Aft Wall Research Articles

Effect of Axisymmetric Aft Wall Angle Cavity in Supersonic Flow Field

Abstract Cavity plays a significant role in scramjet combustors to enhance mixing and flame holding of supersonic streams. In this study, the characteristics of axisymmetric cavity with varying aft wall angles in a non-reacting supersonic flow field are experimentally investigated. The experiments are conducted in a blow-down type supersonic flow facility. The facility consists of a supersonic nozzle followed by a circular cross sectional duct. The axisymmetric cavity is incorporated inside the duct. Cavity aft wall is inclined with two consecutive angles. The performance of the aft wall cavities are compared with rectangular cavity. Decreasing aft wall angle reduces the cavity drag due to the stable flow field which is vital for flame holding in supersonic combustor. Uniform mixing and gradual decrease in stagnation pressure loss can be achieved by decreasing the cavity aft wall angle.

Investigations of self-excited vibration in splitter plate with a cavity in the supersonic mixing layer

The combustion efficiency strongly depends on the mixing process of fuel–air for a combined cycle engine. For an efficient mixing process, self-excited vibration of splitter plate is more practical than that of forced vibration. We have carried out a comprehensive numerical and experimental study of self-excited vibration. Finite element method (FEM) is applied to analyze the natural modal of vibration. Large eddy simulations (LES) are employed for the mechanistic study of self-excited vibration and their influence factors are evaluated from the experimental studies. Displacements are accurately measured by a non-intrusive laser vibrometer. It is found that the first-order vibration shape is quasi-two-dimensional and responsible for improved mixing. We found that self-excited vibration of the splitter plate is motivated by a cavity due to the acoustic self-oscillation. Moreover, self-excited vibration depends on the gap distance between fixed rod and splitter plate, static pressure difference between upper and lower nozzle outlet, length to depth ratio (K) and aft wall angle (θ). At zero gap distance, the frequency is up to 2.5 times higher than that of 3 mm gap distance. So, reduction in gap distance can efficiently increase the frequency of self-excited vibration, which is an encouraging point for the future study.

Numerical Modeling of Transpiration-Cooled Turbulent Channel Flow with Comparisons to Experimental Data

Transpiration cooling is a promising active cooling technique that provides coolant through a porous wall at a rate sufficient to maintain temperatures below a desired limit. The present study uses ANSYS® CFX to numerically investigate the influence of cooling gas injection on the temperature, velocity, and wall skin friction in the boundary layer of a subsonic turbulent hot-gas channel flow for various blowing ratios (), hot-gas temperatures (), and Mach numbers (, ). Here, a cooled permeable porous ceramic matrix composite sample with a hot-gas channel flow is simulated using a monolithic and two-domain approach. For the two-domain approach, separate solvers are used for the hot-gas and porous domains, respectively. These are applied alternatively and coupled to each other by boundary conditions imposed at the respective interface. The simulations are validated against experimental data and provide complementary insight into the effects of the cooling, which cannot be assessed from experimental measurements alone. The results show that, although a monolithic approach would be more convenient from a numerical point of view, a two-domain approach has proven to be superior in matching experimental data for this study. Furthermore, improved thermal boundary-layer profiles downstream of the porous sample are obtained by including the aft wall in the coupling process. This study adds to the limited theoretical and numerical investigations found in the literature on transpiration cooling.

The three-dimensional characteristics of the unsteady wall-pressure in a low-Mach-number rectangular cavity flow with Rossiter model oscillation

This study is focused on examining the three-dimensional characteristics of the unsteady wall-pressure in a low-Mach-number rectangular cavity with Rossiter model oscillation. The effect of cavity width on the resonance is investigated as well. It is observed that not only the resonance magnitude but also the resonance frequency could be altered when the cavity width changes. As the cavity becomes narrow to W/L ≈1, the resonance frequency decreases substantially comparing to that for W/L >1. While for the narrowest cavity with W/L <1, the resonance frequency is the same as that for the wide cavity. Low-frequency disturbances grow stronger as approaching to the side walls. Instantaneous velocity magnitude measurement simultaneously with the unsteady wall-pressure shows the propagation of flow disturbances highly correlated with the pressure resonance. It is observed that for the cavity with W/L ≈1, the pressure resonance is highly correlated with a local propagation of disturbances near the cavity aft wall. The downstream convecting disturbances in the shear layer are found to be weaken in the cases of W/L ≈1. Meanwhile the convection velocity of shear-layer disturbances is decreased, which results in a decrease of the resonance frequency.

