Review—Self-Sustaining Oscillations of Flow Past Cavities
01 Jun 1978-Journal of Fluids Engineering-transactions of The Asme (American Society of Mechanical Engineers)-Vol. 100, Iss: 2, pp 152-165
About: This article is published in Journal of Fluids Engineering-transactions of The Asme.The article was published on 1978-06-01. It has received 885 citations till now.
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TL;DR: A hierarchy of computational approaches that range from semi-empirical schemes that estimate the noise sources using mean-flow and turbulence statistics, to high-fidelity unsteady flow simulations that resolve the sound generation process by direct application of the fundamental conservation principles is discussed in this paper.
520 citations
Cites background from "Review—Self-Sustaining Oscillations..."
...It has long been known that passive devices, such as spoilers and fences near the separation point, and trailing edge ramps can significantly reduce tone amplitudes [380]....
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...[380]), for example to account for the higher sound speed inside the cavity at higher Mach numbers....
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TL;DR: In this article, the authors used numerical simulations to investigate the resonant instabilities in two-dimensional flow past an open cavity and showed a transition from a shear-layer mode to a wake mode for longer cavities and higher Mach numbers.
Abstract: Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier–Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin–Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin–Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering.
495 citations
Cites background from "Review—Self-Sustaining Oscillations..."
...Note that cavity oscillations have been categorized by Rockwell & Naudascher (1978) into ‘fluid-dynamic’ oscillations, and ‘fluid-resonant’ oscillations, where the acoustic resonance of the cavity itself plays an important role, as in a flute or an organ pipe....
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...More details about cavity oscillations in general may be found in review articles (e.g. Rockwell & Naudascher 1978; Blake & Powell 1986); see also the recent work by Howe (1997) for very low Mach number cavity flows, and Crighton (1992) for edge tones....
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TL;DR: In this paper, the authors investigate the effect of the aft ramp angle on the character of the shear layer that spans the cavity in a nonreacting supersonic ship.
Abstract: Experimentalandcomputationalinvestigationsofthee owe eldassociatedwithseveralcavity-basede ameholders in a nonreacting supersonic e ow are described. All cavity e ows were of the open type, that is, length-to-depth ratio L/D<10. Two values of L/D were studied with several offset ratios (OR) and aft ramp angles µ. Results indicate that the aft ramp angle plays an important role in determining the character of the shear layer that spans the cavity. For a rectangular cavity with OR=1 and µ=90 deg, a compression wave forms as the e ow separates from the cavity’ s upstream corner. A strong recompression occurs at the aft wall, and the e ow is visibly unsteady. The pressure on the cavity fore wall decreases steadily and the recompression process occurs more gradually with decreasing aftrampangle.Higherdrag coefe cientsandshorterresidencetimesarefoundin cavitieswithshallower ramp angles.
398 citations
TL;DR: The effects of high-frequency open-loop forcing, fundamental limitations of feedback control for a given configuration of sensors and actuators, and the development of a feedback design methodology that respects the limited range of validity of the available dynamical models are discussed.
Abstract: We review recent advances in understanding, modeling, and controlling oscillations in the flow past a cavity. The fundamental mechanisms underlying cavity flow oscillations have been known for at least 40 years, but suppressing these oscillations in a reliable and robust way is still a challenge today. Interest in controlling the flow past a cavity is motivated by aerospace applications, but in addition, cavity flows provide an attractive canonical problem for exploring general flow control techniques. The focus is on recent advances in modeling these flows, and in controlling them, using both open-loop and closed-loop techniques. A relatively new perspective is that cavity oscillations may not always be self-sustained, but under some flow conditions may be lightly damped resonances, sustained by external disturbances such as boundary layer turbulence. Areas in which our understanding is incomplete, and which deserve further study, are discussed, in particular the effects of high-frequency open-loop forcing, fundamental limitations of feedback control for a given configuration of sensors and actuators, and the development of a feedback design methodology that respects the limited range of validity of the available dynamical models.
388 citations
TL;DR: In this article, an experimental investigation of flow over an axisymmetric cavity showed that self-sustained, periodic oscillations of the cavity shear layer are associated with low cavity drag.
Abstract: An experimental investigation of flow over an axisymmetric cavity shows that self-sustained, periodic oscillations of the cavity shear layer are associated with low cavity drag. In this low-drag mode the flow regulates itself to fix the mean-shear-layer stagnation point at the downstream corner. Above a critical value of the cavity width-to-depth ratio there is an abrupt and large increase of drag due to the onset of the ‘wake mode’ of instability. It is also shown by measurement of the momentum balance how the drag of the cavity is related to the state of the shear layer, as defined by the mean momentum transport $\rho\overline{u}\overline{v}$ and the Reynolds stress $\rho\overline{u^{\prime}v^{\prime}}$, and how these are related to the amplifying oscillations in the shear layer. The cavity shear layer is found to be different, in several respects, from a free shear layer.
361 citations