Cavitation Region Research Articles

Numerical investigation of compressible cryogenic cavitating flows by a modified mass transport model

The objectives of this study are to propose exact numerical methods for the compressible cryogenic cavitating flows and investigate the cavitation behaviors and vortex structures. A numerical modeling framework including large eddy simulations, vapor–liquid equations of state, and a modified mass transport model is presented in this paper. The modified transport model is proposed based on the convective heat transfer in which the convective heat transfer coefficient is associated with the material properties and local temperature. To validate the applicability of the modified model, the liquid nitrogen cavitating flows in the inertial and thermal modes (σ ≈ 0.50, Tthroat = 77.24 K and Tthroat = 85.23 K) are simulated, respectively. Meanwhile, the influence of thermodynamic effects on compressibility is investigated. The numerical method is further utilized to visualize the detailed cavity and vortex structures in different cavitating flow patterns (Tthroat ≈ 77 K, σ = 0.58, 0.39, 0.18). The results show that the predicted cavity structures with the modified mass transport model agree better with the corresponding experimental data. For the thermal mode, since the significant thermal effects restrain the development of cavity, the area of the low sound speed region is smaller than that of the inertial model. The value of the minimum sound speed is larger, so that the Mach number in the cavitation region is reduced. Therefore, the compressibility of the liquid nitrogen cavitation in the thermal mode is weaker. For different cavitating flow patterns, the core region of attached cavities near the throat remains stable during an evolutionary cycle. Compared to the attached cavity region, since some hairpin vortices break into many small-scale discrete vortices, the multi-scale effect of vortex distribution is more remarkable in the shedding cavity region.

A prediction model for suction cavitation erosion in a journal bearing

Suction damage occurred often in dynamically loaded journal bearings. A new theoretical model, including lubrication module, boundary module and prediction module, was proposed to investigate the suction damage in a journal bearing. The results showed that, a stable cavitation area with steady temperature fields formed in the film rupture region, but no suction cavitation erosion occurred in a constant load journal bearing. It was concluded that, suction cavitation erosion occurred within the region of suction cavitation, and higher dimensionless deformation coefficients stood for more muscular suction damage strength within suction cavitation regions. It was beneficial to reduce transient impact force applied to a journal bearing for resisting suction cavitation erosion, leading to an increase of a periodic pressure coefficient.

Grain refinement and mechanism of steel in ultrasound assisted wire and arc additive manufacturing

The small volume molten pool of wire and arc additive manufacturing (WAAM) undergoes complex non-equilibrium solidification process, resulting in the formation of columnar grains that severely limits the mechanical properties of the fabricated components. For refining the microstructure of deposition layers, the optimized ultrasonic equipment and loading methods are installed on a deposition platform, and the ultrasonic energy is coupled synchronously with the WAAM process using ER70S-6 welding wire as the filling material. Experimental results show that high-intensity ultrasound can transform coarse columnar grains into fine equiaxed grains, the area and degree of grain refinement increase with the ultrasonic amplitude. A computational fluid dynamics model of ultrasound assisted WAAM was developed to study the pressure field distribution in the molten pool. The region of acoustic cavitation in molten pools is highly consistent with the region of grain refining, showing that acoustic cavitation is the key factor in changing the grain structure. The collapse of cavitation bubbles generates cyclic alternating local high pressure, resulting in high velocity melt flow near the grain, which causes fatigue fracture of growing columnar grains exceeding the yield strength. A theoretical model is proposed to calculate the maximum length of grain growth under acoustic cavitation, which is in agreement with the experimental statistics of grain size. This work reveals the effect area and mechanism of ultrasound on the rapidly solidified molten pool, and provides a theoretical basis for grain refinement in ultrasound assisted WAAM.

