rayleigh scattering measurements has been developed that utilizes a solid state detector coupled to a custom-designed, low-noise, transimpedance amplifier. The resulting system, based on a PIN photodiode is less expensive, inherently safer, less delicate and, depending on the detected light level, may exhibit higher signal-to-noise ratios than photodetection systems based on photomultiplier tubes. The frequency response of the system was designed to be uniform (3% peak variation) from DC to nearly 100 kHz. Concentration fluctuation spectra of a high scattering cross-section label (jet fluid) gas discharging into a density-matched, low scattering cross-section quiescent reservoir gas were measured using this system. Spectral signal-to-noise ratios as high as 7 decades were achieved under some conditions in parts of the spectrum.https://authors.library.caltech.edu/records/jkd7g-7kt83Viscous and Nuclei Effects on Hydrodynamic Loadings and Cavitation of a NACA 66 (MOD) Foil Section
https://resolver.caltech.edu/CaltechAUTHORS:20160602-174939023
Authors: {'items': [{'id': 'Shen-Y-T', 'name': {'family': 'Shen', 'given': 'Y. T.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1989
DOI: 10.1115/1.3243645
A series of experiments has been conducted on a two-dimensional NACA 66 (MOD) foil to examine the effects of viscosity and nuclei on cavitation inception. In this paper the main discussions center on two foil angles having different types of pressure loadings to represent a propeller blade section operating at design and off-design conditions. At one degree design angle of attack the foil experiences a rooftop-type gradually varying pressure distribution. At three degrees off-design angle of attack the foil experiences a sharp suction pressure peak near the leading edge. Cebeci's viscid/inviscid interactive code is used to compute the viscous scale effects on the development of the boundary layer, lift, drag and pressure distribution on the foil. Chahine's multibubble interaction code is used to compute the effect of nuclei, test speeds, foil size and foil surface on traveling bubble cavitation. Both computer codes are found to agree satisfactorily with the experimental measurements reported here. Two assumptions commonly used to predict full scale surface cavitation from model tests are examined experimentally and theoretically. The first assumption states that cavitation inception occurs when the static pressure reaches the vapor pressure. On the contrary, the experiments showed that the water flowing over the foil surface sustained significant amounts of tension during inception of midchord bubble cavitation as well as leading edge sheet cavitation. The second assumption states that there is no scale effect on the values of negative minimum pressure coefficient. In the case of a rooftop-type pressure loading, the second assumption is supported by the pressure numerical calculations. However, in the case of a pressure loading with a strong suction peak near the leading edge the value of negative minimum pressure coefficient is as much as 12 to 15 percent lower on a model than at full scale.https://authors.library.caltech.edu/records/r0c75-1ez89The effect of walls on a spatially growing supersonic shear layer
https://resolver.caltech.edu/CaltechAUTHORS:ZHUpofa90
Authors: {'items': [{'id': 'Zhuang-Mei', 'name': {'family': 'Zhuang', 'given': 'Mei'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Kubota-Toshi', 'name': {'family': 'Kubota', 'given': 'Toshi'}}]}
Year: 1990
DOI: 10.1063/1.857760
The inviscid instability, with respect to supersonic disturbances of a spatially growing plane mixing layer inside parallel flow guide walls, is investigated using linear stability analysis. For supersonic convective Mach numbers, it is found that the maximum amplification rates of the shear layers approach an asymptotic value and that this maximum amplification rate increases to its maximum value and decreases again as the distance between the walls decreases continuously. Contour plots of the pressure perturbation fields indicate that there are waves propagating outward from the shear layer along the Mach angle, and that the walls provide a feedback mechanism between the growing shear layer and this compression/expansion wave system. The streak lines of the flow confirm that the spreading rate of the shear layer is unusually small for supersonic disturbances.https://authors.library.caltech.edu/records/w50ee-hrf02Mixing at large Schmidt number in the self-similar far field of turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:DAHjfm90
Authors: {'items': [{'id': 'Dahm-W-J-A', 'name': {'family': 'Dahm', 'given': 'Werner J. A.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1990
DOI: 10.1017/S0022112090000738
We present results from an experimental investigation of turbulent transport and molecular mixing of a Sc [dbl greater-than sign] 1 conserved scalar in the fully developed self-similar far field of a steady, axisymmetric, momentum-driven, free turbulent jet issuing into a quiescent medium. Our experiments cover the axial range from the jet exit to 350 diameters downstream, and span the range of Reynolds numbers from 1500 to 20000. Flow visualizations of the scalar concentration field directly verify the presence of an underlying characteristic large-scale organization in the jet far field essentially consistent with a simplified conceptual picture proposed in an earlier study (Dahm & Dimotakis 1987). High-resolution imaging measurements of successive instantaneous scalar concentration profiles in the jet support the presence of such a large-scale organization and provide details of its implications for mixing. These results also establish the proper similarity scaling for the mean concentration in the jet far field and give the scaling constant on the jet centreline as 5.4. We also present conserved scalar concentration p.d.f.s throughout the jet far field, and introduce a chemical reaction method for measuring the p.d.f.s with potentially molecular resolution. The amount of unmixed ambient fluid that reaches the jet centreline is found to decrease with increasing Reynolds number over the range investigated. The distribution of mixed fluid compositions in the concentration p.d.f. also appears to change over this range of Reynolds numbers, indicating that some aspects of large Schmidt number mixing in the jet far field have not yet become Reynolds number independent.https://authors.library.caltech.edu/records/rcm7d-vwk69Similarity of the concentration field of gas-phase turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:20120503-082251230
Authors: {'items': [{'id': 'Dowling-D-R', 'name': {'family': 'Dowling', 'given': 'David R.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1990
DOI: 10.1017/S0022112090000945
This work is an experimental investigation of the turbulent concentration field formed when the nozzle gas from a round, momentum-driven, free turbulent jet mixes with gas entrained from a quiescent reservoir. The measurements, which were made with a non-intrusive laser-Rayleigh scattering diagnostic at Reynolds numbers of 5000, 16000, and 40000, cover the axial range from 20 to 90 jet exit diameters and resolve the full range of temporal and spatial concentration scales. Reynolds-number-independent and Reynolds-number-dependent similarities are investigated. The mean and r.m.s. values of the concentration are found to be consistent with jet similarity laws. Concentration fluctuation power spectra are found to be self-similar along rays emanating from the virtual origin of the jet. The probability density function for the concentration is also found to be self-similar along rays. Near the centreline of the jet, the scaled probability density function of jet fluid concentration is found to be nearly independent of the Reynolds number.https://authors.library.caltech.edu/records/k8vsy-53167Instability of inviscid, compressible free shear layers
https://resolver.caltech.edu/CaltechAUTHORS:20160602-175836795
Authors: {'items': [{'id': 'Zhuang-Mei', 'name': {'family': 'Zhuang', 'given': 'Mei'}}, {'id': 'Kubota-Toshi', 'name': {'family': 'Kubota', 'given': 'Toshi'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1990
DOI: 10.2514/3.10466
The linear spatial instability of inviscid, compressible laminar mixing of two parallel streams, comprised of the same gas, has been investigated with respect to two-dimensional wave disturbances. The effects of the velocity ratio, temperature ratio and the temperature profile across the shear layer have been examined. a nearly universal dependence of the normalized maximum amplification rate on the convective Mach number is found, with the normalized maximum amplification rate decreasing significantly with increasing convective Mach number in the subsonic region. These results are in accord with those of recent growth-rate experiments in compressible turbulent free shear layers and other similar recent calculations.https://authors.library.caltech.edu/records/q2g68-7bp84Some consequences of the boundedness of scalar fluctuations
https://resolver.caltech.edu/CaltechAUTHORS:20120508-154933764
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Miller-P-L', 'name': {'family': 'Miller', 'given': 'Paul L.'}}]}
Year: 1990
DOI: 10.1063/1.857666
Values of the scalar field c(x,t), if initially bounded, will always be bounded by the limits set by the initial conditions. This observation permits the maximum variance ∼(c′^2) to be computed as a function of the mean value c. It is argued that this maximum should be expected in the limit of infinite Schmidt numbers (zero scalar species diffusivity). This suggests that c′/c on the axis of turbulent jets, for example, may not tend to a constant, i.e., independent of x/d, in the limit of very large Schmidt numbers. It also underscores a difficulty with the k^(−1) scalar spectrum proposed by Batchelor [J. Fluid Mech. 5, 113 (1959)].https://authors.library.caltech.edu/records/pkfxg-89x38Stochastic geometric properties of scalar interfaces in turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:MILpofa91a
Authors: {'items': [{'id': 'Miller-P-L', 'name': {'family': 'Miller', 'given': 'Paul L.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1991
DOI: 10.1063/1.857876
Experiments were conducted in which the behavior of scalar interfaces in turbulent jets was examined, using laser-induced fluorescence (LIF) techniques. The experiments were carried out in a high Schmidt number fluid (water), on the jet centerline, over a jet Reynolds number range of 1000<=Re<=24 000. Both two-dimensional scalar data, c(r,t) at fixed x/d, and one-dimensional scalar data, c(t) at fixed x/d and r/x, were analyzed using standard one- and two-dimensional fractal box-counting algorithms. Careful treatment was given to the handling of noise. Both long and short records as well as off-centerline measurements were also investigated. The important effect of threshold upon the results is discussed. No evidence was found of a constant (power-law) fractal dimension over the range of Reynolds numbers studied. On the other hand, the results are consistent with the computed behavior of a simple stochastic model of interface geometry.https://authors.library.caltech.edu/records/sjnk7-y1k13Rotary oscillation control of a cylinder wake
