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.

/pdf/on-self-sustained-oscillations-in-two-dimensional-2hggpd6hyj.pdf

On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities

https://www.win.tue.nl/~sjoerdr/papers/boek.pdf

An introduction to acoustics

http://cwrowley.princeton.edu/papers/Cattafesta-PAS08.pdf

Active control of flow-induced cavity oscillations

▪ Abstract Interaction of a vortex, or combinations of them, with a cylinder, blade, or foil may involve both rapid distortion of the incident vorticity field and shedding of vorticity from the surface of the body. This review focuses on the underlying flow physics, with the aim of clarifying the origin of the induced loading. In the case of near or direct encounter of the incident vortex, the relation between three-dimensional features of the flow structure and the local loading poses interesting challenges for further research. With recently developed simulation and laboratory techniques, opportunities now exist to determine the instantaneous quantitative structure of these complex distortions and to interpret them within a theoretical, vorticity-based framework. Effective control of vortex interactions appears to be attainable. It will be necessary, however, to develop creative strategies, distinct from those traditionally employed for control of unstable shear flows.

Vortex-body interactions

A review of active control of flow-induced cavity oscillations is presented. This paper is motivated by two factors. First, the search for solutions to the practical problem of suppressing oscillations caused by flow over open cavities has generated significant interest in this area. Second, cavity oscillation control serves as a model problem in the growing multidisciplinary field of flow control. As such, we attempt to summarize recent activities in this area, with emphasis on experimental implementation of open- and closed-loop control approaches. In addition to describing successes, failures, and outstanding issues relevant to cavity oscillations, we highlight the characteristics of the various actuators, flow sensing and measurement, and control methodologies employed to date in order to emphasize the choices, challenges, and potential of flow control in this and other applications.

/pdf/review-of-active-control-of-flow-induced-cavity-resonance-25do643ubs.pdf

Review of Active Control of Flow-Induced Cavity Resonance

Self-sustained aero-acoustic pulsations in gas transport systems: Experimental study of the influence of closed side branches

High amplitude vortex-induced pulsations in a gas transport system

A theoretical analysis is presented of the low frequency aero-acoustic behavior of closed side branches along a gas transport pipe. The theory predicts the hydrodynamic conditions for moderate and strong pulsations. A model is proposed which predicts the order of magnitude of the power generated by the aero-acoustic source. The theoretical analysis leads to the design of spoilers which reduce the pulsation level by 30 to 40 dB. The results obtained by theoretical analysis and model experiments (Reynolds number 10-6) have been confirmed in full scale tests (Reynolds number 10-8).

Flow Induced Pulsations in Gas Transport Systems: Analysis of the Influence of Closed Side Branches

The 'whistler nozzle' and horn as aero-acoustic sound sources in pipe systems

Low-Mach-number gas flow in a pipe-line network with side branches closed at one end has induced severe pressure pUlsations in compressor facilities of the gas transport system in The Netherlands. The present investigation considers the case of self sustained resonance in a duct with a side-branch cavity in order to find design rules to avoid flow induced resonance in complex pipe-line networks. The influence of flow rate, side-branch length and acoustic properties of the pipe system is determined experimentally. Resonance occurs for discrete combinations of hydrodynamic and acoustic phase conditions, analogous to those found for a deep cavity on a solid wall in free space. Pressure pUlsations of large amplitude are obtained only for frequencies fitting the pipe geometry as a whole. While linear stability theory can be used to predict mean flow conditions for which self sustained resonance may occur, under resonant conditions the shear layer instability is essentially non linear, resulting in a saturation of the sound source. This opens the possibility of predicting the influence of the acoustical properties of the pipe system on the amplitude of the acoustic field with a semi-empirical model.

J.C. Bruggeman

Papers

Self-sustained aero-acoustic pulsations in gas transport systems: Experimental study of the influence of closed side branches

High amplitude vortex-induced pulsations in a gas transport system

Flow Induced Pulsations in Gas Transport Systems: Analysis of the Influence of Closed Side Branches

The 'whistler nozzle' and horn as aero-acoustic sound sources in pipe systems

Self Sustained Low Frequency Resonance in Low-Mach-Number Gas Flow Through Pipe-Lines with Side-Branch Cavities: A Semi-Empirical Model