Cavity Trailing Edge Research Articles

Numerical investigation on influence of different geometrical cavity flame holders and flame stabilization mechanism in hydrogen fueled multi-strut-based scramjet combustor

A detailed computational study was carried out to explore how the presence of a cavity and cavity angles influence the performance and combustion mechanisms within a scramjet combustor. This research utilized ANSYS Fluent 2021 to create hexahedral meshes and simulate the complex fluid dynamics in scramjet engines equipped with dual strut injectors. The simulations addressed various critical aspects of fluid mechanics, including the continuity, momentum (via the Navier-Stokes equations), and energy equations. These equations were used to model non-reacting flows, while the species transport equations were applied to analyse reacting flows. The turbulence within the combustor was modelled using the standard K-ε model, a common choice for such high-speed aerodynamic evaluations. The study's primary goal was to assess the effects of different geometrical configurations of cavity flame holders on the performance characteristics of an H2-based scramjet combustor overall efficiency, focusing on mixing efficiency, combustion performance, and total pressure loss. Both configurations with and without a cavity were examined. Key findings indicate that incorporating a cavity into the combustor design leads to developing a robust recirculation zone within the cavity area. This recirculation zone is pivotal in enhancing fuel-air mixing and combustion efficiency, with cavity-based combustors showing an earlier onset of combustion and achieving peak combustion efficiencies around 90–95%. The extent of the recirculation region is notably influenced by the proximity of the strut injector to the cavity's length. Additionally, the presence of a shear layer within the cavity generates a weak shock at the cavity's trailing edge. This shock interaction can adversely affect scramjet combustor performance, especially at higher cavity angles (α) and specific geometric configurations, such as an L/D ratio of 4 and α=30°. The L/D ratio of 4 and α=30° combustor model results in a 13.8% increase in turbulence intensity, which raises air-fuel mixing and there by improves mixing and combustion efficiency by 14.23% and 13.34%, compared to the other depth of the cavities. This advantage is critical, especially considering the compact length of the combustor, which is a desirable attribute in scramjet design.

Effects of Ramp-Induced Incident Shock Impingement Under Various Slope Angle Configurations in a Strut-Based/Cavity-Assisted Supersonic Combustor

ABSTRACT The present study conducts a numerical investigation into the effects of ramp-induced shock impingement with different opening angles on the global flow-field and combustion behaviors within a strut-based/cavity-assisted scramjet combustor. To reproduce the intricate physical phenomena and all inflow properties of the primary channel, two-dimensional Reynolds-averaged Navier–Stokes (RANS) equations integrated with the shear stress transport (SST) k − ω turbulence model is implemented. The current approach considers several slope angle configurations of the ramp since they generate various shock systems that differ in magnitudes and coherent structures, substantially affecting the combustion efficiencies and chemical species distributions. Results reveal that an optimal shock angle of the ramp in correspondence with the current operating conditions can be achieved at 20°, which increases the combustion efficiency by 11% at the end of the cavity trailing edge without provoking excessive loss of total mass-flux-weighted pressure recovery. However, further expanding the slope angle above the optimum will trigger the flow blockage mechanism which enlarges the recirculation region on both sides of the ramp, simultaneously initiate their local thickening processes and create shock train areas owing to intensive interactions with the turbulent boundary layers. These phenomena are known for decreasing the total pressure ratio, adversely affecting all performance metrics of interest and destabilizing the main channel flow path.

Thermochemical non-equilibrium hypersonic flow over a rectangular cavity embedded on a compression ramp

This paper reports a systematic computational investigation that elucidates the fundamental thermochemical non-equilibrium physics that occurs when air at Mach number of 11 encounters a rectangular cavity of aspect ratio L/D = 2.0 embedded on a 25° compression ramp. The mechanistic details of this highly complex flow phenomenon are obtained by solving the compressible form of the Navier–Stokes equations in two dimensions using a finite-volume open-source library. Chemical and thermal non-equilibrium processes are treated using a five-species, 12-reaction chemical kinetics, and a two-temperature model, respectively. Following a detailed validation and grid sensitivity study, two simulations are conducted, one with isothermal boundary conditions and the other with conjugate heat transfer (CHT) to identify the effect of energy transmission to the material on surface heat flux. Fast Fourier transforms and near-wall velocity profiles inside and in the neighborhood of the cavity are used to identify primary oscillatory modes and shear layer dynamics. Two new descriptive states defined as “states I and II,” representative of the minimum and maximum deflection of the shear layer, are used to discuss the dynamical behaviors in the cavity, including the separation region before the cavity, trailing edge effects, frequency analysis of probe data collected at several key locations, and the effect of CHT on surface heat flux. It is found that the flow features at the cavity's center strongly influence the separation upstream of the cavity, and the transrotational temperature near the cavity's trailing edge is strongly correlated with the oscillations of the shear layer.

