Abstract
A two-dimensional direct numerical simulation (DNS) of receptivity of a flat-plate boundary layer to temperature spottiness in Mach 6 free stream is carried out. The influence of spottiness parameters to the receptivity process is studied. It is shown that the temperature spots propagating near the upper boundary-layer edge generate mode F. Further downstream mode F is synchronized with unstable mode S (Mack second mode) and excites the latter via the inter-modal exchange mechanism. A theoretical model describing the excitation of mode F by the temperature spots is developed using the biorthogonal eigenfunction decomposition method. The DNS results agree with the theoretical predictions. If the temperature spots are initiated in the free stream and pass through the bow shock, the dominant receptivity mechanism is different. The spot-shock interaction leads to excitation of acoustic waves, which penetrate into the boundary layer and excite mode S. Numerical simulations shows that this mechanism provides the instability amplitudes an order of magnitude higher than in the case of receptivity to the temperature spots themselves.
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