Supersonic Open Cavity Research Articles

Global and local modal characteristics of supersonic open cavity flows

Flows past cavities at high-speeds have become increasingly important in applications such as flame-holding and propulsion unstart control. Recently maturated linear techniques have helped discern the underlying mechanisms in the subsonic and low supersonic speed regime (M∞<1.4). Here, we combine these linear methods with fully non-linear two- and three-dimensional simulations to assimilate the significant changes observed when the Mach number is increased further to the 1.4≤M∞≤3 range. The resolvent method is first employed to analyze cavity-shear layer coupled oscillations and modal characteristics, which are found to differ in key respects from those reported at lower Mach numbers. At higher speeds, more 2D coupled modes are obtained with the dominant modes containing secondary waves displaying elaborate patterns. The role of the shear layer on the cavity dynamics is then examined with local spatial stability analyses. In addition to the well-known Kelvin–Helmholtz instability encountered in the subsonic and transonic regimes, forward-propagating (k+) supersonic shear layer instabilities are detected at higher speeds. These are associated with Mach wave reflections between the shear layer and the cavity floor and may introduce higher order coupled modes. Furthermore, 3D modal analysis indicates a shift toward the dominance of 3D modes compared to 2D modes; although consistent with compressible free shear layer observations, 2D cavity modes remain significant to higher convective Mach number. When the Reynolds number is increased, resolvent-based mode shapes and frequencies continue to compare favorably with Dynamic Mode Decomposition of large-eddy simulations because of inviscid instability dominance.

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.

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.

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.

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.

On the feedback mechanism in supersonic cavity flows

Self-sustained oscillations in supersonic cavity flows are investigated by implicit large-eddy simulations of a supersonic flow (M∞ = 2.0, ReD = 105) past a three-dimensional rectangular cavity with length-to-depth ratio of 2. Both turbulent and laminar inflows are considered, and a variation of boundary-layer thickness in the turbulent inflow case is conducted. An additional simulation of turbulent free shear layer is also performed to illustrate the relationship between shedding vortices and acoustic excitations. Feedback mechanism is identified as the dominant mechanism driving the self-sustained oscillations in supersonic open cavity flows, regardless of the upstream turbulent state and the boundary-layer thickness. The generation of discrete vortices in the cavity shear layer is shown to be highly associated with acoustic excitations rather than natural instabilities of the cavity shear layer. Simulation results support that the primary noise source arises from the successive passage of large-scale vortices over the cavity trailing edge. The effects of upstream boundary layer on the shear-layer characteristics and acoustic fields will also be discussed.

Characteristics of Oscillations in Supersonic Open Cavity Flows

Characteristics of oscillations in supersonic open cavity flows are investigated numerically using hybrid RANS/LES (Reynolds-Averaged Navier-Stokes/Large Eddy Simulation) method. The oscillation regimes and feedback mechanisms for the supersonic cavity flows are identified and analyzed. The calculation captures a mixed shear-layer/wake oscillation mode in the flow of Ma = 1.75, where these two modes occur alternately. The shear-layer mode and wake mode are driven by vortex convection-acoustic feedback and absolute instability, respectively. In particular, the results indicate that the feedback-acoustic-wave in the shear-layer mode is probably generated by the reflection of the downstream-traveling pressure wave, associated with the shed vortex in the shear layer, on the aft wall. The cavity flow of Ma = 2.52 is then simulated to see the influence of Mach number. It is found that the increase of Mach number may decrease the amplitude of the fluctuations in the shear layer, inhibiting the transition to wake mode. Furthermore, the influence of upstream injection is also studied, where the results show that the injection only weakens the oscillations and faintly shifts the resonant frequencies.

Comparison of three Navier-Stokes equation solvers for supersonic open cavity simulations

Comparison of three Navier-Stokes equation solvers for supersonic open cavity simulations

Algebraic turbulence model simulations of supersonic open-cavity flow physics

A time-accurate double thin-layer Navier-Stokes computation is performed for an unsteady supersonic open cavity with a length-to-depth ratio of 2. The results are used to determine the flow-physics mechanisms responsible for the cavity oscillation cycle. A new cycle is described and compared to previous descriptions. It is found that a shed vortex impinges on the cavity aft lip and forms a pressure pulse that augments or forces, at the vortex shedding frequency, an internal upstream moving wave that has been reflected from the aft corner. This upstream moving wave eventually reflects off the cavity forward wall and forces the shedding of a new vortex. It was found, however, that the reflected wave dissipates before it reaches the aft wall. Instead, a second wave forms beneath the shed vortex and eventually reflects from the aft corner and is forced at the shedding frequency by the shed vortex wave, completing the cycle.

Comparison of Baldwin-Lomax Turbulence Models for Two-Dimensional Open Cavity Computations

A study of several modified versions of the Baldwin-Lomax turbulence model applied to two-dimensional supersonic open cavity flowfields are performed. The results are clearly affected by the levels of turbulent viscosity imposed in the cavity.

Numerical study of a supersonic open cavity flow and pressure oscillation control

Numerical experiments were performed to study the flow structure and suppression of pressure oscillation in the M.,. = 2.5 supersonic open cavity flow problem. Numerical results show that the large pressure variation within the cavity induced by the shear layer's wavy shape produces new vortices not found in situations where M.,. < 2.5. The proposed suppression device imposes mass transfer on the cavity's aft bulkhead. The proposal is tested by replacing one of the cavity walls with a porous wall and adjoining vent chamber. Constant porosity and uniform vent-chamber pressure are assumed. Replacing the aft bulkhead with a porous wall/vent chamber combination provides the most effective suppression of pressure fluctuation, even with a porosity value as small as 0.1 for vent-chamber pressure ranging from p, to l.5py.. Mass transfer at the cavity floor and forward bulkhead are also examined, but they are found to be less effective.