Subsonic Cavity Flow Research Articles

The convexity for compressible subsonic cavity flows

AbstractIn this paper, we first show that the flow velocity takes its maximum on the free boundary, provided that the nozzle wall and obstacle satisfy some corresponding geometric assumptions. Second, the convexity of the free boundary to the compressible subsonic cavity flows will be established. Finally, the optimal regularity of the free boundary at the detachment point is obtained.

Subsonic cavity flow control with Micro-Magneto-Mechanical Systems (MMMS) microvalves

Flow control consists in modifying a flow natural state in order to converge towards another state which is considered as favorable, as drag or noise radiation might be reduced. In this paper, open-loop flow control experiments are carried out on a subsonic open-cavity flow. In the case of unstable flow control, the control focus is brought onto the flow fluctuations modifications rather than modification of the mean flow properties. Therefore, the forcing flexibility using arbitrary signals and the forcing linearity are essential for such flow control cases. In that sense, a linear array of Micro Magneto-Electro-Mechanical Systems actuators has been implemented to perform open-loop flow control experiments on an open-cavity. The actuators are able to generate both quasi-steady and pulsed jets with linear behavior. We proved the microvalves efficiency to damp the cavity oscillations. The quasi-steady jets reached a reduction of 20 dB in the cavity fundamental amplitude sound pressure level. Pulsed jets enabled an additional cavity tone amplitude reduction, which depends on the pulsating frequency and on the forcing amplitude. These results are a first step towards the implementation of the closed-loop control of the open-cavity flow.

Data-driven and physical property-based hydro-acoustic mode decomposition

A data-driven and physical property-based hydrodynamic and acoustic mode decomposition method combining dynamic mode decomposition and Helmholtz decomposition is proposed. It allows decomposition and fast prediction of hydrodynamic and acoustic components of the flow field. The method is tested by a two-dimensional subsonic open cavity flow and a supersonic cold jet, and the hydrodynamic and acoustic features are revealed. For the cavity flow, it is found that the acoustic velocity inside the cavity is composed of several pairs of standing waves. The propagating trajectory of the acoustic waves in the cavity is well captured. The dynamic relation between the hydrodynamic and acoustic motion is investigated. For the supersonic jet, the method successfully identifies the screech in the far field and the “trapped wave” within the potential core.

Waves model of three-dimensional cavity flow and its oscillation mode evolution

The high-speed flow passing through an open cavity will generate complex wave structures. The propagation and evolution of these waves can lead to the self-sustained oscillation of the cavity flow and cause strong noise. The cavity noise may contain multiple acoustic modes with discrete frequencies in the spectrum. A clear understanding of the evolution of the oscillation mode will provide a theoretical basis for the study of the noise control method. By analyzing the waves scattering process at both ends of the cavity at subsonic speed and supersonic speed and considering the three-dimensional spanwise flow, the three-dimensional wave model for subsonic cavity flow and supersonic cavity flow are established respectively. The model involves the nonlinear interaction between different waves in the cavity, which may produce other components different from the Rossiter mode. Based on the pressure signal data measured from the experiments on cavity flow for Mach numbers 0.9 and 1.5, the parameters in the model are linearly estimated. The pressure signals are analyzed by using FFT, bispectral analysis, and continuous wavelet transform. The results show that there are nonlinear interactions between the main oscillation modes, thus producing strong harmonics. The mode-switch phenomenon is observed in both the subsonic case and the supersonic case. The mode-switching exhibits low-frequency behavior and shows randomness as a whole.