A new cavitation model for simulating steady and unsteady cavitating flows

Cavitation is a widespread hydrodynamic phenomenon. Accurate prediction of cavitating flow is beneficial to the design and maintenance of the hydrodynamic machinery. A new cavitation model reducing the number of parameters needed to obtain satisfactory results is presented and validated. Based on the multiphase flow equation, the mass transfer due to cavitation is expressed as a source term in the gas-phase continuity equation. The mass transfer rate is derived from the assumption that the number of cavitation nuclei per unit volume in the mixture varies linearly with the vapor volume fraction during the evaporation stage. The mass transfer rate also incorporates the simplified Rayleigh–Plesset equation and the critical radius of cavitation nuclei. The new cavitation model considers the influence of non-condensable gas and the molar density of non-condensable gas is the only physical parameter that must be specified. The cavitating flow over a hydrofoil, cavitating flow in a sharp-edged orifice, and the unsteady cavitating flow in a venturi tube were predicted to verify the validity of the new cavitation model. The simulation results of the new model are in good agreement with the experimental results. The new cavitation model predicts cavitation regions and periodic unsteady cavitating flows.

Research on Cavitation Wake Vortex Structures Near the Impeller Tip of a Water-Jet Pump

Cavitation can cause noise in the water-jet pump. If cavitation occurs in the water-jet pump, the hydraulic components in the pump are prone to erosion. The surface erosion reduces energy delivery efficiency and increases maintenance costs. The decline in pump performance will lead to the instability of the entire energy system. In this paper, the cavitation flow structure of the water-jet pump is studied by the method of numerical simulation and experiment, which provides a reference for the prediction and improvement of cavitation. Based on the closed test platform, in order to reveal the physical process of cavitation evolution, high-speed photography is used to capture the complex cavitation flow phenomenon in the pump. After that, the cavitation vortex structure was further explored by numerical simulation. Through the simulation of the impeller blade tip leakage flow and the Tip Leakage Vortex Cavitation (TLVC) characteristics under different cavitation conditions, the flow mechanism of the impeller blade tip leakage flow and the separation vortex induced by the cavitation region under different cavitation conditions were revealed. The main factors affecting the development of the cavitation wake vortex structures were summarized.

Analyzing the Inlet Gas Void Fraction on the Flow Characteristics for a Multiphase Pump Based on Cavitation Cases

Inlet gas void fraction (IGVF) affects the cavitation evolution in a multiphase pump and easily results in a drop of the head and efficiency when cavitation is more serious. In this paper, a numerical method was performed to qualitatively and quantitatively analyze the effect of the inlet gas void fraction on the pressure and velocity characteristics of the multiphase pump at different cavitation stages. The results show that with the increase of IGVF and the development of cavitation, the pressure in the impeller flow passage is reduced, and the pressure corresponding to the cavitation region drops sharply to the saturated vapor pressure. With the decrease of the cavitation coefficient, and due to the expulsion effect of the cavitation bubbles, the relative velocity in the cavitation region becomes larger. Because of the large pressure gradient at the end of the cavitation bubbles, the kinetic energy of the fluid is insufficient to overcome the effect of the inverse pressure gradient, resulting in a backflow vortex. Investigations on cavitation evolution in the multiphase pump at different IGVFs are of great significance for improving its performance.

Effects of in-nozzle flow and low-pressure zone on spray targeting of multi-hole gasoline direct injection injector

The purpose of this study is to reveal the effect of In-nozzle flow and low-pressure zone near the nozzle exit on the spray targeting of a multi-hole gasoline direct injection injector using a numerical simulation. The change in spray targeting owing to in-nozzle flow and the distribution of the liquid phase at the hole exit were analyzed through a multi-phase flow simulation. A volume of fluid (VOF) model was used to effectively distinguish between the liquid flow and cavitation region. Results of the numerical simulation at the internal and near-field show that cavitation occurred at a sharp corner with a high flow velocity. Moreover, the low liquid rebounded at the hole wall, and the flow was bent in the direction of cavitation. The results of the spray simulation indicated that an increase in the injection pressure reduced the minimum pressure in the low-pressure zone. By contrast, the increase in ambient pressure not only decreased the pressure in the low-pressure zone, but also caused the low-pressure zone to form closer to the hole exit. In most cases, the low-pressure zone had a greater effect on plume bending than on in-nozzle flow, which was further enhanced as the ambient pressure increased.

