CaltechAUTHORS: Book Chapter
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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 28 May 2024 19:48:44 -0700Shock Tubes in Rarefied Gas Flow Research
https://resolver.caltech.edu/CaltechAUTHORS:20170801-130148469
Year: 1968
In any real fluid motion there exists regions in space-time
in which the fluid is far from thermodynamic equilibrium.
The relative extent of these non-equilibrium regions is
determined by the ratio of the molecular relaxation times and the corresponding length scales to the macroscopic time and space scales appropriate to the flow. Gas flow within such non-equilibrium regions is properly called "rarefied". In recent years the shock tube has become a rather efficient tool in the investigations of rarefied gas flows and I intend to illustrate progress in this use through a discussion of some very recent and typical investigations of the GALCIT group carried out under NASA sponsorship.https://resolver.caltech.edu/CaltechAUTHORS:20170801-130148469Interaction Effects on the Drag of Bluff Bodies in Tandem
https://resolver.caltech.edu/CaltechAUTHORS:20170731-102517069
Year: 1978
DOI: 10.1007/978-1-4684-8434-2_10
The objective of this study is to obtain better understanding of the flow over two tandemly positioned bluff bodies in close enough proximity to strongly interact with each other. This interaction is often beneficial in that the drag of the overall system is reduced. Prototypes for this problem come from tractor-trailer and cab-van combinations, and from various add-on devices designed to reduce their drag.
The primary object of the present investigation is an axisymmetric configuration which seems to have first been studied by Saunders (1966). A disc of diameter d_1 is coaxially placed in front of a flat-faced cylinder of diameter d_2. For a given ratio d1/d2, there is a value of gap ratio, g*/d_2, for which the drag of the forebody system is a minimum. In the most optimum configuration, d_1/d_2 = 0.75, g*/d_2 = 0.375, and the corresponding forebody drag coefficient is 0.02, a remarkable reduction from the value of 0.75 for the cylinder alone. For each value of d_1/d_2, the minimum drag configuration, g*/d_2, appears to correspond to a minimum dissipation condition in which the separation stream surface just matches (joins tangentially onto) the rearbody. Support for this idea is furnished by comparison with some results derived from free-streamline theory and from flow visualization experiments. However, when g*/d_2 exceeds a critical value of about 0.5, the value of C_(Dmin) is almost an order of magnitude higher than for subcritical optimum gap ratios. The increase seems to be connected with the onset of cavity oscillations.
For non-axisymmetric geometry (square cross-sections) the separation surface cannot exactly match the rearbody and the subcritical minimum values of drag are higher than for circular cross-sections.https://resolver.caltech.edu/CaltechAUTHORS:20170731-102517069The plane mixing layer flow visualization results and three dimensional effects
https://resolver.caltech.edu/CaltechAUTHORS:20201007-081705336
Year: 1981
DOI: 10.1007/3-540-10289-2_27
The turbulent mixing layer between two streams of different velocities continues to play a central role in research aimed at improved understanding of turbulent shear flows in general, At present, not all researchers are in agreement as to what various experiments imply about the structure of mixing layers at high Reynolds number. The views which are held differ on the question as to how and to what extent three dimensionality develops in these flows and whether the Characteristic spanwise organized large vortex structures (rollers) continue to be a dominant feature. The traditional view, as extended to the contemporary scene, is that ultimately (i.e., sufficiently far downstream or at sufficiently high Reynolds number) the flow will be completely disorganized. The view put forward by "eddy chasers" is that such vortex structures are primary elements, characteristic of the underlying mean vorticity field, which is particularly simple for the mixing layer, and that, as long as the velocity difference is maintained, there is a mechanism to regenerate these primary structures by what, for convenience, may be called a Kelvin-Helmholtz instability, The heart of the controversy then is whether, or to what extent, secondary and higher instabilities will ultimately break down, completely disorganize or prevent formation of organized primary structures. In a plane mixing layer, the primary structures would, ideally, be two dimensional, containing the basic single component of vorticity while secondary and higher modes of instability would introduce three dimensionality and the other two components of vorticity into the flow. An interesting question follows: to what extent do such secondary instabilities change the properties (e.g., the growth rate; the Reynolds stress) that the mixing layer would have in ideal two dimensional development? In this paper we examine several aspects of this question and discuss some recent relevant experiments.https://resolver.caltech.edu/CaltechAUTHORS:20201007-081705336The Structure and Control of a Turbulent Reattaching Flow
https://resolver.caltech.edu/CaltechAUTHORS:20170731-100953029
Year: 1988
DOI: 10.1007/978-3-642-83281-9_34
An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp-edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge. It was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the "shedding" type instability of the entire bubble. A method of frequency scaling is proposed which correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.https://resolver.caltech.edu/CaltechAUTHORS:20170731-100953029Discussion on "phenomenological modeling: Present and future"
https://resolver.caltech.edu/CaltechAUTHORS:20141201-142720632
Year: 1990
DOI: 10.1007/3-540-52535-1_66
This afternoon all of the models that we have heard fall in the same
class; namely, local closures. First-order local closure (K-theory or
eddy diffusivity) models the momentum fluxes as down-gradient of
the mean momentum. The second-order local closure models the
third moments as down-gradient of the local second moments, or
local mean variables.
