Supersonic Cavity Flow Research Articles

Control of Pressure Oscillations Induced by Supersonic Cavity Flow

A control method is developed to suppress pressure oscillations induced by supersonic cavity flow using high-speed upstream injection. The injection is generated with a large blowing coefficient through a channel, which guides the airflow blowing off in the leading edge of the cavity. Wind tunnel experiments are performed to validate the method with Mach 1.8 and 2.0 flows over a length-to-depth 6 cavity. Six dynamic pressure transducers are used to characterize the cavity oscillation. A remarkable suppression of pressure fluctuation is realized in the controlled cavity flow. The overall sound pressure level can be reduced by larger than 10 dB in both configurations. The most remarkable reduction in pressure fluctuation occurs near the cavity trailing edge, wherein flow-induced oscillation is strongest. Almost no cavity tones are observed near the cavity trailing edge under control. Further, the control method is proven effective and stable over time. The feedback loop is interrupted in the controlled cavity flow with a weak impact of shear vortices on the cavity aft wall. The control method may be a practical and potential candidate for use in engineering applications because it does not need additional gas supply in the control system.

Flow Field and Combustion Field Control Using Pylons Installed Upstream of a Cavity in Supersonic Flow

Flow Field and Combustion Field Control Using Pylons Installed Upstream of a Cavity in Supersonic Flow

Nonlinear Characteristics of a Rectangular Cavity in Supersonic Flow

Time-resolved schlieren images and unsteady pressure measurements were employed to understand the flowfield characteristics of a rectangular cavity at Mach number 1.8 with a unit Reynolds number of . It was illustrated that the vortex dynamics in the cavity shear layer were found to be nonlinear, where the vortex speed was observed to be varied from to with an average value of . Unsteady pressure measurements inside the cavity revealed six tones, among which the third tone was dominant. The uniqueness of the flow features both outside as well as inside the cavity was studied for one vortex shedding cycle. Analyses using different techniques divulged that the occurrence of the first and second tones was due to wave propagation and vortex shedding, respectively. On the other hand, frequency that corresponds to one time period was correlated to the third tone. Further investigations unveiled that the entrainment of freestream between shear-layer vortices generated type 5 acoustic waves, which were responsible for the production of other cavity tones. The wavelet analyses indicated that the frequencies were amplitude modulated in time, and the wavelet spectra of the second and third tones contain high-energy events along with sporadic distribution of low-energy events without mode switching phenomenon.

Flow Characteristics of a Supersonic Open Cavity

An open cavity flow exhibits intense self-sustained oscillations. This transient behavior stimulates violent pressure fluctuations because of multiple-order cavity tones. Detached eddy simulation was conducted to simulate the cavity flow at the freestream Mach number of 1.19. In order to improve the understanding of the shear layer convection processes and frequency characteristics the velocity of the flow field at cavity mid-span was studied using the dynamic mode decomposition (DMD) algorithm. The first three modes of the supersonic cavity flow were extracted to describe the flow configuration at the dominant frequencies. The two-vortices, three-vortices, and four-vortices are the corresponding first three DMD modes. The simplified mode structures are proposed to explain the flow dynamics in a supersonic cavity. When a feedback compression wave encounters the extrusion wave at a special location, an “analogous sonic boom” phenomenon appears causing violent noise in the cavity.

Mode Behavior in Supersonic Cavity Flows

The behavior of oscillation modes in a supersonic flow of a Mach number of 1.58 over a cavity of and was studied using wind-tunnel experiments. Through instantaneous shadowgraph visualization, the leading-edge shock and trailing-edge shock were both found to be at antiphase to one another for the first mode. Simultaneous unsteady pressure measurements were carried out at the front and aft walls of the cavity. From a wavelet analysis, the dominant mode was found to be the second mode, which prevailed for a major duration of the measurement time. Mode switching was observed between the first and second modes of oscillation. The mean velocity of the vortical disturbance convecting downstream and the acoustic disturbance travelling upstream inside the cavity was determined from a cross-correlation analysis. An examination using wavelet coherence revealed the coherence and phase difference existing between the pressure signal at the front and aft walls. The phase difference estimated from the measurements indicated the strong influence of cavity acoustic resonance on the cavity oscillations. However, the phase difference values showed a slight deviation between the acoustic resonance and cavity modes owing to the standoff distance between the trailing-edge shock and the aft wall.