Experimental study of supersonic flow over cavity with aft wall offset and cavity floor injection

Experimental studies were conducted at Mach 1.71 supersonic flow over open cavity to find the best configuration of combined passive (aft wall offset) and active methods (cavity floor injection) to optimize the noise suppression. Cavity of length to depth ratio 3 with 0%, 5% and 10% aft wall offset were analyzed with the combined effect of 2 bar, 4 bar and 6 bar injection pressures, injected from locations of 25%, 50% and 75% of cavity length from the front wall. Experimental methodology includes instantaneous schlieren visualization and unsteady pressure measurements. In all the schlieren images, the presence of shear layer and various cavity flow features are clearly visible. Suppression in tonal amplitude for increase in offset and injection pressure is observed for various injection locations. Injection location at 75% cavity length from front wall was found to provide the maximum suppression. Spectrogram plots clearly indicate the redistribution of energy among tones and broadband noises. Increase in the number of tones is observed for higher injection pressure and aft wall offset combination. Nature of acoustic wave is confirmed from correlation and coherence plot. Over All Sound Pressure Level (OASPL) and normalized mean pressure plot are used to perform comparative study for various configurations.

Entrainment characteristics of cavity shear layers in supersonic flows

Two-dimensional large eddy simulations have been carried out to investigate entrainment characteristics of cavity shear layers in supersonic flows, which is crucial to the understanding and modeling of cavity-stabilized combustion for scramjet applications. Effects of free-stream Mach number and cavity configuration are investigated. The stability and growth rate of the cavity shear layers are found to strongly depend on the compressibility effects, similar to those observed in the free shear layers. However, the growth rate of the cavity shear layers is much higher under low supersonic conditions and decreases more rapidly with increasing Mach number when compared to that of free shear layers. Cavities with larger length-to-depth ratio or larger aft angle appear to have greater shear-layer growth rate. Entrainment of the free-stream fluids into the cavity recirculation zone is to a large extent determined by the interactions between the shear layer and the cavity aft wall. The mass flux entrainment ratio of the cavity shear layers appears lower than that of free shear layers and basically ranges from 0.6 to 1.7 under the present conditions.

Loads and Acoustics Prediction on Deployed Weapons Bay Doors

Unsteady separated flow from deployed weapons bay doors can interact with the highly unsteady flow in the open bay cavity, which is known to exhibit strong acoustic content and could lead to fluid-resonance and high-intensity acoustic noise. The culmination of these unique flow physics can potentially excite structural modes of the doors, aircraft surfaces, or externally carried munitions and fuel tanks and can ultimately lead to aeroelastic instabilities, such as buffet, flutter, limit-cycle oscillations, or fatigue-induced failures. A hybrid Reynolds-averaged Navier–Stokes large eddy simulation (RANS/LES) method with low-dissipation schemes is developed to improve flow and acoustics predictive capabilities for supersonic weapons bays. Computational simulations are conducted for a weapons cavity with different deployed bay doors configurations, including the effect of dynamically moving doors, to assess the tonal content and unsteady aerodynamic loads on the doors. Wind tunnel testing is also carried out to provide unsteady experimental data for use in validating the high-fidelity simulation capability. The simulation results in terms of unsteady pressure, velocity fluctuations, and pressure resonant frequencies are computed and presented. The results suggest that the deployed doors energize the shear layer and cause it to go deeper into the cavity and produce higher unsteady fluctuations on the weapons cavity floor and aft wall. The deployed doors also cause a shift in the dominant resonant modes.

