Cavity Flow Field Research Articles

Data-based analysis of multimodal partial cavity shedding dynamics

Time-resolved X-ray densitometry void fraction measurements and accompanying acoustic emissions have revealed that partial cavity shedding on a hydrofoil can be multimodal, with spontaneous changes in shedding sequence (referred to here as cavitation style) for fixed inlet flow conditions. These spontaneous, intermittent transitions between two physically different shedding styles are examined using data-based image analysis of the cavity flow fields in order to extract the associated physical mechanism leading to style transition. Three data-driven decomposition techniques are compared: proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), and cluster-based reduced-order modeling (CROM), with a primary focus on the latter. The results highlight the utility of CROM over DMD and POD in the context of intermittent event analysis, both in terms of mode interpretability and transition mechanism identification. A frequency-based analysis of the CROM output revealed the existence of a shared harmonic between the different physical cavitation modes which gave further insight to the transition process. The data-based analysis ultimately illuminates the underlying flow mechanism that leads to the style transition, namely the key role of maximum cavity length buildup as it interacts with the vapor cloud collapse downstream of the hydrofoil.

Ignition dynamics of a pulse detonation igniter in a supersonic cavity flameholder

The dynamics of using a pulse detonation wave to ignite a supersonic cavity flameholder was investigated. Fueling conditions ranging from lean to rich were investigated. Simultaneous 40-kHz schlieren imaging, OH planar laser-induced fluorescence, and chemiluminescence imaging were used to elucidate the fluid dynamics and combustion physics throughout the transient disruption of the ignition event. For moderate and rich fueling conditions, the ignition event stabilized combustion away from the detonation plume until the detonation products were fully exhausted. Ignition of leaner conditions required a low-level flame kernel to persist in the cavity step until the recirculating cavity flow field was re-established. The initial ignition mechanism was shown to be primarily driven by hot products from the detonation plume shedding into the low velocity region near the aft facing step in the cavity, followed by propagation and stabilization of a flame to a favorable region of the cavity. Quenching of cavity combustion at lean overall cavity mixtures was shown to be due to temporary backpressuring of cavity fuel injectors and cavity evacuation from the detonation pressure impulse.

Aeroacoustic Investigation of Passive and Active Control on Cavity Flowfields Using Delayed Detached Eddy Simulation

In the present study, CFD simulation with delayed detached eddy simulation (DDES) is performed to investigate an open cavity at Mach 0.85. A clean cavity and cavity with passive and active control methods, including sawtooth spoiler, flat-top spoiler, crossflow rod, and steady leading edge blowing, are analyzed. The results obtained from all the control methods are compared with clean cavity, and all the flow control methods show positive effect on the overall sound pressure level reduction with the decrement up to 8 dB. The effect of active control on sound pressure level in the cavity is much better than that of passive control, with the magnitude of tone noise decreasing by 20-30 dB. The main focus of this investigation is to test the noise suppression effect by passive and active control methods.

Experimental study of forward-facing cavity with energy deposition in hypersonic flow conditions

Controlling nose surface peak heat fluxes is crucial to the design of hypersonic vehicles. In this study, we report a novel technique of convective heat flux reduction on the nose surface of a spherically blunted cone by employing a forward-facing cavity combined with heat energy deposition inside the cavity. The heat deposition is achieved by the exothermic reaction of a chromium film coated on the cavity surface. The experiments are performed in hypersonic shock tunnels using air as the test gas at free stream stagnation enthalpy conditions of 2.2 ± 0.08 MJ/kg (H1), 3.2 ± 0.018 MJ/kg (H2), and 5.4 ± 0.02 MJ/kg (H3), for a geometry of cavity length to diameter ratio of 1. Schlieren images of the flow are captured using a high-speed camera and a high-power pulsed diode laser light source. The surface heat flux measurements were performed using calibrated platinum thin film sensors. We observed that the heat deposition altered the cavity flow field significantly by lowering the flow oscillation frequency and increasing the shock standoff distances with higher oscillation amplitudes. The overall surface mean heat flux reduction is increased from ≈13% to ≈49% compared to the blunt body geometry, whose nose radius is 30 mm and enhanced with reference to the cavity without heat deposition from ≈15% to ≈35%. Chromium film surface reactions are studied using X-ray Photoelectron spectroscopy, and the results confirm that the exothermic surface reactions of the Cr thin film are attributed to the formation of oxides and nitrides of Cr.

Relation Between the Finite-Time Lyapunov Exponent and Acoustic Wave

The relation between different time scales of the finite-time Lyapunov exponent (FTLE) and the acoustic wave is studied. Gonzalez et al.’s proposition (“Finite-Time Lyapunov Exponent-Based Analysis for Compressible Flows,” Chaos, Vol. 26, No. 8, 2016, Paper 083112), stating that the joint FTLE is equal to dilatation that measures the aeroacoustic waves, is extended. A rigorous mathematical proof is provided for the two-dimensional problem. An analytical sound field induced by a dipole source and a numerical simulated flowfield of an open cavity are used to verify the relation between the FTLE and dilatation. Based on a heuristic analysis and a numerical visualization, it was also found that the long time FTLE has similar patterns to the schlieren and can reveal both the near-field flow structures and the far-field acoustic waves. The work shows that the FTLE can be useful in studying acoustics.

