Numerical Simulation Of Cavitation Research Articles

Study on the application of vortex generator to suppress ship propeller cavitation excitation

ABSTRACT A triangular vortex generator (VG) is used to suppress the hull vibration induced by propeller cavitation in this paper. Firstly, the numerical simulation method is used to simulate the original flow field around the ship, and the high wake area caused by the separation of the flow at the stern is analysed, which leads to serious cavitation on the propeller and induced hull vibration. Then, the effects of the installation angle and position of the VG on the wake of the ship are analysed, and the appropriate design parameters of the VG are obtained. Through the numerical simulation of cavitation on the propeller, the effects of VG on cavity shape and hull fluctuating pressure induced by cavitation are analysed. The VG can improve the non-uniformity of wake field, delay cavitation generation, and weaken the cavitation on the propeller tip area. The maximum reductions of the first, second, and third harmonics of fluctuating pressure are about −41.2%, −36.6%, and −78.5%, respectively. Finally, the vibration of each cabin of the full scale vessel is significantly reduced after the installation of the VG, especially in the cabin near the propeller. The reduction range is up to −54.5%, and a good vibration reduction effect is obtained.

Characteristics of cavitating flows in cryogenic pump units in a combined cycle power system based on liquefied natural gas and solar energy

The cavitation of cryogenic pump units is an important cause for the low cold energy recovery efficiency and low utilization efficiency of solar energy in combined cycle power systems based on liquefied natural gas (LNG) and solar energy. Through numerical simulation of cavitation in cryogenic pump units, the research established the correlation mechanism between cavitation evolution and energy characteristics. By using the transport-based cavitation model considering the thermomechanical effect of cryogenic media, a novel numerical simulation model for two-phase flows during cryogenic cavitation was developed. In addition, the numerical model of cavitation was verified to be reliable. The cavitation performance of the two-stage LNG submerged pump in cryogenic pump units was studied under multiple conditions. In addition, the evolution of cavitating flow regimes under conditions of different flow rates and its influences on flow patterns of various flow passage components were investigated by combining analysis on temperature changes and flow fields. The entropy production diagnostic model (EPDM) was also adopted to discuss the distribution of the irreversible entropy production rate (EPR) in the evolution process of cavitation. Results show that the interfacial EPR that characterizes the phase-transition intensity after inception of cavitation exerts significant influences. Analysis of influencing mechanisms of changes in temperature, changes in flow fields, and distribution characteristics of energy consumption can provide a theoretical basis for further optimization.

Numerical Simulation of Cavitation and Hydrodynamic Characteristics of Supercavitating Projectiles in the Shear Flow

To investigate the cavitation and hydrodynamic characteristics of supercavitating projectiles in the shear flow, the Mixture multiphase and Schnerr-Sauer cavitation models are employed to simulate the underwater projectiles. The inflow average velocity is 600 m/s, and the shear rates range from 0 to 7500 s−1. In the uniform flow, the supercavity enveloping projectiles is vertically symmetrical. The drag is dominated by pressure drag, and the lift coefficient is 0. However, the supercavity is asymmetric in the shear flow, which deviates towards the low-speed side of projectiles. This is because the flow around projectiles runs faster on the high-speed side, and the vortices on the low-speed side entrain more fluid from the high-speed side. Thus, the projectiles suffer from normal shear stress orientating towards the low-speed side, and the lift coefficient turns negative. When the shear rate further increases, the projectile shoulder contacts water on the high-speed side, and the viscosity around projectiles is enhanced, resulting in the significant augmentation of the drag coefficient. As the water pressure is strongly larger than saturated vapor pressure on the low-speed side, the normal component of pressure acts more intensely towards the low-speed side of projectiles, and the lift coefficient is further decreased.

