Cavity Flow Oscillations Research Articles

Influence of back pressure adjustment of porous media on cavity flow noise control

Control of self-sustained oscillation and noise reduction poses a significant challenge. The present study employs Implicit Large Eddy Simulation at a Mach number of 0.85 to investigate the influence of a porous cavity floor on flow dynamics. By substituting the solid floor with porous media, the fundamental pressure–velocity relationship within the medium is established according to Darcy's law. Findings reveal marked suppression of wall pulsations, accompanied by a 10 dB decrease in sound pressure levels. The porous medium induces blowing and suction effects, effectively modulating large-scale re-circulation and mitigating shear layer instability, thereby approximating free mixing layer characteristics and suppressing cavity flow oscillations. At an optimized porosity for maximum noise reduction, altering back pressure at the cavity floor induces a transition in the local flow regime from suction-dominated to blowing-dominated state. Excessive reduction of back pressure promotes suction; conversely, increased pressure intensifies blowing, further attenuating feedback mechanisms and enhancing noise reduction. To explore noise reduction mechanisms, mode decomposition analyses demonstrate the efficacy of porous media in disrupting large-scale coherence structures within shear layer and redistributing energy from dominant modes to a broader frequency spectrum that engages smaller flow structures. This energy reallocation mechanism contributes to the mitigation of cavity flow noise and deepens insights into the role of porous media in flow modulation and noise control.

A single-degree-of-freedom mathematical model for energy harvesting from cavity flow oscillations

Flow-induced vibrations have proven to be a valuable power source for wireless sensors and MEMS devices. In this study, the energy harvesting aspects of an open cavity flow is explored and modelled as a single-degree-of-freedom system. Experiments with flow over a cavity of length-to-depth ratio of 3, with an incoming velocity of 30 m/s was taken as a case study to validate the model. It was observed in experiments that three distinct flow oscillation frequencies of significant amplitudes, corresponding to the different modes of oscillation were generated. To harvest this oscillatory power, a piezoelectric cantilever beam, mounted on the aft wall was subjected to unsteady pressure forces. To harvest efficiently, the natural frequency of the cantilever beam was tuned to lock in with the flow oscillation frequencies. The frequencies of cavity oscillations agreed well with Rossiter’s model in the experiments. A single-degree-of-freedom system assuming lumped parameters was used for modelling. The forcing term was described using a periodic function that uses empirical values for amplitude. The model was able to predict the overall trend and values of the average power and RMS voltage generated across a resistor load with reasonable accuracy. Using the model, the effects of cavity length, incoming velocity and resonant frequency were also studied.

Characteristics of Oscillation in Cavity of Helmholtz Nozzle Generating Self-excited Pulsed Waterjet

Cavity flow oscillations in the axisymmetric cavity are critical to the operating efficiency of self-excited pulsed waterjets, which are widely employed in many practical applications. In this study, the behaviors of a turbulent flow in axisymmetric cavities causing cavity flow oscillations are investigated based on wall pressure characteristics. Experiments are performed using four Helmholtz nozzles with varying length-to-radius ratios at flow velocities of 20–80 m/s. Three orders of hydrodynamic modes in axisymmetric cavity are obtained through the spectral analysis of wall pressure. Based on the experimental results, the empirical coefficient of Rossiter’s formula is modified, and the values of the parameter phase lag and the ratio of convection velocity to free stream velocity are obtained as 0.061 and 0.511, respectively. In addition, the spectral peak with a relatively constant frequency shows that the flow-acoustic resonance is excited significantly. A modified model is introduced based on the fluidic networks to predict the lock-on frequency. The results obtained can provide a basis for the structural optimization of the nozzle to improve the performance of self-excited pulsed waterjets.

