On the pressure oscillations inside a deep cavity excited by a grazing airflow

An experimental and theoretical study of the pressure oscillations generated by the flow over a deep cavity is presented. Such a configuration, which is akin to a Helmholtz resonator, arises in many applications, for instance when a window or the sunroof of an automobile remains open. The linear resonator model is fully validated by experiments. The linear stability characteristics of the free shear layer in the neck of the cavity are retrieved from neck wall pressure measurements. An efficient sound reduction scheme is proposed, which is based on the use of piezo-electric actuators placed upstream of the neck. These elements act as small discrete flaps which force the shear layer in the neck to oscillate at a frequency distinct from the cavity resonance frequency. A quasi complete attenuation of the peak pressure may then be achieved. The classical linear stability analysis of the free shear layer is successful in accounting for the experimental observations and it leads to the identification of the physical mechanism responsible for the efficiency of the sound reduction scheme. Moreover linear stability theory yields limitations to the efficiency of the technique in the form of an energy criterion involving the Strouhal number. (C) 2003 Elsevier SAS. All rights reserved.