Closed-loop feedback control of bluff body aeroacoustics

Abstract

Searching for a unified methodology for controlling the aeroacoustics of common fluid-structure interaction has been a major topic of interest among aeroacoustics research community. The presence of fluid-structure interaction obstructs and disturbs the flow of fluid, which results in different types of developing flow instabilities and unsteadiness according to the geometries of discontinuities. This interaction can lead to undesirable sound, vibration and aerodynamics forces acting on the structure, causing its damage or fatigue. Through investigation on canonical flows such as a flow past bluff body, provides fundamental concepts governing the aeroacoustic behaviours. Despite its simple geometry, studying such flows is a building block to provide further insight into the modelling and control of more complicated flows. In this study of a flow past a square cylinder at Reynolds number ReL = 200 and Mach number Ma = 0.2, the phenomenon of vortex shedding and acoustic generation mechanism are examined numerically. A low-dissipation, highly accurate direct aeroacoustic simulation (DAS) solver has been employed to resolve the fully compressible Navier-Stokes equations for the two-dimensional simulation of square cylinder flow at low and high Reynolds numbers. The capability and accuracy of the employed DAS solver are demonstrated. Based on the flow realizations of the DAS solutions, the proper orthogonal decomposition (POD) and dynamic mode decomposition method (DMD) has been employed to determine the minimum degrees of freedom required to represent the flow and acoustics field at high resolution. For the current non-linear flow problem, the dominant POD modes are retained as the POD bases and the Galerkin procedure is adopted to project the compressible Navier-Stokes equations onto this low-dimensional space, resulting in a reduced order model (ROM), thereby simplifying the problem into a finite-dimensional non-linear dynamical system in time. The long-time stability of the ROM to calculate periodic solution is evaluated and alternatively a calibration technique to stabilise the reduced order model is employed. Prior to the control design, a control strategy has been developed to reduce the square cylinder aeroacoustics by manipulating the vortex shedding. A review of the popular actuation technologies for low-to-moderate speed flow control applications is offered and the blowing/suction actuator is focused in the present study. The actuation configurations are determined from the DMD modes and the effect of different control parameters on the square cylinder aeroacoustics is investigated. The calibrated ROM is then implemented into the closed-loop control as control plant function. A ROM based, full-state feedback control is implemented to reduce the noise generated from the square cylinder using blowing/suction actuation. The control algorithm is successfully simulated using the DAS solver and an 8.76 dB reduction is achieved in terms of the oveall acoustics power.