https://resolver.caltech.edu/CaltechAUTHORS:20160602-180432495
Authors: {'items': [{'id': 'Tokumaru-P-T', 'name': {'family': 'Tokumaru', 'given': 'P. T.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1991
DOI: 10.1017/S0022112091001659
Exploratory experiments have been performed on circular cylinders executing forced rotary oscillations in a steady uniform flow. Flow visualization and wake profile measurements at moderate Reynolds numbers have shown that a considerable amount of control can be exerted over the structure of the wake by such means. In particular, a large increase, or decrease, in the resulting displacement thickness, estimated cylinder drag, and associated mixing with the free stream can be achieved, depending on the frequency and amplitude of oscillation.https://authors.library.caltech.edu/records/x68bp-7ax94Reynolds number dependence of scalar fluctuations in a high Schmidt number turbulent jet
https://resolver.caltech.edu/CaltechAUTHORS:MILpofa91b
Authors: {'items': [{'id': 'Miller-P-L', 'name': {'family': 'Miller', 'given': 'Paul L.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1991
DOI: 10.1063/1.858043
The scalar rms fluctuations in a turbulent jet were investigated experimentally, using high-resolution, laser-induced fluorescence techniques. The experiments were conducted in a high Schmidt number fluid (water), on the jet centerline, over a jet Reynolds number range of 3000<=Re<=24 000. It was found that the normalized scalar rms fluctuations c[script ']/ c-bar decrease with increasing flow Reynolds number, at least for the range of Reynolds numbers investigated. Since c[script ']/ c-bar is a measure of the inhomogeneity of the scalar field, this implies that high Schmidt number turbulent jets become more homogeneous, or better mixed, with increasing Re. These findings need to be assessed in the context of the documented Reynolds number independence of flame length for Re>3000 or 6500.https://authors.library.caltech.edu/records/jsjt4-nga89Fractals, Dimensional Analysis and Similarity, and Turbulence
https://resolver.caltech.edu/CaltechAUTHORS:20141020-132024116
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1991
[no abstract]https://authors.library.caltech.edu/records/y0es7-tpb78An Adaptive Lagrangian Method for Computing 1D Reacting and Non-reacting Flows
https://resolver.caltech.edu/CaltechAUTHORS:20141021-162327176
Authors: {'items': [{'id': 'Lappas-T', 'name': {'family': 'Lappas', 'given': 'Tasso'}}, {'id': 'Leonard-A', 'name': {'family': 'Leonard', 'given': 'Anthony'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1993
DOI: 10.1006/jcph.1993.1037
A method for computing one-dimensional unsteady compressible
flows. with and without chemical reactions is presented. This work has focused on the accurate computation of the discontinuous waves that arise in such flows. The main feature of the method is the use of an
adaptive Lagrangian grid. This allows the computation of discontinuous waves and their interactions with the accuracy of front-tracking algorithms. This is done without the use of additional grid points representing shocks, in contrast to conventional front-tracking
schemes. The Lagrangian character of the present scheme also allows contact discontinuities to be captured easily. The algorithm avoids interpolation across discontinuities in a natural and efficient way. The method has been used on a variety of reacting and non-reacting flows
in order to test its ability to compute accurately and in a robust way complicated wave interactions.https://authors.library.caltech.edu/records/qh6zr-y7n76The lift of a cylinder executing rotary motions in a
uniform flow
https://resolver.caltech.edu/CaltechAUTHORS:20120308-070414615
Authors: {'items': [{'id': 'Tokumaru-P-T', 'name': {'family': 'Tokumaru', 'given': 'P. T.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1993
DOI: 10.1017/S0022112093002368
The mean lift coefficient of a circular cylinder executing rotary motions in a uniform flow is investigated. These motions include steady rotation, and rotary oscillations with a net rotation rate. Results for the steadily rotating cylinder show that for a given rotation rate, larger cylinder aspect ratios yield higher lift coefficients. It was also found that the addition of forced rotary oscillations to the steady rotation of the cylinder increases the lift coefficient in the cases where the wake would normally be separated in the steadily rotating case, but decreases it otherwise. In addition, a method for estimating the mean lift of a rotating cylinder is presented. Estimates based on this method compare favourably with similar data published for steadily rotating cylinders.https://authors.library.caltech.edu/records/ypcw8-brn96Image correlation velocimetry
https://resolver.caltech.edu/CaltechAUTHORS:20160602-181135884
Authors: {'items': [{'id': 'Tokumaru-P-T', 'name': {'family': 'Tokumaru', 'given': 'P. T.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1995
DOI: 10.1007/BF00192228
This paper focuses on the correlation of two successive scalar images for the purpose of measuring imaged fluid motions. A method is presented for deforming, or transforming, one image to another. Taylor series expansions of the Lagrangian displacement field are used, in conjunction with an integral form of the equations of motion, to approximate this transformation. The proposed method locally correlates images for displacements, rotations, deformations, and higher-order displacement gradient fields, and applies a global minimization procedure to insure a global consistency in the results. An integral form of the equations of motion is employed. No explicit spatial or temporal differentiation of the image data is required in estimating the displacement field. As a consequence, this method is appropriate for both continuous-scalar as well as discrete-particle-image data. Successive two-dimensional digital CCD images of fluid motion marked with dye, are used to verify the capabilities of the method. The utility of the method is also illustrated using a pair of Voyager 2 images of Jupiter.https://authors.library.caltech.edu/records/95js3-rfb82Instability of wake-dominated compressible mixing layers
https://resolver.caltech.edu/CaltechAUTHORS:ZHUpof95
Authors: {'items': [{'id': 'Zhuang-Mei', 'name': {'family': 'Zhuang', 'given': 'Mei'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1995
DOI: 10.1063/1.868692
The instability of supersonic mixing layers, with velocity profiles possessing a wake component, is investigated using linear, inviscid, spatial theory. The mean-velocity profile is represented by a hyperbolic-tangent profile plus a wake component. Such profiles are encountered in the initial region of experimental supersonic shear-layer flows, as well as in envisaged hypersonic propulsion systems in which ingested boundary layers generate substantial wake components. Shear-layer and wake instability modes previously found in incompressible mixing layers are also found in compressible mixing layers. The existence of a wake component in the velocity profile renders the mixing layer more unstable at all free-stream Mach numbers. For convective Mach numbers exceeding unity, the shear-layer mode splits into two supersonic modes, and the mixing layer becoming more unstable with increasing wake deficit. The wake mode becomes less unstable and eventually stable with increasing compressibility, i.e., increasing convective Mach numbers.https://authors.library.caltech.edu/records/7bmz8-6k512On strained flames with hypergolic reactants: The H_2/NO/F_2 system in high-speed, supersonic and subsonic mixing-layer combustion
https://resolver.caltech.edu/CaltechAUTHORS:20160602-184120224
Authors: {'items': [{'id': 'Egolfopoulos-F-N', 'name': {'family': 'Egolfopoulos', 'given': 'Fokion N.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Bond-C', 'name': {'family': 'Bond', 'given': 'Chris'}}]}
Year: 1996
DOI: 10.1016/S0082-0784(96)80129-9
We report on a numerical investigation on the dynamics and structure of strained, non-premixed and
premixed, H_2/NO/F_2 flames. This is an important, hypergolic reacting system that has been used in high-
power chemical-laser systems and, presently, relied upon in experimental studies of high-speed, supersonic and subsonic, turbulent shear-layer mixing and combustion. The study included a detailed description of
the chemical kinetics and molecular transport for the H_2
/NO/F_2 system and was conducted for a wide
range of reactant concentrations and inert diluents, with flow/chemical parameters chosen to correspond
to specific chemically reacting, supersonic mixing-layer experiments. Both non-premixed and premixed
flames were studied using opposed-jet flow configurations. The results confirmed the experimental conclusion that, even at low reactant concentrations, the H_2/NO/F_2
system can sustain high Damköhler number chemical activity at high strain rates with room-temperature free streams. A comparison between the results of the present numerical simulations and the experimental chemically reacting mixing-layer studies, however, indicates that the predominant fraction of product formation in high-speed, turbulent, mixing layers must take place in a mode in which the reactants are in premixed, rather than in non-premixed,
strained diffusion flames.https://authors.library.caltech.edu/records/31hqr-04698Measurements of scalar power spectra in high Schmidt number turbulent jets
https://resolver.caltech.edu/CaltechAUTHORS:20120208-132613256
Authors: {'items': [{'id': 'Miller-P-L', 'name': {'family': 'Miller', 'given': 'Paul L.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1996
DOI: 10.1017/S0022112096001425
We report on an experimental investigation of temporal, scalar power spectra of round, high Schmidt number (Sc ≃ 1.9 × 10^3), momentum-dominated turbulent jets, for jet Reynolds numbers in the range of 1.25 × 10^4 ≤ Re ≤ 7.2 × 10^4. At intermediate scales, we find a spectrum with a slope (logarithmic derivative) that increases in absolute value with Reynolds number, but remains less than 5/3 at the highest Reynolds number in our experiments. At the smallest scales, our spectra exhibit no k^(−1) power-law behaviour, but, rather, seem to be approximated by a log-normal function, over a range of scales exceeding a factor of 40, in some cases.https://authors.library.caltech.edu/records/zystd-4vk88Mixing in turbulent jets: scalar measures and isosurface geometry
