Pressure fluctuations beneath instability wavepackets and turbulent spots in a hypersonic boundary layer

Abstract

To investigate the pressure-fluctuation field beneath turbulent spots in a hypersonic boundary layer, a study was conducted on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. Controlled disturbances were created by pulsed-glow perturbations based on the electrical breakdown of air. Under quiet-flow conditions, the nozzle-wall boundary layer remains laminar and grows very thick over the long nozzle length. This allows the development of large disturbances that can be well-resolved with high-frequency pressure transducers. A disturbance first grows into a second-mode instability wavepacket that is concentrated near its own centreline. Weaker disturbances are seen spreading from the centre. The waves grow and become nonlinear before breaking down to turbulence. The breakdown begins in the core of the packets where the wave amplitudes are largest. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction. The turbulent core grows downstream, resulting in a spot with a classical arrowhead shape. Behind the spot, a low-pressure calmed region develops. However, the spot is not merely a localized patch of turbulence; instability waves remain an integral part. Limited measurements of naturally occurring disturbances show many similar characteristics. From the controlled disturbance measurements, the convection velocity, spanwise spreading angle, and typical pressure-fluctuation field were obtained.

Effects of Compressibility and Shock-Wave Interactions on Turbulent Shear Flows

“…After being first observed in DNS, these structures have now also been observed in laboratory experiments by Casper et al (2014). With respect to the part of the growth mechanism that was affected by compressibility, Redford et al (2012) showed how it was the destabilisation of the surrounding boundary layer (Gad-El-Hak's second mechanism) that was affected by Mach number, with a much slower rate of generation of new structures at Mach 6 compared to Mach 3 in their simulations.…”

Transition in Hypersonic Boundary Layers: Role of Dilatational Waves

“…Experimental work comparing the linear growth of second mode with the results of the linear stability theory (LST) has been undertaken both in quiet [12,15,16] and noisy tunnels [13,17,18]. The nonlinear stages of the second-mode evolution and its relevance to turbulence development was investigated both by direct numerical simulations (DNS) [19][20][21][22][23] and experiments [24][25][26][27]. Issues related to the effect of the surface roughness have also been discussed [28][29][30].…”

Thermoacoustic Interpretation of Second-Mode Instability

“…For relatively sharp bodies at hypersonic speeds in particular, the transition is dominated by second-mode waves, or fast-slow modes [3], which are manifest as convectively amplified streamwise-propagating disturbances with a distinctive acoustic signature, typically in the ultrasonic range. Many numerical [4][5][6][7][8][9][10] and experimental [11][12][13][14][15][16][17][18][19][20] works have confirmed the existence of such instabilities.…”