numericalstudyoninfluenceofstructuralvibrationoncavitatingflowaroundaxisymmetricslenderbody

The unsteady behaviors of cloud cavitating flow would lead to structural vibration and deformation that conversely affect its development. The present paper aims to preliminarily discuss the influences of structural vibration on the development of the cavitating flow. Simulations of a slender body are carried out under different vibration amplitudes and frequencies. The results show that the structural vibration causes alternate variation of local attack angle at the head of the body, and thus changes the development of cavitation and re-entrant jet. On the downstream side, the length and thickness of the cavity are larger than that on the upstream side due to larger area of negative pressure. For a large vibration amplitude, alternate variations of the local attack angle change the adverse pressure gradient at the closure of the cavity, and then affect the development of the re-entrant jet, so that the phenomena of local shedding of the cavitation happen, compared with global shedding in the case of no structural vibration. For a frequency larger than 0.05, transverse speed of the vibration is suggested to be a dominant factor in controlling the behavior of the cavitating flow besides the local attack angle, since it causes local cavitating phenomena

A study of the collapse speed of bubble clusters

The collapse process of bubble cluster is closely related to bubble-bubble interaction. Theoretical analysis and numerical simulation are adopted to study the collapse of bubble cluster with various distributions. The key parameters for bubble collapse, including bubble quantity, volume fraction, and dimensionless pressure, are acquired by dimensional analysis. The effects of key parameters on collapse of bubble cluster are investigated by direct numerical simulation. The numerical result shows that the collapsing speed of bubble cluster increases with the increase of bubble quantity and dimensionless pressure, decreases with the increase of volume fraction. A condensation rate is considered on the basis of bubble cluster with primitive cubic distributions. Square pyramid arrangement and random arrangement of bubbles are also simulated. A parameter study of the dimensionless bubble distance bubble cluster with random arrangement shows that a larger distance generally results in a larger collapse speed of bubble cluster. (C) 2020 Elsevier Ltd. All rights reserved

numericalsimulationofbubbledetachmentatsubmergedorificeandanalysisofinterfacestability

This paper presents the experimental and numerical results of the bubble detachment from a submerged orifice at a constant gas flow rate. the compressible large eddy simulation combined with the volume of fluid method is adopted in the simulation and is validated by experiment. the transition criterion from the elongation stage to the detachment is obtained. in the detaching stage in the simulation, the distributions of the pressure and the surface tension on the cylindrical bubble neck are obtained. the rayleigh-plesset equation in the cylindrical coordinate frame is used to describe this process. based on the comparison between the numerical results and the equation analysis, a reference value of the uncertain integral parameter in the equation is determined

A numerical model for cloud cavitation based on bubble cluster

The cavitation cloud of different internal structures results in different collapse pressures owing to the interaction among bubbles. The internal structure of cloud cavitation is required to accurately predict collapse pressure. A cavitation model was developed through dimensional analysis and direct numerical simulation of collapse of bubble cluster. Bubble number density was included in proposed model to characterize the internal structure of bubble cloud. Implemented on flows over a projectile, the proposed model predicts a higher collapse pressure compared with Singhal model. Results indicate that the collapse pressure of detached cavitation cloud is affected by bubble number density. Keywords: Cavitation model, Bubble number density, Bubble cluster, Collaps

Study on the energy-focusing mechanism of spatial bubble clusters

Cavitation research has important implications in fields such as mechanical drag reduction, material processing, and new medical device development. Bubble cluster formation, development, and collapse are critical steps in the cavitation process. High-precision numerical simulations have shown that the collapse of bubble clusters exhibits a characteristic energy focusing from the outside to the inside. This study proposes a focus-type model for the energy transfer in bubble clusters to analyze the formation mechanism of collapse pressure and improve the accuracy of quantitative predictions. The model comprises multiple bubbles (alpha) radiating energy and a bubble (beta) receiving energy. Through numerical simulation, the energy transfer law during bubble interaction is studied, showing that relative energy transfer decreases as the dimensionless distance increases, which corresponds with the theoretical model. The study further analyses the relationship between energy transfer in basic and composite bubble cluster structures. Additionally, the study observed the pressure focusing effect of the bubble clusters and found a strong correlation between the focusing effect and dimensionless distance