Reverse Flow in Submerged Journal Bearings

In this study, the Navier–Stokes equations, solved by a computational fluid dynamics (CFD) method with the two-phase flow, are used to analyze the flow in submerged journal bearings. The “tongue”-shaped reverse flow, which was observed experimentally near the end of the cavitation zone, is predicted with the presented model. The effects of various design and operational parameters, including eccentricity ratio, supply pressure, rotational speed, oil viscosity, and radial clearance, on the location, length, and frequency of the reverse flow are presented and explained. The load-carrying capacity of the journal bearing is found to fluctuate with the same pattern as that of the reverse flow, which may result in a vibration of the entire shaft–bearing system. Comparisons with experimental results of pressure distribution validate the present model in solving reverse flow near the cavitation region.

Comprehensive Investigations on Fluid Flow and Cavitation Characteristics in Rotating Disk System

The present work numerically investigates the flow behavior of Newtonian fluid between rotating disk systems. Rotating disk flow is an embranchment of fundamental flow research, which forms the theoretical basis for the flow media in many disk-type hydraulic machinery, and has been widely applied in energy, aerospace, vehicles, medical, and chemical areas. The Reynolds equation model of the rotating disk flow is established based on a series of assumptions, and considers the characteristics of hexahedral surface textures on the friction disk, and the control equation is dimensionless. The velocity, pressure, and pressure coefficient in oil film are solved by finite difference algorithm, and the shear cavitation incipient position is predicted. Graphical visualization and numerical data about cavitation characteristics are also presented. Furthermore, the cavitation process is researched in CFX and the experiment, eventually verifying the correctness of the numerical analysis. The pressure perturbation caused by texture boundaries and the low pressure at the vortex center induce the initial generation of cavitation, and cavitation effect can obviously enhance lubricating performance. The cavitation region extends from micro-dimples towards non-textured region when texture rate increases. This study can guide design of surface structures such as mechanical seals, wet clutches, and disk turbines.

Investigation on leakage vortex cavitation and corresponding enstrophy characteristics in a liquid nitrogen inducer

A modified thermal cavitation model was employed to study the thermodynamic cavitation and effects of tip leakage vortex (TLV) on hydraulic losses in a three-blade inducer. Comparative analyses for TLV cavitation under different temperatures were presented. The tip leakage flow was spirally distributed, and its shape was negatively correlated with the nitrogen temperature. Thermodynamic effects reduced the vapor volume in the cavitation region and vorticity primarily at the leading edge, thus inhibiting the development of TLV. Enstrophy analysis was proposed to reveal the energy losses on TLV, and the enstrophy dissipation rate showed good predictability for TLV evolution. With the increasing temperature, the intensity of energy losses was reduced due to the thermal effects combined with weak cavitation intensity. Moreover, the change of temperature mainly influenced the primary tip leakage vortex (PTLV) but had minimal impact of the secondary tip leakage vortex. The energy losses were positive correlated with TLV, and PTLV was offset to the hub of the inducer due to the thermodynamic effects. Thus, the tip leakage can be considered as a key parameter to minimize the occurrence of thermodynamic cavitation in a liquid nitrogen inducer toprovide a reference for the design of the inducer.