Spatiotemporal correlation and control of wall pressure fluctuations induced by subsonic cavity flows

Strong wall pressure fluctuations are generated mostly by a high-speed flow passing over a cavity and should be accurately predicted and controlled for the structural design of aircraft. In this work, a spatiotemporal analysis was performed to estimate wall pressure fluctuations and reveal the mechanism of flow control by a sawtooth-like vortex generator installed on the leading edge or by the use of a rounded trailing edge in suppressing wall pressure fluctuations. Both the cavity tones and overall sound pressure level were analyzed under different flow speeds and flow control approaches. To investigate the power transfer in the temporal and spatial domains, power spectral density contours were drawn and discussed. For the mode separation, the phased array technique with an improved beamforming algorithm was adopted to calculate the wavenumber map and then to separate the acoustic modes from the convective modes by utilizing the spatial correlation of the wall pressure fluctuations. The mode separation results show that the convective modes contribute mainly to the low-frequency pressure fluctuations, while the acoustic modes contribute to both the low-frequency and high-frequency pressure fluctuations. Flow control by the rounded trailing edge approach is more effective than that by the serrated structure approach in reducing the cavity acoustic environment, which enables wall pressure fluctuations to transfer from the high-frequency range to the low-frequency range compared with the “no control” case.

An implicit kinetic inviscid flux for predicting continuum flows in all speed regimes

In this study, the kinetic inviscid flux (KIF) is improved and coupled with an implicit strategy. The KIF is a recently proposed numerical method, which is a dynamic combination of the kinetic flux vector splitting (KFVS) method and the totally thermalized transport (TTT) method. The inherent microscopic mechanism of the KFVS makes the KIF good at solving shock waves and avoiding the numerical shock instability phenomenon. When developing the implicit KIF, it is noticed that, in boundary layers, the KFVS part of the KIF not only reduces the accuracy but also seriously reduces the Courant–Friedrichs–Lewy (CFL) number. As a result, a new weight is proposed in this paper to combine the KFVS method with the TTT method properly. Besides admitting the use of larger CFL numbers, this new weight also contributes to more accurate numerical results like pressure, friction coefficient, and heat flux when solving shock waves, boundary layers, and complex supersonic/hypersonic flows. To examine the validity, accuracy, and efficiency of the proposed method, six numerical test cases covering the whole speed regime are conducted, including the hypersonic viscous flow past a cylinder, the hypersonic double-cone flow, the hypersonic double-ellipsoid flow, the laminar shock-boundary layer interaction, the supersonic flow around a ramp segment and the subsonic lid-driven cavity flow.

Compressible subsonic cavity flow in a nozzle

This paper concerns the existence and uniqueness of a two-dimensional compressible subsonic cavity flow behind a given obstacle with a free surface in an infinitely long nozzle. More precisely, for any given atmospheric pressure pe > 0, we first show that there exists a critical value mcr for the incoming mass flux m0 such that if m0 < mcr, then there exists a unique smooth subsonic cavity flow behind the given obstacle, and the free surface initiates smoothly from the vertex of the obstacle and goes to infinity at the far field. Second, we establish some fundamental properties of the cavity flow, such as the positivity of the horizontal velocity and the asymptotic behavior of the flow at the far field. Finally, we obtain that the vertical velocity is positive under the corresponding assumptions on the upper nozzle wall and the upper surface of the obstacle.

Spatial-temporal patterns of three-dimensional subsonic turbulent cavity flow

Three-dimensional, high Reynolds number cavity flow is investigated. The cavity was exposed to a free-stream Mach number of 0.40 and a Reynolds number (based on cavity depth) of 1.6 × 106, with a thick incoming turbulent boundary-layer. Dominant modes and structures were studied by conducting unsteady numerical flow simulations (CFD), and applying Proper-Orthogonal-Decomposition (POD) in both two and three-dimensions. Fully three-dimensional POD analysis of cavity flow has been rarely conducted in other studies, and it is demonstrated that two-dimensional POD analysis is valid in order to emphasize patterns which are limited in space, but could be misleading without the comparison with the three-dimensional POD analysis. The analysis sheds new light on three-dimensional cavity flow by disclosing novel non-intuitive dominant structures and modes. Stretched streamwise vortices were found to be dominant, at least as the spanwise structures that originate from the classical two-dimensional shear-layer instability. Furthermore, the effect of symmetry plane assumption, which is usually disregarded, was studied rigorously for the first time. POD decomposition of symmetric and anti-symmetric flow modes revealed that anti-symmetric modes contain a significant portion of the total energy; however, they only interact weakly with the symmetric modes.

