Feedback Mechanism in Supersonic Laminar Cavity Flows

Abstract

T HE resonance noise in supersonic cavity flows has been studied extensively, but the dominant mechanism and factors responsible for this phenomenon remain unclear [1]. In 1964, Rossiter [2] proposed that the resonance noise was a result of a feedback mechanism between the discrete vortices in the cavity shear layer and acoustic disturbances inside the cavity. The feedback mechanism consists of four key procedures: 1) the cavity shear layer at the leading edge is excited by acoustic disturbances, which lead to the generation of shedding vortices; 2) the shedding vortices grow as they convect to downstream, and eventually they impinge on the cavity trailing edge and pass the cavity; 3) a feedback compression wave is generated near the trailing edge due to the passage of large-scale vortices; and 4) the feedback compression wave will propagate upstream and excite the shear layer again. Then, the feedback cycle is closed. However, Heller and Bliss [3] stated that discrete vortices were not usually observed in supersonic cavity flows, and they pointed out that the periodic mass addition and removal were responsible for the acoustic radiation. Tam et al. [4] did numerical simulations and reported that the feedback compression waves were caused by the reflection of Mach waves at the bottom aft wall. Nishioka et al. [5] pointed out that the feedback compression waves were generated instantly when higher-speed fluid injection occurred near the cavity trailing edge. At Mach number of 5.0, the cavity oscillation frequencies are accurately predicted using simple “closed-box” acoustic theory [6]. For the supersonic laminar cavity flows, Krishnamurty [7] observed experimentally that laminar inflow produced louder resonance noise than that with turbulent inflow. In Heller et al.’s [8] study, no resonance was observed in the turbulent cavity flow at Mach number of 3.0; however, a strong resonant peak occurred in the laminar cavity flow. The adverse effects of cavity tones become more severe in supersonic laminar cavity flows, but few investigations are conducted to address the mechanism underlying self-sustained oscillations in the supersonic laminar cavity flows. The generation of resonance noise in supersonic laminar cavity flows might be driven by the feedback mechanism as described by Rossiter [2], but more efforts are required to put insight into the supersonic laminar cavity flows. The aim of the present study was to address the mechanism driving the self-sustained oscillations in supersonic laminar cavity flows. Implicit large eddy simulations (ILESs) are conducted. One typical feedback cycle is visualized with phase-averaged flowfields. The features of discrete vortices in the cavity shear layer are discussed.