Self-sustained oscillations of turbulent flows over an open cavity

Abstract

Low Mach number turbulent flows over a cavity have been studied in a variety of parameters, Reynolds number ($Re_θ$), the length (L) and depth (D) of cavity and ratio of length to depth (L/D), in order to investigate quantitative characteristics of large-scale vortical structures responsible for self-sustained oscillations. Wind tunnel experiments with particle image velocimetry (PIV) were conducted in the range of 1< L/D < 4 when the incoming boundary layer is turbulent at $Re_θ$ =830 and 1810. The mechanism of self-sustained oscillations in laminar cavity flows has been well characterized; however, the occurrence of self-sustained oscillations in turbulent cavity flows has only previously been characterized by direct observation of flows. Here, the quantitative characteristics of vortical structures in turbulent flows over an open cavity were determined, and then statistical properties were examined for evidence of self-sustained oscillations. To measure the fluctuating pressure related to self-sustained oscillation over cavities, microphones were used, which were installed on the vertical face of the downstream corner. Although the energy spectra of wall pressure fluctuations showed no evidence of self-sustained oscillations, the formation of large-scale vortical structures was observed in the instantaneous velocity fields by applying Galilean decomposition and low-pass filtering. Statistical analyses were applied based on the instantaneous velocity fields of PIV data. The spatial distributions of vertical velocity correlations (v-v) showed alternating patterns that reflect the organized nature of the large-scale vortical structures. Self-sustained oscillation modes according to the inflow parameters and cavity geometries were determined by a modified version of Rossiter’s equation, which were consistent with the number of vortical structures existing between leading and trailing edge of the cavity. Furthermore the numbers of vortical structures determined i...