Reduced-Order Modeling of Cavity Flow Oscillations across Multi-Mach Numbers Using Deep Learning

The reduced-order model can accurately and efficiently predict unsteady problems in many aerospace engineering applications. The traditional reduced-order model based on proper orthogonal decomposition (POD) and Galerkin projection has poor robustness and large error in predicting complex problems. In this paper, a reduced-order model combining POD and deep learning is proposed to predict cavity flow oscillations under different flow conditions. Firstly, POD modes and corresponding coefficients are obtained by POD. Then, two deep learning frameworks, including multilayer perceptron (MLP) and long short-term memory (LSTM) neural networks, are used to predict the future POD coefficients, respectively. Finally, the cavity flow oscillations across multi-Mach numbers are predicted by the POD modes and the future coefficients. The results show that both of these frameworks can accurately predict cavity flow oscillations when the flow conditions change, and the time cost is reduced by order of magnitude. In addition, due to the performance of LSTM is better than that of MLP, its calculation speed is faster.

Energy harvesting from cavity flow oscillations

This study investigates the energy harvesting prospects of self-sustained flow oscillations emanating from grazing flow over a rectangular cavity by employing experimental and computational methods. Two cavity geometries with length-to-depth ratios of 2 and 3, exposed to an incoming flow of 30 m/s, were selected for the purpose. The power spectral density of the baseline cavity flows showed the presence of high-amplitude peaks whose frequencies agreed to those estimated from Rossiter’s feedback model. For energy harvesting, a piezoelectric beam was placed perpendicular to the aft wall and its natural frequency tuned to match closely with the dominant frequencies of the cavity flow oscillations. From the experiments, an average and maximum instantaneous power of 21.11 and 284.18 µW was recorded for the cavity with L/ D = 2 whereas for the cavity with L/ D = 3 the corresponding values were 32.16 and 403.46 µW respectively. Time-frequency analysis showed the forcing of the beam at the cavity oscillation frequency and the substantial increase in the amplitude of beam vibrations when this frequency was close to the natural frequency of the beam.

Suppression of the cavity oscillation using high-speed mass injection

The cavity flow oscillation is a typical unsteady, nonlinear flow phenomenon, accompanied by resonance with strong pressure oscillation and intense turbulence. A control method is proposed to suppress the cavity oscillation using high-speed mass injection. Wind-tunnel experiments are performed to validate the control method. A length-to-depth six cavity is employed in the experiments with a Mach number 1.8 of the incoming flow. The upstream injection is generated in the leading edge of the cavity with a large momentum coefficient (more than 10%). Effects of the high-speed mass injection on the cavity oscillation are validated by a significant reduction of the overall sound pressure level inside the cavity. The broadband cavity noise is reduced by more than 10 dB, with a great suppression of the cavity tones. Owing to the high-speed mass injection, the cavity oscillation has been effectively suppressed in the supersonic condition. Because there is no need of additional gas supply in the control system, the control strategy is considered to be a potential and practical candidate for engineering applications.

Flow–acoustic resonance in a cavity covered by a perforated plate

To explain the large-scale hydrodynamic instability along a cavity-backed perforated plate in a flow duct, a two-dimensional multimodal analysis of flow disturbances is performed. First, a hole-by-hole description of the perforated plate shows a spatially growing wave with a wavelength close to the plate length, but much larger than the period of perforation. To better understand this problem and also cavity flow oscillations, we then combine the travelling mode and global mode analyses of the flow where the plate is represented by a homogeneous impedance. The spatially growing wave is, from a homogeneous point of view, essentially a Kelvin–Helmholtz instability wave, strongly distorted by evanescent acoustic waves near the cavity downstream edge. The phase difference of the unstable hydrodynamic mode at the two edges is found to be a bit larger than , whereas the upstream-travelling evanescent waves reduce the total phase change around the feedback loop, so that the phase condition of the global mode can still be satisfied. This particular case indicates the significant effects of those evanescent waves on both the amplitude and phase of cavity flow disturbances. The criterion of the global instability is discussed: the loop gain being larger or smaller than unity determines whether the global mode is unstable or stable. A global mode in the stable regime, which has so far received little attention, is explored by investigating the system response to external forcing. It is shown that sound can be produced when a lightly damped flow–acoustic resonance is excited by a vortical wave.

