Optimal Control of Aeroacoustic Noise
Generated by Cylinder Vortex Interaction

S. Scott Collis
Department of Mechanical Engineering and Materials Science

Kaveh Ghayour
Department of Computational and Applied Mathematics and
Department of Mechanical Engineering and Materials Science

Matthias Heinkenschloss
Department of Computational and Applied Mathematics
Rice University

International Journal of Aeroacoustics, Vol. 1 (2002), No. 2, pp. 97-114.

Abstract

This paper presents an optimal control formulation and solution for an idealized Blade Vortex Interaction (BVI) problem. This problem consists of the interaction of an inviscid vortex pair with a circular cylinder in a steady Mach 0.3 uniform flow with wall-normal velocity used as control on the cylinder surface. This model problem captures the fundamental noise generation process of the BVI phenomena while mitigating many of the complexities of the full rotorcraft problem. As such, it serves as a stepping stone towards optimal control of realistic BVI flows. The optimal control problem is solved using a gradient based method where gradient information is computed from a continuous adjoint analysis of the governing unsteady Euler equations. The BVI wave packet is targeted by defining an objective function that measures the square amplitude of pressure fluctuations in an observation region over a time interval of interest. The computed control actuation reduces BVI noise to 6% of its uncontrolled value which is a reduction in sound pressure level of 12db. The optimal control, unlike most common mitigation methods, does not target the interaction directly - instead, the computed boundary control, when processed by the potential flow about the cylinder, produces a wave packet of the correct amplitude and phase to approximately cancel the BVI noise in the observation region.

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