A Design for High-Speed Journal Bearings with Reduced Pad Size and Improved Efficiency

Improving efficiency is a general task in the design process of high-speed journal bearings. A specific fixed-pad bearing geometry featuring reduced pad length and additional design measures with the intention of reducing frictional power loss is investigated, experimentally and theoretically, for a journal diameter of 500 mm up to surface speeds of 94 m/s and unit loads of 5.0 MPa. To model fluid flow in the bearing outside the lubricant gap, an extension to Elrod’s cavitation algorithm based on assuming the inertia of fluid flow is proposed. Validation of the extended thermo-elasto-hydrodynamic lubrication (TEHL) model shows good agreement between measurement and prediction in wide operating ranges, however, with systematic tendencies of the remaining deviations. Furthermore, measured local pressure and film thickness distributions indicate a complex formation of cavitation with an influence of axial flow that is not covered by pure Couette-flow in the cavitation region. Measured as well as predicted data prove increased bearing efficiency for high rotor speeds. To provide understanding on the impact of the applied design measures improving efficiency, their combination is separated into the individual ones. Reduced axial and peripheral pad length both contribute almost equally to the reduction in power loss and improve its value by 37% compared to the standard design. Finally, further steps to deeper identify the behavior of the bearing are comprehensively discussed.

Experimental research on sealing performance of liquid film seal with herringbone-grooved composite textures

Based on the mechanical seal experiments, sealing performances of the mechanical face seals with the herringbone-grooves (HG) only and with composite textures of herringbone grooves and circular dimple textures (HGC) were investigated respectively. The reserve suction effect of HG observed in the experiments provided the evidence for the main inducing mechanism of zero leakage in liquid film sealing. The cavitation region of HGC decreases with the rotating speed, and the HG has the reserve effect. The HGC has lower friction torque and larger film thickness compared to HG, indicating better friction performance. Because of the circular dimples, the temperature difference in sealing rings is reduced and the corresponding thermal performance of the mechanical seal has been improved.

Oil Cavitation Morphology Analysis of Textured Cylindrical Rotating Friction Pair on Circular-Disk End Faces

Cavitation is a destructive hydraulic phenomenon; it causes damage to small hydraulic machinery and large water-conservancy projects, and cavitation and cavitation corrosion problems are long-standing problems for hydraulic machinery experts. Micro-textures are often machined onto the end face near the indirect contact point to increase the oil film bearing capacity and reduce the transfer torque in hydraulic machinery, but the pressure disturbance caused by the texture dimple and boundary can induce cavitation. To delay or eliminate cavitation on the textured surface of equipment such as mechanical seals and dynamic bearings, the cavitation effect of a cylindrical texture on a rotating frictional pair was investigated from theoretical, imitational, and experimental perspectives. Low pressure is extremely important for inducing cavitation; once cavitation conditions are met, a full bubble can be formed in about 0.015 s. Cavitation mainly occurs at the outer radius of the rotating end face, i.e., in the high-speed region. There is a mutual mass transfer between gas and liquid phases when cavitation occurs, but the generation of cavitation bubbles is faster than its collapse, thus causing an increasing cavitation region.

Dynamic fluid-structure interaction of graded foam core sandwich plates to underwater blast

• Performing fluid-structure interaction analysis on fully clamped graded foam core sandwich. • Introducing cavitation evolution under the planar underwater impulsive loading. • Showing nonuniformed distribution of the cavitated bubbles during its extension and collapse. • Proposing the determining factors of cavitation evolution for the graded sandwich. • Showing superiority of the core gradient design for eliminating cavitation. The aim of this paper is to address characteristics of cavitation evolution caused by the one-dimensional fluid-structure interaction as the graded foam core sandwich panels are subjected to underwater impulsive loadings. A transparent underwater shock tube is adopted to generate controllable and planar blast impulses exerted on the fully clamped sandwich panels. The effects of the impulsive intensity, core strength, core density gradient, and boundary condition are carried out to examine the cavitation evolution during the dynamic fluid-structure interaction. The results show that the initiation, extension, and collapse of the cavitation are significantly determined by the propagations of the breaking front and the closing front, which have different correlations with the considering effects. Due to the dynamic core compression and unloading, the distinct elevating speed of the closing front is firstly found in the fluid-structure interaction under the impulsive loadings. During the cavitation evolution, the non-uniformed distribution of the cavitated bubbles indicates that the cavitation region closer to the fluid-solid interface experiences more considerable pressure drops. The dynamic response of the weakest layer of the core shows predominant influences on the propagation trajectories of wave fronts and reducing of the cavitation ratio, indicating that the core gradient design can efficiently decrease the transmitted impulse and has superiority to the uniform cores with the identical areal mass.

