Upstream Cavity Research Articles

Investigation on ignition enhancement characteristics by high-energy spark in a scramjet combustor under low flight Mach conditions

Ignition characteristics of the long electrode distance high-energy spark igniter (LHSI) in a cavity-stabilized scramjet combustor at low flight Mach has been numerically and experimentally investigated and compared with those of conventional lower energy spark igniter (SI). The effects of ignition and injection schemes on ignition performance were analyzed and the enhancement of greater spark energy were verified. The results show that the widest ignition boundary of SI is observed when the kerosene is injected in far field due to the sufficient evaporation of kerosene droplets and the disturbance caused by upstream cavity. Due to the greater energy deposition by LHSI, the ignition performance is markedly improved and the ignition boundary of combustor is extended from 0.131–0.148 to 0.091–0.224 under the scheme of I1+L2. The increased energy deposition of spark plasma ignites more combustible mixture and enhances the initial combustion, leading to a larger high-temperature zone. This is crucial for ensuring the survival of the flame kernel against dissipation of airflow in the scramjet combustor operating at low fight Mach numbers due to the short duration of spark discharge.

Hydrodynamic Porosity: A New Perspective on Flow through Porous Media, Part II

In this work, we build upon our previous finding that hydrodynamic porosity is an exponential function of pore-scale flow velocity (or interstitial Reynolds number). We previously discovered this relationship for media with a square cavity geometry—a highly idealized case of the dead-ended pore spaces in a porous medium. Thus, we demonstrate the applicability of this relationship to media with other cavity geometries. We do so by applying our previous analysis to rectangular and non-rectangular cavity geometries (i.e., circular, and triangular). We also study periodic flow geometries to determine the effect of upstream cavities on those downstream. We show that not only does our exponential relationship hold for media with a variety of cavity geometries, but it does so almost perfectly with a coefficient of determination (R2) of approximately one for each new set of simulation data. Given this high fit quality, it is evident that the exponential relationship we previously discovered is applicable to most, if not all, unwashed media.

Investigation of Cavitation Flow and Entropy Production Characteristics in a Dual-Rotor Turbine Flowmeter

Flow meters are extensively utilized in fields such as chemical engineering, petroleum, and aerospace, and are an indispensable component of modern industry. This paper examines the metrological properties of a dual-rotor turbine flow meter within its measurable flow range through experimental approaches and investigates the cavitation flow dynamics within the flow meter using numerical methods. First, the flow characteristics curve of the dual-rotor turbine flow meter was established experimentally, and the accuracy of numerical simulation results was validated. Secondly, the transient characteristics of the cavitation cavity were revealed using the Z-G-B cavitation model and dynamic mesh technology. Finally, entropy production theory was applied to investigate the energy losses caused by cavitation, analyzing the contributions of different types of energy losses during the cavitation process. Flow calibration experiments and numerical simulations reveal an increase in the meter coefficient of the dual-rotor turbine flow meter in high-flow cavitation zones, indicating that the displayed flow rate is slightly higher during cavitation compared to non-cavitating flows. Transient cavitation flow undergoes three stages: attachment, development, and collapse. At 323 K, the volume fractions of upstream and downstream cavities increase by 38.9% and 48.3%, respectively, with the cavitation cycle duration being 1.21 times that at 298 K. At 343 K, these increases are 75.3% and 239.2%, with the cycle duration being 2.63 times that at 298 K. Among the various sources of loss, the contribution from losses due to pulsating velocity gradients is the most significant, with maximum proportions of 81.95%, 85.1%, and 87.11% at 298 K, 323 K, and 343 K, respectively.