Control of Supersonic Cavity Flow Using Plasma Actuators

Cavity flows are present in a wide range of aerospace applications. The pressure fluctuations associated with cavity resonance have made them the target of significant flow control efforts aimed at suppressing resonance. A potential scramjet application for cavity flows may require resonance enhancement in addition to resonance suppression. Localized arc filament plasma actuators have demonstrated the ability to control a high subsonic cavity flow. This work investigates the control authority of the localized arc filament plasma actuators in a supersonic () cavity flow. The localized arc filament plasma actuators significantly suppress the primary cavity resonance of a naturally and strongly resonating cavity. Additionally, the trend in effectiveness suggests that introducing mode competition through the excitation of the Kelvin–Helmholtz instability, and thereby influencing the shear-layer structure formation process, is the likely control mechanism. The effects of two-dimensional and three-dimensional excitation are explored. Although two-dimensional excitation achieves the greatest resonance suppression, three-dimensional excitation shows similar suppression with significantly less sensitivity to the excitation Strouhal number, making it more desirable for practical applications. In a weakly resonating cavity, the flow significantly responds to excitation near the resonance Strouhal numbers. However, electromagnetic interference makes it difficult to quantify the level of resonance enhancement.

Mode transition and oscillation suppression in supersonic cavity flow

Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation (DNS). For an uncontrolled cavity, as the thickness of the boundary layer declines, transition of the dominant mode from the steady mode to the Rossiter II mode and then to the Rossiter III mode is observed due to the change of vortex-corner interactions. Meanwhile, a low frequency mode appears. However, the wake mode observed in a subsonic cavity flow is absent in the current simulation. The oscillation frequencies obtained from a global dynamic mode decomposition (DMD) approach are consistent with the local power spectral density (PSD) analysis. The dominant mode transition is clearly shown by the dynamic modes obtained from the DMD. A passive control technique of substituting the cavity trailing edge with a quarter-circle is studied. As the effective cavity length increases, the dominant mode transition from the Rossiter II mode to the Rossiter III mode occurs. With the control, the pressure oscillations are reduced significantly. The interaction of the shear layer and the recirculation zone is greatly weakened, combined with weaker shear layer instability, responsible for the suppression of pressure oscillations. Moreover, active control using steady subsonic mass injection upstream of a cavity leading edge can stabilize the flow.

Hydroacoustic analysis of open cavity subsonic flow based on multiple parameter numerical models

The prediction of the flow-induced noise level is a key issue in the fluid–dynamic acoustics. In the hydroacoustics field, the complicated feedback induced by the flow past open cavities can amplify the convection instability in the shear layer which further leads to important noise radiations. The noise consists of intense narrowband and broadband components. In this paper, the level of the noise radiated by a subsonic cavity flow is calculated by using numerical flow computations based on the large eddy simulation (LES) and by solving the Ffowcs Williams-Hawkings equation. A series of three-dimensional open cavity models with overset grids and appropriate boundary conditions are developed for the hydroacoustic numerical computation. The self-sustained oscillation characteristics of the cavity flow are investigated, together with the mechanisms of the cavity noise generation. The distinguishing features of the flow-induced noise of the underwater structure cavities are studied with respect to the parameters of the cavity models, such as the free stream velocity, the dimensions of the cavity mouth, the angle of the cavity neck, the horizontal and vertical porous cavity models and the actual submarine open cavity model with an incoming flow attack angle. It is shown that it may be feasible to reduce the flow-induced noise by appropriate optimal parameters of the underwater structure cavities.