Numerical Simulation of Cavitation and FlashBoiling in GDI Nozzle and Spray

This work aims at using Computational Fluid Dynamics (CFD) method to establish a gasoline direct injection (GDI) engine nozzle and combustion chamber model to simulate cavitation and flash boiling phenomena and analyze how these phenomena affect the engine performance. FLUENT 15.0 is used to simulate the flow of fuel. The cavitation phenomenon in GDI nozzle was simulated comprehensively, and the influences of parameter values such as inlet pressure and outlet pressure on cavitation were studied in this work. The results show that high injection pressure can promote the occurrence of cavitation and high outlet pressure has an inhibitory effect on cavitation. However, the effect of cavitation on atomization cannot be seen intuitively only through the simulation of the internal nozzle. The two-dimensional inter nozzle model established in this work is a foundation for the establishment of external nozzle model. It can provide specific boundary conditions at nozzle outlet. In addition, a three-dimensional external nozzle model was established to simulate the flash boiling spray in the combustion chamber. Based on the mechanism, it can be found that flash boiling spray improves the atomization quality. After verifying the simulation results with relevant experiments, these models could bring great convenience to the study of cavitation and flash boiling with sufficient reliability in further study.

Study on Setting Elevation of Double-Suction Centrifugal Pump Based on Cavitation Characteristics

Setting elevation is an important factor affecting the safe operation of pumps, and the empirical calculation method in engineering cannot fully reflect the cavitation characteristics. In this paper, numerical simulation of cavitation in double-suction centrifugal pump with clearances is carried out. The cavitation characteristics under different flow rates was analyzed based on the setting elevation from the empirical method. It was found that there was cavitation inception under both large flow rate and small flow rate. The cavitation characteristics of the pump with different net positive suction head available (NPSHa) is analyzed. It is found that there is no obvious change in the pump characteristics from cavitation inception to critical cavitation, but after the point of the critical cavitation is reached, a large number of cavities plug the blade passage, and the efficiency decreases rapidly. The results show that in engineer although critical net positive suction head (NPSHc) is considered and the safety margin is increased, it is difficult to ensure that there is no cavitation inception happened in the pump. The cavitation characteristics analyzed by numerical simulation in this paper can provide guidance for the selection of suitable setting elevation.

Numerical Simulation of Cavitation and Damping Force Characteristics for a High-Speed Supercavitation Vehicle

In this study, an attempt has been made to investigate the supercavitation and hydrodynamic characteristics of high-speed vehicles. A homogeneous equilibrium flow model and a Schnerr–Sauer model based on the Reynolds-averaged Navier–Stokes method are used. Grid-independent inspection and comparison with experimental data in the literature have been carried out to verify the accuracy of numerical methods. The effect of the navigation speed and angle of attack on the cavitation morphology and dynamic characteristics has been investigated. It has been demonstrated that the angle of attack has a remarkable influence on the wet surface and hydrodynamic force, whereas navigation speed has little effect on the position force of the vehicle under the circumstance of no wet surface. The hydrodynamic force changes periodically with the swing of the vehicle, but its maximum is greater than that for the direct navigation state at the same attack angle. Moreover, the damping effect obviously affects the hydrodynamic force amplitude and movement trend.

Influence of wall roughness on cavitation performance of centrifugal pump

Cavitation is a phenomenon that occurs easily during rotation of fluid machinery and can decrease the performance of a pump, thereby resulting in damage to flow passage components. To study the influence of wall roughness on the cavitation performance of a centrifugal pump, a three-dimensional model of internal flow field of a centrifugal pump was constructed and a numerical simulation of cavitation in the flow field was conducted with ANSYS CFX software based on the Reynolds normalization group k-epsilon turbulence model and Zwart cavitation model. The cavitation can be further divided into four stages: cavitation inception, cavitation development, critical cavitation, and fracture cavitation. Influencing laws of wall roughness of the blade surface on the cavitation performance of a centrifugal pump were analyzed. Research results demonstrate that in the design process of centrifugal pumps, decreasing the wall roughness appropriately during the cavitation development and critical cavitation is important to effectively improve the cavitation performance of pumps. Moreover, a number of nucleation sites on the blade surface increase with the increase in wall roughness, thereby expanding the low-pressure area of the blade. Research conclusions can provide theoretical references to improve cavitation performance and optimize the structural design of the pump.

