Aft Wall Research Articles

Performance evaluation of a scramjet engine utilizing varied cavity aft wall divergence with parallel injection in a reacting flow field

Abstract The effect of implications of the dual cavity with aft wall divergence in a parallel injection reacting flow has been numerically investigated. This research intends to emphasize the behaviour of the supersonic flow under varying divergence angles of the cavity aft wall. A two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equation and an SST k-ω turbulence model with a single-step chemical reaction for hydrogen-air are utilized for the simulation. Followed by the strut injector, the cavities are positioned symmetrically inside the combustor. The bottom cavity aft wall divergence varies, whereas the top wall is mounted with a rectangular cavity. The performance of cavity locations is compared with the baseline DLR model. From the evaluation of numerical outcomes along with different cavity configurations, it has been noted that the 15-degree cavity divergence angle enhances the recirculation zone which leads to improved mixing performance. Also, the cavity improves the combustion stability by increasing the flow residence time. From this numerical analysis, it is associated that an almost 20 % reduction in combustor length is achieved, however, an 18 % rise in the pressure loss is noted because of emanating shock waves from the cavity edges.

Flow control in a confined supersonic cavity flow using subcavity

The effects of the front wall and aft wall sub-cavities in the flow field of a confined supersonic deep cavity are numerically investigated. The turbulent simulations are carried out by deploying a finite volume-based explicit density-based solver in the OpenFOAM framework in conjunction with the k − ω SST (Shear Stress Transport) turbulence model. A cavity with a length-to-depth ratio of three placed in a confined passage is considered in the study. The freestream Mach number at the entrance of the passage is approximately 1.71. The addition of the sub-cavity of lengths ranging between 0.2 and 0.3 times the length of the main cavity in the front wall and the aft wall, significantly affects the frequencies of cavity oscillations as obtained from the spectral signature. The front wall sub-cavity of length ratio 0.2 reduces the dominant frequency by almost 60 percent as compared to the baseline cavity. The analysis and comparison of the flow field using the numerical schlieren in both configurations reveal a significant alteration in the flow field. The flow visualization provides a distinct understanding of the attenuation and enhancement of pressure oscillations obtained through spectral analysis in the presence of sub-cavities.

ROLE OF DUAL CAVITY ON THE COMBUSTION PERFORMANCE OF A SPLITTER PLATE–ASSISTED SUPERSONIC COMBUSTOR

An unsteady-state simulation using Reynolds-averaged Navier-Stokes was conducted to explore the performance of an ethylene-fueled scramjet combustor employing a dual cavity floor injection strategy and a splitter plate. The validity of this approach was confirmed by comparing it to experimental data, which showed a reasonable match between the observed experimental results and the simulation. A thorough analysis of the injection system's performance is conducted using variables such as the scramjet combustor's total pressure loss, wall pressures, and combustion efficiency. The findings revealed that the leading edge cavity shock waves' interaction from both the walls created a highpressure area close to the cavities, which forces the fuel jets in the direction of the combustor walls. Fuel distribution into the combustor is thereby prompted by the intense interactions that arise between the cavity's aft walls and fuel jets. Moreover, the recirculation zones within the cavity are significantly expanded, improving both the performance of combustion and mixing within the cavity. The combustion efficiency in the current study has shown a notable increase of 13% compared to our previously investigated configuration involving a splitter plate combined with a single cavity combustor.

Experimental Study on Combustion Modes and Oscillations in a Cavity-Based Scramjet Combustor

This study conducted experiments to investigate flame behaviors in a cavity-based scramjet combustor operating under Mach 2.92 supersonic inflow. Pressure measurements and flame chemiluminescence observations were combined to study the initial flame formation and combustion mode transitions. In addition, flame luminosity standard deviation and fast Fourier transform power spectral density (PSD) were post-processed to assess the impact of equivalence ratio and aft wall height on combustion modes as well as oscillations. It is indicated that increasing the equivalence ratio leads to a transition from lifted shear-layer mode to ramjet mode. Elevating the cavity aft wall height improves the combustor’s self-ignition capability and promotes the transition from lifted shear-layer mode to ramjet mode as long as flame stabilization is achieved. It is found that increasing the equivalence ratio and aft wall height also results in more intense combustion oscillations; besides, raising the cavity aft wall height could accentuate the predominant frequency of low-frequency oscillations in the combustor. Finally, it is demonstrated that the application of spark plasma effectively suppresses these low-frequency combustion oscillations, which is a promising active control method in the scramjet engine.