MCGA-assisted ignition process and flame propagation of a scramjet at Mach 2.0

The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc (MCGA) at Mach 2.0 were investigated. Effects of equivalence ratios on the MCGA-assisted ignition process and flame propagation of the scramjet were recorded by two high-speed cameras from different view angles. The discharge characteristics of MCGA are also collected synchronously with the high-speed cameras. The distributions of temperature, velocity, and equivalence ratios in non-reactive flows of the cavity were simulated by Reynolds Averaged Navier-Stokes (RANS) model. The results show that MCGA can achieve reliable ignition with the Global Equivalence Ratios (GER) between 0.06 and 0.17. The ignition process is composed of flame kernel generation, flame development, and stable combustion. The time from flame kernel generation to the establishment of global flame decreases as GER decreases from 0.17 to 0.08. In the streamwise direction, the flame first develops to the Cavity Leading Edge (CLE) because of the influence of the cavity recirculation zone and then uplifts into the cavity shear layer, and finally develops to the Cavity Trailing Edge (CTE). In the spanwise direction, the flame width is less than 50% of the width of the cavity before developing to CLE and begins to develop towards the two sides of the combustor after reaching CLE, which is affected by the angular recirculation zone on both sides of CLE. The ignition processes by MCGA in the scramjet combustor are significantly affected by local distributions of equivalence ratios and velocity in the cavity.

Pressure oscillation suppression and mode transition for supersonic cavity flows controlled by upstream mass injections

Direct numerical simulations were performed to investigate an active control strategy for supersonic (Mach 1.8 and 2.2) flows past a rectangle cavity with a length-to-depth ratio of 4. A steady mass injection is applied upstream of the cavity as the active control technique. The pressure oscillations are significantly suppressed by two mechanisms: (1) thickening and lifting of the cavity shear layer to alleviate downstream impingement with the cavity trailing edge and (2) weakening of the cavity shear layer instability. When the initial boundary layer thickness of the supersonic cavity flow is relatively small, a stronger mass injection leads to increased cavity shear layer thickening and uplift, increased weakening of the shear layer instability, and higher suppression of the pressure oscillations. When the Mach number equals 1.8, the dominant flow mode changes from the Rossiter II mode to the Rossiter III mode under active control, which is detected by the dynamic mode decomposition. However, the mode transition under active control substantially differs if the initial boundary layer thickness is relatively large, for which the pressure oscillation suppression controlled by the high-velocity upstream mass injection is not better than a low-velocity injection, owing to a higher shear layer instability. Mechanism (2) listed above is therefore more important than mechanism (1).

Effect of different typical high speed train pantograph recess configurations on aerodynamic noise

For high-speed trains, the aerodynamic noise becomes an essential consideration in the train design. The pantograph and pantograph recess are recognised as important sources of aerodynamic noise. This paper studies the flow characteristics and noise contributions of three typical high-speed train roof configurations, namely a cavity, a ramped cavity and a flat roof with side insulation plates. The Improved Delayed Detached-Eddy Simulation approach is used for the flow calculations and the Ffowcs Williams & Hawkings aeroacoustic analogy is used for far-field acoustic predictions. Simulations are presented for a simplified train body at 1/10 scale and 300 km/h with these three roof configurations. In each case, two simplified pantographs (one retracted and one raised) are located on the roof. Analysis of the flow fields obtained from numerical simulations clearly shows the influence of the train roof configuration on the flow behaviour, including flow separations, reattachment and vortex shedding, which are potential noise sources. A highly unsteady flow occurs downstream when the train roof has a cavity or ramped cavity due to flow separation at the cavity trailing edge, while vortical flow is generated by the side insulation plates. For the ramped cavity configuration, moderately large pressure fluctuations appear on the cavity outside walls in the upstream region due to unsteady flow from the upstream edge of the plate. The raised pantograph, roof cavity, and ramped cavity are identified as the dominant noise sources. When the retracted pantograph is located in the ramped roof cavity, its noise contribution is less important. Furthermore, the insulation plates also generate tonal components in the noise spectra. Of the three configurations considered, the roof cavity configuration radiates the least noise at the side receiver in terms of A-weighted level.