Mitigation of wind tunnel wall interactions in subsonic cavity flows

The flow over an open aircraft bay is often represented in a wind tunnel with a cavity. In flight, this flow is unconfined, though in experiments, the cavity is surrounded by wind tunnel walls. If untreated, wind tunnel wall effects can lead to significant distortions of cavity acoustics in subsonic flows. To understand and mitigate these cavity–tunnel interactions, a parametric approach was taken for flow over an L/D = 7 cavity at Mach numbers 0.6–0.8. With solid tunnel walls, a dominant cavity tone was observed, likely due to an interaction with a tunnel duct mode. An acoustic liner opposite the cavity decreased the amplitude of the dominant mode and its harmonics, a result observed by previous researchers. Acoustic dampeners were also placed in the tunnel sidewalls, which further decreased the dominant mode amplitudes and peak amplitudes associated with nonlinear interactions between cavity modes. This indicates that cavity resonance can be altered by tunnel sidewalls and that spanwise coupling should be addressed when conducting subsonic cavity experiments. Though mechanisms for dominant modes and nonlinear interactions likely exist in unconfined cavity flows, these effects can be amplified by the wind tunnel walls.

Noise control of subsonic cavity flows using plasma actuated receptive channels

We introduce a passive receptive rectangular channel at the trailing edge of an open rectangular cavity to reduce the acoustic tones generated due to coherent shear layer impingement. The channel is numerically tested at Mach 0.3 using an unsteady three-dimensional large eddy simulation. Results show reduction in pressure fluctuations in the cavity due to which sound pressure levels are suppressed. Two linear dielectric barrier discharge plasma actuators are placed inside the channel to enhance the flow through it. Specifically, acoustic suppression of 7 dB was obtained for Mach 0.3 flow with the plasma actuated channel. Also, the drag coefficient for the cavity reduced by over three folds for the channel and over eight folds for the plasma actuated channel. Such a channel can be useful in noise and drag reduction for various applications, including weapons bay, landing gear and branched piping systems.

Effects of Scaling on High Subsonic Cavity Flow Oscillations and Control

The effects of scaling on cavity oscillations and control have been studied by measuring the unsteady pressure on the floor of three cavities of different scales. The cavities have a length-to-depth ratio of 5 and a length-to-width ratio of 2, and the corresponding linear dimensions are in the ratio . The experiments were conducted with clean cavities and cavities fitted with leading-edge sawtooth spoilers so as to study the influence of scaling on clean cavities as well as the effectiveness of the passive control method on different sized cavities. The results showed significant variation of certain spectral characteristics of the clean cavities. The control effectiveness of the spoilers also showed variations with a change in scale of the model. It is recommended that, before implementing a passive control device for practical applications, the device should be tested in the possible range of cavity length-to-boundary-layer-thickness ratio () that can be experienced in actual flight.

Characterising low-speed, transitional cavity flow

In various studies involving subsonic transition cavity flows, terms like ‘transitional-open’ and ‘transitional-closed’ cavity flow regimes are used in the literature. However, the exact fluid dynamic criteria to distinguish them have not been firmly established. The present work attempts to clarify this. Experiments to measure static pressure and fluctuating pressure distribution in a rectangular cavity with length-to-depth (L/D) ratio of 3 to 20 were performed. Based on pressure measurements on the front, bottom and rear faces of the cavity, additional criteria are established to clearly distinguish the range of critical L/D (or (L/D)cri) where these different transitional cavity flow regimes occur. The present work ascertains that the (L/D)cri for open and transitional-open cavity flow is 6·0-6·5, for transitional-open and transitional-closed cavity flow is 9-10 and for transitional-closed and closed cavity flow is 14-15. Flow visualisation qualitatively supports the flow pattern in the different cavity flow regimes.

