Disturbances In Hypersonic Boundary Layer Research Articles

Interaction and breakdown induced by multiple optimal disturbances in hypersonic boundary layer

The present paper finds that the coexistence of multiple primary instability waves may cause a non-trivial nonlinear interaction and breakdown process, which has not been reported before. In the considered Mach 6 flat-plate boundary layer, a global resolvent analysis reports three optimal disturbances (local maxima): a high-frequency planar wave, a low-frequency oblique wave and a stationary streak. For the dominant planar and oblique waves, a parabolised stability equation analysis identifies the initial non-modal transient growth and downstream modal growth. Initiated by these two optimal disturbances jointly, the complete linear and nonlinear instability processes until breakdown to turbulence are shown with direct numerical simulation. Owing to the transient growth, the oblique wave may be more significant than the planar wave in the breakdown. The oblique wave and scales of nonlinear interactions are pronounced in the outer layer, whose significance may not be comprehensively characterised by the wall pressure measurement. Fourier modes characterising the oblique-wave oblique breakdown, the planar-wave fundamental resonance, the planar-wave subharmonic resonance and the combination resonance related to a detuned mode are observed successively. The detuned mode seems to dominate the near-wall dynamics in the late nonlinear stage, characterised by $\varLambda$ -like structures. Meanwhile, the existence of this detuned mode is independent of the initial amplitude ratio and the absolute amplitude of the oblique and planar waves. Weakly nonlinear stability analyses demonstrate that the detuned mode is mainly a consequence of the secondary instability under the combination of planar and oblique primary waves. Wave vector plots reveal the resonant state of multiple triads. Energy budget and amplitude-correction analyses provide a clear physical image of energy transfer.

Experimental study of the influence of small angles of attack and cone nose bluntness on the stabilization of hypersonic boundary layer with passive porous coating

The present work was devoted to an experimental study of small angles of attack and cone nose bluntness on the efficiency of stabilization of high-frequency disturbances in hypersonic boundary layer with a passive porous coating. The experiments were carried out on a cone with an apex half-angle 7° and nose bluntness radii R = 0.03, 0.5 and 1 mm installed under angles of attack α = 0–1° at the freestream Mach number M∞ = 5.8. High-frequency pressure fluctuations on the sides of the cone with the solid and porous surfaces were measured. It is shown that at all angles of attack and cone bluntness, the passive porous coating permits an efficient suppression of disturbances in hypersonic boundary layer both on the windward and leeward sides of the cone.

A numerical investigation of the evolution of 2-D disturbances in hypersonic boundary layers and the effect on the flow structure due to the existence of shocklets

The evolution of 2-D disturbances in hypersonic boundary layer with Mach number 6,8, and 10 was investigated numerically by three different numerical schemes. At the entrance, second mode T-S waves with different amplitudes were introduced, and the relation between the Mach number and the amplitude of the disturbance when shocklets started to appear was investigated. By comparing the disturbance velocity profiles with those provided by linear stability theory, the effects of shocklets on flow structures were also investigated.

Development of Natural Disturbances in a Hypersonic Boundary Layer on a Sharp Cone

Experimental data on the location of the laminar—turbulent transition and development of natural disturbances in a laminar hypersonic boundary layer on a sharp thermally insulated cone with a half–angle of 7° are presented. The existence of the second mode of disturbances is confirmed. It is shown that the transition is determined by the first mode of disturbances. The experimental data are in good agreement with theoretical calculations.