Analysis of Flow Oscillation Due to Sidewall of Three-Dimensional Supersonic Open Cavity Flow

Unsteady turbulent flow simulations were performed based on the Reynolds-averaged Navier–Stokes (RANS) equations to investigate flow oscillation due to three-dimensional (3D) configuration of a Mach 1.5 supersonic open cavity flow with a length-to-depth ratio of 3. Two-dimensional (2D) and 3D unsteady simulation results were analyzed and compared with experimental data and Rossiter’s empirical prediction data. The three-dimensional cavity width-to-depth ratio (W/D) was 1, 3.8 and 7.6. Computational results indicated that pressure oscillation in the 2D flow was generated by a single-flow structure, whereas a multiple-flow structure generated multiple oscillation peaks in the 3D flow. The flow structure in the 3D cavity was investigated. For the 2D flow case, the cavity internal pressure wave was directly synchronized with the free shear layer. In the 3D flow case, an unstable spanwise flow due to the sidewall was observed. This spanwise fluctuation produced additional pressure oscillations coupled with the streamwise internal pressure wave. The numerical results indicate that the spanwise flow reduces the propagation speed of the internal pressure waves and the intensity of the corresponding pressure fluctuation.

Effects of subcavity in supersonic cavity flow

An experimental and numerical study is conducted on a rectangular open cavity with a length to depth ratio of 2 at Mach number 1.71 by placing a subcavity at different locations. The subcavity at the front wall has already been established as a passive control device experimentally. In addition, it has been observed that it can act as a passive resonator. However, in the current study, it is found that the location of the subcavity and its dimensions play a crucial role in determining the types of oscillations existing inside the cavity. Cavity models with a subcavity length to main cavity length of 0.2 (l/L = 0.20) were investigated by placing the subcavity at the front wall, aft wall, and simultaneously at both front and aft walls. High speed schlieren visualization revealed the presence of different shock features associated with the cavity flow field. Statistical techniques such as fast Fourier transform, spectrogram, coherence, and correlation are employed to analyze the unsteady pressure data. Numerical computations are carried out to validate the experimental results and also to explore the flow physics. The front wall subcavity acts as a passive control device with a maximum reduction of 34.1 dB in the sound pressure level for the most dominant tone, and there is also a notable reduction in the overall sound pressure level by 11.7 dB. In the case of front wall subcavity, the acoustic wave gets inclined as it interacts with the subcavity, thereby displacing the shear layer to form a dome-shaped structure. The aft wall subcavity acts as a passive resonator with distinct fluid-resonant oscillations and the respective modal frequencies differ widely from those predicted using Rossiter’s expression. The shear layer interacts with a recirculation region formed inside the subcavity at the aft wall, thereby mitigating the effect due to direct impingement of the shear layer on the aft wall. The subcavity at both walls acts as a passive suppression device with a reduction of 34.9 dB in the sound pressure level for the most dominant mode and also with a reduction of 14.5 dB in the overall sound pressure level.

Investigations on flow and growth properties of a cavity-actuated supersonic planar mixing layer downstream a thick splitter plate

Mixing enhancement is vital to combustion efficiency in a rocket-based combined cycle engine. Presently, we propose a cavity as an actuator and investigate the cavity-actuated supersonic mixing layer experimentally using particle image velocimetry (PIV) and nanoparticle-based planar laser scattering (NPLS). Large eddy simulation (LES) is conducted to obtain the detailed flow structures and pressure spectra of the supersonic mixing layer. Results indicate that the vortex evolution of the supersonic mixing layer is locked strongly to the excitation frequencies from the self-sustaining oscillation of supersonic cavity flow in both the near and far flowfields. Multiple coherent structures of different sizes coexist in the initial flowfields, and then only large ones survive downstream. Pressure spectra can reflect the vortex evolution entirely. The third mode of the pressure spectrum gradually becomes dominant due to having the highest energy content. Curvature shocklets are observed in this case at the low convective Mach number of 0.22. The growth rate in the linear process is slightly larger than that of the case without a cavity while the velocity thickness is 3.34 times as large as that. The normalized growth rate sees a massive increase and is 8.95 times as large as the typical result of a free supersonic shear layer.