Numerical investigation on flow and aerothermal characteristics of rarefied hypersonic flows over slender cavities

Flow and aerothermal characteristics of rarefied hypersonic flows passing through slender cavities are investigated comprehensively using the direct simulation Monte Carlo (DSMC) method by various cases, such as the cavity with different depth-to-width ratios and different inclination angles, and the freestream at different altitudes and Mach numbers. The cavity depth-to-width ratio has a great influence on flow structures and gas density inside the cavity, but a slight effect on gas temperature inside the cavity and on heat flux over cavity surfaces. In comparison to the vertical cavity, the upstream-inclined cavity reduces the gas density and the high-temperature area inside the cavity, showing a decrease rate of gas density of about 53 % for φ = -45°, while the downstream-inclined cavity greatly raises the gas density and slightly expands the high-temperature area, reaching a growth rate of gas density of about 114 % for φ = 45°. As the freestream altitude increases from 50 to 80 km, the main vortex expands remarkably while the heat flux near the top of the aft wall decreases sharply, and a noteworthy finding is that the dimensionless temperature contours inside the cavity for the cases of H = 50, 60, and even 70 km seem to be unchanged. Compared with the case of Ma = 6, the maximum density inside the cavity is increased by about 5.5 times for the case of Ma = 30, and it is about 7.3 times for the maximum temperature inside the cavity. However, raising the freestream Mach number seems to not alter the streamline patterns inside the cavity. The three-dimensional effect reduces the maximum density inside the cavity by about 14.5 % to 17.4 % and results in a smaller main vortex compared with the two-dimensional (2D) cavity, and moreover, the slender cavity can weaken the three-dimensional effect due to its larger depth-to-width ratio and the relatively short cavity width. Besides, the vortex-transport theory is proposed, which can well explain the change of gas density inside the cavity for all cases considered in this work.

Effects of various aftwall edge ramp on open cavities in a confined supersonic flow

An experimental study on the supersonic flow of Mach number 1.81 Over the rectangular cavity of L/D = 3 with different aft wall angles such as Baseline case 90° and aft wall edge ramp of 30°. Measurements of acoustic oscillations were carried out on the wall and floor of the cavity using unsteady pressure transducers. Schlieren flow visualization indicates the presence of a shear layer and the other shock features that are associated with the cavity flow field. In order to obtain the experimental results, data analysis was performed in the unsteady data Using statistical analytical techniques i.e., Fast Fourier Transform, Short Time Fourier Transform, and Coherence. After careful examination of the power spectra of the cavities, it is concluded that as the aft wall angle decreases by introducing the aft wall ramp, the amplitude decreases. It is clearly observed that the complete escape of vortices in Low angled cavity. From the correlation plot, the existence of the acoustic wave inside the cavity for all the cases is deduced. The maximum OASPL and static pressure values are observed at the aft wall. These results, the strength of the self-sustained oscillations and acoustic disturbance and mode switching were observed in the higher angled cavity 90°. Also, the mode switching co-existence was reduced in the aft wall edge ramp angle of 30°.

Aeroacoustic investigation of transonic flow behavior in M219 deep cavity with passive flow control configurations

This study aims to investigate the effect of front and aft wall modifications and their combinations on the sound pressure level response of the deep configuration of the M219 cavity at transonic speed. Experimental results for the baseline geometry of the cavity were validated with the high-fidelity lattice Boltzmann method computational fluid dynamics solver, SIMULIA PowerFLOW®. In this study, inclined and parallel cuts were applied to the front and aft walls of the M219 cavity to determine their efficiency in reducing the overall sound pressure level (OASPL). The most effective configurations on reducing OASPL at the front and aft walls were combined to form mixed configurations. All front wall and aft wall configurations as well as mixed configurations were validated through grid independence studies. Simulations showed that modifications to the aft wall were more effective in reducing noise than modifications to the front wall. The spectral analysis of pressure oscillations provided information about the broadband and tonal pressure responses of the mixed configurations. The findings showed that the mixed configurations were effective in reducing the frequency and amplitude of the Rossiter modes. The most successful configurations on noise reduction also showed a decrease in the drag coefficient and the average static pressure distribution along the cavity.