https://resolver.caltech.edu/CaltechAUTHORS:CATjfm96
Authors: {'items': [{'id': 'Catrakis-H-J', 'name': {'family': 'Catrakis', 'given': 'Haris J.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1996
DOI: 10.1017/S002211209600078X
Experiments have been conducted to investigate mixing and the geometry of scalar isosurfaces in turbulent jets. Specifically, we have obtained high-resolution, high-signal-to-noise-ratio images of the jet-fluid concentration in the far field of round, liquid-phase, turbulent jets, in the Reynolds number range 4.5 × 10^3 ≤ Re ≤ 18 × 10^3, using laser-induced-fluorescence imaging techniques. Analysis of these data indicates that this Reynolds-number range spans a mixing transition in the far field of turbulent jets. This is manifested in the probability-density function of the scalar field, as well as in measures of the scalar isosurfaces. Classical as well as fractal measures of these isosurfaces have been computed, from small to large spatial scales, and are found to be functions of both scalar threshold and Reynolds number. The coverage of level sets of jet-fluid concentration in the two-dimensional images is found to possess a scale-dependent-fractal dimension that increases continuously with increasing scale, from near unity, at the smallest scales, to 2, at the largest scales. The geometry of the scalar isosurfaces is, therefore, more complex than power-law fractal, exhibiting an increasing complexity with increasing scale. This behaviour necessitates a scale-dependent generalization of power-law-fractal geometry. A connection between scale-dependent-fractal geometry and the distribution of scales is established and used to compute the distribution of spatial scales in the flow.https://authors.library.caltech.edu/records/m3yh7-72j12Scale distributions and fractal dimensions in turbulence
https://resolver.caltech.edu/CaltechAUTHORS:CATprl96
Authors: {'items': [{'id': 'Catrakis-H-J', 'name': {'family': 'Catrakis', 'given': 'Haris J.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1996
DOI: 10.1103/PhysRevLett.77.3795
A new geometric framework connecting scale distributions to coverage statistics is employed to analyze level sets arising in turbulence as well as in other phenomena. A 1D formalism is described and applied to Poisson, lognormal, and power-law statistics. A d-dimensional generalization is also presented. Level sets of 2D spatial measurements of jet-fluid concentration in turbulent jets are analyzed to compute scale distributions and fractal dimensions. Lognormal statistics are used to model the level sets at inner scales. The results are in accord with data from other turbulent flows.https://authors.library.caltech.edu/records/93cfn-y7c25Cover illustration: Non-premixed hydrocarbon flame
https://resolver.caltech.edu/CaltechAUTHORS:20120118-105435182
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1997
DOI: 10.1088/0951-7715/10/1/001
This year's cover illustration, reproduced here as figure 1, depicts an image formed by a short-time (1/1000 s) exposure of a non-premixed hydrocarbon flame. The flow is driven by the buoyancy forces generated by the density difference from the combustion heat release and resulting temperature rise. The Reynolds number for this buoyancy-induced, turbulent flow is relatively low, estimated at a few thousand.https://authors.library.caltech.edu/records/3d4y0-zz484Unsplit Schemes for Hyperbolic Conservation Laws with Source Terms in One Space Dimension
https://resolver.caltech.edu/CaltechAUTHORS:20160602-181443806
Authors: {'items': [{'id': 'Papalexandris-M-V', 'name': {'family': 'Papalexandris', 'given': 'Miltiadis V.'}}, {'id': 'Leonard-A', 'name': {'family': 'Leonard', 'given': 'Anthony'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1997
DOI: 10.1006/jcph.1997.5692
The present work is concerned with an application of the theory of characteristics to conservation laws with source terms in one space dimension, such as the Euler equations for reacting flows. Space-time paths are introduced on which the flow/chemistry equations decouple to a characteristic set of ODE's for the corresponding homogeneous laws, thus allowing the introduction of functions analogous to the Riemann invariants in classical theory. The geometry of these paths depends on the spatial gradients of the solution. This particular decomposition can be used in the design of efficient unsplit algorithms for the numerical integration of the equations. As a first step, these ideas are implemented for the case of a scalar conservation law with a nonlinear source term. The resulting algorithm belongs to the class of MUSCL-type, shock-capturing schemes. Its accuracy and robustness are checked through a series of tests. The stiffness of the source term is also studied. Then, the algorithm is generalized for a system of hyperbolic equations, namely the Euler equations for reacting flows. A numerical study of unstable detonations is performed.https://authors.library.caltech.edu/records/yjytt-wzf43Non-premixed hydrocarbon ignition at high strain rates
https://resolver.caltech.edu/CaltechAUTHORS:20160602-183737190
Authors: {'items': [{'id': 'Egolfopoulos-F-N', 'name': {'family': 'Egolfopoulos', 'given': 'Fokion N.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1998
DOI: 10.1016/S0082-0784(98)80456-6
We report on the results of numerical-simulation investigations of ignition characteristics of hydrocarbon-fuel blends expected from thermal cracking of typical jet fuels, at conditions relevant to high-Mach-number, air-breathing propulsion. A two-point-continuation method was employed, with a detailed description of molecular transport and chemical kinetics, focusing on the effects of fuel composition, reactant temperature, additives, and imposed strain rate. It captured the entire S-curve that describes the processes of vigorous burning extinction, and ignition. The results demonstrate that ignition of such fuel blends is dominated by the synergistic behavior of CH_4 and C_2H_4. A fuel temperature of T_(fuel)=950 K was employed throughout. At higher air temperatures (T_(air)=1200 K), addition of small amounts of CH_4 to C_2H_4 molerately inhibits C_2H_4 ignition, while at lower T_(air)=1050 K, CH_4 promotes ignition. Large amounts of CH_4, however, inhibit C_2H_4 ignition at all T_(air)s. Ignition promotion was also investigated through the independent addtion of H_2 and F_2 in the reactant streams. H_2 addition (e.g., 2–10%) produces a two-stage ignition and sustains higher ignition strain rates. Small amounts of F_2 (1%) result in F-radical production, contributing to efficient fuel consumption, enhancing ignition characteristics. Ignition strain rates of σign≅4000 s^(−1), as compared to σ_(ign) ≅ 250 s^(−1) for pure C_2H_4, can be attained with such additives at lower temperatures (T_(air)=1050 K).https://authors.library.caltech.edu/records/v2x2y-e2380Shape Complexity in Turbulence
https://resolver.caltech.edu/CaltechAUTHORS:CATprl98
Authors: {'items': [{'id': 'Catrakis-H-J', 'name': {'family': 'Catrakis', 'given': 'Haris J.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 1998
DOI: 10.1103/PhysRevLett.80.968
The shape complexity of irregular surfaces is quantified by a dimensionless area-volume measure. A joint distribution of shape complexity and size is found for level-set islands and lakes in two-dimensional slices of the scalar field of liquid-phase turbulent jets, with complexity values increasing with size. A well-defined power law, over 3 decades in size (6 decades in area), is found for the shape complexity distribution. Such properties are important in various phenomena that rely on large area-volume ratios of surfaces or interfaces, such as turbulent mixing and combustion.https://authors.library.caltech.edu/records/faz3p-1ng55Turbulent shear-layer mixing at high Reynolds numbers: effects of inflow conditions
https://resolver.caltech.edu/CaltechAUTHORS:20120119-070649934
Authors: {'items': [{'id': 'Slessor-M-D', 'name': {'family': 'Slessor', 'given': 'M. D.'}}, {'id': 'Bond-C-L', 'name': {'family': 'Bond', 'given': 'C. L.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 1998
DOI: 10.1017/S0022112098002857
We report on the results from a set of incompressible, shear-layer flow experiments, at high Reynolds number (Re_δ ≡ ρΔUδ_T(x)/μ ≃ 2×10^5), in which the inflow conditions of shear-layer formation were varied (δ_T is the temperature-rise thickness for chemically-reacting shear layers). Both inert and chemically-reacting flows were investigated, the latter employing the (H_2+NO)/F_2 chemical system in the kinetically-fast regime to measure molecular mixing. Inflow conditions were varied by perturbing each, or both, boundary layers on the splitter plate separating the two freestream flows, upstream of shear-layer formation. The results of the chemically-reacting 'flip experiments' reveal that seemingly small changes in inflow conditions can have a significant influence not only on the large-scale structure and shear-layer growth rate, as had been documented previously, but also on molecular mixing and chemical-product formation, far downstream of the inflow region.https://authors.library.caltech.edu/records/7c9vs-tq914Effects of Additives on the Non-Premixed Ignition of Ethylene in Air
https://resolver.caltech.edu/CaltechAUTHORS:20160602-183442047
Authors: {'items': [{'id': 'Egolfopoulos-F-N', 'name': {'family': 'Egolfopoulos', 'given': 'Fokion N.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2000
DOI: 10.1080/00102200008947302
The ignition characteristics of heated C_2H_4
counterflowing against heated air were numerically investigated in the presence of additives such as NO, F_2, and H_2. C_2H_4 and air temperatures were chosen to resemble conditions relevant to high-Mach number, air-breathing propulsion. The numerical simulations were conducted along the stagnation streamline of the counterflow and included detailed descriptions
of chemical kinetics and molecular transport. It was found that addition of NO at concentrations of about 10,000 ppm (1%), results in a substantial increase of the
ignition strain rate, from 300 /s to values up to 32,000/s. This ignition promotion is caused by enhanced radical production that is initiated through the interaction
between NO and HO_2. A further increase in the NO amount leads to reduced improvements. Small additions of F_2
and H_2 were also found to promote ignition, but to lesser extent compared to NO. Results also show that with the addition of F_2 in the presence of NO, ignition promotion is further enhanced, and for F_2 and NO concentrations larger than 25,000 ppm, the system becomes hypergolic.