세장비 인 초음속 공동유동에서 차원 효과로 인한 유동 진동 주파수 분석

Unsteady turbulent flow simulations were performed to investigate flow oscillation frequency due to three dimensional effect in supersonic cavity flow of length-to-depth ratio of 3, width-to-depth ratio of 1 and Mach 1.5. Two- and three-dimensional unsteady simulations result were compared with experimental data and prediction from the Rossiter equation. Pressure oscillation in three-dimensional cavity and the corresponding frequency were well matched with the empirical data. The flow structure corresponding to each mode was investigated. The results show that two dimensional flow is in the wake mode and three-dimensional flow is mainly in the shear layer mode. The oscillation frequency spectrum clearly indicate the three-dimensional effect due to the sidewall.

Numerical analysis of supersonic flows over an aft-ramped open-mode cavity

The characteristics of supersonic cavity flows were investigated using large eddy simulation together with the acoustic analogy method. A fifth-order hybrid compact-weighted essentially non-oscillatory scheme was applied to calculate the convective flux, and a sixth-order compact scheme was used for the viscous flux. Farassat's Formula 1A was used to solve the Ffowcs William–Hawkings equations to obtain the far-field acoustic pressure fluctuations. The effects of cavity configuration and flow Mach number on the pressure waves generated by the interaction of shear vortices and cavity were compared. The ramped rear-step of the cavity can increase the more-organized level of flow coherent structures, and decrease the static pressure distribution on the cavity bottom-wall. The attenuation of the interactions between the shear layer vortex and the aft-ramped wall of the cavity can reduce the feedback of pressure waves upstream. The energy of the main frequency significantly decreases for the far-field acoustics in the upstream flow over the ramped rear-step cavity. Thus, the effectiveness of noise reduction by the rear-ramp is considerable for the area upstream of the cavity for the present conditions, but it is not the case opposite to and downstream of the cavity. The present conclusions are valuable for evaluating the performance of noise suppression by cavities embedded in supersonic inflows in engineering applications.

Experimental Study of Subcavity in Supersonic Cavity Flow

An experimental study is conducted at Mach number 1.71 on rectangular open cavity of length-to-depth ratio () 2 with subcavity at the front wall. Subcavity at the front wall has been established as a passive control device. In the current study it is found that subcavity at the front wall can also act as a passive resonator. The dominance of fluid-dynamic or fluid-resonant oscillations accordingly causes the subcavity at the front wall to act as a passive control device or as a passive resonator in supersonic cavity flow. In the current study the presence of the fluid-dynamic and fluid-resonant cavity oscillations is clearly brought out and the inherent differences between the two types of oscillations are also highlighted. Fluid-resonant cavity oscillations are not often observed experimentally in most of the experimental works related to supersonic cavity flow. A total of six cavity models with different subcavity lengths are investigated and are also designated by the ratio of subcavity length to main cavity length (, 0.10, 0.20, 0.25, 0.30, and 0.40). High-speed schlieren flow visualization and unsteady pressure measurements are employed to gain further insight into the flow physics. High-speed schlieren flow visualization clearly indicates the presence of six different types of waves and five different flow features that are associated with the cavity flow field. Statistical analysis techniques, namely, fast Fourier transform, spectrogram, correlation, and coherence, are employed for analyzing the unsteady pressure data. Fluid-dynamic-type cavity oscillations are dominant in , 0.10 and 0.20 cavities. Pressure oscillations inside the cavity are suppressed as is increased from 0.00 to 0.20. As is increased from 0.20 to 0.25, the type of cavity oscillations changes from fluid-dynamic to fluid-resonant. This change in the type of cavity oscillations results in increased pressure oscillations inside the cavity for , 0.30, and 0.40. Distinct cavity tones at certain frequencies in power spectrum plots are classified into fluid-dynamic mode and fluid-resonant mode. Modal frequencies of fluid-resonant modes differ widely from those predicted by modified Rossiter formula. Correlation analysis suggests the presence of fluid-resonant behavior in , 0.30, and 0.40 cavities. With reference to cavity, there is maximum reduction of 30.2 dB in sound pressure level for the second fluid-dynamic mode, and there is also 4.4 dB reduction in overall sound pressure level for the cavity.