Enhancing flow boiling using a microchannel with pillar–cavity mixed structures: A lattice Boltzmann study

Enhancement of flow boiling in microchannels through adjusting surface structures has attracted much attention in recent years. However, most of the existing studies focus on homogeneous surface structures. In the present study, a novel vertical microchannel with pillar–cavity mixed structures is conceived to enhance flow boiling heat transfer. In the mixed microchannel, cavities and pillars are distributed on the vertical sidewalls of the upstream and downstream flow channel, respectively. A multicomponent phase-change lattice Boltzmann model is employed to investigate the flow boiling performance of the mixed microchannel. Numerical results show that the cavities in the mixed microchannel can supply effective nucleation sites for timely departure of bubbles, while the pillars in the mixed microchannel can suppress the expansion of the vapor film from the outlet toward the inlet. Moreover, the bubbles from the upstream cavities can entrain the cold liquid to disrupt the vapor film covering the downstream pillars for the rewetting of the heated surface. As a result, the flow boiling performance can be significantly enhanced by the synergistic effect of the pillar and cavity structures, and the best flow boiling performance can be achieved by controlling the ratio of the number of cavities to the total number of structures in the mixed microchannel to optimize the synergistic effect. The influences of the structural parameters of pillars and cavities on the flow boiling performance have also been studied. It is found that the height of the pillars and the depth of the cavities have important influences on the flow boiling performance, while the boiling performance is not sensitive to the width of the pillars.

Windage Torque Reduction in Low-Pressure Turbine Cavities. Part I: Concept Design and Numerical Investigations

Abstract The windage torque on rotational walls has a negative effect on the performance of the low-pressure turbine. In this paper, three novel flow control concepts (FCCs) were proposed to reduce the windage torque within a turbine stator well, with upstream and downstream cavities connected by an interstage labyrinth seal. The swirl and flow pattern inside a reference turbine cavity was first investigated and the potential locations for the FCCs were identified using numerical simulations. FCC1 was a circumferential row of leaned deflectors downstream of the labyrinth seal. FCC2 was a set of deflector vanes and platform to optimize the ingress swirl at high radius in the upstream cavity. FCC3 combined the two flow concepts and the superposition resulted in a stator well windage torque reduction of 70% when compared to the baseline design. The FCCs also showed performance benefits at off-design conditions and over a range of secondary flow rates to the cavity. In Part II, the numerical analysis and performance of the FCCs are validated in an experimental rig using additively manufactured components.

Adaptation of von Karman Institute for Fluid Dynamics’ Isentropic Compression Tube Facility for High-Speed Low-Pressure Turbines Testing

Abstract This paper presents the commissioning of a newly designed High-Speed Low-Pressure Turbine (LPT) stage, operating at transonic exit Mach numbers and low Reynolds numbers, typical of modern geared turbofan (GTF) applications. The work falls within the scope of the Clean Sky 2 project SPLEEN (Secondary and Leakage Flow Effects in High-speed Low-pressure Turbines), which consists of an extensive experimental characterization of LPTs for GTFs. Geometries and measurements will constitute a valuable open-access database for the validation of simulation methods and data analysis tools. At first, the characteristics of the research turbine are illustrated. The LPT stage is designed with an equal number of stationary vanes and shrouded rotor blades. The upstream and downstream hub cavities are purged and feature engine-realistic rim seals. The nominal operating condition of the stage is reported, along with a set of off-design conditions, obtained by varying rotor speed and purge mass flows. The second part of the paper describes the substantial revamping of the CT3 large-scale compression tube at Von Karman Institute for Fluid Dynamics, traditionally employed for high-pressure turbine testing, now adapted to host a high-speed LPT stage. The third section is an overview of the time-averaged and fast-response instrumentation. The test article is heavily instrumented, to maximize the amount of acquired data, while minimizing the number of blowdown tests. The dataset for each operating conditions includes aerothermal measurements in the annulus flow, in the hub cavities and in the shroud labyrinth seal. In addition, this section presents the design of an in-house traversing system for continuous probe traversing during the test. Finally, the outcome of the commissioning phase is discussed, with particular emphasis on the operating conditions stability, as well as on the inlet and cavity injection uniformity. The commissioning of a traversing system for continuous probe traversing during the short-duration test is also discussed.