The present investigations suggest that the use of C_2H_4, NO, and F_2 may permit ignition at conditions of relevance to SCRAMJET's.https://authors.library.caltech.edu/records/wdhmk-wpg15The mixing transition in turbulent flows
https://resolver.caltech.edu/CaltechAUTHORS:DIMjfm00
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2000
DOI: 10.1017/S0022112099007946
Data on turbulent mixing and other turbulent-flow phenomena suggest that a (mixing) transition, originally documented to occur in shear layers, also occurs in jets, as well as in other flows and may be regarded as a universal phenomenon of turbulence. The resulting fully-developed turbulent flow requires an outer-scale Reynolds number of Re = U[delta]/v [greater, similar] 1–2 × 104, or a Taylor Reynolds number of ReT = u[prime prime or minute] [lambda]T/v [greater, similar] 100–140, to be sustained. A proposal based on the relative magnitude of dimensional spatial scales is offered to explain this behaviour.https://authors.library.caltech.edu/records/51hzf-dxw06Turbulent shear-layer mixing: growth-rate compressibility scaling
https://resolver.caltech.edu/CaltechAUTHORS:SLEjfm00a
Authors: {'items': [{'id': 'Slessor-M-D', 'name': {'family': 'Slessor', 'given': 'M. D.'}}, {'id': 'Zhuang-Mei', 'name': {'family': 'Zhuang', 'given': 'M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2000
DOI: 10.1017/S0022112099006977
A new shear-layer growth-rate compressibility-scaling parameter is proposed as an alternative to the total convective Mach number, Mc. This parameter derives from considerations of compressibility as a means of kinetic-to-thermal-energy conversion and can be significantly different from Mc for flows with far-from-unity free-stream-density and speed-of-sound ratios. Experimentally observed growth rates are well-represented by the new scaling.https://authors.library.caltech.edu/records/x1r20-zhd94Unseeded molecular flow tagging in cold and hot flows using ozone and hydroxyl tagging velocimetry
https://resolver.caltech.edu/CaltechAUTHORS:20111209-083545829
Authors: {'items': [{'id': 'Pitz-R-W', 'name': {'family': 'Pitz', 'given': 'Robert W.'}}, {'id': 'Wehrmeyer-J-A', 'name': {'family': 'Wehrmeyer', 'given': 'Joseph A.'}}, {'id': 'Rlbarov-L-A', 'name': {'family': 'Rlbarov', 'given': 'Lubomir A.'}}, {'id': 'Oguss-D-A', 'name': {'family': 'Oguss', 'given': 'Douglas A.'}}, {'id': 'Batliwala-F', 'name': {'family': 'Batliwala', 'given': 'Farrokn'}}, {'id': 'DeBarber-P-A', 'name': {'family': 'DeBarber', 'given': 'Peter A.'}}, {'id': 'Deusch-S', 'name': {'family': 'Deusch', 'given': 'Stefan'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2000
DOI: 10.1088/0957-0233/11/9/303
Two complementary unseeded molecular flow tagging techniques for gas-flow velocity field measurement at low and high temperature are demonstrated. Ozone tagging velocimetry (OTV) is applicable to low-temperature air flows whereas hydroxyl tagging velocimetry (HTV) is amenable to use in high-temperature reacting flows containing water vapour. In OTV, a grid of ozone lines is created by photodissociation of O_2 by a narrowband 193 nm ArF excimer laser. After a fixed time delay, the ozone grid is imaged with a narrowband KrF laser sheet that photodissociates the ozone and produces vibrationally excited O_2 that is subsequently made to fluoresce by the same KrF laser light sheet via the O_2 transition B^3Σ_u^-(v'=0,2) ← X^3Σ_g^-(v"=6,7). In HTV, a molecular grid of hydroxyl (OH) radicals is written into a flame by single-photon photodissociation of vibrationally excited H_2O by a 193 nm ArF excimer laser. After displacement, the OH tag line position is revealed through fluorescence caused by OH A^2Σ^+_-X^2Π (3←0) excitation using a 248 nm tunable KrF excimer laser. OTV and HTV use the same lasers and can simultaneously measure velocities in low and high temperature regions. Instantaneous flow-tagging grids are measured in air flows and a flame. The velocity field is extracted from OTV images in an air jet using the image correlation velocimetry (ICV) method.https://authors.library.caltech.edu/records/fg8wp-ezg75Temporal coherence of individual turbulent patterns in atmospheric seeing
https://resolver.caltech.edu/CaltechAUTHORS:KERao00
Authors: {'items': [{'id': 'Kern-Brian-Daniel', 'name': {'family': 'Kern', 'given': 'Brian'}}, {'id': 'Laurence-T-A', 'name': {'family': 'Laurence', 'given': 'Ted A.'}}, {'id': 'Martin-D-Christopher', 'name': {'family': 'Martin', 'given': 'Chris'}, 'orcid': '0000-0002-8650-1644'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2000
We used a variation of the generalized scidar (scintillation detection and ranging) technique to examine the temporal coherence of turbulent patterns at different altitudes in the atmosphere above Palomar Observatory. This enables us to test the validity of a frozen turbulence hypothesis in the local reference frame of the moving atmosphere. The data set analyzed here contains three turbulent patterns, each at a different altitude, which remain internally coherent over time scales of 0.28 0.41 s. This measurement is significant, because it is made on a 5-m aperture, allowing moving patterns to be tracked over time scales longer than their own lifetimes.https://authors.library.caltech.edu/records/zhk11-arn63A Comparative Numerical Study of Premixed and Non-Premixed Ethylene Flames
https://resolver.caltech.edu/CaltechAUTHORS:20160602-182617580
Authors: {'items': [{'id': 'Egolfopoulos-F-N', 'name': {'family': 'Egolfopoulos', 'given': 'Fokion N.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2001
DOI: 10.1080/00102200108952135
Detailed numerical simulations of premixed and non-premixed C_2H_4/air flames were conducted, using
six available kinetic mechanisms. The results
help assess differences between these mechanisms and
are of interest to proposed hydrocarbon-fueled SCRAMJET concepts, in which C_2H_4 can be expected to be a major component of the thermally cracked fuel. For premixed flames, laminar flame speeds were calculated and compared with available experimental data. For non-premixed flames,
ignition/extinction Z-curves were calculated for conditions of relevance to proposed SCRAMJET concepts. Results revealed a large variance in predictions of the kinetic mechanisms examined. Differences in laminar flame speeds as high as factors of 2.5 were found. For the conditions
investigated, computed ignition and extinction strain rates for non-premixed flames differed by factors as high as 300 and 3, respectively. This indicates that while there are differences in high-temperature kinetics that control flame propagation and extinction, discrepancies in low-temperature kinetics that control ignition can be even more significant. Sensitivity- and species-consumption
analyses indicate uncertainties in fuel kinetics and, most importantly, on the oxidation of C_2H_3 and
the production of CH_2CHO, whose kinetics are not well known and can crucially affect production of
the important H radicals. These findings stress the
need for experimental data in premixed and non-premixed configurations that can be used to assess these phenomena and provide the basis for a comprehensive validation.https://authors.library.caltech.edu/records/5yfaf-9fq90Flow structure and optical beam propagation in high-Reynolds-number gas-phase shear layers and jets
https://resolver.caltech.edu/CaltechAUTHORS:DIMjfm01
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Catrakis-H-J', 'name': {'family': 'Catrakis', 'given': 'H. J.'}}, {'id': 'Fourguette-D-C', 'name': {'family': 'Fourguette', 'given': 'D. C.'}}]}
Year: 2001
We report on the structure of the scalar index-of-refraction field generated by turbulent, gas-phase, incompressible and compressible shear layers and incompressible jets, and on associated beam-propagation aero-optical phenomena. Using simultaneous imaging of the optical-beam distortion and the turbulent-flow index-of-refraction field, wavefront-phase functions were computed for optical beams emerging from the turbulent region in these free-shear flows, in an aero-optical regime producing weak wavefront distortions. Spatial wavefront-phase behaviour is found to be dominated by the large-scale structure of these flows. A simple level-set representation of the index-of-refraction field in high-Reynolds-number, incompressible shear layers is found to provide a good representation of observed wavefront-phase behaviour, indicating that the structure of the unsteady outer boundaries of the turbulent region provides the dominant contributions.https://authors.library.caltech.edu/records/7j565-01n35Transition stages of Rayleigh–Taylor instability between miscible fluids
https://resolver.caltech.edu/CaltechAUTHORS:COOjfm01
Authors: {'items': [{'id': 'Cook-A-W', 'name': {'family': 'Cook', 'given': 'Andrew W.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2001
DOI: 10.1017/S0022112001005377
Direct numerical simulations (DNS) are presented of three-dimensional, Rayleigh–Taylor instability (RTI) between two incompressible, miscible fluids, with a 3:1 density ratio. Periodic boundary conditions are imposed in the horizontal directions of a rectangular domain, with no-slip top and bottom walls. Solutions are obtained for the Navier–Stokes equations, augmented by a species transport-diffusion equation, with various initial perturbations. The DNS achieved outer-scale Reynolds numbers, based on mixing-zone height and its rate of growth, in excess of 3000. Initial growth is diffusive and independent of the initial perturbations. The onset of nonlinear growth is not predicted by available linear-stability theory. Following the diffusive-growth stage, growth rates are found to depend on the initial perturbations, up to the end of the simulations. Mixing is found to be even more sensitive to initial conditions than growth rates. Taylor microscales and Reynolds numbers are anisotropic throughout the simulations. Improved collapse of many statistics is achieved if the height of the mixing zone, rather than time, is used as the scaling or progress variable. Mixing has dynamical consequences for this flow, since it is driven by the action of the imposed acceleration field on local density differences.https://authors.library.caltech.edu/records/8pg1r-fae06Unsplit algorithms for multidimensional systems of hyperbolic conservation laws with source terms
https://resolver.caltech.edu/CaltechAUTHORS:20160602-163756768
Authors: {'items': [{'id': 'Papalexandris-M-V', 'name': {'family': 'Papalexandris', 'given': 'M. V.'}}, {'id': 'Leonard-A', 'name': {'family': 'Leonard', 'given': 'A.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2002
DOI: 10.1016/S0898-1221(02)00128-1
This work describes an unsplit, second-order accurate algorithm for multidimensional systems of hyperbolic conservation laws with source terms, such as the compressible Euler equations for reacting flows. It is a MUSCL-type, shock-capturing scheme that integrates all terms of the governing equations simultaneously, in a single time-step, thus avoiding dimensional or time-splitting. Appropriate families of space-time manifolds are introduced, along which the conservation equations decouple to the characteristic equations of the corresponding 1-D homogeneous system. The local geometry of these manifolds depends on the source terms and the spatial derivatives of the flow variables. Numerical integration of the characteristic equations is performed along these manifolds in the upwinding part of the algorithm. Numerical simulations of two-dimensional detonations with simplified kinetics are performed to test the accuracy and robustness of the algorithm. These flows are unstable for a wide range of parameters and may exhibit chaotic behavior. Grid-convergence studies and comparisons with earlier results, obtained with traditional schemes, are presented.https://authors.library.caltech.edu/records/7y3jz-zna98Scalar concentration measurements in liquid-phase flows with pulsed lasers
https://resolver.caltech.edu/CaltechAUTHORS:20160602-163406315
Authors: {'items': [{'id': 'Shan-Jerry-W', 'name': {'family': 'Shan', 'given': 'Jerry W.'}}, {'id': 'Lang-D-B', 'name': {'family': 'Lang', 'given': 'Daniel B.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2004
DOI: 10.1007/s00348-003-0717-7
he suitability of pulsed lasers for laser-induced-fluorescence (LIF) measurements of the local concentration of scalars in liquid-phase flows is investigated. Experiments were performed to measure the fluorescence intensity of aqueous solutions of rhodamine-6G chloride excited by a Q-switched, frequency-doubled, Nd:YAG laser. The fluorescence intensity is found to be linear with dye concentration, but not with illumination power density. The fluorescence intensity saturates at laser power densities easily exceeded by short-pulse-duration lasers. A procedure for calibrating and normalizing the raw image data is discussed which relies only on weak absorption and the linearity of fluorescence intensity with concentration. This procedure enables quantitative concentration measurement with pulsed-laser-induced fluorescence.https://authors.library.caltech.edu/records/7ndxb-7tp39Particle streak velocimetry and CH laser-induced fluorescence diagnostics in strained, premixed, methane–air flames
https://resolver.caltech.edu/CaltechAUTHORS:20160602-182208042
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Goodwin-D-G', 'name': {'family': 'Goodwin', 'given': 'David G.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2005
DOI: 10.1016/j.proci.2004.08.105