Oscillatory mode transition for supersonic open cavity flows

The transition in the primary oscillatory mode in an open cavity has been experimentally investigated and the associated characteristics in a Mach 1.71 flow has been analyzed. The length-to-depth (L/D) ratios of the rectangular cavities are varied from 1.67 to 3.33. Unsteady pressure measurement and flow visualization are employed to understand the transitional flow physics. Flow visualization revealed the change in oscillation pattern from longitudinal mode to transverse mode and is also characterized by the presence of two bow shocks at the trailing edge instead of one. The transition is found to occur between L/D 1.67 and 2, marked by a change in the feedback mechanism, resulting in a shift from the vortex circulation driven transverse feedback mode to the oscillating shear layer driven longitudinal feedback mode. Cavities oscillating in the transition mode exhibit multiple tones of comparable strength. Correlation analysis indicated the shift in the feedback mechanism. Wavelet analysis revealed the temporal behaviour of tones during transition. Tone switching is observed in deeper cavities and is attributed to the occurrence of two bow shocks as evident from the temporo-spectral characteristics of transition that affects the shear layer modal shape.

Flow-induced vibration in the compressible cavity flow

The cavity plays an important role in the fuel-air mixing and combustion stability inside the hypersonic scramjet. However, the high levels of time-dependent loading resulting from the supersonic cavity flow can cause intense structural vibration even damage. Experiments and numerical simulations were performed to understand the complex fluid-structure interaction in this paper. A cantilever plate with a cavity was installed as a splitter plate in the supersonic mixing layer wind tunnel. The response displacements of this cantilever plate were measured by a non-intrusive laser vibrometer. Large eddy simulation (LES) was applied to calculate the aerodynamic loading. Results show that the St number of time-dependent surface-averaged pressure difference agrees well with semi-empirical relation of Heller used to predict the resonance mode. The cantilever plate exhibits a directly dependent response to self-oscillation of supersonic cavity flow. Measurement results of displacement indicate that the vibration shape of this plate is dominantly two-dimensional.

Numerical Simulation of Aero-Optical Effects in a Supersonic Cavity Flow

A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes turbulence model is applied to compute the wave-front aberrations in an optical beam passing through a supersonic open cavity flow. The turbulence model blends a Reynolds-averaged Navier–Stokes-type closure near solid walls with a subgrid model in the freestream based on the ratios of estimated inner and outer turbulent length scales. The cavity geometry is modeled using an immersed boundary method, and an auxiliary flat-plate simulation is performed to replicate the effects of the wind-tunnel boundary layer on the computed optical path difference. Two-dimensional proper orthogonal decomposition modes of the optical wave front are computed; these compare favorably with wind-tunnel data, despite uncertainties about inflow turbulence levels and boundary-layer thicknesses over the wind-tunnel window. Dynamic mode decomposition of a planar wave front spanning the entire cavity reveals that wave-front distortions are driven by shear-layer oscillation at the Rossiter frequencies; these disturbances create eddy shocklets that propagate into the freestream and create additional optical path disturbances.

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.