Numerical investigation on flow characteristics of low-speed flow over a cavity with small aspect ratio

Numerical simulations of low-speed flow over a two-dimensional cavity at different cavity length-to-depth ratio (R) and freestream Reynolds number (Re) are performed by using the Nektar++ method. This work focuses on flow characteristics of vortices inside cavity, such as evolution process, number, and velocity due to variations in the R and Re. From the evolution process shown by the cavity flow at R = 1 and Re = 1612, it was found that both the main and corner vortices originate from the wall vortex appeared on the cavity wall, and the main vortex reaches stable faster than two corner vortices. There are seven vortices of similar size and shape arranged vertically inside the cavity for the case of R = 0.1 and Re = 1612, number of these vortices decreases to one while velocity of fluid inside the cavity increases gradually as the R increases from 0.1 to 0.9. The increase of Re results in more vortices and a larger velocity of fluid inside the cavity, furthermore, a sequence of small-scale vortical structures downstream of the cavity trailing edge are generated under the condition of Re = 25792. In addition, sidewall effects on cavity flow characteristics are investigated by employing a three-dimensional cavity, and the analysis shown that sidewall effects on flow pattern of fluid in the spanwise plane near the sidewall are not negligible.

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.

Numerical study of cavitation regimes in flow over a circular cylinder

Cavitating flow over a circular cylinder is investigated over a range of cavitation numbers for both laminar and turbulent regimes. We observe non-cavitating, cyclic and transitional cavitation regimes with reduction in freestream $\sigma$. The cavitation inside the Karman vortices in the cyclic regime, is significantly altered by the onset of "condensation front" propagation in the transitional regime. At the transition, an order of magnitude jump in shedding Strouhal number is observed as the dominant frequency shifts from periodic vortex shedding in the cyclic regime, to irregular-regular vortex shedding in the transitional regime. In addition, a peak in pressure fluctuations, and a maximum in St versus $\sigma$ based on cavity length are observed at the transition. Shedding characteristics in each regime are discussed using dynamic mode decomposition. It is demonstrated that the condensation fronts observed in the transitional regime are supersonic (referred as "condensation shocks"). In the presence of NCG, multiple condensation shocks in a given cycle are required for complete cavity condensation and detachment, as compared to a single condensation shock when only vapor is present. This is explained by the reduction in pressure ratio across the shock in the presence of NCG effectively reducing its strength. In addition, at $\sigma=0.85$ (near transition from the cyclic to the transitional regime), presence of NCG suppresses the low frequency irregular-regular vortex shedding. Vorticity transport at $Re=3900$, in the transitional regime, indicates that the region of attached cavity is nearly two-dimensional with very low vorticity, affecting Karman shedding in the near wake. In addition, the boundary-layer separation point is found to be strongly dependent on the amounts of vapor and gas in the freestream for both laminar and turbulent regimes.

Investigation of Spectral Characteristics by Passive Control Methods Past a Supersonic Cavity

Numerical investigations of the leading-edge sawtooth spoiler and slanted aft wall were performed to explore the mechanisms of the suppression of pressure fluctuations in an open-type cavity flow, focusing on the pressure fluctuation spectra and their spatial distributions. The rectangular cavity has a length-to-depth ratio of six, and the freestream Mach number is 1.5. Improved delayed detached-eddy simulation was used to predict the unsteady cavity flows and pressure fluctuations. Both control methods exhibited effectiveness in pressure fluctuation suppression, but their respective mechanisms are quite different. It was observed that the leading-edge sawtooth spoiler lifts the shear layer and reduces its interaction with the aft wall. The energy distribution of Rossiter modes changes significantly, which indicates that the pressure fluctuation feedback loop inside the cavity has changed. However, the slanted aft wall merely alters the cavity flowfield. Unlike the leading-edge method, the Rossiter mode distribution and the feedback mechanism remain unchanged. The slanted aft wall mainly suppresses the upstream-traveling perturbation generated by the cavity trailing edge but has little influence on the downstream-traveling perturbation.