The influence of cavity parameters on the combustion oscillation in a single-side expansion scramjet combustor

Cavity has been validated to be efficient flameholders for scramjet combustors, but the influence of its parameters on the combustion oscillation in scramjet combustor has barely been studied. In the present work, a series of experiments focusing on this issue have been carried out. The influence of flameholding cavity position, its length to depth ratio L/D and aft wall angle θ and number on ethylene combustion oscillation characteristics in scramjet combustor has been researched. The obtained experimental results show that, as the premixing distance between ethylene injector and flameholding cavity varies, the ethylene combustion flame will take on two distinct forms, small-amplitude high frequency fluctuation, and large-amplitude low frequency oscillation. The dominant frequency of the large-amplitude combustion oscillation is in inverse proportion to the pre-mixing distance. Moreover, the influence of cavity length to depth ratio and the aft wall angleθexists diversity when the flameholding cavity position is different and can be recognized as unnoticeable compared to the impact of the premixing distance. In addition, we also find that, when the premixing distance is identical and sufficient, increasing the number of tandem flameholding cavities can change the dominant frequency of combustion oscillation hardly, let alone avoid the combustion oscillation. It is believed that the present investigation will provide a useful reference for the design of the scramjet combustor.

Numerical study on passive convective mass transfer enhancement

Passive mixing mechanisms are widely used for heat and mass transfer enhancement. Vortices generated in flowfield lead to gradients that favour convective mass transfer. Computations on enhancement of convective mass transfer of sublimating solid fuel by baroclinic torque generated vortices in the wake of a swept ramp placed in high speed flow is presented here. Advection Upstream Splitting Method (AUSM) based computational scheme employed in the present study, to solve compressible turbulent flow field involving species transport, could capture the complex flow features resulted by vortex boundary layer and shock boundary layer interactions. Convective mass transfer is found to get improved in regions near boundary layer by horseshoe vortex and further transported to other regions by counter rotating vortex pair. Vortices resulted by flow expansion near aft wall of wedge and recompression wave-boundary layer interactions also promotes convective mass transport. Extensive computations have been carried out to reveal the role of vortices dominance at various lateral sweep angles in promotion of convective mass transfer in turbulent boundary layer.

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.

Prediction of particle distribution and particle impact erosion in inclined cavities

The cavity is one of the basic structures in pipe systems, but it is vulnerable in erosive environments. Cavities can occur in various places throughout industrial processes, each making a difference in the distribution of erosion. The aim of this study is to investigate gas (air)-solid flow properties and erosion characteristics in inclined cavities. A two-way URANS (SST)-DPM method was adopted. The simulations were carried out for three Stokes numbers (St), eight height differences (h) from 0 to 0.8H, and ten inclination angles from 0° to 90°. Results indicated that gas flow properties change little with inclination angle (φ). However, a variation in φ resulted in a change in the distribution of the particles and erosion on the aft wall, especially when St were 1.2 and 9.1. The particle number (N) in the zone 10mm from the aft wall decreases linearly with φ for both previously mentioned St, and the slopes of N-φ curves increase exponentially with h. The maximum erosion rate (Em) on the aft wall also decreases linearly with φ for the St of 1.2, and the negative slope of Em-φ curve follows an exponentially increasing with h. But Em decreases quadratically with φ when St is 9.1, and the function coefficient appears a Gaussian relationship with h.

Effect of cavity fore wall modifications in supersonic flow

Understanding the complex behaviour of the cavity flow is essential for the design of supersonic combustor. The characteristics of the axisymmetric aft ramp cavity with fore wall modification have been experimentally studied in a blow-down type supersonic flow facility. The facility consists of a conventional CD nozzle that issues a flow Mach number of 1.88 to a supersonic combustor of circular cross section, which is placed immediately downstream of the nozzle. The axisymmetric cavities are incorporated within the combustor. The cavities are of open type and their length is kept constant while their depth varies. The aft wall of the cavities is inclined with three ramp angles and the fore wall is provided with a constant fillet radius of 3 mm. The performance of the cavity is analysed based on wall static pressures, momentum flux distribution at the exit of the combustor and stagnation pressure loss across the combustor. The study reveals that fore wall fillet cavities provide less cavity drag and stagnation pressure loss for various aft ramp angles under identical operating conditions.

