Abstract
In this study direct numerical simulations are employed to investigate the effects of various parameters on the laminar-flow-control capabilities of narrowly spaced streaks in a supersonic boundary layer at Mach$2.0$. Previous work by Sharmaet al.(J. Fluid Mech., vol. 873, 2019, pp. 1072–1089) has found these streak modes, excited by a spanwise blowing-and-suction strip, to be highly effective at delaying pure oblique-type breakdown. In the present work it is shown that spectrum-enriching subharmonic modes, relevant with increasing running-length Reynolds number, do not destroy the controlling mechanism, and also a complex breakdown scenario, triggered by a multi-frequency point source, is found to be effectively controlled. Moreover, the control-streak excitation by roughness elements is compared in detail with the blowing-and-suction method, revealing relevant differing features.
Highlights
The aerodynamic properties of a body moving at supersonic velocities are considerably impacted by the laminar-to-turbulent transition of the flow around it
This mean-flow distortion (MFD) was extracted from a precursor direct numerical simulation (DNS), in which only the control mode (0, 5) was forced
Streaks with five times the wavenumber of the fundamental oblique breakdown mode (1, 1), i.e. (0, 5), have been introduced using either blowing and suction or surface roughness elements to control the breakdown in various scenarios
Summary
The aerodynamic properties of a body moving at supersonic velocities are considerably impacted by the laminar-to-turbulent transition of the flow around it. In order to have a beneficial effect on the transition scenario, forced control streaks need to have a significantly higher spanwise wavenumber than the naturally occurring streak mode, generated by the oblique waves. Due to the growing boundary-layer thickness, the amplified frequencies and wavelengths decrease with downstream distance This natural cascade could lead to a multiple oblique breakdown scenario combined with several subharmonic resonance triads at different downstream locations. At this point it is not clear if the control streaks, excited at low running length and decaying in streamwise direction, can effectively control a natural transition process extending over a relatively long distance, and how sensitive the control technique is to a more realistic disturbance background. The excitation of control streaks by a spanwise surface-roughness row is investigated
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