Numerical investigations of the energy performance and pressure fluctuations for a waterjet pump in a non-uniform inflow

As a way of exploitation and utilization of ocean energy, the waterjet pump is used in a wide range of high-speed marine vessels over 30 knot. This paper aims to investigate the mechanism of the energy loss and pressure fluctuations caused by the non-uniform inflow for a waterjet pump. Unsteady internal flows inside the waterjet pump are simulated using the Reynolds-averaged Navier-Stokes equations with the SST k-omega turbulence model. The predicted pump head and efficiency are in reasonable accordance with the experimental data. The inflow non-uniformity would decrease the hydraulic head, efficiency and increase the axial force fluctuations in the impeller, causing large pulsations in the unsteady energy performance. Based on analyses of the energy loss, the turbulent kinetic energy production and the diffusion of the Reynolds stress are major sources of the energy loss in the waterjet pump. The non-uniform inflow induces a dramatic energy loss in the intake duct and diffuser with an apparent flow separation observed near the trailing edge of the diffuser blade. Due to the inflow non-uniformity, the pressure fluctuates violently at the impeller rotating frequency (f(n)) in the intake duct, impeller and near the diffuser inlet, but a dominant frequency of 2f(n), is generated by the unsteady flow separation near the diffuser outlet. (C) 2020 Elsevier Ltd. All rights reserved

Mechanism analyses of the unsteady vortical cavitation behaviors for a waterjet pump in a non-uniform inflow

The waterjet pump is widely applied in the high-speed marine vessels to exploit various kinds of resources in the vast ocean. The transient cavitating flows in a waterjet pump are numerically investigated under a non-uniform inflow for the purpose of revealing the correlation mechanism between the cavitation and the vorticity diffusion as well as the exited pressure fluctuations. The unsteady numerical simulation is conducted by using the Reynold-Averaged Navier-Stokes (RANS) method coupled with a homogenous cavitation model. Both the hydrodynamic performance and the cavitation performance are well predicted by the present numerical approach when compared with the available experimental data. The cavitation occurrence would cause larger pulsations to the hydrodynamic characteristics and the nonuniformity together with perpendicularity at the impeller inlet plane. As the blade passes through the non-uniform inflow, the instantaneous cavitation dynamics behaviors include the cavity generation, development and extinction, and the dominant frequency corresponds to the impeller rotating frequency. Based on analyses of the boundary vorticity flux, the cavitation is an important mechanism for vorticity diffusion from the blade into the mainstream with the major contributor of the variable density due to cavitation. Furthermore, combined computational and theoretical analysis illustrates that the cavity volume variations would cause the flow-rate fluctuations and the cavity volume acceleration is the major source for the pressure fluctuations inside the mixed-flow waterjet pump

Investigations into the unsteady internal flow characteristics for a waterjet propulsion system at different cruising speeds

Unsteady turbulent flows in a waterjet propulsion system are investigated at various cruising speeds with the emphasis on pressure fluctuations. The numerical methodology is based on the Reynolds-Averaged Navier-Stokes (RANS) equation with the SST k-omega turbulence model and a sliding mesh technique. The head and efficiency of the waterjet pump are predicted fairly well compared with the available experimental data. The pressure fluctuates intensively in the impeller and the dominant frequency is the impeller rotating frequency with the largest amplitude near the impeller inlet. Besides, two dominant frequency components exist in the intake duct and the diffuser. A high-frequency component is caused by the rotor-stator interaction, and another component is generated by the unsteady vortex evolution in the diffuser passage and would propagate upstream to the impeller and the intake duct. Analyses based on the vorticity transport equation demonstrate the great contribution of the vortex stretching term to the vorticity distribution and evolution in the diffuser. Finally, at the cruising speed of 45 knot, the flows inside the duct are strongly affected by the impeller rotation and present a periodic prewhirl motion with the dominant frequency of the impeller rotating frequency