Influence of cavity floor injection strategy on mixing improvement study of a splitter plate-assisted supersonic combustor

In the current research, an effort has been made to investigate the splitter plate-based scramjet combustor by inserting a cavity in the lower wall and observing how the different geometric parameters of the combustor, namely the cavity's aft wall angle, the bottom wall's divergence angle, and the splitter plate's angle of attack, affect the mixing performance of the supersonic combustor. Initially, experimental data from the published literature for a splitter plate is used to validate the implemented computational approach. Next, different aft wall angles of the cavity are compared, and the optimal value is found to be 120° The current investigation demonstrates that geometric modification significantly affects the performance of a splitter plate-assisted supersonic combustor. An increase in the cavity's aft wall angle improves the combustion efficiency. A further study is conducted on the splitter plate + cavity at various divergence angles of the bottom wall. According to our study, the separation region on the top wall increases as the divergence angle increases, resulting in a reduction in combustion performance. Finally, while simulating different angles of attack of splitter plate, it has been noticed that, for a negative angle of attack (-1°), the cavity provides a recirculation region and a sufficient shear mixing layer. This allows adequate air-fuel mixing to sustain combustion.

Active control of pressure fluctuations in an incompressible turbulent cavity flow

Large-eddy simulations of incompressible turbulent boundary layer flows over an open cavity are conducted to investigate the effects of wall-normal steady blowing on the suppression of pressure fluctuations on the cavity walls. The length (L) to depth (D) ratio (L/D) of the cavity is 2 and the Reynolds number based on the cavity depth (ReD) is 12,000. Wall-normal steady blowing is imposed through a limited transverse slot in an upstream region of the cavity leading edge while varying the momentum coefficient (Cμ). As the value of Cμ increases, the pressure fluctuations on the cavity walls decrease mostly due to weakened impingement of turbulent vortical structures residing in the shear layer on the aft wall by lifting effects, reaching a maximum reduction of −7.61% compared to a baseline when Cμ=0.015. However, as the value of Cμ increases further, the pressure fluctuations increase gradually, exceeding that of a baseline when Cμ>0.05. Because intensified turbulent vortical structures in the shear layer with an increase of Cμ lead to strong interactions of small-scale vortices with the cavity walls with an accompanying large motion of the primary recirculation zone within the cavity, additional pressure fluctuations generated on the cavity walls deteriorate the control performance due to the blowing.

Effects of multiple subcavities with floor subcavity in supersonic cavity flow

Experimental and computational analysis has been carried out by many researchers on supersonic cavity flow, but detailed analysis based on Rossiter's model still requires some insight. In the current study an open rectangular cavity with a length to depth ratio of 2 (L/D = 2) and Mach number at the inlet as 1.71, was considered as a baseline configuration for experimental analysis. Statistical techniques such as power spectral density (PSD), correlation, and overall sound pressure level (OASPL) were carried out on the unsteady pressure data, to analyze the aero-acoustic flow physics. High-speed schlieren images were processed to obtain spatially coherent modes by proper orthogonal decomposition (POD). The analysis was extended for different dimensions of subcavities on the aft, floor, and front wall. This detailed analysis of these configurations with different dimensions and combinations revealed the various flow features and mode frequencies in supersonic cavity. As the front wall subcavity act as a passive control device, reducing the overall sound pressure level inside the cavity whereas, the aft wall subcavity acts as a passive resonator with distinct harmonic fluid-resonant modes, a similar phenomenon was observed for floor subcavity at different locations. A novel method was employed to analyze Rossiter's model and its applicability in estimating experimental modes was verified, as it accurately predicted the dominant frequencies with a maximum of 2.74% uncertainty among all the configurations.