Anechoic wind tunnel noise tests for high-speed train bogie aerodynamic noise characterization

At speeds normally reached by high-speed trains (above 300 km/h), aerodynamic noise becomes significant. The bogie region is one of the main sources of aerodynamic noise in high-speed trains due to incoming flow impinging the bogie components and the cavity steps. Noise tests were performed in the Railway Technical Research Institute's anechoic wind tunnel placed in Tokyo, Japan, for different flow speeds from 100 km/h up to 250 km/h using simple shapes representing bogie components, and a 1:5 scale simplified bogie installed in a cavity. The noise spectra radiated by the simple shapes placed at different heights relative to the upstream cavity step are presented, showing the relation between the component geometry and position in the cavity with the produced noise. In addition, the measured noise spectra are shown for different configurations of the 1:5 scale simplified bogie: bare, only with front motor and gearbox, only with rear motor and gearbox and with all of them. The presented results aim to provide a better understanding of how the aerodynamic noise is produced in the bogie region of a high-speed train, and to be used for the prediction models adjustment or validation.

Experimental study on ignition and blowout characteristics of dual combined flame stabilizers in scramjet

The ignition and blowout characteristics have a significant impact on the performance and operating limit of the scramjet. In this study, direct-connected ground tests of a typical scramjet combustor have been conducted to simulate the high-altitude flight condition of Mach 6. The incoming air has a total temperature of 1689 K and a dynamic pressure of 30 kPa. The ignition and blowout characteristics of the dual strut/cavity combined flame stabilizers in the scramjet are obtained. The immediate response of pressure at key positions is used to analyze the forward propagation characteristics of pressure during the ignition process and the pressure variation during the flame blowout process. The flame distribution and temperature variation during the ignition and blowout processes are obtained by high-speed photography, OH⁎ spontaneous radiation photography, spectrometer and multispectral imaging equipment. Results indicate that the process from local ignition to global ignition successfully has durations lasting for hundreds of milliseconds, comprising four stages with two pressure lift stages. The flame diffusing forward and upstream cavity igniting successfully are the main processes, constituting 60% and 30% of the duration, respectively. During the ignition process, the pressure response time of the two-stage cavity differs by several milliseconds. And the flame is transmitted forward to the upstream cavity through combustor walls. As the ignition process progresses under high equivalent ratio (ER) conditions, flame stabilization mode shifts from stabilizing flame in the recirculation zone of the cavity to stabilizing flame in the shear layer of the strut. Upon completion of global ignition, the flame temperature at the shear layer of the cavity upstream exceeds 2700 K, but decreases to 2125 K as the core reaction zone is shifted from the top of the strut to the tail and bottom of the cavity when the ER decreases. Two pressure drops occur during the flame blowout process, and the critical ER of the global blowout is approximately 0.33. The primary shear layer flame is blown out during the first pressure drop, and the intensity of the cavity flame decreases. Finally, the second pressure drop occurs as the flame in the cavity moves downstream and extinguishes.

Numerical investigation on the effects of HC geometry on the flow and heat transfer in the disk cavity system at the turbine vane root

In this study, we numerically investigate the flow and heat transfer in the sealing system at the vane root of a heavy-duty turbine with the aim of determining the optimal geometry of honeycomb cells within a certain parameter range. First, we analyze the impact of honeycomb cell geometry on flow in the sealing gaps, including flow rotation, energy dissipation in the teeth chamber and honeycomb cell, and frictional losses. Next, we examine the heat transfer capacity between the leakage flow and the surfaces of the rotor or stator, and establish the relationship between the stream function and the average heat transfer coefficient. Third, we compare the predicted core swirl ratio, the most important aerodynamic parameter in disk cavities, with those from equations in the literature. The decrease in core swirl ratio in the upstream disk cavity indicates that the use of different honeycomb cell geometries may lead to excessive axial thrust. Finally, we study the local and average heat transfer capacities on the turbine disk and introduce a correction factor (0.12–0.21) into the correlation to make it applicable to analogous turbine disks or rotor–stator cavities in other turbo machines with low radii honeycomb labyrinth seals. Based on our findings, we provide recommendations for the optimal honeycomb cell geometries for the two sealings for different targets. These results serve as a reference for designers of honeycomb labyrinth seals for gas turbines and other turbo machines.