We present the use of simultaneous particle streak velocimetry (PSV) and CH planar laser-induced fluorescence (PLIF) diagnostics in the study of planar, strained, premixed, methane–air flames, stabilized in a
jet-wall stagnation flow. Both PSV and PLIF data are imaged at high spatial resolution and sufficiently
high framing rates to permit an assessment of flame planarity and stability. Concurrent measurements
of mixture composition, (Bernoulli) static-pressure drop, and stagnation-plate temperature provide accurate boundary conditions for numerical simulations. The new PSV implementation is characterized by very
low particle loading, high accuracy, and permits short recording times. This PSV implementation and analysis methodology is validated through comparisons with previous laminar flame-speed data and detailed numerical simulations. The reported diagnostic suite facilitates the investigation of strained hydrocarbon–air flames, as a function of nozzle-plate separation to jet-diameter ratio,
L/d, and equivalence ratio, ɸ. Methane–air flames are simulated using a one-dimensional streamfunction approximation, with full chemistry (GRI-Mech 3.0), and multi-component transport. In general, we find good agreement between experiments and simulations if boundary conditions are specified from measured velocity fields. Methane–air flame strength appears to be slightly overpredicted, with the largest disagreements for lean flames.https://authors.library.caltech.edu/records/sh5sq-zne63Turbulent mixing
https://resolver.caltech.edu/CaltechAUTHORS:DIMarfm05
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2005
DOI: 10.1146/annurev.fluid.36.050802.122015
The ability of turbulent flows to effectively mix entrained fluids to a molecular scale is a vital part of the dynamics of such flows, with wide-ranging consequences in nature and engineering. It is a considerable experimental, theoretical, modeling, and computational challenge to capture and represent turbulent mixing which, for high Reynolds number (Re) flows, occurs across a spectrum of scales of considerable span. This consideration alone places high-Re mixing phenomena beyond the reach of direct simulation, especially in high Schmidt number fluids, such as water, in which species diffusion scales are one and a half orders of magnitude smaller than the smallest flow scales. The discussion below attempts to provide an overview of turbulent mixing; the attendant experimental, theoretical, and computational challenges; and suggests possible future directions for progress in this important field.https://authors.library.caltech.edu/records/pvcct-ydr21Impinging laminar jets at moderate Reynolds numbers and separation distances
https://resolver.caltech.edu/CaltechAUTHORS:BERpre05
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Sone-K', 'name': {'family': 'Sone', 'given': 'Kazuo'}}, {'id': 'Mattner-T-W', 'name': {'family': 'Mattner', 'given': 'Trent W.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Goodwin-D-G', 'name': {'family': 'Goodwin', 'given': 'David G.'}}, {'id': 'Meiron-D-I', 'name': {'family': 'Meiron', 'given': 'Dan I.'}, 'orcid': '0000-0003-0397-3775'}]}
Year: 2005
DOI: 10.1103/PhysRevE.72.066307
An experimental and numerical study of impinging, incompressible, axisymmetric, laminar jets is described, where the jet axis of symmetry is aligned normal to the wall. Particle streak velocimetry (PSV) is used to measure axial velocities along the centerline of the flow field. The jet-nozzle pressure drop is measured simultaneously and determines the Bernoulli velocity. The flow field is simulated numerically by an axisymmetric Navier-Stokes spectral-element code, an axisymmetric potential-flow model, and an axisymmetric one-dimensional stream-function approximation. The axisymmetric viscous and potential-flow simulations include the nozzle in the solution domain, allowing nozzle-wall proximity effects to be investigated. Scaling the centerline axial velocity by the Bernoulli velocity collapses the experimental velocity profiles onto a single curve that is independent of the nozzle-to-plate separation distance. Axisymmetric direct numerical simulations yield good agreement with experiment and confirm the velocity profile scaling. Potential-flow simulations reproduce the collapse of the data; however, viscous effects result in disagreement with experiment. Axisymmetric one-dimensional stream-function simulations can predict the flow in the stagnation region if the boundary conditions are correctly specified. The scaled axial velocity profiles are well characterized by an error function with one Reynolds-number-dependent parameter. Rescaling the wall-normal distance by the boundary-layer displacement-thickness-corrected diameter yields a collapse of the data onto a single curve that is independent of the Reynolds number. These scalings allow the specification of an analytical expression for the velocity profile of an impinging laminar jet over the Reynolds number range investigated of 200<=Re<=1400.https://authors.library.caltech.edu/records/e2x06-hwr02Imaging through turbulence with a quadrature-phase optical interferometer
https://resolver.caltech.edu/CaltechAUTHORS:KERao05
Authors: {'items': [{'id': 'Kern-B', 'name': {'family': 'Kern', 'given': 'Brian'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}, {'id': 'Martin-D-Christopher', 'name': {'family': 'Martin', 'given': 'Chris'}, 'orcid': '0000-0002-8650-1644'}, {'id': 'Lang-D-B', 'name': {'family': 'Lang', 'given': 'Daniel B.'}}, {'id': 'Thessin-R-N', 'name': {'family': 'Thessin', 'given': 'Rachel N.'}}]}
Year: 2005
DOI: 10.1364/AO.44.007424
We present an improved technique for imaging through turbulence at visible wavelengths using a rotation shearing pupil-plane interferometer, intended for astronomical and terrestrial imaging applications. While previous astronomical rotation shearing interferometers have made only visibility modulus measurements, this interferometer makes four simultaneous measurements on each interferometric baseline, with phase differences of π/2 between each measurement, allowing complex visibility measurements (modulus and phase) across the entire input pupil in a single exposure. This technique offers excellent wavefront resolution, allowing operation at visible wavelengths on large apertures, is potentially immune to amplitude fluctuations (scintillation), and may offer superior calibration capabilities to other imaging techniques. The interferometer has been tested in the laboratory under weakly aberrating conditions and at Palomar Observatory under ordinary astronomical observing conditions. This research is based partly on observations obtained at the Hale Telescope.https://authors.library.caltech.edu/records/0ffcm-5w434Planar shock cylindrical focusing by a perfect-gas lens
https://resolver.caltech.edu/CaltechAUTHORS:DIMpof06
Authors: {'items': [{'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Samtaney-Ravi', 'name': {'family': 'Samtaney', 'given': 'R.'}, 'orcid': '0000-0002-4702-6473'}]}
Year: 2006
DOI: 10.1063/1.2186553
We document a gas lensing technique that generates a converging shock wave in a two-dimensional wedge geometry. A successful design must satisfy three criteria at the contact point between the gas lens and the wedge leading edge to minimize nonlinear reflected and other wave effects. The result is a single-point solution in a multidimensional parameter space. The gas lens shape is computed using shock-polar analysis for regular refraction of the incident shock at the gas lens interface. For the range of parameters investigated, the required gas-lens interface is closely matched by an ellipse or hyperbola. Nonlinear Euler simulations confirm the analysis and that the transmitted shock is circular. As the converging transmitted shock propagates down the wedge, its shape remains nearly uniform with less than 0.1% peak departures from a perfect circular cylinder segment. Departure from the design criteria leads to converging shocks that depart from the required shape. The sensitivity to incident shock Mach number, as well as the qualitative effects of the presence of boundary layers are also discussed.https://authors.library.caltech.edu/records/bf4p4-r5n15Particle velocimetry in high-gradient/high-curvature flows
https://resolver.caltech.edu/CaltechAUTHORS:20110119-112708491
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'J. M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2006
DOI: 10.1007/s00348-006-0137-6
Particle-velocimetry techniques typically rely on the assumption that particle velocities match fluid velocities. However, this assumption may be invalid if external forces or inertia cause the particle motion to differ from that of the flow. In this paper, particle motion through premixed stagnation flames is modeled, including Stokes-drag and thermophoretic forces. The finite time interval employed in particle-tracking techniques can act as a low-pass filter in flow regions with
large curvature in the velocity field. To account for this
effect, the modeled-particle-tracking profile for a specified time interval is estimated from the predicted particle position in time and compared to measurements. The
implementation presented here is applicable to other
simulated flow fields and allows direct comparisons with
particle-velocimetry measurements. Expressions are also
derived that allow particle-tracking data to be corrected
for these effects.https://authors.library.caltech.edu/records/dms99-49s53Reynolds-number effects and anisotropy in transverse-jet mixing
https://resolver.caltech.edu/CaltechAUTHORS:SHAjfm06
Authors: {'items': [{'id': 'Shan-Jerry-W', 'name': {'family': 'Shan', 'given': 'Jerry W.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2006
DOI: 10.1017/S0022112006001224
Experiments are described which measured concentration fields in liquid-phase strong transverse jets over the Reynolds-number range 1.0×10^3 ≤ Rej ≤ 20×10^3. Laser-induced-fluorescence measurements were made of the jet-fluid-concentration fields at a jet-to-freestream velocity ratio of Vr =10. The concentration-field data for far-field (x/dj =50) slices of the jet show that turbulent mixing in the transverse jet is Reynolds number dependent over the range investigated, with a scalar-field PDF that evolves with Reynolds number. A growing peak in the PDF, indicating enhanced spatial homogenization of the jet-fluid concentration field, is found with increasing Reynolds number. Comparisons between transverse jets and jets discharging into quiescent reservoirs show that the transverse jet is an efficient mixer in that it entrains more fluid than the ordinary jet, yet is able to effectively mix and homogenize the additional entrained fluid. Analysis of the structure of the scalar field using distributions of scalar increments shows evidence for well-mixed plateaux separated by sharp cliffs in the jet-fluid concentration field, as previously shown in other flows. Furthermore, the
scalar field is found to be anisotropic, even at small length scales. Evidence for local anisotropy is seen in the scalar power spectra, scalar microscales, and PDFs of scalar increments in different directions. The scalar-field anisotropy is shown to be correlated to the vortex-induced large-scale strain field of the transverse jet. These experiments add to the existing evidence that the large and small scales of high-Schmidt-number turbulent mixing flows can be linked, with attendant consequences for the universality of small scales of the scalar field for Reynolds numbers up to at least Re=20×10^4.https://authors.library.caltech.edu/records/c9kp2-zzt83Premixed laminar C_1–C_2 stagnation flames: Experiments and simulations with detailed thermochemistry models
https://resolver.caltech.edu/CaltechAUTHORS:20160602-162343641
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2007
DOI: 10.1016/j.proci.2006.07.110
A generally accepted mechanism for the combustion of C_1 and C_2 hydrocarbons is still elusive. This paper discusses a technique that can further validate and constrain such mechanisms, towards the development of a comprehensive model for small hydrocarbon combustion. The approach relies on detailed measurements of strained premixed flames in a jet-wall stagnation flow. This geometry yields a flow with boundary conditions that can be reliably specified, facilitating simulation and detailed comparisons with experiment. The diagnostics are optimized for accuracy, minimal flame disturbance, and rapid simultaneous recording of velocity and CH radical profiles. Flame simulations rely on a one-dimensional hydrodynamic model, a multi-component transport formulation, and several detailed chemistry models. Direct comparisons between experiment and simulation allow for an assessment of the various models employed. Experimental data for methane, ethane, and ethylene flames are compared to numerical simulations using several thermochemistry models. GRI-Mech 3.0, a C_3 model by Davis et al. (DLW99), and two versions of the San Diego mechanism are utilized. While GRI-Mech 3.0 and the DLW99 models accurately predict experiment in some cases, the 2005/03/10 revision of the San Diego mechanism is found to give the best overall agreement with experiment for methane, ethane, and ethylene flames.https://authors.library.caltech.edu/records/371b1-74k22Verification of a fluid-dynamics solver using correlations with linear stability results
https://resolver.caltech.edu/CaltechAUTHORS:20160602-161905791
Authors: {'items': [{'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'G.'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Pantano-Carlos', 'name': {'family': 'Pantano', 'given': 'C.'}, 'orcid': '0000-0003-3971-2278'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2008