Numerical Investigation of Tip Leakage Vortex Cavitating Flow in a Waterjet Pump with Emphasis on Flow Characteristics and Energy Features

The Delayed Detached Eddy Simulation (DDES) turbulence model was coupled with a homogeneous cavitation model to analyze the tip-leakage vortex (TLV) cavitating-flow characteristics in a waterjet pump. The numerical results agree well with experimental data. The results show that the vortex evolution in the waterjet pump has three stages, which is similar to that around a hydrofoil, but the vorticity variations in the waterjet pump are more complicated. The relative-vorticity-transport equation was then applied to find the reason for the differences between the vorticity variation observed in the waterjet pump and that around a hydrofoil. The results indicate that the drastic fusion process of the TSV cavity and the TLV cavity in the waterjet pump resulted in the formation of triangular cavitation region near the blade tip that is difficult to reproduce by stationary hydrofoil simulation. This fusion process caused the local variation of fluid volume and further affected the vorticity transport. The entropy-production evaluation method considering the phase transition was then used to analyze the dissipation losses in the complex cavitation region. The results indicate that the drastic fusion process of the TSV cavity and the TLV cavity significantly influenced the entropy production rate distributions and enhanced the disturbance of the flow field. In addition, severe phase transition occurs in the drastic fusion region accompanied by huge phase-transition losses.

An h-adaptive local discontinuous Galerkin method for second order wave equation: Applications for the underwater explosion shock hydrodynamics

The underwater explosion (UNDEX) phenomenon involves hydrodynamic behaviors with a wide range of spatial scales, from macroscopic cavitation regions to refined areas with fluid-structure interactions around the shock wavefront. Correct description of the refined shock wavefront is important for determining the shock wave position and pressure peak. In this work, the local discontinuous Galerkin (LDG) method for the two-dimensional wave equation, which allows for the local triangular meshes refinement and merging during the shock wave propagation process, is presented to model the UNDEX shock hydrodynamics. This article is an extension of our previous work, where we discretize the UNDEX total wave formulation in the LDG framework and simulate the UNDEX shock wave and cavitation phenomenon. In order to save computational cost (including CPU time and computational storage) and capture the shock wavefront more accurately, the LDG method is extended to be in combination with the mesh adaption technique. The troubled-cell indicator, depending on the local jump of the pressure, is adopted to identify elements which require to be refined or coarsened. Typical numerical examples are tested to assess the capability of the present h-adaptive LDG method for capturing the UNDEX shock wave. The numerical results clearly demonstrate that the mesh adaption technique enables us to dynamically increase the mesh resolution in close proximity to the flow discontinuities with a large pressure jump. The local increase of the mesh resolution results in a substantial increase in the accuracy of the pressure peak. In terms of the computational efficiency, the h-adaptive LDG method can save nearly half of the calculation time in comparison to the non-adaptive LDG method on uniform meshes, whose mesh resolution is the same as that of the maximum level of refinement of the h-adaptive LDG method.

Delayed detached eddy simulation and vorticity analysis of cavitating flow around a marine propeller behind the hull

Delayed detached eddy simulation (DDES) together with a homogeneous cavitation model is employed to investigate the cavitating flow around a marine propeller behind the hull. The cavitation influence on the simulation errors distribution and vortex dynamics is attached particular attention. The results indicate the predicted cavitation pattern and pressure fluctuation agree fairly well with the experimental results. Moreover, the three equations method is utilized to assess the simulation accuracy by calculating DDES errors. It is indicated that the propeller cavitation region exhibits large error values, especially for the re-entrant jet and tip vortex cavitation region. That demonstrates the occurrence of cavitation has a considerable impact on the errors distribution. In addition, the vorticity transport equation is applied to illustrate the cavitation-vortex interaction. From the results, the vortex stretching term is dominant among the terms, and the effect of the vortex dilation term is to transport vorticity from the sheet cavitation region to the tip vortex cavitation region. Finally, the hull wake influence on cavitation is also involved in the paper. It is noted that high wake region has a great contribution to the generation and development of cavitation.