Experimental investigation of starting characteristics and wave propagation from a shallow open cavity and its acoustic emission at supersonic speed

Experiments were carried out on a shallow open cavity (L/D = 5) at a supersonic Mach number (M = 1.8) to understand its transient starting characteristics, wave propagation (inside and outside the cavity) during one vortex shedding cycle, and acoustic emission. Starting characteristics and wave propagation were visualized through time resolved schlieren images, while acoustic emissions were captured through unsteady pressure measurements. Results showed a complex shock system during the starting process which includes characteristics of the bifurcated shock system, shock train, flow separation, and shock wave boundary layer interaction. In one vortex shedding cycle, vortex convection from cavity leading edge to cavity trailing edge was observed. Flow features outside the cavity demonstrated the formation and downstream movement of a λ-shock due to the interaction of shock from the cavity leading edge and shock due to vortex and generation of waves on account of shear layer impingement at the cavity trailing edge. On the other hand, interesting wave structures and its propagation were monitored inside the cavity. In one vortex shedding cycle, two waves such as a reflected compression wave from a cavity leading edge in the previous vortex shedding cycle and a compression wave due to the reflection of Mach wave at the cavity trailing edge corner in the current vortex shedding cycle were visualized. The acoustic emission from the cavity indicated that the 2nd to 4th modes/tones are dominant, whereas the 1st mode contains broadband spectrum. In the present studies, the cavity feedback mechanism was demonstrated through a derived parameter coherence coefficient.

Flow between the train underbody and trackbed around the bogie area and its impact on ballast flight

The aerodynamic behaviour of flow past a simplified high-speed train bogie including the ground underneath with ballast particles at scale 1:10 is studied numerically. It is found that the flow around the bogie is highly unsteady due to strong flow separations and flow interactions developed there. Generally, the ballast particles distributed inside the wheels are situated in the stronger turbulent flow and are subject to much higher aerodynamic forces than the particles located outside the wheels. Moreover, these aerodynamic forces increase when the ballast particles are located downstream of the bogie cavity and reach the peak values close to the bogie cavity trailing edge. The force time-series are produced based on the simulations of an array of the ballast particles in a wind-tunnel setup and it shows that the ballast flight is apt to happen as the rear part of the bogie cavity passing by the ballast bed. When the ballast particles become airborne, the fluctuating forces generated increase significantly. Therefore, the stronger unsteady flow developed around the bogie cavity, especially in the cavity trailing edge region, will produce larger fluctuating forces on the ballast particles, which will be more likely to cause ballast flights for high-speed railways.

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.

Generation of Acoustic Disturbances in Supersonic Laminar Cavity Flows

The generation of acoustic disturbances in supersonic laminar cavity flows is investigated by large-eddy simulations of supersonic laminar flow (M = 1.2, 2.0, and 3.0) past a rectangular cavity with a length-to-depth ratio of 2. Results suggest that well-originated large-scale vortical structures with strong spanwise coherence are present in the shear layer. Compressibility effects have significant impacts on the shear-layer development and the fluctuation properties. The dominant mechanism for the acoustic radiation in supersonic laminar cavity flows is shown to be associated with the successive passage of large-scale vortices over the cavity trailing edge. It is found that Mach waves radiated from the cavity shear layer may have significant contributions for the noiseradiation in terms of enhancing the strength of the feedback compression waves.

The effect of acoustic forcing on an airfoil tonal noise mechanism

The response of the boundary layer over an airfoil with cavity to external acoustic forcing, across a sweep of frequencies, was measured. The boundary layer downstream of the cavity trailing edge was found to respond strongly and selectively at the natural airfoil tonal frequencies. This is considered to be due to enhanced feedback. However, the shear layer upstream of the cavity trailing edge did not respond at these frequencies. These findings confirm that an aeroacoustic feedback loop exists between the airfoil trailing edge and a location near the cavity trailing edge.