Effect of heat transfer on an angled cavity placed in supersonic flow

Experimental and numerical studies on the effect of heat transfer to an open cavity placed in supersonic crossflow are presented. Cavity floor is subjected to the desired amount of heat flux and the consequent effects in the flow dynamics inside the cavity and its neighborhood are systematically assessed. Cavity flowfield has been visualized using high-speed Schlieren imaging and pressure on the cavity floor has been measured using both steady and unsteady pressure transducers. Two-dimensional unsteady compressible turbulent flow field has been numerically simulated using a Harten-Lax-van Leer-Contact (HLLC) scheme based unstructured finite volume method solver. The numerical method has been validated using both experimental data available in the literature as well as the wall pressure measured in the present study. Significant changes in flow dynamics such as the growth of the recirculation region, shearlayer oscillation, shearlayer impingement on the aft wall of the cavity, and frequency of cavity induced oscillations have been observed. Experimental observations are well complemented by the numerical studies and could reveal the physics of alteration in cavity flow dynamics with the heat addition.

Flowfield structure characteristics of the hypersonic flow over a cavity: From the continuum to the transition flow regimes

The cavity configurations produced by discontinuities at thermal protection system coated at surface of reentry space vehicles can lead to separation, attachment, and even transition of gas flows around the vehicle's surface, which can bring serious aerodynamic heating during the space vehicles reentering the Earth's atmosphere. In this paper, the continuum, slip, and transition flow regimes are chosen to represent reentry atmosphere environment and then a numerical investigation of hypersonic gas flows over a family of cavities is performed by using the direct simulation Monte Carlo method to reveal flow structures in the cavity flowfield under different length-to-depth ratios and freestream incoming Mach numbers. In particular, when the freestream varies from the continuum to the transition flow regimes, the minimal length-to-depth ratios required for cavity flow becoming the closed type are 8, 6, and 5, respectively. The effect of the freestream incoming Mach number on the cavity flow characteristics is getting weaker when the freestream varies from the continuum to the transition flow regime. The two significant findings are that the cavity flow characteristics under hypersonic conditions are primarily determined by cavity geometry rather than freestream incoming Mach number and secondly, effect of the freestream incoming Mach number on cavity flow type is very slight if the freestream is in the transition flow regime.

Effects of subcavity in supersonic cavity flow

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

Numerical analysis on the transient cavity flow field during liquid nitrogen jet fracturing

Liquid nitrogen jet fracturing is expected to provide a novel treatment for reservoir stimulation. To verify its feasibility, the flow fields in cavity at different times are simulated by CFD method. Then the transient temperature and pressure distributions are analyzed. Based on the cavity pressure distributions, the pressure boosting effect is evaluated. The results show that the nitrogen gas close to the cavity entrance is easily pushed out and the temperature in this region decreases quickly during liquid nitrogen jet fracturing. However, the temperature at the cavity bottom increases firstly at the squeezing action of liquid nitrogen jet and then decreases due to the heat transfer induced by the low temperature liquid nitrogen jet. Compared with water jet, the liquid nitrogen jet can generate equivalent pressure boosting effect in the cavity. Due to the compressibility of nitrogen gas, the pressure boosting process of liquid nitrogen jet fracturing falls behind the hydrajet fracturing. This study identifies the pressure boosting capability of liquid nitrogen jet and can further provide theoretical basis for the researches of this fracturing treatment.

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 Study of Subcavity in Supersonic Cavity Flow

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

Self-sustained oscillation for compressible cylindrical cavity flows

The presence of a cavity changes the mean and fluctuating pressure distributions. Similarities are observed between a cylindrical cavity and a rectangular cavity for a compressible flow. The type of cavity flow field depends on the diameter-to-depth ratio and the length-to-depth ratio. The feedback loop is responsible for the generation of discrete acoustic tones. In this study, the self-sustained oscillation for a compressible cylindrical cavity flow was investigated experimentally. For open-type cavities, the power spectra show that the strength of resonance depends on the diameter-to-depth ratio (4.43–43.0) and the incoming boundary layer thickness-to-depth ratio (0.72–7.0). The effective streamwise length is used as the characteristic length to estimate the Strouhal number. At higher modes, there is a large deviation from Rossiter’s formula for rectangular cavities. The gradient-based searching method was used to evaluate the values of the empirical parameters. Less phase lag and a lower convection velocity are observed.