Investigation into cavitation damage potentiality using pressure pulsation phenomena in a low head Francis turbine for small hydropower schemes

This study investigated unsteady pressure pulsations in a low-head Francis turbine under the effects of strong cavitation for different plant discharge factors and submergence levels of the draft tube at normal water temperature. The effect of the submergence water level of the draft tube at a dominating frequency was found to be negligible underrated-load and upper-part-load plant-operating conditions. However, the amplitude increased as the submergence of the head of the draft tube increased. At a high plant discharge coefficient, i.e., under overload conditions, the maximum amplitude was found at the blade-passing frequency. Additionally, a cavitation region was observed in the vicinity of the blades of the Francis runner, indicating the start of cavitation damage. Moreover, the results revealed the characteristics of low-frequency high amplitudes, which were predominant in the draft tube for the lower-part-load condition. According to the relevant literature, such amplitudes can cause instability in the hydraulic system of a powerplant, potentially resulting in fatigue-related damage to the turbine during operation. This study's predicted numerical results indicated the potentiality of cavitation damage in low-head Francis runners for different plant-operating conditions.

Numerical simulation of cryogenic cavitating flow in LRE oxygen turbopump inducer

Turbopump is a critical component of modern liquid rocket engine (LRE), which employs cryogen as propellant. Its working performance is closely relevant to the reliability of LRE. The performance improvement of turbopump is limited by cryogenic cavitation condition. While the cryogenic cavitating flow characteristic is not well understood till now. A novel numerical method for cryogenic cavitation simulation was developed based on a transport-based cavitation model in the present investigation. The real thermodynamic properties of fluids and an additional energy source item which takes the heat transfer during phase change into account were imbedded into the code, and the empirical constants of the cavitation model are unnecessary to alter. It is found that a moderate reduction of released heat by vapor condensation in the energy source item provides better prediction accuracy for temperature distributions, and an optimal reduction proportion has been suggested. The cavitating flow features inside a three-bladed inducer using liquid oxygen as working fluid have been investigated in detail based on the proposed method, thermal effect reduces both the cavitation regions and the vapor volume fraction inside cavities remarkably, producing less blocking effect on the blade flow channels, which delays the head breakdown. Higher temperature shows stronger thermal effect and better cavitation performance. A classical semiempirical theoretical model is introduced to quantitatively predict the influence of thermal effect on cavitation performance. The predicted results are in consistent with the calculated results. The developed numerical model shows favorable universality to act as a reliable tool for cryogenic turbopump design.

Numerical simulations to study nonlinear wave-based interaction and turbulent magnetic field amplification in the laboratory and astrophysical plasmas

A theoretical model is proposed to study the interaction of high-frequency oblique whistler wave (OWW) and low-frequency kinetic Alfvén waves (KAWs). In our previous investigation (Dewan et al 2020 Phys. Plasmas 27 032111), we established the existence of these two plasma modes (OWW and KAW) by virtue of parametric decay instability. For the present investigation, a numerical simulation technique is employed to solve the coupled system of equations. The ponderomotive force exerted by OWW excites the low-frequency KAW. This quasi-static force induces the genesis of density cavitation (regions of accumulation and depletion) in low-frequency KAW in the magnetized plasma. The simulation results give the localization of the OWW, which amplifies with time. At a later time, the structures become chaotic. The energy cascade is presented in terms of the ensemble-averaged power spectrum. We have also developed a semi-analytical model for this wave–wave interaction mechanism to understand the underlying physics of the field localization process.