摘要:

Single‐ and double‐pulsed visualizations were employed to study the characteristics and evolution of large‐scale structures in compressible mixing layers with convective Mach numbers (Mc) of 0.51 and 0.86. Instantaneous images and spatial correlations based on large ensembles of images show that large‐scale structures which span the entire mixing layer thickness are a more dominant feature atMc=0.51 than atMc=0.86. Double‐pulsed images in theMc=0.51 developing region show the structures’ formation to proceed as the roll‐up of a wavy mixing region, in agreement with the Kelvin–Helmholtz picture of structure formation derived from studies of incompressible mixing layers. However, little indication of roll‐up was present forMc=0.86. In theMc=0.51 fully developed region, the large‐scale structures evolve similarly to those of incompressible mixing layers, even undergoing pairing processes. Similar structure evolution processes were not encountered inMc=0.86; instead, it was more difficult to even track large‐scale motions between the initial and delayed images. Convective velocities derived from space‐time correlations of ensembles of double‐pulsed images are in good agreement with the theoretical convective velocity at the center of the mixing layer, but are higher than the theoretical value toward the high‐speed side of the mixing layer and lower toward the low‐speed side. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868609

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

As part of an investigation on hypersonic boundary layer transition, the first successful instantaneous hot‐wire and fluctuating pressure measurements have been conducted in the Ludwieg‐tube facility of DLR at a free‐stream Mach number ofM∞=5. The disturbance amplitudes [root‐mean‐square (RMS) values] of the fluctuations of Pitot pressure and mass flow are smaller than the values measured in conventional, i.e., continuously driven or blowdown, hypersonic wind tunnels. Hot‐wire measurements in the laminar boundary layer on a flat plate in the same wind tunnel at Mach 5 did not show the expected dominance of high‐frequency second mode instabilities, predicted by linear stability theory and found in other experiments. On the other hand, the growth of natural disturbances within a broad frequency band up to 50 kHz could be observed on the flat plate. The measured amplification rates are in good agreement with theoretical results for three‐dimensional first mode disturbances. The theoretical investigations were based on a local and nonlocal instability analyses of similarity profiles for flat plate boundary layer flow atM∞=5. Whereas the local analysis is limited to parallel model flows, nonlocal analysis additionally takes into account the nonparallel basic flow and the nonuniform disturbance flow evolution. Surprisingly, the first rather than second mode instability wave growth is found to be dominant in the Ludwieg‐tube experiment. This result deviates from measurements obtained in other wind tunnels. It indicates that not only transition locations but also instability wave growth for hypersonic boundary layer flows are strongly dependent on the disturbance environment of the individual wind tunnel. Due to the very low initial, high‐frequency disturbance amplitude level in the free stream of the Ludwieg‐tube, second mode instabilities may not grow to significant values or not be present at all. As such a free‐stream disturbance spectrum can also be expected under free‐flight conditions, the Ludwieg tube may be a suitable wind tunnel to adequately simulate boundary layer flow transition along smooth surface, hypersonic flight vehicles. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868610

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities are considered in a fluid layer of thicknessthaving perturbations of arbitrary wave numberkat either one or both interfaces. The evolution of the perturbation amplitudes &eegr;1,2(&tgr;), &tgr; =time, is given analytically in terms of a coupling angle &thgr; which measures the strength of the coupling between interfaces 1 and 2. A new type of freeze‐out in shocked layers is reported according to which, the proximity of the two interfaces can, under proper conditions, lead to the complete freeze‐out of one, but not both, perturbations. For example, to freeze the first interface one needs &eegr;1(0)/&eegr;2(0)= sin &thgr;. Freeze‐out cannot be achieved in the RT case; instead, one can kill one of the modes. For example, setting &eegr;1(0)/&eegr;2(0)= tan(&thgr;/2) will kill the exponentially growing mode, leaving only the oscillatory mode at both interfaces. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868611

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In an attempt to assess the suggestion that submerged gas injection can under certain conditions be described as exhibiting chaotic dynamics, a short analysis has been undertaken on the results of a set of experiments over a limited set of conditions including the variation of chamber volume, injection nozzle diameter, liquid viscosity, and gas flow rate at a temperature of 20 °C. A convenient noninvasive approach is demonstrated using the upstream pressure signal to reconstruct the return map attractors. The classic period doubling sequence leading to irregular behavior is observed only over certain system parameter ranges. The important parameters believed to be responsible for the deterministic chaotic behavior and their relationship are briefly discussed. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868612

出版商:AIP    

年代:1995

数据来源: AIP