Sound-source distribution in the bogie section of a train determined by simultaneous measurement by pressure-sensitive paint and a microphone

Sound-pressure level of acoustic waves from the flow around a 1/8-reduced-scale simplified train model and pressure-fluctuation distribution of the bottom surface of the bogie were measured simultaneously by a microphone and a pressure-sensitive paint (PSP). A high-pressure-fluctuation area was observed on the upstream side on the bogie bottom surface at the peak sound frequency. The phase distribution of the peak frequency of the PSP data was observed to be uniform in the spanwise direction and delayed in the downstream direction. This result indicates that propagation speed of peak surface pressure fluctuation was 66 % of the freestream wind velocity. Thus, the measured peak sound frequency was found to be the same as the theoretical cavity peak frequency given by the Rossiter equation with that propagation speed as a vortex convection velocity. Therefore, the peak sound is concluded to be generated from acoustic feedback in the cavity, which is the gap between the upstream cavity edge and the bogie. Moreover, the difference between the measured and correlated peak sound levels of the bottom surface of the bogie, was no more than 3 dB, where the correlated sound level was calculated by using the Lighthill-Curle equation with coherent output power data.

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.

Experimental investigation of the underwater ventilated tail cavity at different angles of attack

The research on the structure and dynamic evolution of ventilated tail cavity is an important basis for designing the tail structure of an underwater vehicle. In this paper, through water tunnel experiments, the characteristics of the ventilated tail cavity at different angles of attack α are studied. Using a high-speed camera and pressure measurement system, the transient evolution characteristics of the tail cavity under different Froude numbers Fr and ventilation flow coefficient CQv are obtained. The results show that: under the influence of Fr and CQv, the underwater ventilating tail cavity mainly includes four forms: foam cavity (FC), intact cavity (IC), partially broken cavity (PBC), and pulsating foam cavity (PFC); For the intact cavity of the vehicle with the angle attack, the upstream cavity is more stable and long, and more likely to have the re-entrant jet closure. However, the downstream cavity is more broken and isolated, and this phenomenon alleviates as CQv or Fr number increases; For the intact cavity, with CQv or Fr number increasing, the vehicle bottom pressure fluctuation increases, and the influence of gravity on the vehicle bottom pressure fluctuation is smaller than that of gas-liquid shear on the bottom pressure fluctuation; The energy provided by jet under the 4° attack angle vehicle is lower than that under the 0° attack angle vehicle.

Evaluating of the effect of the cavitation-vortex interaction on the energy conversion in a two-stage LNG cryogenic submerged pump

The quantitative investigation on the unsteady interaction of the thermodynamic cavitation-vortex dynamics has been conducted via the developed entropy production diagnostic model in a two-stage liquefied natural gas cryogenic submerged pump. A thermodynamic cavitation modeling framework and Liutex method are applied to capture the characteristics of the formation, development and collapse of the cavities and vortex. It is found that the increasing flow rate and temperature both intensify the thermodynamic effect of cavitation. As the increase of flow rate, the thermodynamic effect suppresses the growth of cavities, thus raising the maximum temperature drop by 0.65 K at NPSH3% and 0.87 K at NPSH5% with the same temperature. Massive bubbles collapse causes the twist, deformation and breakdown of U-type vortex at the interface, which directly leads to the significant increase of interface entropy production rate in heat transfer entropy production rate (HTEPR). As the increase of temperature, the thermodynamic effect both shortens the cavity size and development period under the same flow rate. The strong shear and rotation regions drive upstream and the pressure rebounds rapidly downstream that weakens the backflow. The shrinkage of upstream cavity and the accelerated collapse of small-scale cloud cavity reduce the HTEPR and viscous flow entropy production rate

Brush seal design and secondary air system modification of a heavy-duty gas turbine to improve the output power

The overall performance of heavy-duty gas turbines is strongly affected by the mass flow distribution of the secondary air system. Reducing the mass flow of the secondary air is a remarkable method to improve the performance of a gas turbine. The aim of this article is to implement a brush seal as an alternative to labyrinth seal. Based on the experiences gained from the integrity of brush seals into a gas turbine, installing a brush seal would not necessarily improve turbine performance. This is because of SAS mass flow redistribution, which may not lead to overall SAS mass flow reduction. Therefore, some other components in the SAS arrangement should be modified as controlling variables to overcome this problem. The effect of these modifications on the airflow distribution is investigated using an in-house network analysis code. To establish an optimum solution (optimized brush seal geometry and controlling variables), the network analysis code is coupled with an in-house optimization code that benefits from the Genetic Algorithms and Artificial Neural Networks. Some constraints including upstream and downstream cavity purge flow and vane cooling air are considered in the optimization process. The final network results show a 33.27% reduction in overall SAS mass flow. To ensure an improvement in the performance of the new three-stage turbine, a CFD analysis is conducted, which indicates 1.0% more power with respect to the original turbine. In the end, the aerodynamic and mechanical behavior of the brush seal is analyzed using CFD and FEM.

