Abstract: This research investigates the effects of spanwise and streamwise coherent structures in a turbulent mixing layer on the deflection of a thin light beam which is transmitting transversely through the mixing layer from the high-speed side to the low speed side. Both equal and unequal density mixing layers of varying pressures and velocities are studied, using a lateral effect detector to dynamically track the motion of a He-Ne laser beam. Beam deflections in the streamwise direction are found to be associated mainly with the spanwise coherent structures; at low Reynolds Numbers the beam deflection is directly related to the part of a spanwise structure through which the beam passes. Maximum deflections are associated with the trailing edge of the spanwise coherent structures. Spanwise deflections are caused mainly by the streamwise coherent structures and as such exhibit large variations across the span of the flow. With the development of the streamwise structures, spanwise deflections are found to exceed streamwise deflections. Mixing transition, as scaled using the momentum thickness of the high-speed side, is found to cause a peak in therms fluctuations of both the streamwise and spanwise deflections.

ID: CaltechAUTHORS:20141210-134738343

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Abstract: The effects of a periodic disturbance, applied to the one of the the free streams, on large-scale structure and mixing processes in chemically reacting shear layers and wakes were investigated over a range of Reynolds numbers above and below the mixing transition range. Two different methods were employed to measure the amount of chemical product and thus the extent of molecular-scale mixing. Absorption by reacted phenolphthalein provided cross-stream average product thickness and laser induced fluorescence intensity provided the product concentration distribution. These methods provided, in addition, effective flow visualization of the large-scale structures and of their response to the periodic forcing. The detailed effects of periodic forcing on distribution of mixing along a free shear layer are complex, but the predominant, overall effect is to increase mixing at low Reynolds number (in respect to the mixing transition) and to decrease it at high Reynolds number.

ID: CaltechAUTHORS:20141210-133626177

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Abstract: The interaction of an unseparated supersonic turbulent boundary layer with a compression corner produces an extremely rapid rise in pressure at the corner, followed by a more gradual increase to the final pressure. In this paper, the flow in the corner region is analyzed by an integral method with the objective of predicting the initial pressure rise. Comparisons with experimental pressure rise data are presented for cases covering supersonic and hypersonic flows of practical interest. Also presented are some calculations and comparisons of downstream pressure distributions obtained by using the predicted corner rise as the first point in an existing inviscid method.

ID: CaltechAUTHORS:20141222-110906638

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Abstract: For many years experimental research in turbulence was devoted to the measurement of various correlations and special functions which had evolved from the statistical theories and from engineering computing methods based on the hierarchy of Reynolds equations. A recent change in direction toward a more deterministic description of turbulent structure has been initiated by the discovery of large coherent structures in several turbulent shear flows. The new point of view suggests that with every shear flow (jet, boundary layer, mixing layer, etc.) is associated an identifiable, characteristic structure; the development of the flow is controlled by the interactions of these structures with each other. An understanding of their properties should give insight into actual physical processes in turbulent flows, such as entrainment, transport, mixing, noise production, gustiness, etc. and should lead to improved methods for analyzing and computing them. Experiments designed to study these properties are aided by recent developments in instrumentation technology such as computer-aided control of the experiments, but the venerable technique of flow visualization is still an indispensable aid.

ID: CaltechAUTHORS:20141222-105100548

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Abstract: Experimental results are presented for the effects of Mach number, Reynolds number, and corner angle on flare-induced separation of a supersonic, turbulent boundary layer. In particular, measurements were obtained for the variation with flare angle, α, of the ratio ℓ_0/δ_0 of the upstream interaction length to the boundary-layer thickness at the beginning of the interaction for Mach numbers 2≤M≤4.5, boundary-layer thickness Reynolds numbers 10^5 < R_δ < 10^6, and adiabatic wall conditions. The model consisted of a hollow cylinder of 12-in. diameter and 51-in. length. Flares of angle 9°≤α≤40° were attached to the cylinder model at either of two location, viz., at x_c= 14 or 28 in. downstream from the sharp leading edge. Measurements consisted chiefly of surface-pressure distributions. Profiles for the undisturbed (flare-off) boundary-layer were also obtained. By varying the several parameters upstream interaction lengths as large as ℓ_0/δ_0 = 30 were observed. It was found that ℓ_0/δ_0 decreases with increasing Mach number and Reynolds number and, of course, increases with flare angle. It was also found that, for constant α, when ℓ_0/δ_0 is plotted vs the local skin-friction coefficient, C_(f0), the Mach-number dependence disappears. From this observation, a simple correlation formula was obtained and used to compare results from other investigations, and also to correlate incipient separation data. The present results complement the incipient-separation data obtained previously by us in the next higher decade of Reynolds number and further confirm the trends established there. It was also found that, for large α, the separated region upstream of the flare has free-interaction characteristics similar to those of upstream-facing steps at high Reynolds number.

ID: CaltechAUTHORS:20141223-095148400

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Abstract: An experimental investigation of the steady, laminar near-wake flow field of a two-dimensional, adiabatic, circular cylinder with surface mass transfer has been made at a free-stream Mach number of 6.0. The pressure and mass-concentration fields associated with the transfer of argon, nitrogen or helium into the near wake were studied for mass transfer from the forward stagnation region, and from the base. For sufficiently low mass transfer rates from the base, for which a recirculating zone exists, the entire near-wake flow field correlates with the momentum flux, not the mass flux, of the injectant, and the mass-concentration field is determined by counter-current diffusion into the reversed flow. For mass addition from the forward stagnation region, the pressure field is undisturbed and the mass-concentration field is nearly uniform in the region of reversed flow. The axial decay of argon mass concentration in the intermediate wake, downstream of the neck, is explained with the aid of an integral solution in the incompressible plane, from which the location of the virtual origin for the asymptotic far-wake solution has been derived as one result.

ID: CaltechAUTHORS:20141210-132533513

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