Experimental study on the characteristics of axisymmetric cavity actuated supersonic flow

The effective means of air fuel mixing and flame holding can be achieved by incorporating cavity in supersonic combustor. Understanding the complex flow field of cavity flow is essential for the design of supersonic combustor. An attempt is made to understand the characteristics of supersonic flow past axisymmetric cavity, and a series of nonreacting experiments are carried out in a blow-down type supersonic flow facility. The facility consists of a supersonic nozzle, issues a flow Mach number of 1.80 into a circular cross sectional supersonic combustor in which axisymmetric cavity is placed. Cavity of two consecutive aft wall angles is the key parameter for the study. The performance of the cavity is investigated based on the static pressure measurement, momentum flux distribution at the exit plane of the combustor, and the stagnation pressure loss of the flow. Wall static pressure distribution revealed that pressure increases with decrease in the secondary aft wall angle below 45° due to stronger recompression of shear layers. Moreover, decreasing primary aft wall angle provides a uniform mixing profile along with decrease in stagnation pressure loss across the combustor.

Heat flux reduction mechanism induced by a combinational opposing jet and cavity concept in supersonic flows

The thermal protection on the surface of hypersonic vehicles attracts an increasing attention worldwide, especially when the vehicle enters the atmosphere at high speed. In the current study, the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Menter׳s shear stress transport (SST) model have been employed to investigate the heat flux reduction mechanism induced by the variations of the cavity configuration, the jet pressure ratio and the injectant molecular weight in the combinational opposing jet and cavity concept. The length of the cavity is set to be 6mm, 8mm and 10mm in order to make sure that the cavity configuration is the “open” cavity, and the jet pressure ratio is set to be 0.4, 0.6 and 0.8 in order to make sure that the flow field is steady. The injectant is set to be nitrogen and helium. The obtained results show that the aft angle of the cavity only has a slight impact on the heat flux reduction, and the heat flux peak decreases with the decrease of the length of the cavity. The design of the thermal protection system for the hypersonic blunt body is a multi-objective design exploration problem, and the heat flux distribution depends on the jet pressure ratio, the aft wall of the cavity and the injectant molecular weight. The heat flux peak decreases with the increase of the jet pressure ratio when the aft angle of the cavity is large enough, and this value is 45°.

Aft wall offset effects on open cavities in confined supersonic flow

The effect of aft wall offset in open cavities in a Mach 1.71 flow is investigated experimentally. Two different rectangular cavities with length to depth ratios of 2 and 3 are investigated to validate the effects over the range of the open cavities. Three different offsets of 0%, 5% and 10% are selected for exploring the effects of aft wall offset in cavity flow fields. For investigating the flow physics, unsteady pressure measurements and schlieren flow visualisation techniques are employed. All the cases tested, exhibited the characteristic cavity flow features. Schlieren flow visualisation clearly indicated the presence of shear layer and other shock features that are associated with the cavity flow field. Statistical analysis techniques viz. Fast Fourier Transforms, correlation, coherence and spectrogram are utilised for analysing the unsteady pressure data. Experimental results confirmed the reduction in tonal amplitude in offset cavities for both the length to depth ratios. The 10% offset case provided the maximum reduction in both the length to depth ratio, 2 as well as length to depth ratio, 3 cavity. Spectrogram studies indicated the presence of temporal mode switching in the higher offset value cases. Higher number of modes are also noted to be excited for the offset cases as evident from the spectrogram. 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.

Complex Geometry Effects on Cavity Resonance

The flow over an aircraft bay is often represented using a rectangular cavity; however, this simplification neglects many features of actual flight geometry that could affect the unsteady pressure field and resulting loading in the bay. To address this shortcoming, a complex cavity geometry was developed to incorporate more realistic aircraft-bay features including shaped inlets, internal cavity structure, and doors. A parametric study of these features was conducted based on fluctuating pressure measurements at subsonic and supersonic Mach numbers. Resonance frequencies and amplitudes increased in the complex geometry compared to a simple rectangular cavity that could produce severe loading conditions for store carriage. High-frequency content and dominant frequencies were generated by features that constricted the flow such as leading-edge overhangs, internal cavity variations, and the presence of closed doors. Broadband frequency components measured at the aft wall of the complex cavities were also significantly higher than in the rectangular geometry. These changes highlight the need to consider complex geometric effects when predicting the flight loading of aircraft bays.