Aerodynamic and heat transfer performances of a highly loaded transonic turbine rotor with upstream generic rim seal cavity

In turbine disk cavity, rim seals are fitted between the stator and its adjacent rotor disk. A coolant air injected through the turbine disk cavity to prevent the ingress of mainstream hot gases. The purpose of this paper is to investigate numerically the effect of the upstream purge flow on the aero and thermal performances of a high pressure turbine rotor. The investigations are conducted on a generic rim seal cavity inspired from a realistic turbofan engine. Four purge fractions (PF) equal to 0.2%, 0.5%, 1.0% and 1.5% of the mainstream are considered. The simulations are done by solving the three-dimensional Reynolds averaged Navier-Stokes and energy transport equations. The results include the effect of the PF on the cooling effectiveness, the sealing effectiveness, the secondary flows with losses and the heat transfer behavior, within the cavity and across the rotor passage. The low PF of 0.2% provided a low cooling effectiveness, a moderate sealing effectiveness and minimum losses. The high PF of 1.5% gave a high cooling effectiveness, a best sealing effectiveness and maximum losses. The medium PF of 1.0% supplied a compromise between the aerodynamic and thermal design needs with good cooling and sealing efficiencies and a tolerable level of losses.

Numerical investigation of combustion characteristics of upstream crescent cavities in a scramjet combustor

This study investigates the combustion performance of a number of novel, crescent shaped cavities placed directly in front of the fuel injection point in a scramjet combustor. Cavities are commonly used for flameholding in scramjet combustors, however they are almost exclusively placed downstream of the fuel injection point and their effect on mixing performance is limited. The crescent cavities in this study were previously found to enhance mixing in chemically frozen flow and the purpose of this study is to investigate whether this performance enhancement translates to chemically reacting flow. The performance of the cavities was numerically studied using unsteady Reynolds-Averaged Navier–Stokes (URANS) modelling with hydrogen as the fuel and an isothermal 1800 K combustor wall, and performance was compared to a no-cavity baseline. The cavities were found to enhance combustion by up to 114% and heat release by up to 143% compared to the baseline when hybrid fuelling is employed and by 47.4% and 54.7%, respectively, when no hybrid fuelling is used. Total pressure loss in the cavity cases was similar to the baseline case and viscous drag was lower in the cavity cases due to the presence of combustion and fuel near the wall. Flame penetration was lower in the cavity cases than in the baseline however as a result of the increased fuel concentration on the wall, while mean heat flux is lower in the cavity cases than in the baseline. Ignition is also found to occur more rapidly in the cavity cases and the flow inside the cavity is oscillatory in nature, as was the case in chemically frozen flow. Given that the crescent cavities in this study significantly enhance combustion performance with limited drawbacks, especially when hybrid fuelling is used, they are an attractive option for operational scramjet combustor design.

Effect of cavity arrangement on the ignition mode of vaporized kerosene in supersonic flow

The ignition characteristics and flame stabilization of vaporized kerosene are experimentally investigated in a direct-connected model engine. The incoming flow of Ma 2.0 is generated by an air heater at a total pressure of 0.98 MPa and a stagnation temperature of 920 K, which simulates flight conditions at a height of 17 km and flow of Ma 4.0. Vaporized China RP-3 aviation kerosene is employed as fuel in a single-side expansion combustor. The result shows that the ignition characteristics are closely related to the cavity location and configuration. Four modes of ignition are classified in this paper, and an orthogonal distribution of the mode with the injection pressure and temperature variations is studied in detail. Ignition in the downstream single cavity is more difficult than that in the upstream cavity but more easily generates an intensive flame stabilized in the cavity shear layer. The existence of an ignition threshold is responsible for this difference. Only if the injection condition exceeds the critical point can the flame be established and directly reach an intensive state. In a tandem dual cavity, the ignition mode distribution is similar to that of a single cavity under lean fuel conditions. Without a shock train in the flow field, the coupling effect of the two cavities on the ignition mode transition is not obvious.