DOI: 10.1016/j.jcp.2008.01.055
A novel method is described for verification of fluid-dynamics solvers based on correlations with solutions from linear stability analysis. A difficulty with the linear stability analysis solutions for spatially developing flows is that flow fields typically exhibit exponentially growing features compromising the performance of classical error metrics. This motivates the construction of a projection-based metric that only assumes the shape of the solution and not the growth rate of the perturbations, thus also allowing the latter to be determined. The proposed correlation metric complements classical error metrics, such as p-norms, and can also be used for time-dependent problems with realistic boundary conditions. We demonstrate how the present method can be applied in the verification of an Euler solver for the instability behavior of laminar compressible free and confined shear layers.https://authors.library.caltech.edu/records/80j2y-2k488Mixing Measurements in a Supersonic Expansion-Ramp Combustor
https://resolver.caltech.edu/CaltechAUTHORS:20160602-161703534
Authors: {'items': [{'id': 'Bonanos-A-M', 'name': {'family': 'Bonanos', 'given': 'Aristides M.'}}, {'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2008
DOI: 10.1007/s10494-008-9133-7
This paper reports results on molecular mixing for injection via an expansion-ramp into a supersonic freestream with M_1 = 1.5. This geometry produces a compressible turbulent shear layer between an upper, high-speed "air" stream and a lower, low-speed "fuel" stream, injected through an expansion-ramp at α = 30° to the high-speed freestream. Mass injection is chosen to force the shear layer to attach to the lower guide wall. This results in part of the flow being directed upstream, forming a recirculation zone. Employing the hypergolic hydrogen-fluorine chemical reaction and pairs of "flip" experiments, molecular mixing is quantified by measuring the resulting temperature rise. Initial experiments established the fast-chemistry limit for this flow in terms of a Damköhler number (Da). For Da ≥ 1.4, molecularly mixed fluid effectively reacts to completion. Parameters varied in these experiments were the measurement station location, the injection velocity of the (lower) "fuel" stream, the stoichiometry for the flip experiments, and the density ratio of the fuel and air streams. As expected, mixing increases with increasing distance from the injection surface. The mixed fluid fraction increases by 12% when changing the fuel-to-air stream density ratio from 1 to 0.2. Comparisons with measurements at subsonic (high-speed) "air" stream velocities show that the trend of decreasing mixing with increasing speed documented in free-shear layer flows is also encountered in these flows. The current geometry produces higher mixing levels than do free shear layers.https://authors.library.caltech.edu/records/08smf-kef95Satellite remote sounding of mid-tropospheric CO_2
https://resolver.caltech.edu/CaltechAUTHORS:CHAgrl08
Authors: {'items': [{'id': 'Chahine-M-T', 'name': {'family': 'Chahine', 'given': 'M. T.'}}, {'id': 'Chen-Luke', 'name': {'family': 'Chen', 'given': 'Luke'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul'}}, {'id': 'Jiang-Xun', 'name': {'family': 'Jiang', 'given': 'Xun'}, 'orcid': '0000-0001-8932-3807'}, {'id': 'Li-Qinbi', 'name': {'family': 'Li', 'given': 'Qinbi'}}, {'id': 'Pagano-T-S', 'name': {'family': 'Pagano', 'given': 'Thomas'}}, {'id': 'Randerson-J-T', 'name': {'family': 'Randerson', 'given': 'James T.'}, 'orcid': '0000-0001-6559-7387'}, {'id': 'Yung-Y-L', 'name': {'family': 'Yung', 'given': 'Yuk L.'}, 'orcid': '0000-0002-4263-2562'}]}
Year: 2008
DOI: 10.1029/2008GL035022
Human activity has increased the concentration of the earth's atmospheric carbon dioxide, which plays a direct role in contributing to global warming. Mid-tropospheric CO_2 retrieved by the Atmospheric Infrared Sounder shows a substantial spatiotemporal variability that is supported by in situ aircraft measurements. The distribution of middle tropospheric CO_2 is strongly influenced by surface sources and large-scale circulations such as the mid-latitude jet streams and by synoptic weather systems, most notably in the summer hemisphere. In addition, the effects of stratosphere-troposphere exchange are observed during a final stratospheric warming event. The results provide the means to understand the sources and sinks and the lifting of CO_2 from surface layers into the free troposphere and its subsequent transport around the globe. These processes are not adequately represented in three chemistry-transport models that have been used to study carbon budgets.https://authors.library.caltech.edu/records/trkpy-epq28LES approach for high Reynolds number wall-bounded flows with application to turbulent channel flow
https://resolver.caltech.edu/CaltechAUTHORS:PANjcp08
Authors: {'items': [{'id': 'Pantano-Carlos', 'name': {'family': 'Pantano', 'given': 'C.'}, 'orcid': '0000-0003-3971-2278'}, {'id': 'Pullin-D-I', 'name': {'family': 'Pullin', 'given': 'D. I.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}, {'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'G.'}, 'orcid': '0000-0003-4024-4571'}]}
Year: 2008
DOI: 10.1016/j.jcp.2008.04.015
We describe a large-eddy simulation approach for turbulent channel flow using the stretched-vortex subgrid-scale model. The inner region of the turbulent boundary layer is not included in the modeling of this attached, wall-bounded flow. Appropriate boundary conditions and closure are derived using a combination of elements from asymptotic expansions, matching, and well-established wall-modeling approaches. The modeling approach for this application combines the stretched-vortex subgrid model with a localized wall-shear-stress treatment that relates the instantaneous wall-parallel velocity to the shear stress via the log-law, as appropriate for this (near-) zero pressure gradient flow. The impermeability boundary condition is built into the method such that only the outer-flow solution is simulated, obviating the need to impose the stiff no-slip condition at the wall. This formulation attempts to minimize numerical and modeling errors introduced by the boundary-condition treatment, while preserving the fundamental elements required to predict low-order statistics of these flows. We present simulation results for turbulent channel flow up to Reynolds number based oil the wall-friction velocity of 10^6. These compare favorably with results from large-scale DNS and experimental correlations.https://authors.library.caltech.edu/records/775va-2jy03Premixed laminar C_3H_8- and C_3H_6-air stagnation flames: experiments and simulations with detailed kinetic models
https://resolver.caltech.edu/CaltechAUTHORS:20090521-164859696
Authors: {'items': [{'id': 'Benezech-L-J', 'name': {'family': 'Benezech', 'given': 'Laurent J.'}}, {'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2009
DOI: 10.1016/j.proci.2008.06.117
A validation methodology based on the comparison of flame simulations relying on reacting-flow models with experiment is applied to C3 flames. The work reported targets the assessment of the modeled reactions and reaction rates relevant to C3-flame propagation in several detailed combustion kinetic models. A better knowledge of C3 hydrocarbon combustion chemistry is required before attempting to bridge the gap between the reasonably well-understood oxidation of C1–C2 hydrocarbons and the more complex chemistry of heavier hydrocarbons in a single kinetic model. Simultaneous measurements of velocity and CH-radical profiles were performed in atmospheric propane(C3H8)- and propylene(C3H6)-air laminar premixed stagnation flames stabilized in a jet-wall configuration. These nearly flat flames can be modeled by one-dimensional simulations, providing a means to validate kinetic models. Experimental data for these flames are compared to numerical simulations that rely on a 1D hydrodynamic model, a multi-component transport formulation including thermal diffusion, and five different detailed-chemistry models, in order to assess the adequacy of the models employed. The 2005/12 release of the San Diego mechanism is found to have the best overall performance. The logarithmic sensitivities of the simulated flame locations to variations in the kinetic rates are calculated via the "brute-force" method for six representative flames. The kinetic mechanisms used in the analysis are GRI-Mech 3.0 (for reference), the C3 mechanism by Davis-Law-Wang (1999), the latest release (2005/12) of the C1–C3 San Diego mechanism, the 2006/06 release of the C3–C4 unsaturated hydrocarbon mechanism by Battin-Leclerc et al., and Version 0.5 of the C1–C3 mechanism by Konnov (2000).https://authors.library.caltech.edu/records/47zr2-35567Molecular Mixing and Flowfield Measurements in a Recirculating Shear Flow. Part II: Supersonic Flow
https://resolver.caltech.edu/CaltechAUTHORS:20090828-231037589
Authors: {'items': [{'id': 'Bonanos-A-M', 'name': {'family': 'Bonanos', 'given': 'Aristides M.'}}, {'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2009
DOI: 10.1007/s10494-009-9199-x
Fundamental aspects of mixing between two gaseous streams in a complex geometry are studied and discussed. In the present paper, a supersonic top-stream is expanded over a 30° ramp, through which a secondary lower-stream is injected. The mass flux through the secondary stream is purposely insufficient to provide the entrainment requirements of the resulting shear layer, causing it to attach to the lower guidewall. Part of the shear layer fluid is directed upstream forming a recirculation zone, with enhanced mixing characteristics. The pressure coefficient of the device is quantified as a function of velocity ratio. The effect of heat release on the pressure coefficient is also reported. Molecular mixing was measured employing "flip" experiments based on the hypergolic hydrogen-fluorine chemical reaction. The amount of mixing for the expansion-ramp geometry is found to be higher than in classical free shear layers. However, as in free shear layers, the level of mixing decreases with increasing top-stream velocity. Results for a similar configuration with subsonic/transonic flow in the top stream are reported in Part I of this two-part series.https://authors.library.caltech.edu/records/kyg5n-xtk83Molecular Mixing and Flowfield Measurements in a Recirculating Shear Flow. Part I: Subsonic Flow
https://resolver.caltech.edu/CaltechAUTHORS:20090828-231037748
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Johnson-M-B', 'name': {'family': 'Johnson', 'given': 'Michael B.'}}, {'id': 'Bonanos-A-M', 'name': {'family': 'Bonanos', 'given': 'Aristides M.'}}, {'id': 'Slessor-M-D', 'name': {'family': 'Slessor', 'given': 'Michael'}}, {'id': 'Su-W-J', 'name': {'family': 'Su', 'given': 'Wei-Jen'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2009
DOI: 10.1007/s10494-009-9200-8
The mixing and flowfield of a complex geometry, similar to a rearward-facing step flow but with injection, is studied. A subsonic top-stream is expanded over a perforated ramp at an angle of 30°, through which a secondary stream is injected. The mass flux of the second stream is chosen to be insufficient to provide the entrainment requirements of the shear layer, which, as a consequence, attaches to the lower guidewall. Part of the flow is directed upstream forming a re-entrant jet within the recirculation zone that enhances mixing and flameholding. A control-volume model of the flow is found to be in good agreement with the variation of the overall pressure coefficient of the device with variable mass injection. The flowfield response to changing levels of heat release is also quantified. While increased heat release acts somewhat analogously to increased mass injection, fundamental differences in the flow behaviour are observed. The hypergolic hydrogen-fluorine chemical reaction employed allows the level of molecular mixing in the flow to be inferred. The amount of mixing is found to be higher in the expansion-ramp geometry than in classical free-shear layers. As in free-shear layers, the level of mixing is found to decrease with increasing top-stream velocity. Results for a similar configuration with supersonic flow in the top stream are reported in Part II of this two-part series.https://authors.library.caltech.edu/records/ccbc8-s1m38Shock focusing in a planar convergent geometry: experiment and simulation
https://resolver.caltech.edu/CaltechAUTHORS:20100209-094356527
Authors: {'items': [{'id': 'Bond-C', 'name': {'family': 'Bond', 'given': 'C.'}}, {'id': 'Hill-D-J', 'name': {'family': 'Hill', 'given': 'D. J.'}}, {'id': 'Meiron-D-I', 'name': {'family': 'Meiron', 'given': 'D. I.'}, 'orcid': '0000-0003-0397-3775'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2009
DOI: 10.1017/S0022112009991492
The behaviour of an initially planar shock wave propagating into a linearly convergent wedge is investigated experimentally and numerically. In the experiment, a 25° internal wedge is mounted asymmetrically in a pressure-driven shock tube. Shock waves with incident Mach numbers in the ranges of 1.4–1.6 and 2.4–2.6 are generated in nitrogen and carbon dioxide. During each run, the full pressure history is recorded at fourteen locations along the wedge faces and schlieren images are produced. Numerical simulations performed based on the compressible Euler equations are validated against the experiment. The simulations are then used as an additional tool in the investigation.