B23 亜音速キャビティ流れにおける振動現象のスペクトル解析(B2圧縮性流れII)

B23 亜音速キャビティ流れにおける振動現象のスペクトル解析(B2圧縮性流れII)

Contributions of turbulence to subsonic cavity flow wall pressures

Poisson-type pressure sources in the flow over open cavities were examined for several subsonic free stream conditions. Wall-mounted pressure transducers and two-component particle image velocimetry (PIV) were employed synchronously to obtain data at Mach 0.19, 0.29, 0.39, 0.58, and 0.73 for a rectangular cavity with a length-to-depth ratio of 6. The modified quadratic stochastic estimation was used to yield a time-resolved, low-dimensional representation of the velocity field using the PIV and pressure measurements. A conformal transformation was applied to the cavity geometry to allow the integration of Poisson’s equation giving a time-resolved estimate of the surface pressure fluctuations. The use of Poisson’s equation allowed the contribution of the mean-shear and nonlinear velocity sources of the surface pressure loads to be evaluated independently, showing that the resonant features of the cavity flow (Rossiter modes) are dominant in the mean-shear contribution and that nonlinear interactions become increasingly significant above Mach 0.4.

Properties of subsonic open cavity flow fields

Flow over an open cavity was studied for several different subsonic free stream Mach numbers ranging from 0.19 to nearly 0.73. Velocity field information was acquired through an application of particle image velocimetry, while the fluctuating surface pressure was acquired through a linear array of surface pressure sensors. These data were acquired on the centerline of the cavity which had a length to depth ratio of 6 and a turbulent boundary layer upstream of its leading edge. Over the range of free stream Mach numbers the fluctuating surface pressure spectra in the cavity exhibited different behavior ranging from no apparent resonance to resonance being dominated by the second or third Rossiter modes. The broadband levels of surface pressure spectra with strong resonant tones collapse with scaling by the flow dynamic pressure. Velocity measurements reveal that the center of circulation of the flow within the cavity moves from the aft wall towards the center of the cavity with increasing Mach number. The trend in the mean flow was also apparent in the mean square fluctuating velocities although to a lesser extent. Application of the proper orthogonal decomposition was performed independently on the velocity fields from each of the different free stream cases yielding a spatially dependent basis set. Using a similarity parameter between these spatial orthogonal basis functions it was shown that the first two modes for all of the cases were quantitatively similar. Furthermore this analysis showed some higher less energetic modes that were similar between the cases.

Analysis of the spectral relationships of cavity tones in subsonic resonant cavity flows

The understanding of the self-sustained flow-acoustic coupling mechanism in flows over shallow rectangular cavities is of great interest owing to its various practical applications. The ability to understand and predict the resonant frequencies in such flows has recently been advanced through contributions from signal processing theory and by viewing the Rossiter tones as the product of an amplitude modulation process between a fundamental aeroacoustic loop frequency (fa) and a modulating lower frequency. The results obtained using this approach applied to detailed and high-quality spectral data of shallow cavity flow over the Mach number range of 0.20–0.65 are presented and discussed. The new approach, while not a predictive technique, is used to clearly identify all the tones (Rossiter modes, their harmonics, and harmonics of fa) observed in the pressure spectra and to show relationships between the tones. The asymptotic growth with Mach number of fa and the small-step changes of the modulating lower frequency over the Mach number range studied provide insight into the variation of the Rossiter mode parameters. The results also indicate that the empirical parameters in the Rossiter equation vary with Mach number for fixed cavity geometry.

Using Linear Stability Analysis as a Tool to Evaluate Jet and Cavity Flow Control Situations

Flow control has the potential to improve the efficiency of aerospace systems. The design of a flow control system requires knowledge of flow phenomena including information on the evolution of naturally occurring disturbances in the flow. Natural flow disturbances can evolve at various frequencies and growth rates and can assume various mode shapes. Knowledge of the behavior of natural flow instabilities could aid actuator selection and strategy for flow control. In this paper we use linear stability analysis as a tool to analyze jet and cavity flow control situations. We present results from a linear stability study of the resonant acoustic mechanisms in subsonic impinging jets and subsonic cavity flows using the compressible Rayleigh equation. In an impinging jet, the subsonic instabilities are known to be responsible for the acoustic feedback, which is an essential link in setting up the resonances associated with this flow. The instabilities of the helical type are also relevant from the standpoint o...