Suppression of Cavity Flow Oscillations via Three-Dimensional Steady Blowing

This paper presents an investigation of the flow physics and control of cavity flow oscillations using a three-dimensional (3-D) array of steady jets located near the cavity leading edge. The array injects 3-D, steady flow normal to the freestream to suppress the fluctuating surface pressure in a rectangular cavity with a length-to-depth ratio of 6 for Mach 0.3 to 0.7. Sixteen configurations are assessed for their suppression as a function of the aggregate momentum coefficient , spatial duty cycle , and dimensionless wavelength . Significant reductions of fluctuating surface pressure are observed. Schlieren and particle image velocimetry are performed to investigate the baseline and controlled flows. Companion large-eddy simulations with spanwise periodic boundary conditions at Mach 0.6 generally agree with the experiments, despite a significant difference in the Reynolds numbers. The simulations show that control reduces the pressure fluctuations inside the entire cavity, albeit at a higher level of . Spanwise wavelike structures produced by the control input distort the shear layer, inhibit the growth of large-scale vortical structures, and reduce the strength and length scale of turbulent fluctuations near the impingement region. A slot-configuration design guide is provided, which compares favorably with the limited available data in the literature.

Heat flux characteristics within and outside a forward facing cavity in a hypersonic flow

A forward facing cavity effect on heat flux variations inside the cavity and over the nose surface is studied over a typical projectile geometry using the shock tunnel experiments. The flow oscillation will tend to oscillate the shock, which in turn varies the heat flux over the nose surface as well as inside the cavity. The higher length to diameter (L/D) ratio of the cavity plays an important role in nose surface heat flux reduction and investigated for L/D = 1, 2 and 3 by a uniquely designed model. The cavity flow oscillation period is determined, using the base pressure and surface heat transfer signals measured inside the cavity with reference to the pitot probe steady test time and shock oscillation period. The frequency analysis for oscillations is performed using the fast Fourier transformation. Experimental results showed that, the mean nose surface heat flux reduced by ≈4.6% to ≈7%, ≈6% to ≈8% and 3% to ≈31.4 and the mean shock standoff distance (l) is increased by ≈13%, ≈25% and ≈35% for L/D = 1, 2 and 3 configurations respectively with reference to the baseline geometry (without cavity geometry). As L/D increases, the nose surface location on which the heat flux reduction is observed moves towards the cavity lip and more surface area is covered with lower heat flux. Cavity region heat flux variations are studied by incorporating the thin film sensors over the cavity surface and it is observed that, a significant ≈32.5% to ≈36%, and ≈28% to ≈58% lower mean heat flux for L/D = 2 and 3 respectively with reference to the baseline geometry stagnation point heat flux.

On shock train interaction with cavity oscillations in a confined supersonic flow

The interaction between shock train and cavity flow oscillations has been investigated experimentally in an open jet facility. Mach 1.71 flow has been passed over a set of rectangular cavities with L/D ratios varying between 5 and 10. Unsteady pressure measurement and schlieren flow visualization is employed to gain insight into the flow physics. Flow visualization reveals the presence of shock train coupled shear layer oscillations at different levels. Confinement variation establishes the importance of cavity depth in affecting the shock train structure, as the increase in depth resulted in decrease in boundary layer thickness. Shock train strength is found to decrease with decrease in L/D and the weak shock system promotes development of generic cavity oscillations and flow features through longitudinal mechanism. The shock train is found to be oscillatory in nature and the mean position of the bifurcated shock shifts downstream with decrease in L/D. Large scale structures and strengthened oscillations increase the mean and RMS pressure level of the cavities respectively. These large scale structures are incoherent for cavities with hardly any oscillations and coherent for cavities with sustained oscillations. Mode switching and temporal variations in pressure fluctuations are absent for shock train coupled cavity oscillations.