The linearly convergent geometry strengthens the incoming shock repeatedly, as waves reflected from the wedge faces cross the interior of the wedge. This investigation shows that aspects of this structure persist through multiple reflections and influence the nature of the shock-wave focusing. The shock focusing resulting from the distributed reflected waves of the Mach 1.5 case is distinctly different from the stepwise focusing at the higher incoming shock Mach number. Further experiments using CO_2 instead of N_2 elucidate some relevant real-gas effects and suggest that the presence or absence of a weak leading shock on the distributed reflections is not a controlling factor for focusing.https://authors.library.caltech.edu/records/4te8g-pjp86Large-eddy simulation of mixing in a recirculating shear flow
https://resolver.caltech.edu/CaltechAUTHORS:20100513-111738152
Authors: {'items': [{'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Bonanos-Aristides-M', 'name': {'family': 'Bonanos', 'given': 'Aristides'}, 'orcid': '0000-0003-2145-4354'}, {'id': 'Pantano-Carlos', 'name': {'family': 'Pantano', 'given': 'Carlos'}, 'orcid': '0000-0003-3971-2278'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2010
DOI: 10.1017/S0022112009992965
The flow field and mixing in an expansion-ramp geometry is studied using large-eddy
simulation (LES) with subgrid scale (SGS) modelling. The expansion-ramp geometry
was developed to investigate enhanced mixing and flameholding characteristics while
maintaining low total-pressure losses. Passive mixing was considered without taking
into account the effects of chemical reactions and heat release, an approximation
that is adequate for experiments conducted in parallel. The primary objective
of the current work is to validate the LES–SGS closure in the case of passive
turbulent mixing in a complex configuration and, if successful, to rely on numerical
simulation results for flow details unavailable via experiment. Total (resolved-scale
plus subgrid contribution) probability density functions (p.d.f.s) of the mixture fraction
are estimated using a presumed beta-distribution model for the subgrid field. Flow
and mixing statistics are in good agreement with the experimental measurements,
indicating that the mixing on a molecular scale is correctly predicted by the LES–
SGS model. Finally, statistics are shown to be resolution-independent by computing
the flow for three resolutions, at twice and four times the resolution of the coarsest
simulation.https://authors.library.caltech.edu/records/pvqa4-5s935Experimental investigation of converging shocks in water with various confinement materials
https://resolver.caltech.edu/CaltechAUTHORS:20101026-111445870
Authors: {'items': [{'id': 'Eliasson-Veronica', 'name': {'family': 'Eliasson', 'given': 'V.'}}, {'id': 'Mello-M', 'name': {'family': 'Mello', 'given': 'M.'}, 'orcid': '0000-0003-2129-9235'}, {'id': 'Rosakis-A-J', 'name': {'family': 'Rosakis', 'given': 'A. J.'}, 'orcid': '0000-0003-0559-0794'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'P. E.'}}]}
Year: 2010
DOI: 10.1007/s00193-010-0276-9
Fluid-solid coupling typically plays a negligible role in confined converging shocks in gases because of the rigidity of the surrounding material and large acoustic impedance mismatch of wave propagation between it and the gas. However, this is not true for converging shocks in a liquid. In the latter case, the coupling can not be ignored and properties of the surrounding material have a direct influence on wave propagation. In shock focusing in water confined in a solid convergent geometry, the shock in the liquid transmits to the solid and both transverse and longitudinal waves propagate in the solid. Shock focusing in water for three types of confinement materials has been studied experimentally with schlieren and photoelasticity optical techniques. A projectile from a gas gun impacts a liquid contained in a solid convergent geometry. The impact produces a shock wave in water that develops even higher pressure when focused in the vicinity of the apex. Depending on the confining material, the shock speed in the water can be slower, faster, or in between wave speeds in the solid. For solid materials with higher wave speeds than the shock in water, regions in the water is put in tension and cavitation occurs. Materials with slower wave speeds will deform easily.https://authors.library.caltech.edu/records/ffqs6-wcg77LES of an inclined sonic jet into a turbulent crossflow at Mach 3.6
https://resolver.caltech.edu/CaltechAUTHORS:20110118-091059381
Authors: {'items': [{'id': 'Ferrante-Antonino', 'name': {'family': 'Ferrante', 'given': 'Antonino'}, 'orcid': '0000-0002-5336-572X'}, {'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2011
DOI: 10.1080/14685248.2010.522580
We have performed large-eddy simulation with subgrid scale (LES-SGS) stretched-vortex model of an inclined sonic jet into a supersonic crossflow at Mach 3.6. The main flow features generated by the gas-dynamic interactions of the jet with the supersonic crossflow, such as barrel shock, shear layer, and counter-rotating vortex pair, are numerically captured by the employed LES-SGS. The transition and spatial development of the jet into a supersonic crossflow have been shown to be strongly dependent on the inflow conditions of the crossflow. This result indicates that correct turbulent inflow conditions are necessary to predict the main flow characteristics, dispersion and mixing of a gaseous jet in a supersonic, turbulent crossflow using LES-SGS. This work presents a methodology for the generation of realistic synthetic turbulent inflow conditions for LES of spatially developing, supersonic, turbulent, wall-bounded flows. The methodology is applied to the study of a supersonic turbulent flow over a flat wall interacting with an inclined jet. The effects of inflow conditions on the spatial development of the inclined jet are discussed, and then the results are compared with the available experimental data. Also, the dominant vortical structures generated by the jet/turbulent boundary layer interaction are identified as sheets, tilted tubes and discontinuous rings, and a visualization of their spatiotemporal development is provided. The identified vortical structures are shown to be enveloped by the helium mass-fraction isosurface, thus showing the important role of those structures in the dispersion of a gaseous jet in a supersonic crossflow.https://authors.library.caltech.edu/records/910gh-ben65Experiments and modelling of premixed laminar stagnation flame hydrodynamics
https://resolver.caltech.edu/CaltechAUTHORS:20110921-082924026
Authors: {'items': [{'id': 'Bergthorson-J-M', 'name': {'family': 'Bergthorson', 'given': 'Jeffrey M.'}}, {'id': 'Salusbury-S-D', 'name': {'family': 'Salusbury', 'given': 'Sean D.'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2011
DOI: 10.1017/jfm.2011.203
The hydrodynamics of a reacting impinging laminar jet, or stagnation flame, is studied experimentally and modelled using large activation energy asymptotic models and numerical simulations. The jet-wall geometry yields a stable, steady flame and allows for precise measurement and specification of all boundary conditions on the flow. Laser diagnostic techniques are used to measure velocity and CH radical profiles. The axial velocity profile through a premixed stagnation flame is found to be independent of the nozzle-to-wall separation distance at a fixed nozzle pressure drop, in accord with results for non-reacting impinging laminar jet flows, and thus the strain rate in these flames is only a function of the pressure drop across the nozzle. The relative agreement between the numerical simulations and experiment using a particular combustion chemistry model is found to be insensitive to both the strain rate imposed on the flame and the relative amounts of oxygen and nitrogen in the premixed gas, when the velocity boundary conditions on the simulations are applied in a manner consistent with the formulation of the streamfunction hydrodynamic model. The analytical model predicts unburned, or reference, flame speeds that are slightly lower than the detailed numerical simulations in all cases and the observed dependence of this reference flame speed on strain rate is stronger than that predicted by the model. Experiment and simulation are in excellent agreement for near-stoichiometric methane–air flames, but deviations are observed for ethylene flames with several of the combustion models used. The discrepancies between simulation and experimental profiles are quantified in terms of differences between measured and predicted reference flame speeds, or position of the CH-profile maxima, which are shown to be directly correlated. The direct comparison of the measured and simulated reference flame speeds, ΔS_u, can be used to infer the difference between the predicted flame speed of the combustion model employed and the true laminar flame speed of the mixture, ΔS^O_f, i.e. ΔS_u=ΔS^O_f, consistent with recently proposed nonlinear extrapolation techniques.https://authors.library.caltech.edu/records/5n370-rz233Scalar excursions in large-eddy simulations
https://resolver.caltech.edu/CaltechAUTHORS:20161116-160411835
Authors: {'items': [{'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2016
DOI: 10.1016/j.jcp.2016.08.035
The range of values of scalar fields in turbulent flows is bounded by their boundary values, for passive scalars, and by a combination of boundary values, reaction rates, phase changes, etc., for active scalars. The current investigation focuses on the local conservation of passive scalar concentration fields and the ability of the large-eddy simulation (LES) method to observe the boundedness of passive scalar concentrations. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. The present study characterizes passive-scalar excursions in LES of a shear flow and examines methods for diagnosis and assesment of the problem. The analysis of scalar-excursion statistics provides support of the main hypothesis of the current study that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) provides insufficient dissipation to produce a sufficiently smooth scalar field. In the LES runs three parameters are varied: the discretization of the convection terms, the SGS model, and grid resolution. Unphysical scalar excursions decrease as the order of accuracy of non-dissipative schemes is increased, but the improvement rate decreases with increasing order of accuracy. Two SGS models are examined, the stretched-vortex and a constant-coefficient Smagorinsky. Scalar excursions strongly depend on the SGS model. The excursions are significantly reduced when the characteristic SGS scale is set to double the grid spacing in runs with the stretched-vortex model. The maximum excursion and volume fraction of excursions outside boundary values show opposite trends with respect to resolution. The maximum unphysical excursions increase as resolution increases, whereas the volume fraction decreases. The reason for the increase in the maximum excursion is statistical and traceable to the number of grid points (sample size) which increases with resolution. In contrast, the volume fraction of unphysical excursions decreases with resolution because the SGS models explored perform better at higher grid resolution.https://authors.library.caltech.edu/records/cy5je-f9r52Wall-Modeled Large-Eddy Simulation of Autoignition-Dominated Supersonic Combustion
