TRUE SOURCES OF LINEAR SOUND IN PLANE COUETTE FLOW

The sound generation by pure vortex mode disturbances in an two-dimensional (2D) unbounded inviscid plane Couette flow is investigated. We present results by Kelvin-mode analysis as well as numerical simulations of the Euler equations, while focusing on the dynamics in the spectral plane. Our results show a dominance of the anisotropic linear sound generation in subsonic shear flows by vortices inside the boundaries of rapid distortion theory (RDT). The linearly generated, highly directional field is comparable to the hydrodynamic field, which physical headstone is the mode coupling, induced by the nonnormality in shear flow systems at moderate shear rates of the velocity. Comparisons of the classical acoustic analogy (AA) approach by Lighthill (1952) with the herein presented results identify the inability of AAs to capture the shear-induced anisotropy of the generated waves in the spectral plane.