串联离心泵过流部件空化及磨损的数值模拟

串联离心泵过流部件空化及磨损的数值模拟

Numerical simulation of cavitation and atomization using a fully compressible three-phase model

The aim of this paper is to present a fully compressible three-phase (liquid, vapour and air) model and its application to the simulation of in-nozzle cavitation effects on liquid atomization. The model employs a combination of homogeneous equilibrium barotropic cavitation model with an implicit sharp interface capturing VoF approximation. The numerical predictions are validated against the experimental results obtained for injection of water into the air from a step-nozzle, which is designed to produce asymmetric cavitation along its two sides. Simulations are performed for three injection pressures, corresponding to three different cavitation regimes, referred to as cavitation inception, developing cavitation and hydraulic-flip. Model validation is achieved by qualitative comparison of the cavitation, spray pattern and spray cone angles. The flow turbulence in this study is resolved using the Large Eddy Simulation approach. The simulation results indicate that the major parameters that influence the primary atomization are cavitation, liquid turbulence and, to a smaller extent, the Rayleigh-Taylor and Kelvin-Helmholtz aerodynamic instabilities developing on the liquid/air interface. Moreover, the simulations performed indicate that periodic entrainment of air into the nozzle occurs at intermediate cavitation numbers, corresponding to developing cavitation (as opposed to incipient and fully-developed cavitation regimes); this transient effect causes a periodic shedding of the cavitation and air clouds and contributes to improved primary atomization. Finally, the cone angle of the spray is found to increase with increased injection pressure but drops drastically when hydraulic-flip occurs, in agreement with the relevant experiments.

Numerical Simulation of Cavitation in Mixed Flow Pump

The purpose of this study is to investigate the performance and three-dimensional behavior of the flow in a mixed flow pump and the way cavitation phenomenon is affected by different parameters such as fluid temperature, pump speed and flow rate. Computational fluid dynamic software FLUENT 6.3 was utilized to simulate the whole flow field of the pump. RNG k-e model combined with standard wall functions is chosen to deal with the turbulent feature of the problem. The studied pump has four blades mounted on a conical hub which form the rotary part and nine static vanes afterward as the stationary part. So the rotor-stator interaction was treated with a Multiple Reference Frame (MRF) technique. Different cases were analyzed for different flow rates and different pump speeds. While the flow rates variation and the pump revolution change cavitation occurrence widely, the temperature variations caused by weather changes during a year has little effect on cavitation. The cavitation region which is defined by the saturation pressure in that temperature was shown for various cases on a blade

Multiphase Thermodynamic Tables for Efficient Numerical Simulation of Cavitating Flows: A Novel Look-Up Approach Toward Efficient and Accurate Tables

In this paper a numerical simulation of unsteady sheet cavitation is presented as it occurs on an NACA-0015 hydrofoil. The computational approach is based on the Euler equations for unsteady compressible flow, using an equilibrium cavitation model of Schnerr, Schmidt, and Saurel. It was found that for a computational method that directly uses the thermodynamic closure relations, more than 90% of the computational time was spent in computations associated with these relations. To circumvent this problem, the computationally costly method is replaced by using precomputed multiphase thermodynamic tables containing the same information, with no need to determine the flow phase. The thermodynamic computations using this approach are almost instantaneous. However, preparing these multiphase tables is not straightforward. The main difficulty is due to the slope discontinuities in the phase-transitioning regions of the multiphase tables. In these regions, the conventional look-up approaches are inaccurate and inefficient (their accuracy is insensitive to data grid refinement). To remove this bottleneck, phase-oriented interpolations in transition cells is developed as a novel approach that allows for preparing much more accurate and efficient (much smaller size) multiphase thermodynamic tables compared to conventional interpolation approaches.

Experimental real-time tracking and diffusion/mechanics numerical simulation of cavitation in gas-saturated elastomers

The effect of a gas decompression after saturation on rubbers was investigated with both an experimental and a numerical approach. Experimental results have been obtained with a commercial transparent rubber under hydrogen and a tensile machine fitted with a pressure chamber that allows a spatial and temporal tracking of damage. The influence of the decompression rate and cavity radius have been studied and compared with a numerical model based on the theory of the hollow sphere and the implementation of a Fickean law to have gas diffusion in the material. This multi-scale model was used to temporally predict the response of a spherical cavity in a hyperelastic incompressible material submitted to a coupled gas/mechanical loading. The aim of the study was to understand the gas exchanges between the cavity and the material and to temporally predict with a simple critical stretch ratio criterion when a small cavity can growth to a visible size. The influence of decompression rate, pressure level, cavity radius and position in the sample were discussed and compared with experiments.