There is another completely different class of modeling or class of
closure, and that is non-local turbulence closure. I mentioned
before about the transilient matrix that describes the mixing
between different points separated a finite distance in space. One
can parameterize this matrix in terms of mean flow state or mean
flow instability. When you do that, you can then make forecasts of
the mean field in a turbulent flow that takes into account this nonlocal
mixing.
That has been done. For the ocean, we found results as good as
third-order local closure. For the atmosphere, results were as good
as second-order local closure. We've used it in three-dimensional
weather forecast models covering the whole United States. This is a
new concept of non-local closure, which is different from all the
other local closures.
When would you want to consider using a non-local kind of closure?
Well, if any of you are dealing with turbulent flow that has a
spectrum of eddy sizes where your greatest energy is in the largest
wavelengths, or if you are dealing with turbulent flow that has large
structures in it that are causing non-local mixing, then you might
want to consider a non-local turbulence closure.https://resolver.caltech.edu/CaltechAUTHORS:20141201-142720632Phenomenological modeling: Present and future. Comment 1
https://resolver.caltech.edu/CaltechAUTHORS:20141201-142123436
Year: 1990
DOI: 10.1007/3-540-52535-1_63
Professor Launder's position paper on phenomenological modelling is an impressive survey and valuable account of the status of second-moment closure, principally as applied to Reynolds Stresses. In this respect, it supplements and updates the monograph of Professor Rodi (1980), in which the emphasis is on the status of first-order closures, in particular the κ-ε model, as of 10 years ago.
Between them the two works provide an excellent reference source containing the equations; the rationale for modelling decisions that are made; tables of the constants that have been selected; displays of flow computations and their comparison with experimental measurements for a varied number of flows; and extensive reference lists.https://resolver.caltech.edu/CaltechAUTHORS:20141201-142123436Discussion on the utility of dynamical systems approach
https://resolver.caltech.edu/CaltechAUTHORS:20141201-140640640
Year: 1990
DOI: 10.1007/3-540-52535-1_50
I have a rather provocative question to ask the speakers. What I am wondering is:
When you apply the proper orthogonal decomposition - I know this is a procedure
you use and it is probably standard - you remove the mean flow and you only look at
the perturbations. Why do not you include the mean flow, because if you did you
would suppress the cubic terms, and also you would be getting something that would
be closer to what people call coherent structures, such as hairpin vortices.https://resolver.caltech.edu/CaltechAUTHORS:20141201-140640640Structure in the Near Field of the Transverse Jet
https://resolver.caltech.edu/CaltechAUTHORS:20170726-131022892
Year: 1991
DOI: 10.1007/978-3-642-76087-7_17
Photographs of a jet issuing from a wall into a crossflow display the four types of vortical structures which exist in the near field: namely, the jet shear layer vortices, the nascent far field vortex pair, the near-wall horseshoe vortices, and a system of vortices in the wake of the jet. It is shown that the wake vorticity is not "shed" from the jet but is formed from vorticity which originated in the wall boundary layer. The sources of vorticity for the other types of structures are also briefly discussed.https://resolver.caltech.edu/CaltechAUTHORS:20170726-131022892Panel Discussion: Direct Numerical Simulation or Experiments?
https://resolver.caltech.edu/CaltechAUTHORS:20141201-135146578
Year: 1991
DOI: 10.1007/978-1-4615-3750-2_30
I have been tapped as the moderator for this panel, and I hope that everybody
else is prepared to say a few things. I think that our job as panellists is to provoke a little discussion from the audience, and I would like to say a few words to get things started. First,
to clarify the subject of the discussion a little bit, I think that what is implied in the title by
"experiments" are laboratory experiments. I think that the direct numerical simulations are, in
fact, also experiments, and that we should be talking about using direct numerical simulation
in the same way that we use the laboratory, to produce flows that can be studied and brought
into the research picture.https://resolver.caltech.edu/CaltechAUTHORS:20141201-135146578The mixing transition in free shear flows
https://resolver.caltech.edu/CaltechAUTHORS:20141201-133432952
Year: 1991
DOI: 10.1007/978-1-4615-3750-2_1
The term "mixing transition" denotes an increase in molecular mixedness observed in a
shear How which has earlier experienced the conventional (momentum) transition from laminar
flow. First defined by Konrad (1976), from measurements of concentration in a free shear layer,
the transition has been described and measured by a number of other methods, in particular by
flow visualization, by measurement of chemical reaction product and by hot-wire anemometry, in
aqueous as well as gaseous flows. In this presentation we review some of the measurements
and try to assess what insight they may give on several questions that occur. 1. What is the
relation of the mixing transition to Reynolds number and to other events: the momentum transition;
vortex pairing; development of streamwise vortex structure? 2. How much does interfacial
area increase during the transition? 3. How fast docs this occur? 4. Docs chaotic advection
play a role? The answers arc tentative and incomplete.https://resolver.caltech.edu/CaltechAUTHORS:20141201-133432952Uses of flow visualization in research
https://resolver.caltech.edu/CaltechAUTHORS:20141201-132549656
Year: 1992