Aeroacoustic modal analysis of underexpanded pipe jets with and without an upstream cavity

The investigation of the aeroacoustics of an underexpanded pipe-cavity jet is carried out experimentally. Two different aspect ratios of the cavity are tested for a wide range of nozzle pressure ratios. Both internal and externally radiated pipe-cavity acoustics are studied. Linear and higher-order spectral analyses are implemented on the unsteady cavity pressure to comprehend the nature of the cavity acoustics and nonlinear interactions of different acoustic modes of the pipe–cavity system. Results show that an increase in depth leads to an enhancement in the nonlinear interactions. Furthermore, the power spectral and overall sound pressure level analyses of pipe and pipe-cavity jet noise radiation are carried out. High-speed schlieren imaging techniques are used to understand jet dynamics. Highly unsteady motion of the jet initial shear layer is observed due to an upstream disturbance of the cavity. In addition, proper orthogonal and dynamic mode decomposition methods are used to extract the spatial and dynamic modes of the jet structure. These methods are used to segregate the cavity associated jet dynamics and screech dynamics.

Upward Transport of Trichloroethylene Vapour in a Dry Soil Column

Experimental and modelling studies were performed to assess the predictability of gas phase transport models based on the Fickian-type diffusion expression and dusty-gas model (DGM) equations for the upward transport of trichloroethylene (TCE) vapour at low vapour source densities in a dry soil column. The TCE vapour concentrations measured upstream in the column were only 6% and 15.7% of the TCE saturated vapour concentration (i.e., TCE vapour densities of 0.031 and 0.08 g/L, respectively) when TCE liquid existed in the upstream cavity as a vapour source. A good fit between the model-predicted profile of the downstream TCE vapour concentration and the observed data can only be attained by using a tortuosity value that is 0.2–0.3 times of that computed by the Millington and Quirk’s (1961) empirical equation. This result revealed that diffusion dominates over advection for the aforementioned transport conditions. Furthermore, the differences between the concentration evolution profiles away from the source predicted by the Fickian and DGM-based models present a reverse trend depending on whether the downstream boundary is covered (impermeable) or uncovered (opened to the air). The results revealed that the predicted upward transport of dense vapour at low source densities with a covered downstream boundary predicted by the two models differed considerably. • Trichloroethylene (TCE) vapour was <20% saturated in the experimental soil column containing the TCE liquid. • The fitted effective molecular diffusion coefficient was less than the estimated coefficient by the usual method. • A Fickian-type diffusion model may under- or over-predict vapour concentrations for upward transport of dense vapour.

Experimental investigation of internal two-phase flow structures and dynamics of quasi-stable sheet cavitation by fast synchrotron x-ray imaging

The quasi-stable sheet cavitation produced in a small Venturi channel is investigated using a fast synchrotron x-ray imaging technique aided with conventional high speed photography. The use of x rays instead of visible light solves cavitation opacity related issues, and x-ray phase contrast-based edge enhancement enables high-definition visualization of the internal two-phase morphology. The simultaneous acquisition of time-resolved velocity and void fraction fields through post-processing of the recorded x-ray images reveals, for the first time, the complex diphasic flow structures inside the sheet cavity, which is essentially divided into six characteristic parts. Distinct from the current mainstream view, the globally steady sheet cavitation is found to be characterized by a weak but constantly existing re-entrant flow that can penetrate the entire cavity. The turbulent velocity fluctuations inside the sheet cavity are also investigated. The turbulence level in the reverse flow region is observed to be as low as in the outer main flow, demonstrating the relatively steady status of the re-entrant flow. Unlike the streamwise and cross-stream fluctuations, the shear stress appears to be weakly correlated with the velocity gradient. The collapse of the vapor phase and the vaporization at the upstream cavity interface are found to be the primary causes of shear stress intensification.