Numerical Investigation of Cavity Flow Field Characteristics in Supersonic Flow

The cold flow field in a two dimensional cavity of supersonic combustor has been simulated numerically by using the compressible flow Navier-Stokes equation with the κ-ω SST turbulence model. The flow field structure of different cavity aft wall slope angle (16°,30° and 90°) , different fore aft wall height ratio (1 and 2) and different length depth ratio (3 and 5) are analyzed. The conclusions are as follows: As cavity aft wall slope angle decreases, the compression wave formed at cavity leading separation corner shifts into expansion wave, the shear layer moves into cavity gradually; As cavity fore aft wall height ratio increases from one to two, the expansion wave formed at cavity leading separation corner strengthens and there is no compression wave formed at;As cavity length depth ratio increases from three to five, the compression or expansion wave formed at cavity leading separation corner weakens, cavity bottom wall pressure tends to be constant and aft wall pressure rises.

Experimental validation of CFD simulations of free-fall lifeboat launches in regular waves

This paper presents results from model tests and CFD simulations of lifeboat launches in regular waves. To the authors’ knowledge, this is the first time CFD simulations of lifeboat launches in waves have been validated. The predicted accelerations agreed well with the measured ones. The pitch velocity was slightly overestimated due to a slight difference in the geometric features at the bow between the CFD model and the physical model. The simulations provided in general accurate or conservative estimates of the local pressure at various locations on the hull except on one location on top of the canopy where the pressure was slightly under-predicted. Furthermore, it has been shown that to improve the predictions of the pressure loads on the aft wall of the lifeboat, the compressibility of air has to be taken into account in the simulations in order to capture the behaviour of the air-pocket behind the lifeboat.

Experimental study of particle erosion in a cavity with a height difference between its walls

Experiments were carried out to analyze the effects of cavity height on cavity erosion under gas–solid flow. Spatial distributions of particles and erosion rates were documented for cavities with a variable difference in their leading and aft wall heights in experiments using two particle sizes. Results showed that the number of particles found inside the cavity decreased dramatically, as the difference in cavity wall height increased. Besides, the micro-erosion appearance on the top area of the aft wall flattened out with the increase of height difference (h). The erosion curves also indicated that the erosion rate on the top area of the aft wall for the cases of h≠0 is larger than that on the same y position for the case of h=0. Meanwhile, the maximum erosion rate is more serious for larger particle. The experiment also indicated that increasing the height difference between cavity walls reduced erosion of the aft wall. And the maximum erosion rate was markedly reduced, when the height difference between cavity walls was altered from 0 to about 0.2H.

Effect of aft wall offset and ramp on pressure oscillation from confined supersonic flow over cavity

A study is conducted on supersonic flow of Mach number 1.65 over rectangular cavity of length to depth ratio (L/D) 3 with aft wall ramp angles of 30°, 60° and 90° for with and without aft wall offset. Unsteady pressure measurement and schlieren flow visualization is employed to gain insight into the flow physics. Schlieren visualization revealed shock train like pattern for the cavities without offset. These shock structures are unsteady and oscillated longitudinally inside the test section over the cavity. The shock pattern disappeared as the blockage produced due to cavity pressure rise and shear layer oscillation is minimized by offsetting the cavity aft wall. Experimental investigations confirmed that the size of the cavity relative to the test section could have significant effect on the nature of flow field over the cavity and also on the acoustic oscillations within the cavity. For 90° ramp angle the dominant mode frequency in pressure oscillations shifted from second to third mode as the cavity aft wall is offset; however the shift in dominant mode frequency is not observed for the 60° cavity with aft wall offset. The forward traveling acoustic wave inside the cavity is confirmed to be present for 60° and 90° ramps while it is found to be absent for 30° ramp angled cavities. Analysis of the unsteady pressure measured inside the cavity revealed that the shock train has significant effect on the nature of acoustic oscillations from within the cavity. Aft wall offset is found to be effective in avoiding the occurrence of shock train and it is also found to increase the level of cavity pressure fluctuations. The highest pressure occurred at the aft wall and static pressure at all locations inside the cavity decreased as the aft wall ramp angle is reduced.