Flow visualization has important applications in engineering studies of complex flow fields and it is an
indispensable tool in fluid-mechanics research. Some examples of the role which it has played in research
discovery are discussed.https://resolver.caltech.edu/CaltechAUTHORS:20141201-132549656Instability and Turbulence in Shear Flows
https://resolver.caltech.edu/CaltechAUTHORS:20170731-142554277
Year: 1993
DOI: 10.1016/B978-0-444-88889-1.50008-X
Increasing attention is being paid to the large scale structure of turbulence and to the so-called "coherent" vortical structures which have been disclosed and studied for a number of turbulent shear flows in laboratory experiments and in numerical simulations. The coherent structures develop from the instability waves which create the flow; they portray the genesis of the turbulence in the primary instability of the global vorticity distribution. This is not in the sense of classical laminar instability, whichinitiates transition to turbulence, but as thedriving instability in the fully developed turbulence. That instability provides the link to the amplitude of the turbulent motion, which in classical turbulent modelling must be calibrated empirically as a fundamental step or steps in the closure of the Reynolds averaged equations of motion. It also rationalizes the dependence on various parameters, such as compressibility, clarifies response to external disturbances, and suggests the possibility of "turbulence control". The global instability is being incorporated into new models of turbulent shear flow development and the coherent structures form the basis for new, Lagrangian models of chemical mixing and reaction in these flows.https://resolver.caltech.edu/CaltechAUTHORS:20170731-142554277A Plating Method for the Construction of High-Precision Nozzles
https://resolver.caltech.edu/CaltechAUTHORS:20170901-105248315
Year: 1994
A research program to study the effects of density ratio and exit Mach number on the development of a compressible, axisymmetric jet has been undertaken, using different gases to change the density of the jet. However, each gas and Mach number combination requires a unique nozzle contour, and the manufacture of the planned number of nozzles by traditional methods would have been prohibitively expensive. A plating technique to inexpensively manufacture high precision nozzles was developed, and a prototype nozzle constructed. Data are presented which shows the flow quality to be as good as or better than the nozzles used by previous experimenters.https://resolver.caltech.edu/CaltechAUTHORS:20170901-105248315Flow visualization as a basic research tool
https://resolver.caltech.edu/CaltechAUTHORS:20141201-131151732
Year: 1995
In this presentation, we briefly describe some of the research projects in
GALCIT in which flow visualization played a central role.https://resolver.caltech.edu/CaltechAUTHORS:20141201-131151732Jets and mixing layers
https://resolver.caltech.edu/CaltechAUTHORS:20181119-155309490
Year: 2004
DOI: 10.1017/cbo9780511610820.002
Laser-induced fluorescence (LIF) diagnostics and highspeed, real-time digital image acquisition techniques are combined to map the composition field in a water mixing layer. A fluorescent dye, which is premixed with the lowspeed freestream fluid and dilutes by mixing with the highspeed fluid, is used to monitor the relative concentration of high-speed to low-speed fluid in the layer.
The three digital LIF pictures shown here were obtained by imaging the laser-induced fluorescence originating from a collimated argon ion laser beam, extending across the transverse dimension of the shear layer, onto a 512–element linear photodiode array. Each picture represents 384 contiguous scans, each at 400 points across the layer, for a total of 153 600 point measurements of concentration. The vertical axis maps onto 40 mm of the transverse coordinate of the shear layer, and the horizontal axis is time increasing from right to left for a total flow real time of 307 msec. The pseudocolor assignment is linear in the mixture fraction (ξ) and is arranged as follows: red-unmixed fluid from the low-speed stream (ξ=0); blue-unmixed fluid from the high-speed stream (ξ=1); and the rest of the spectrum corresponds to intermediate compositions.
Figures 1 and 2, a single vortex and pairing vortices, respectively, show the composition field before the mixing transition. The Reynolds number based on the local visual thickness of the layer and the velocity difference across the layer is Re=1750 with U_2/U_1=0.46 and U_1=13 cm/sec. Note the large excess of high-speed stream fluid in the cores of the structures.https://resolver.caltech.edu/CaltechAUTHORS:20181119-155309490Structure and Mixing in Turbulent Shear Flows
https://resolver.caltech.edu/CaltechAUTHORS:20141114-143226217
Year: 2014
The broad problem being addressed in our research is to identify and describe the primary vortical ("coherent", "organized") large structures in various turbulent shear flows; to determine how they contribute to the
mixing processes; and to make use of them in modelling and in possibly controlling or modifying those flows. Accumulating experimental evidence suggests that these primary vortical structures are different in different
shear flows. Conclusions which follow from these views are that (i) there cannot be a universal turbulence model for these different flows; and (ii) existence of such structures implies the possibility of their manipulation
or control and thus modification of the flow itself. These organized structures (and their response to excitation) are manifestations of instability response to natural or imposed disturbances and thus may be important
in cooperation with other, physical processes, e.g. , acoustic coupling, rate controlled chemistry, etc., in problems like combustion chamber instability.https://resolver.caltech.edu/CaltechAUTHORS:20141114-143226217