Large-Eddy Simulations of cavitation in a square surface cavity

We report on the development and application of a multiphase approach to the prediction of cavitation induced by high-speed flow over and within a square surface cavity. The approach entails employing a full cavitation model in conjunction with Large-Eddy Simulations in order to capture the initiation and development of bubble formations in turbulent-flow conditions. The incipient formation of the bubble cloud, and the flow processes of vortex shedding and shear-layer oscillations are tracked using the Volume of Fluid method. The validity of the computational approach was assessed by comparisons with experiments on cavitating flow over a hydrofoil. Application to the case of flow over and within a two-dimensional square cavity with cavitation clearly reveal the presence of traveling cavitation at the corner of the cavity trailing edge, and vortex cavitation within the cavity. It is shown that the collapse of cavitation bubbles results in an impact frequency that is higher than the frequency of the shear-layer oscillations. This implies that structural damage due to cavitation is likely to be most severe at the corner formed at the intersection of the cavity’s trailing edge and the flat surface upstream of it.

Cavitation instabilities in hydraulic machines

Cavitation instabilities in hydraulic machines, hydro turbines and turbopump inducers, are reviewed focusing on the cause of instabilities. One-dimensional model of hydro turbine system shows that the overload surge is caused by the diffuser effect of the draft tube. Experiments show that this effect also causes the surge mode oscillations at part load. One dimensional model of a cavitating turbopump inducer shows that the mass flow gain factor, representing the cavity volume increase caused by the incidence angle increase is the cause of cavitation surge and rotating cavitation. Two dimensional model of a cavitating turbopump inducer shows that various modes of cavitation instabilities start to occur when the cavity length becomes about 65% of the blade spacing. This is caused by the interaction of the local flow near the cavity trailing edge with the leading edge of the next blade. It was shown by a 3D CFD that this is true also for real cases with tip cavitation. In all cases, it was shown that cavitation instabilities are caused by the fundamental characteristics of cavities that the cavity volume increases with the decrease of ambient pressure or the increase of the incidence angle.

Three-dimensional flow features in a nominally two-dimensional rectangular cavity

The object of the present paper was to look at the three-dimensional features of a flow in a nominally two-dimensional rectangular cavity having the length-to-depth ratio equal to 3, with laminar incoming boundary layer. The motivation was to give a contribution to the understanding of the origin of the spanwise flow structures observed in cavity flows. Particle image velocimetry was used to highlight the flow structure at low Reynolds number in the plane parallel to the cavity external flow and normal to the cavity bottom (vertical plane) and in planes parallel to cavity bottom (horizontal planes). The results demonstrate that the present measurements are qualitatively and quantitatively in agreement with published studies having as object the observation of the cavity flow behavior in the vertical plane, assuming the cavity to be nominally two-dimensional. Results in the planes parallel to the cavity bottom show the three-dimensional nature of the flow, characterized by the discontinuous presence of jet-like flows directed towards the cavity leading edge and towards the cavity trailing edge. The origin and nature of these discontinuous “jets” are examined and discussed in the paper. The flow structures are characterized by a wavelength scaling with the cavity length in proximity of the leading edge and with the cavity depth in the region of the trailing edge. Isolated three-dimensional flow structures consisting of side by side counter-rotating vortices have been also observed in the horizontal planes and tentatively interpreted as secondary flows induced by the instability of the main recirculation bubble.

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.

NUMERICAL AND EXPERIMENTAL STUDY ON UNSTEADY SHEDDING OF PARTIAL CAVITATION

Periodically unsteady shedding of partial cavity and forming of cavitation cloud have a great influence on hydraulic performances and cavitation erosion for ship propellers and hydro machines. In the present study, the unsteady cavitating flow around a hydrofoil has been calculated by using the single fluid approach with a developed cavitation mass transfer expression based on the vaporization and condensation of the fluid. The numerical simulation depicted the unsteady shedding of partial cavity, such as the process of cavity developing, breaking off and collapsing in the downstream under the steady incoming flow condition. It is noted that good agreement between the numerical results and that of experiment conducted at a cavitation tunnel is achieved. The cavitating flow field indicates that the cavity shedding was mainly caused by the re-entrant jet near cavity trailing edge, which was also clearly recorded by high-speed photographing.