Direct numerical simulation of supersonic internal flow in a model scramjet combustor under a non-reactive condition

A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case.

Numerical Simulation about the Characteristics of the Store Released from the Internal Bay in Supersonic Flow

To understand the influence of the initial release conditions on the separation characteristics of the store and improve it under high Mach number (Ma = 4) flight conditions, the overset grid method and the Realizable turbulence model coupled with an equation with six degrees of freedom are used to simulate the store released from the internal bay. The motion trajectory and the attitude angle of the store separation under the conditions of different centroid, velocity, height and control measures are given by the calculated result. Through analysis, the position of the centroid will affect the separation of the store, which needs to be considered in the design. Increasing the launching height is conducive to the separation of the store. If the store has an initial velocity, it can leave the internal bay more quickly and reduce the probability of collision with the wall. Cylindrical rod and slanted aft wall control measures can improve the attitude of the store and make the store fall more smoothly.

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.

WITHDRAWN: Effects of the penetration height of ethylene transverse jets on flame stabilization behavior in a Mach 2 supersonic crossflow

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM.

Effects of the penetration height of ethylene transverse jets on flame stabilization behavior in a Mach 2 supersonic crossflow

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM.

Experimental study on the flame stabilization mode in the rear-wall-expansion cavity

The present study experimentally investigates the transition of flame stabilization mode at Ma = 2.92 using a model scramjet combustor. The combustion mode is analyzed by pressure measurement and quasi-one-dimensional analysis. The effects of both equivalence ratio and aft wall offset on flame behaviors are further characterized by CH* chemiluminescence. The results show that the transition from cavity stabilized combustion mode to jet-wake stabilized combustion mode is realized by increasing the equivalence ratio. And the quasi-one-dimensional analysis reveals that all cases work in the Early Scram mode and approach thermal chocking after increasing equivalence ratio. As the equivalence ratio increases, the amplitudes of the flamebase and heat release gradually increase, and their distributions gradually migrate downstream. The amplitude of flamebase and heat transfer remains prominent with the consistent rise in equivalence ratio. At the same time, the spatial distribution of heat release tends to spread out upstream and occasionally appear upstream of the leading edge of the cavity, indicating that the flame stabilization mode transforms from cavity stabilized combustion mode to jet-wake stabilized combustion mode. The spatial variation of local heat release rate is responsible for the transition of flame stabilization mode. A slight aft wall offset promotes the transition from cavity stabilized combustion mode to jet-wake stabilized combustion mode at a higher equivalence ratio.

Experimental investigation on the flame stabilization modes of liquid kerosene in a cavity-based scramjet combustor

The flame stabilization modes of liquid kerosene is investigated in a model scramjet combustor with flight condition of Ma 6. The effects of equivalence ratio and cavity configuration schemes on the flame stabilization modes are studied. Flame propagations and the evolution of the flow field from the ignition to the establishment of stable flame in the supersonic flow are acquired through high-speed photography and schlieren photography. Only cavity-recirculation-region stabilized combustion can be obtained in the single-cavity scramjet combustor. The intensity of combustion is weak and is termed as local ignition with low combustion efficiency. In the tandem-cavity scramjet combustor, the flame stabilization mode is largely determined by the total equivalence ratio. The flame could spread into the mainstream and even propagate against the supersonic flow as the equivalence ratio increases. The positive feedback mechanism between combustion and precombustion shock train is responsible for the evolutions of the flame stabilization modes, where the interactions between shock waves and turbulent boundary layer play a key role in the flame propagation process. The large-scale eddies in the shear layer of the upstream cavity shed at the cavity aft wall and could enhance the turbulent levels of the incoming flow of the downstream cavity. Consequently, an intense kerosene flame can be ignited in the tandem-cavity combustor. The flame could not spread to the mainstream in the single-cavity combustor due to the lower turbulence level weakening the interaction of combustion products in the recirculation zone with unburned reactants in the supersonic flow.