Review of Flow-Excited Resonance of Acoustic Trapped Modes in Ducted Shallow Cavities

Flow-excited resonances of acoustic trapped modes in ducted shallow cavities are reviewed in this paper. The main components of the feedback mechanism which sustains the acoustic resonance are discussed with particular emphasis on the complexity of the trapped mode shapes and the strong three-dimensionality of the cavity flow oscillations during the resonance. Due to these complexities of the flow and sound fields, it is still difficult to theoretically or numerically model the interaction mechanism which sustains the acoustic resonance. Strouhal number and resonance amplitude charts are therefore included to help designers avoid the occurrence of resonance in new installations, and effective countermeasures are provided which can be implemented to suppress trapped mode resonances in operating plants.

Numerical investigation of the passive control of cavity flow oscillations by a dimpled non-smooth surface

Abstract Computational investigations are conducted to determine the effectiveness of a passive control technique, which was employed to decay the pressure oscillations induced by a subsonic flow over a cavity. This work focuses on a cavity with a small opening but a large volume. The passive control technique is employed by introducing a dimpled non-smooth surface, which is installed at the upstream of the cavity. Large eddy simulation is used to investigate the flow field and flow instability around the cavity for the smooth and non-smooth cases. Experiments are conducted in an acoustic wind tunnel for the smooth case to validate the computational scheme. Flow visualizations revealed that the dimpled surface located upstream effectively suppresses cavity flow oscillations. Finally, the control mechanism of cavity oscillation with the dimpled non-smooth surface is also determined based on the comparison of the flow field structure between the smooth and non-smooth cases.

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.

Feedback control of cavity flow oscillations using simple linear models

AbstractUsing data from direct numerical simulations, linear models of the compressible flow past a rectangular cavity are found. The emphasis is on forming simple models which capture the input–output behaviour of the system, and which are useful for feedback controller design. Two different approaches for finding a linear model are investigated. The first involves using input–output data of the linearized cavity flow to form a balanced, reduced-order model directly. The second approach is conceptual, and involves modelling each element of the flow physics separately using simple analytical expressions, the parameters of which are chosen based on simulation data at salient points in the cavity’s computational domain. Both models are validated: first in the time domain by comparing their impulse responses to that of the full system in direct numerical simulations; and second in the frequency domain by comparing their frequency responses. Finally, the validity of both linear models is shown most clearly by using them for feedback controller design, and then applying each controller in direct numerical simulations. Both controllers completely eliminate oscillations, and demonstrate the advantages of model-based feedback controllers, even when the models upon which they are based are very simple.

A Review of Control Methods for Cavity Flows and Feasibility of Laser Energy Deposition as an Actuator

Studies related to supersonic cavity flow are very important for aeronautics applications. Understanding the physics of cavity flows is crucial for both fluid mechanics and engineering applications. Supersonic cavity flow and control techniques for this type of flow have been studied for many decades. A review of literature on numerical and experimental cavity flow is provided and the flow mechanism inside the cavity is explained using example studies from literature. It is necessary to understand the complex nature of the flow before developing control methods for the cavity flow oscillations. The literature on the development of methods to control the flow oscillations in cavities which are harmful for aircraft is also reviewed. Feasibility of laser energy deposition as an actuator for flow control is investigated demonstrating examples from literature.

Control of Cavity Flow Oscillations by High Frequency Forcing

Time resolved two-dimensional particle image velocimetry (2DPIV) experiments have been conducted to contribute to the understanding of the physics governing the suppression mechanism of cavity flow self-sustained oscillations by means of high frequency excitation of the cavity shear layer. High frequency excitation was introduced by the spanwise coherent vortex shedding in the wake of a cylindrical rod positioned just upstream the cavity entrance, at the edge of the incoming boundary layer. The effectiveness of this suppression was demonstrated for a cavity having the length-to-depth ratio equal to three, in incompressible flow. The spatial and time resolved PIV measurements of the whole flow field in the plane normal to the cavity floor, linear stability analysis of the measured shear layer mean velocity profiles, and preliminary PIV measurements in a plane parallel to the cavity allowed us to offer a better insight into the involved physical mechanisms in suppressing cavity self-sustained oscillations.