Numerical Simulation of Cavitation by Sub-Surface Vortex (2nd Report, Theoretical Modeling of Unsteady Vortex Cavitation)

Numerical Simulation of Cavitation by Sub-Surface Vortex (2nd Report, Theoretical Modeling of Unsteady Vortex Cavitation)

Numerical simulation of cavitation around a two-dimensional hydrofoil using VOF method and LES turbulence model

In this paper simulation of cavitating flow over the Clark-Y hydrofoil is reported using the large eddy simulation (LES) turbulence model and volume of fluid (VOF) technique. We applied an incompressible LES modelling approach based on an implicit method for the subgrid terms. To apply the cavitation model, the flow has been considered as a single fluid, two-phase mixture. A transport equation model for the local volume fraction of vapour is solved and a finite rate mass transfer model is used for the vapourization and condensation processes. A compressive volume of fluid (VOF) method is applied to track the interface of liquid and vapour phases. This simulation is performed using a finite volume, two phase solver available in the framework of the OpenFOAM (Open Field Operation and Manipulation) software package. Simulation is performed for the cloud and super-cavitation regimes, i.e., σ=0.8, 0.4, 0.28. We compared the results of two different mass transfer models, namely Kunz and Sauer models. The results of our simulation are compared for cavitation dynamics, starting point of cavitation, cavity’s diameter and force coefficients with the experimental data, where available. For both of steady state and transient conditions, suitable accuracy has been observed for cavitation dynamics and force coefficients.

Numerical Simulation of Cavitation in a Centrifugal Pump at Low Flow Rate

Based on the full cavitation model which adopts homogeneous flow supposition and considering the compressibility effect on cavitation flow to modify the re-normalization group k-∊ turbulence model by the density function, a computational model is developed to simulate cavitation flow of a centrifugal pump at low flow rate. The Navier-Stokes equation is solved with the SIMPLEC algorithm. The calculated curves of net positive suction head available (NPSHa) HNPSHa agree well with the experimental data. The critical point of cavitation in centrifugal pump can be predicted precisely, and the NPSH critical values derived from simulation are consistent with the experimental data. Thus the veracity and reliability of this computational model are verified. Based on the result of numerical simulation, the distribution of vapor volume fraction in the impeller and pressure at the impeller inlet are analyzed. Cavities first appear on the suction side of the blade head near the front shroud. A large number of cavities block the impeller channels, which leads to the sudden drop of head at the cavitation critical point. With the reduction of NPSHa, the distribution of pressure at the impeller inlet is more uniform.

Numerical Simulation of Cavitation by Sub-Surface Vortex (1st Report, High-Precision Numerical Simulation Scheme for Vortical Flow)

For the safety operation of a large-scale sodium-cooled fast reactor in Japan (JSFR), structural integrity has to be maintained. Therefore, several thermal-hydraulics phenomena, which may reduce the integrity, have been studied. One of them is the cavitation due to sub-surface vortex which can damage structural surfaces. Such a sub-surface vortex is induced by high-speed suction flow into an outlet pipe in the reactor vessel. However, the onset condition of the cavitation can not be clarified easily because the cavitation shows highly complicated behaviors associated with phase change. Therefore, the authors are developing a CFD code to simulate the cavitation accurately and to investigate the onset condition of the cavitation. In this paper, as the first part of the development, a high-precision simulation scheme for vortical flows is presented. In this scheme, a vortical flow is identified in each cell and a vortical velocity distribution is determined locally to be consistent with the local velocity distribution. Then, the calculations of momentum transport through cell faces are performed in consideration with the vortical velocity distribution. In addition, a pressure distribution near the vortex center is also considered to simulate accurately the mechanical balance between pressure gradient and centrifugal force. As a fundamental verification of the developed scheme, the reproducibility of the burgers vortex is checked. Furthermore, inviscid vortex attenuation in two-dimensional domain is simulated. As a result, it is confirmed that the developed scheme can simulate vortical flows more accurately than conventional schemes.

