NONLINEAR RECEPTIVITY TO OBLIQUE VORTICAL MODES IN FLOW PAST AN ELLIPTIC LEADING EDGE

Nonlinear boundary-layer receptivity to pairs of unsteady oblique freestream vortical modes is studied in direct numerical simulation of flow over a flat plate with an elliptic leading edge. The freestream is perturbed by three types of oblique Fourier modes, differing in the magnitude of the three vorticity components. The vortical modes excite steady boundary-layer streaks. The associated receptivity mechanism, described in detail, is quadratic in the forcing amplitude. Elliptic leading edges with two different aspect ratios are considered. We find that – and explain why – the streak amplitudes in nonlinear receptivity are largely unaffected by the leading-edge bluntness for the types of external disturbances studied. As linear receptivity is the predominant mechanism at low forcing frequencies, the nonlinear mechanism comes into play when high-frequency vortices are present in the freestream. Nonlinear receptivity is therefore expected to contribute to the excitation of boundary-layer streaks by freest...

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