Active and Passive Control of Flow Past a Cavity

Abstract

Flow past open cavities is well known to give rise to highly coherent and self-sustained oscillations, leading to undesirable aeroacoustic resonance. Cavity flows are encountered not only in engineering applications but also our daily life, for example, the weapon bays, landing gears and wheel wells of aircrafts, in the depressions of submarine and ship hulls, in the pantograph recess of high-speed train, in the sunroof of cars or in the closed side branches of pipelines. Periodic and intense aeroacoustic vibrations deriving from the self-sustained oscillations of cavity flows can give rise to structural fatigue, optical distortion and store separation problems, especially for high-speed aircrafts. For a typical open-cavity flow, the induced acoustic level exceeds 160dB at transonic Mach numbers (MacManus & Doran, 2008) and it still reaches approximately 130dB at around 100 ∼110km/h for passenger vehicles because the passenger compartment acts as a Helmholtz resonator (Gloerfelt, 2009). Cavity-like geometries are also observed in places such as urban canyons, rivers and lakes. For these environmental fields, cavity flows affect the mass transfer processes of various pollutants and chemical toxic substances that occur between the cavity and the main flow (Chang et al., 2006). In the last decade, open cavities have attracted many researchers engaged in scramjet engines with regard to mixing and flame-holding enhancement for supersonic combustion (Asai & Nishioka, 2003; Kim et al., 2004). Because of these issues across a wide range of applications, cavity flows have been of practical and academic interests for more than a half-century. The flow-induced oscillations in an open cavity arise from a feedback loop formed as a result of successive events that take place in sequence. Figure 1 illustrates the schematic of cavity flows with an acoustic resonance. A boundary layer of thickness δ separates at the upstream edge of the cavity of length L and depthD. The resulting separating shear layer is convectively unstable due to Kelvin-Helmholtz instability, and it soon rolls up into vortices. Every time the organized vortical structures collide the downstream corner, the expansion waves are radiated from the corner owing to the vorticity distortion at lowMach numbers (Yokoyama & Kato, 2009), while as Mach number increases, the compression waves are generated near the downstream corner, especially for supersonic flows (Nishioka et al., 2002). It should be noted that the hypersonic shear layers do not always roll up into isolated vortices, just forming wavy patterns. The strength of these induced waves is determined by the relative position of the traveling vortices and the downstream corner. Rockwell & Knisely (1978) classified the vortex-corner interactions into four possible events on the basis of flow visualizations: Complete Escape (CE), Partial Escape (PE), Partial Clipping (PC) and Complete Clipping (CP). The incident acoustic waves propagate inside the cavity towards the upstream corner and 17