https://resolver.caltech.edu/CaltechAUTHORS:20170608-100338274
Authors: {'items': [{'id': 'Candler-G-V', 'name': {'family': 'Candler', 'given': 'Graham V.'}}, {'id': 'Cymbalist-N', 'name': {'family': 'Cymbalist', 'given': 'Niccolo'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2017
DOI: 10.2514/1.J055550
Simulations of combustion in high-speed and supersonic flows need to account for autoignition phenomena, compressibility, and the effects of intense turbulence. In the present work, the evolution-variable manifold framework of Cymbalist and Dimotakis ("On Autoignition-Dominated Supersonic Combustion," AIAA Paper 2015-2315, June 2015) is implemented in a computational fluid dynamics method, and Reynolds-averaged Navier–Stokes and wall-modeled large-eddy simulations are performed for a hydrogen–air combustion test case. As implemented here, the evolution-variable manifold approach solves a scalar conservation equation for a reaction-evolution variable that represents both the induction and subsequent oxidation phases of combustion. The detailed thermochemical state of the reacting fluid is tabulated as a low-dimensional manifold as a function of density, energy, mixture fraction, and the evolution variable. A numerical flux function consistent with local thermodynamic processes is developed, and the approach for coupling the computational fluid dynamics to the evolution-variable manifold table is discussed. Wall-modeled large-eddy simulations incorporating the evolution-variable manifold framework are found to be in good agreement with full chemical kinetics model simulations and the jet in supersonic crossflow hydrogen–air experiments of Gamba and Mungal ("Ignition, Flame Structure and Near-Wall Burning in Transverse Hydrogen Jets in Supersonic Crossflow," Journal of Fluid Mechanics, Vol. 780, Oct. 2015, pp. 226–273). In particular, the evolution-variable manifold approach captures both thin reaction fronts and distributed reaction-zone combustion that dominate high-speed turbulent combustion flows.https://authors.library.caltech.edu/records/3nxjs-e6s85Incompressible variable-density turbulence in an external acceleration field
https://resolver.caltech.edu/CaltechAUTHORS:20170911-101013585
Authors: {'items': [{'id': 'Gat-Ilana', 'name': {'family': 'Gat', 'given': 'Ilana'}, 'orcid': '0000-0003-0223-0507'}, {'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Chung-Daniel', 'name': {'family': 'Chung', 'given': 'Daniel'}, 'orcid': '0000-0003-3732-364X'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2017
DOI: 10.1017/jfm.2017.490
Dynamics and mixing of a variable-density turbulent flow subject to an externally imposed acceleration field in the zero-Mach-number limit are studied in a series of direct numerical simulations. The flow configuration studied consists of alternating slabs of high- and low-density fluid in a triply periodic domain. Density ratios in the range of 1.05 ⩽ R ≡ ρ_1/ρ_2 ⩽ 10 are investigated. The flow produces temporally evolving shear layers. A perpendicular density–pressure gradient is maintained in the mean as the flow evolves, with multi-scale baroclinic torques generated in the turbulent flow that ensues. For all density ratios studied, the simulations attain Reynolds numbers at the beginning of the fully developed turbulence regime. An empirical relation for the convection velocity predicts the observed entrainment-ratio and dominant mixed-fluid composition statistics. Two mixing-layer temporal evolution regimes are identified: an initial diffusion-dominated regime with a growth rate ~t^(1/2) followed by a turbulence-dominated regime with a growth rate ~t^3. In the turbulent regime, composition probability density functions within the shear layers exhibit a slightly tilted ('non-marching') hump, corresponding to the most probable mole fraction. The shear layers preferentially entrain low-density fluid by volume at all density ratios, which is reflected in the mixed-fluid composition.https://authors.library.caltech.edu/records/hj9ed-rdt84Mixing, scalar boundedness, and numerical dissipation in large-eddy simulations
https://resolver.caltech.edu/CaltechAUTHORS:20180509-105001995
Authors: {'items': [{'id': 'Sharan-Nek', 'name': {'family': 'Sharan', 'given': 'Nek'}, 'orcid': '0000-0002-7274-8232'}, {'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2018
DOI: 10.1016/j.jcp.2018.05.005
Numerical schemes for scalar transport and mixing in turbulent flows must be high-order accurate, and observe conservation and boundedness constraints. Discretization accuracy can be evaluated from the truncation error, and assessed by its dispersion and dissipation properties. Dispersion errors can cause violation of physical scalar bounds, whereas numerical dissipation is key to mitigating those violations. Numerical dissipation primarily alters the energy at small scales that are critical to turbulent mixing. Influence of additional dissipation on scalar mixing in large-eddy simulations (LES) of incompressible temporally evolving shear flow is examined in terms of the resolved passive-scalar field, z. Scalar fields in flows with different mixing behavior, exhibiting both uniform and non-uniform mixed-fluid composition across a shear layer, are compared for different grid resolutions, subgrid-scale models, and scalar-convection schemes. Scalar mixing is assessed based on resolved passive scalar probability density function (PDF), variance, and spectra. The numerical-dissipation influence on mixing is found to depend on the nature of the flow. Mixing metrics sensitive to numerical dissipation are applied to examine the performance of limiting methods employed to mitigate unphysical scalar excursions. Two approaches, using a linear-scaling limiter for finite-volume schemes and a monotonicity-preserving limiter for finite-difference schemes, are studied. Their performance with respect to accuracy, conservation, and boundedness is discussed.https://authors.library.caltech.edu/records/82c5c-etw20Simulating schlieren and shadowgraph images from LES data
https://resolver.caltech.edu/CaltechAUTHORS:20190726-104616679
Authors: {'items': [{'id': 'Luthman-E', 'name': {'family': 'Luthman', 'given': 'Elizabeth'}, 'orcid': '0000-0003-3225-8877'}, {'id': 'Cymbalist-N', 'name': {'family': 'Cymbalist', 'given': 'Niccolo'}}, {'id': 'Lang-Daniel', 'name': {'family': 'Lang', 'given': 'Daniel'}}, {'id': 'Candler-G-V', 'name': {'family': 'Candler', 'given': 'Graham'}}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul'}}]}
Year: 2019
DOI: 10.1007/s00348-019-2774-6
Geometrical optics ray-tracing is used to derive schlieren and shadowgraph images from large-eddy simulation (LES) data of a jet in supersonic crossflow and to compare with experimental data. Including the components of the optical system that forms the image in the simulation is found to be important. The technique produces images that replicate flow physics more faithfully than straight-line path integration and other techniques, and more efficiently than physical-optics techniques. Applications of these simulated images are demonstrated in supersonic flows. Time-correlated pairs of shadowgraph images taken from the LES using this technique are used in conjunction with an image-correlation velocimetry technique to compare the estimated convection velocity field in the LES to that of experiments of the same flow. Agreement between the two is good with a maximum variance of 5% by some metrics. This technique can aid in the validation of LES results, allowing quantitative comparison between experiment and simulation, and to extract information unattainable by experiment alone. Comparisons of simulated and experimental jet penetration into the supersonic freestream are also made.https://authors.library.caltech.edu/records/grdkj-d3b23Turbulent shear-layer mixing: initial conditions, and direct-numerical and large-eddy simulations
https://resolver.caltech.edu/CaltechAUTHORS:20190911-152302664
Authors: {'items': [{'id': 'Sharan-Nek', 'name': {'family': 'Sharan', 'given': 'Nek'}, 'orcid': '0000-0002-7274-8232'}, {'id': 'Matheou-Georgios', 'name': {'family': 'Matheou', 'given': 'Georgios'}, 'orcid': '0000-0003-4024-4571'}, {'id': 'Dimotakis-P-E', 'name': {'family': 'Dimotakis', 'given': 'Paul E.'}}]}
Year: 2019
DOI: 10.1017/jfm.2019.591
Aspects of turbulent shear-layer mixing are investigated over a range of shear-layer Reynolds numbers, Re_δ = ΔUδ/ν, based on the shear-layer free-stream velocity difference, ΔU, and mixing-zone thickness, δ, to probe the role of initial conditions in mixing stages and the evolution of the scalar-field probability density function (p.d.f.) and variance. Scalar transport is calculated for unity Schmidt numbers, approximating gas-phase diffusion. The study is based on direct-numerical simulation (DNS) and large-eddy simulation (LES), comparing different subgrid-scale (SGS) models for incompressible, uniform-density, temporally evolving forced shear-layer flows. Moderate-Reynolds-number DNS results help assess and validate LES SGS models in terms of scalar-spectrum and mixing estimates, as well as other metrics, to R_eδ ≲ 3.3×10^4. High-Reynolds-number LES investigations to R_eδ ≲ 5×10^5 help identify flow parameters and conditions that influence the evolution of scalar variance and p.d.f., e.g. marching versus non-marching. Initial conditions that generate shear flows with different mixing behaviour elucidate flow characteristics in each flow regime and identify elements that induce p.d.f. transition and scalar-variance behaviour. P.d.f. transition is found to be largely insensitive to local flow parameters, such as Re_δ, or a previously proposed vortex-pairing parameter based on downstream distance, or other equivalent criteria. The present study also allows a quantitative comparison of LES SGS models in moderate- and high-Re_δ forced shear-layer flows.https://authors.library.caltech.edu/records/rgpkf-4wt81