Numerical simulation for rarefied hypersonic flows over non-rectangular deep cavities

A comprehensive numerical study is performed to investigate rarefied hypersonic flows past various non-rectangular cavity configurations using the direct simulation Monte Carlo method with the effects of free-stream Mach numbers, high-temperature gas effects, and three-dimensional (3D) effects analyzed in depth. In this work, two groups of non-rectangular cavities are considered: one is the shallow-front type and the other is the shallow-rear type. The primary objective is to obtain insight into the flow characteristics and surface pressure on and heat flux to these non-rectangular cavities. Making the front or rear of the cavity shallower does not alter the flow characteristics inside the upper cavity too much, but it causes the vortex to not fill the entire cavity any longer. Instead, a “dead-water” region is formed in the bottom cavity, and this “dead-water” region becomes expanded as the front or rear of the cavity gets shallower. In addition, making the front or rear of the cavity shallower has little influence on surface pressure on and heat flux to the aft wall of the cavity, while it plays an important role in the distributions of surface pressure and heat transfer coefficients over the cavity floor. For the case of shallower-rear cavity, the surface pressure and heat flux at the right end of the cavity floor are as high as 2 and 20 times the rectangular-cavity value, respectively. Free-stream Mach number and high-temperature gas effects have a negligibly minor influence on flow characteristics inside the cavity, while 3D effects play an important role. In comparison with 2D cavities, 3D relieving effects in finite-span cavities prevent the external stream from penetrating deeper into cavities, leading to much smaller surface pressures on and heat fluxes to the cavity floors.

Numerical study on detonation initiation process in the chamber with characteristic structures of the dual-mode scramjet combustor

In order to improve the performance of traditional ramjet, a ramjet based on pulse detonation (PD-Ramjet) instead of isobaric combustion was proposed. Several special-shaped detonation chambers which retained the characteristic structures of the dual-mode scramjet combustor were designed. Characteristic structures included the expansion channel and the cavity. The filling process and detonation initiation process of the stoichiometric hydrogen/air mixture under the incoming flow condition of the sub-combustion mode were studied in the detonation chambers with characteristic structures by two-dimensional numerical simulation method, and the influences of the characteristic structures on the filling process and detonation initiation process were analyzed. The simulation and analysis results indicated that the hydrogen concentration at both sides of the chamber near the outlet was low due to the structure of the expansion channel, and the pressure and velocity of the detonation wave decreased gradually after the expansion channel. The cavity had a significant influence on the filling process, which resulted in the uneven hydrogen concentration downstream the cavity, especially near the outlet of the chamber. The intensity of the detonation wave attenuated to a certain extent by reason of the cavity, where the peak pressure first decayed due to the sudden expansion of the flow path and then rose owing to the reflection at the cavity aft wall.

The investigation of inclined AFT wall cavities in a circular scramjet combustor

Achieving good mixing with high combustion efficiency, stable flame holding, and low stagnation pressure losses are still open issues in supersonic reacting flows. Mixing and vorticity in supersonic flows are mainly driven by the baroclinic term that is correlated to the density and pressure gradients. An increase of the baroclinic term has been observed to strongly depend on shock waves arising within the combustion chamber. This work numerically investigated the interaction between the fuel jet and the air stream as a function of the aft wall angle inclination. In fact, this parameter has a primary impact on the shock wave inclination arising from the cavity leading edge and therefore on the baroclinic term. Four different combustor geometries have been investigated: without cavity and with three different aft wall inclinations (90°–15°, 60°–15°, 30°–15°). 3D RANS numerical simulations showed that the primary ramp angle has a critical impact on the fuel-air mixing and consequently on the combustion efficiency. A correlation between the combustor geometry and the shock angle inclination has been proposed. Greater combustion efficiency, homogeneous H2O, and temperature distribution across the combustor are found with the 30–15 aft wall cavity configuration. Lowering the primary aft wall of the cavity from 90° to 30°, resulted in the rising of a stronger bow shock, extended recirculation volume within the cavity, and increased fuel-air mixing.