Algebraic turbulence model simulations of supersonic open-cavity flow physics

Abstract

A time-accurate double thin-layer Navier-Stokes computation is performed for an unsteady supersonic open cavity with a length-to-depth ratio of 2. The results are used to determine the flow-physics mechanisms responsible for the cavity oscillation cycle. A new cycle is described and compared to previous descriptions. It is found that a shed vortex impinges on the cavity aft lip and forms a pressure pulse that augments or forces, at the vortex shedding frequency, an internal upstream moving wave that has been reflected from the aft corner. This upstream moving wave eventually reflects off the cavity forward wall and forces the shedding of a new vortex. It was found, however, that the reflected wave dissipates before it reaches the aft wall. Instead, a second wave forms beneath the shed vortex and eventually reflects from the aft corner and is forced at the shedding frequency by the shed vortex wave, completing the cycle. IntroductionT HE basic physical structure of cavity flowfields can be described as either closed, open, or transitional. 1 " 4 Closed cavities are typically long and shallow with a length-to-depth ratio (L/D) greater than 13. These are characterized by a shear layer that impinges on the cavity floor, producing two large recirculation regions. Closed cavities are associated with higher drag coefficients 5 " 8 and heat transfer properties 9 " 11 than those of open cavities; as such, they are less desirable. Open cavities are short and deep with an L/D < 10. They contain shear layers that span the cavity and are more typical of those found in aircraft applications. Open-cavity flowfields are remarkably complicated, with internal and external regions that are coupled via self-sustained shear-layer oscillations. Coherent shed vorticity, unsteady weak shock or pressure waves, and interactions between the shed vortices and the vortices that reside in the cavity also are present. Flowfield characteristics appear to depend primarily on the shape of the cavity and the Mach number, with Reynolds number effects considered to be less important. 12 ' 13 Several issues remain to be understood for open-cavity flowfields. Researchers appear to agree that an oscillating shear layer exists, that the primary and secondary vortices residing within the cavity are driven by the shear layer, that a mass breathing effect occurs within the cavity, and that pressure oscillations exist. However, the mechanisms driving this flowfield have not yet been agreed upon.Because of the unsteady nature of supersonic open-cavity flowfields, the measurement of field properties within the cavity is difficult experimentally. As such, surface properties, such as timeaveraged pressure and time-averaged sound-pressure level (SPL), are usually reported in the literature. Unsteady quantities typically are presented in terms of the spectral SPL. Recent experimental work 12 for subsonic to transonic open cavities provides a benchmark for comparing the effects of different Mach numbers, aspect ratios, flowfield dimensionality (i.e., two-and thr...

Experimental Investigation on the Mechanism of Flow-Type Changes in Supersonic Cavity Flows.

“…The flow field features boundary layer separation, shear layer instability, vortex flow, acoustics radiation, shock/expansion wave and shock-boundarylayer interactions, and self-sustained pressure oscillations. The co-presence of and interaction among these features in such a simple configuration and their potential hazardous effects on the performance, integrity, and stability of the vehicles present a challenging problem and have stimulated extensive experimental, analytical, 8,9,24,[37][38][39][40] and computational [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] investigations over the years. These studies largely concentrated on mean static-pressure distributions and/or unsteady-pressure spectra in cavities.…”

Visualization of wave propagation within a supersonic two-dimensional cavity by digital streak schlieren

“…Due to this interaction, the reflected wave loses most of its kinetic energy and is unable to disturb the shear layer at the front. The wave from the leading edge to the trailing edge is formed due to the large-scale structures generated in the shear layer at the leading edge which produce an acoustic field as mentioned in Tam et al (1996).…”

A computational investigation on the control of cavity-induced pressure oscillations using sub-cavity

“…This completes the formation of the feedback loop that is widely believed to be reason of cavity resonance. The reflected compression wave (RW) will be dissipated near the middle of the cavity according to Nishioka et al [5] and Tam et al [19]. Figures 4(a) shows a time history of static pressure at the position S1 inside the cavity.…”