Transition in Hypersonic Boundary Layers: Role of Dilatational Waves (original) (raw)

Review of supersonic boundary layer instability and transition models

2012

Extensive research has been carried out to study the stability and transition of incompressible boundary layers, but the compressible counterpart has not received adequate attention. The effect of compressibi lity on the structure of boundary layer needs study. The present effort is to study the stability behaviour of the supersonic and hypersonic boundary layers in the presence of adverse pressure gradients and free stream turbulence. Compressible boundary layers have dominantly 2-D instabilities of Tollmein3chlichting (TS) waves in the early transition, which couple with subharmonics in resonance and nonresonance mechani sms . Dominant transttton structures are the A and hairpin vortices (3-D) in staggered (peak-valley) type and commonly occurring with large amplitudes or in aligned pattern when growth rate is supressed by external forcing, implicitly (complaint coatings) or explicitly (suction, favourable pressure gradients, cooling, etc. ). The aim of the present study is to see...

Aerodynamic Heating in Hypersonic Boundary Layers:\ Role of Dilatational Waves

arXiv: Fluid Dynamics, 2016

The evolution of multi-mode instabilities in a hypersonic boundary layer and their effects on aerodynamic heating are investigated. Experiments are conducted in a Mach 6 wind tunnel using Rayleigh-scattering flow visualization, fast-response pressure sensors, fluorescent temperature-sensitive paint (TSP), and particle image velocimetry (PIV). Calculations are also performed based on both parabolized stability equations (PSE) and direct numerical simulations (DNS). It is found that second-mode dilatational waves, accompanied by high-frequency alternating fluid compression and expansion, produce intense aerodynamic heating in a small region that rapidly heats the fluid passing through it. As a result, the surface temperature rapidly increases and results in an overshoot over the nominal transitional value. When the dilatation waves decay downstream, the surface temperature decreases gradually until transition is completed. A theoretical analysis is provided to interpret the temperatur...

Density effects on turbulent boundary layer structure: from the atmosphere to hypersonic flow

his dissertation examines the e↵ects of density gradients on turbulent boundary layer statistics and structure using Particle Image Velocimetry (PIV). Two distinct cases were examined: the thermally stable atmospheric surface layer characteristic of nocturnal or po- lar conditions, and the hypersonic bounder layer characteristic of high speed aircraft and reentering spacecraft. Previous experimental studies examining the e↵ects of stability on turbulent boundary layers identified two regimes, weak and strong stability, separated by a critical bulk stratifi- cation with a collapse of near-wall turbulence thought to be intrinsic to the strongly stable regime. To examine the characteristics of these two regimes, PIV measurements were ob- tained in conjunction with the mean temperature profile in a low Reynolds number facility over smooth and rough surfaces. The turbulent stresses were found to scale with the wall shear stress in the weakly stable regime prior relaminarization at a critical stratification. Changes in profile shape were shown to correlate with the local stratification profile, and as a result, the collapse of near-wall turbulence is not intrinsic to the strongly stable regime. The critical bulk stratification was found to be sensitive to surface roughness and potentially Reynolds number, and not constant as previously thought. Further investigations examined turbulent boundary layer structure and changes to the motions that contribute to turbulent production. To study the characteristics of a hypersonic turbulent boundary layer at Mach 8, sig- nificant improvements were required to the implementation and error characterization of PIV. Limited resolution or dynamic range e↵ects were minimized and the e↵ects of high shear on cross-correlation routines were examined. Significantly, an examination of parti- cle dynamics, subject to fluid inertia,compressibility and non-continuum e↵ects, revealed that particle frequency responses to turbulence can be up to an order of magnitude smaller than estimates made using a standard shock response test. The e↵ect of over-large tripping devices was also found to increase the wake strength of the mean velocity profile as well as freestream turbulence. A final assessment of the data reveals that Morkovin scaling col- lapses the streamwise turbulence profiles with DNS at the same Mach number. Wall-normal turbulence measurements remain compromised by limited particle frequency response.

Pressure fluctuations induced by a hypersonic turbulent boundary layer

Journal of Fluid Mechanics, 2016

Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analysed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean-velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except i...