Experimental investigation of combustion stabilization modes in a cavity-based supersonic combustor with different wall divergence angles

Optical diagnostics and pressure measurements are carried out to investigate the characteristics of cavity-assisted hydrogen jet combustion in a Mach 2.52 supersonic flow which simulates flight Mach 5.5 ∼ 6 conditions. Two combustor configurations with wall divergence angles being 2.25° and 3.5°, and two injection schemes are designed. Main attention is focused on the influences of the combustor wall divergence angles and injection schemes on combustion stabilization modes. The experimental results show that two totally different combustion stabilization modes and transition processes between combustion stabilization modes are detected. Heat release and jet–cavity interactions are supposed to play an important role in the transition of combustion stabilization modes. In addition to injection schemes and the cavity configuration, the combustor wall divergence angle is demonstrated to obviously affect the combustion stabilization modes in the supersonic cavity flows, suggesting that the divergence angle should be seriously considered in the supersonic combustion organization. The contrastive experiments also show that as long as combined cavity shear layer/recirculation combustion stabilization mode is achieved, the divergence angle should be smaller and the injection distance should be shorter with all the possible means to obtain more stable combustion in present condition. Meanwhile, it needs to be more cautious to design injection schemes if the combustor wall divergence angle is larger.

Model reduction for supersonic cavity flow using proper orthogonal decomposition (POD) and Galerkin projection

The reduced-order model (ROM) for the two-dimensional supersonic cavity flow based on proper orthogonal decomposition (POD) and Galerkin projection is investigated. Presently, popular ROMs in cavity flows are based on an isentropic assumption, valid only for flows at low or moderate Mach numbers. A new ROM is constructed involving primitive variables of the fully compressible Navier-Stokes (N-S) equations, which is suitable for flows at high Mach numbers. Compared with the direct numerical simulation (DNS) results, the proposed model predicts flow dynamics (e.g., dominant frequency and amplitude) accurately for supersonic cavity flows, and is robust. The comparison between the present transient flow fields and those of the DNS shows that the proposed ROM can capture self-sustained oscillations of a shear layer. In addition, the present model reduction method can be easily extended to other supersonic flows.

Effects of Active and Passive Control Techniques on Mach 1.5 Cavity Flow Dynamics

Supersonic flow over cavities has been of interest since 1960s because cavities represent the bomb bays of aircraft. The flow is transient, turbulent, and complicated. Pressure fluctuations inside the cavity can impede successful weapon release. The objective of this study is to use active and passive control methods on supersonic cavity flow numerically to decrease or eliminate pressure oscillations. Jet blowing at several locations on the front and aft walls of the cavity configuration is used as an active control method. Several techniques are used for passive control including using a cover plate to separate the flow dynamics inside and outside of the cavity, trailing edge wall modifications, such as inclination of the trailing edge, and providing curvature to the trailing edge wall. The results of active and passive control techniques are compared with the baseline case in terms of pressure fluctuations, sound pressure levels at the leading edge, trailing edge walls, and cavity floor and in terms of formation of the flow structures and the results are presented. It is observed from the results that modification of the trailing edge wall is the most effective of the control methods tested leading to up to 40 dB reductions in cavity tones.

Oscillatory Characteristics of Shallow Open Cavities in Supersonic Flow

Numerical simulations and experiments were carried out to investigate the unsteady flow in a two-dimensional rectangular open cavity with emphasis on the effects of length-to-depth ratio on the shear-layer development and cavity drag. The cavity was exposed to a supersonic flow of Mach 2. The results of the simulations were compared and analyzed with experimental data. Detailed predictions of flow features and frequencies showed a good agreement with the experiments. Particularly, the results indicated how the cavity flowfield changes when the length-to-depth ratio of the cavity is changed. Below a certain value of length-to-depth ratio, self-sustained periodic oscillations are strong and coherent, and the cavity drag is low. Beyond this value, the oscillations are weak, frequencies are mostly broadband, and the drag is high. This transition was found to occur for a cavity of length-to-depth ratio of 5. The investigation has also explored features of cavity shear-layer development, hitherto unexplained, and these are elucidated.

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.