Characteristics of cylindrical cavities in a compressible turbulent flow

The characteristics of flow over rectangular cavities have been well documented. However, few studies have considered cylindrical cavity flows. For compressible cylindrical cavity flows, the type of cavity flowfield is dependent on the diameter-to-depth ratio (D/H), which is similar to the length-to-depth ratio for a rectangular cavity flow. For an open-type cavity flow (D/H≤6.14), there is a slight upstream influence and uniform static pressure distributions are observed inside the cavity; a small adverse pressure gradient ahead of the rear face and a downstream expansion. The leading- and rear-edge expansions are more significant for transitional- (D/H=8.60–21.00) and closed-type (D/H=43.00) cavities. The amplitude of surface pressure fluctuations increases toward the rear face, for all of the test cases, whereas an additional peak near the middle of cavity floor is observed for a closed-type cavity. Notably, the effect of freestream Mach number is pronounced only on the amplitude of peak pressure fluctuations near the rear wall. Similarities between a cylindrical cavity and a rectangular cavity for a compressible flow are also highlighted in this study.

Oscillatory Characteristics of Shallow Open Cavities in Supersonic Flow

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

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.

高功率激光片状放大器片腔内流场模拟及优化

The cleanliness of neodymium glass surface in the slab amplifier is a key indicator of guaranteeing high performance and long lifetime of amplifiers, and the reasonable internal flow field distribution is the prerequisite for keeping clean in the amplifier cavity. By means of computational fluid dynamics method and with the Fluent software, the internal flow field of slab amplifier cavity in the purging process of pure nitrogen is simulated, and the model validity is verified on the prototype system. By adjusting the structure and location arrangement of air inlets and outlets of the amplifier, the optimal flow field distribution is obtained. The experimental results show that compared with those of the non-optimized amplifier, the optimized amplifier not only takes only half time to obtain the same required cleanliness, but also the cleanliness of neodymium glass surface is higher.

Effects of Freestream Turbulence on Cavity Tone and Sound Source

To clarify the effects of freestream turbulence on cavity tones, flow and acoustic fields were directly predicted for cavity flows with various intensities of freestream turbulence. The freestream Mach number was 0.09 and the Reynolds number based on the cavity length was 4.0 × 104. The depth-to-length ratio of the cavity,D/L, was 0.5 and 2.5, where the acoustic resonance of a depth-mode occurs forD/L= 2.5. The incoming boundary layer was laminar. The results for the intensity of freestream turbulence of Tu = 2.3% revealed that the reduced level of cavity tones in a cavity flow with acoustic resonance(D/L=2.5)was greater than that without acoustic resonance(D/L=0.5). To clarify the reason for this, the sound source based on Lighthill’s acoustic analogy was computed, and the contributions of the intensity and spanwise coherence of the sound source to the reduction of the cavity tone were estimated. As a result, the effects of the reduction of spanwise coherence on the cavity tone were greater in the cavity flow with acoustic resonance than in that without resonance, while the effects of the intensity were comparable for both flows.

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.

Cavity ignition in supersonic flow by spark discharge and pulse detonation

Ignition of an ethylene fueled cavity in a supersonic flow was achieved through the application of two energy deposition techniques: a spark discharge and pulse detonator (PD). High-frequency shadowgraph and chemiluminescence imaging showed that the spark discharge ignition was passive with the ignition kernel and ensuing flame propagation following the cavity flowfield. The PD produced a high-pressure and temperature exhaust that allowed for ignition at lower tunnel temperatures and pressures than the spark discharge, but also caused significant disruption to the cavity flowfield dynamics. Under certain cavity fueling conditions a multiple regime ignition process occurred with the PD that led to decreased cavity burning and at times cavity extinction. Simulations were performed of the PD ignition process, capturing the decreased cavity burning observed in the experiments. The PD exhaust initially ignited and burned the fuel within the cavity rapidly. Simultaneously, the momentary elevated pressure from the detonation caused a blockage of the cavity fuel, starving the cavity until the PD completely exhausted and the flowfield could recover. With sufficiently high cavity fueling, the decrease in burning during the PD ignition process could be mitigated. Cavity fuel injection and entrainment of fuel through the shear layer from upstream injection allowed for the spark discharge ignition process to exhibit similar behavior with peaks and valleys of heat release (but to a lesser extent). The results of using the two energy deposition techniques emphasized the importance of cavity fueling and flowfield dynamics for successful ignition.

Flow Field Simulation Analysis of Three-Way Water Hydraulic Valve Based on Surface Micro Modelling

Due to special physical and chemical properties of water medium, three-way water hydraulic valve has many key technical problems that could seriouly affect the operating performance. Safety valve in three-way hydraulic valve is seen as a research object, which is aiming at studying the effect of flow field distribution with surface micro modeling in three-way hydraulic valve cavity based on FLUENT. The simulation results show that, proper surface micro modelling help to improve the three-way water hydraulic valve cavity flow field distribution, different structural parameters affecting different flow field distribution, as for micro modelling design, careful consideration of the selection of structure parameters is necessary.