Corner Singularity Dynamics and Regularity of Compressible Viscous Navier--Stokes Flows

We show unique existence (global in time) and regularity of solutions to the Navier--Stokes equations for compressible viscous flows with nonzero initial and Dirichlet boundary data on polygonal domains. The solution is constructed by the inverse of the sum of two operators taken from Da Prato and Grisvard [J. Math. Pures Appl. (9), 54 (1975), pp. 305--387] and, applying to the inverse the corner singularity result of the Lamé system and the Laplace with parameters, is split into singular and regular parts. The orders of the leading corner singularities for the velocity are the same as those of the Lamé system, and the one for the temperature is the same as that for the Laplace operator. The coefficients of the singularities, called the stress intensity functions, are constructed, and the remainders are shown to have increased regularities. Consequently the derivatives of the flow variables may blow up along the trajectory curves starting near the nonconvex vertices, so the dynamical behaviors will be steeply changed there. The corner singularities are also propagated into the region by the transport character of the continuity equation. The explicit expression of singularities near the corners may be employed in a direct estimation of compressible cavity flow oscillations, for instance, the resonant amplitude tones near corners.

Schlieren Visualization of Shock Wave Phenomena over a Missile-Shaped Body at Hypersonic Mach Numbers

The flow over a missile-shaped configuration is investigated by means of Schlieren visualization in short-duration facility producing free stream Mach numbers of 5.75 and 8. This visualization technique is demonstrated with a 41° full apex angle blunt cone missile-shaped body mounted with and without cavity. Experiments are carried out with air as the test gas to visualize the flow field. The experimental results show a strong intensity variation in the deflection of light in a flow field, due to the flow compressibility. Shock stand-off distance measured with the Schlieren method is in good agreement with theory and computational fluid dynamic study for both the configurations. Magnitude of the shock oscillation for a cavity model may be greater than the case of a model without cavity. The picture of visualization shows that there is an outgoing and incoming flow closer to the cavity. Cavity flow oscillation was found to subside to steady flow with a decrease in the free stream Mach number.

Passive Control Techniques to Alleviate Supersonic Cavity Flow Oscillation

The effectiveness of two passive control techniques for alleviating the pressure oscillation generated in a supersonic cavity flow was investigated numerically. The control devices suggested in the present study include a triangular bump and a subcavity installed near the leading edge of a rectangular cavity. Time-dependent supersonic cavity flow characteristics with turbulent features were examined by using the three-dimensional, mass-averaged Navier-Stokes computation based on a finite volume scheme and large eddy simulation. The results show that the pressure oscillation near the trailing edge dominates overall time-dependent cavity pressure variations. Such an oscillation can be attenuated more significantly in the presence of the subcavity compared with the bump or blowing, and a larger subcavity leads to better control performance.

An Experimental Investigation of Cavity Flow Oscillations and Tones of an In-Flow Microphone

An experiment was conducted to investigate the flow-induced nose-cone cavity oscillations of a B&K in-flow microphone. A 38.1-mm-diameter scale model of the B&K 12.7-mm UA 0386 nose cone was constructed and tested in a 177.8-mm-round free-jet in the Anechoic Chamber Wind Tunnel at NASA Ames Research Center at ReD up to 1.1 × 105. The results indicated that the B&K nose-cone cavity behaves much like a cavity without a screen. Detailed flow velocity measurements over the cavity screen have shown inflection points in the mean velocity profiles and high disturbance intensities in the vicinity of the cavity trailing edge. These results and the high coherence between the strong velocity and acoustic pressure oscillations at the cavity trailing edge implied that the cavity acoustic tones are the consequence of the cavity shear-layer impingement on the cavity trailing edge. The effect of cavity screen appears to be secondary in the velocity range tested. The screen seems mainly to modulate the Strouhal numbers of the higher acoustic modes.