Numerical Simulation of Cavitation and Aeration in Discharge Gated Tunnel of a Dam Based on the VOF Method

Cavitation, usually known as a destructive phenomenon, involves turbulent unsteady two-phase flow. Having such features, cavitating flows have been turned to a challenging topic in numerical studies and many researches are being done for better understanding of bubbly flows and proposing solutions to reduce its consequent destructive effects. Aeration may be regarded as an effective protection against cavitation erosion in many hydraulic structures, like gated tunnels. The paper concerns numerical simulation of flow in discharge gated tunnel of a dam using RNG model coupled with the volume of fluid (VOF) method and the zone which is susceptible of cavitation inception in the tunnel is predicted. Then a vent is considered in the mentioned zone for aeration and the numerical simulation is done again to study the effects of aeration. The results show that aeration is an impressively useful method to exclude cavitation in mentioned tunnels.

Numerical simulation of cavitation in the conical-spray nozzle for diesel premixed charge compression ignition engines

The conical-spray injector is capable of achieving lean mixture with high homogeneity in the cylinder for diesel Premixed charge compression ignition (PCCI) engine with advanced injection timing. To better understand the cavitating flow inside the conical-spray injector, numerical simulations have been conducted by using a mixture multiphase model and a full cavitation model in this study. The results indicate that the cavitation evolution significantly affects the liquid sheet thickness and velocity at nozzle exit, which further change the spray angle and droplet Sauter mean diameter (SMD) dramatically. Based on the cavitation distribution inside the nozzle, the cavitating flow inside the conical-spray nozzle can be classified into four regimes with no cavitation, cavitation inception at inlet, developing cavitation at nozzle exit and super cavitation respectively. The extension of cavitation to nozzle exit in the super cavitation regime significantly improves the fuel atomization by increasing the injection velocity and decreasing the thickness of the liquid sheet. A cavitation map for the conical-spray injector has been developed by sweeping the ambient pressure and injection pressure simultaneously. It is found that the phenomenon of super cavitation only occurs in a narrow region where ambient pressure is very low. Therefore, the start of injection timing should be kept well before top dead center (TDC) to ensure the occurrence of super cavitation inside the nozzle in order to provide more homogeneous fuel/air mixture for diesel PCCI engines.

Numerical Simulation of Cavitation and Aeration in Discharge Gated Tunnel of a Dam Based on the VOF Method

Cavitation, usually known as a destructive phenomenon, involves turbulent unsteady two-phase flow. Having such features, cavitating flows have been turned to a challenging topic in numerical studies and many researches are being done for better understanding of bubbly flows and proposing solutions to reduce its consequent destructive effects. Aeration may be regarded as an effective protection against cavitation erosion in many hydraulic structures, like gated tunnels. The paper concerns numerical simulation of flow in discharge gated tunnel of a dam using RNG model coupled with the volume of fluid (VOF) method and the zone which is susceptible of cavitation inception in the tunnel is predicted. Then a vent is considered in the mentioned zone for aeration and the numerical simulation is done again to study the effects of aeration. The results show that aeration is an impressively useful method to exclude cavitation in mentioned tunnels.

Validation of full cavitation model in cryogenic fluids

Numerical simulation of cavitation in cryogenic fluids is important in improving the stable operation of the propulsion system in liquid-fuel rocket. It also represents a broader class of problems where the fluid is operating close to its critical point and the thermal effects of cavitation are pronounced. The present article focuses on simulating cryogenic cavitation by implementing the “full cavitation model”, coupled with energy equation, in conjunction with iterative update of the real fluid properties at local temperatures. Steady state computations are then conducted on hydrofoil and ogive in liquid nitrogen and hydrogen respectively, based on which we explore the mechanism of cavitation with thermal effects. Comprehensive comparisons between the simulation results and experimental data as well as previous computations by other researchers validate the full cavitation model in cryogenic fluids. The sensitivity of cavity length to cavitation number is also examined.