Cavitation Problem Research Articles

Corner-transport-upwind lattice Boltzmann model for bubble cavitation.

Aiming to study the bubble cavitation problem in quiescent and sheared liquids, a third-order isothermal lattice Boltzmann model that describes a two-dimensional (2D) fluid obeying the van der Waals equation of state, is introduced. The evolution equations for the distribution functions in this off-lattice model with 16 velocities are solved using the corner-transport-upwind (CTU) numerical scheme on large square lattices (up to 6144×6144 nodes). The numerical viscosity and the regularization of the model are discussed for first- and second-order CTU schemes finding that the latter choice allows to obtain a very accurate phase diagram of a nonideal fluid. In a quiescent liquid, the present model allows us to recover the solution of the 2D Rayleigh-Plesset equation for a growing vapor bubble. In a sheared liquid, we investigated the evolution of the total bubble area, the bubble deformation, and the bubble tilt angle, for various values of the shear rate. A linear relation between the dimensionless deformation coefficient D and the capillary number Ca is found at small Ca but with a different factor than in equilibrium liquids. A nonlinear regime is observed for Ca≳0.2.

Experimental study on submerged supersonic gaseous jet induced tail cavity

Multiphase jets and cavitation problems are inevitable for high-speed underwater vehicles propelled by jet engines. Unlike being injected into stagnant water, the gaseous jet behind a underwater vehicle is usually conjugated with a tail cavity. The pulsation and collapse of such cavities can seriously affect the vehicle performance. In this study, the shape character, forming mechanism and control conditions for the supersonic gaseous jet induced tail cavity at the wake of a revolution body are experimentally investigated in a water tunnel. The induced cavity is ventilated only by a convergent-divergent nozzle with a designed Mach number of 2.45. The form of the cavity is recorded through two high-speed cameras both horizontally and vertically under different Froude numbers and ventilation rates. The time averaged form is thus obtained through digital image processing to eliminate the transient characteristics of the cavity. The experiment is conducted with the Froude number ranging from 3.2 to 16.2, and the ventilation rate 0 to 0.5. Due to the high density and velocity ratio between water and gas, the structure of such flow is usually very complicated. Many novel phenomena of the jet-cavity interaction are observed. With increasing stagnation pressure of the central jet, the induced cavities evolves form foamy, intact, partially break, to pulsating foamy closure type. The foamy and intact tail cavities share the same profile and characteristics that of a supercavity. And the pulsating foamy closure type was never observed before in a traditional supercavitating flow. The outline of the pulsating foamy cavity is the same as the foamy cavity's, indicating that they have the similar forming mechanism. A comparison with the jet-cavity interaction model is made and the following conclusions are obtained:the real ventilation rate, which corresponds to the re-entrant jet gas blocked by the cavity boundary, is the key factor in controlling the cavity form. When the gaseous jet is completely blocked by the water-gas interface, an intact or foamy cavity will be formed. A partially break cavity appears only when some fraction of the jet is blocked and this is when some of the strongest interactions between jet and cavity occurs. When little gas was blocked by the interface, a pulsating foamy cavity forms. With the structure of gaseous jet considered, the transition of the induced cavity closure between different types is in favour of the prediction from Paryshev's model of cavity closure to a central jet. The variation of the cavity form, thus the interaction strength between jet and cavity, coincides with the real ventilation rate estimated through the theoretical model.

Fourier-Chebyshev spectral method for cavitation computation in nonlinear elasticity

A Fourier-Chebyshev spectral method is proposed in this paper for solving the cavitation problem in nonlinear elasticity. The interpolation error for the cavitation solution is analyzed, the elastic energy error estimate for the discrete cavitation solution is obtained, and the convergence of the method is proved. An algorithm combined a gradient type method with a damped quasi-Newton method is applied to solve the discretized nonlinear equilibrium equations. Numerical experiments show that the Fourier-Chebyshev spectral method is efficient and capable of producing accurate numerical cavitation solutions.

Instantaneous torque, energy saving and flow rate ripple analysis of a common rail pump equipped with different delivery-pressure control systems

A mechanical model of a high-pressure pump of a common rail fuel injection system is presented and validated by comparison with experimental instantaneous pump shaft torque and pump piston lift data. The instantaneous torque has been measured with a high-performance torque meter installed on a hydraulic rig for testing pieces of injection apparatus. In the model, the mechanics of the piston plunger and the forces exchanged between pistons and cam are simulated, and friction losses between mobile parts are taken into account. The numerical tool is used to investigate the dynamical performance of the high-pressure pump and to analyse the impact of the rail pressure control strategy on instantaneous torque, energy saving and flow rate ripple. The rail pressure control strategy, based on the application of a fuel metering valve at the pump inlet, gives rise to an improved hydraulic efficiency of the injection system at part loads and to a moderate rate of pressure increase in the pumping chamber at part loads. However, the rail pressure control strategy based on the installation of a pressure control valve at one rail extremity leads to a reduction in the pump flow rate ripple and to a diminution in the fatigue stress. Furthermore, cavitation problems can occur during intake and early compression phases of the pump cycle when the fuel metering unit is working.

The vacuum system reform and test of the super-critical 600mw unit

The deficiencies of the designed vacuum system of the super-critical unit is pointed out in this paper, and then it is reformed by the steam ejector. The experimental results show that the vacuum of the condenser can be improved, the coal consumption can be reduced and the plant electricity consumption can be lowered dramatically at a small cost of the steam energy consumption. Meanwhile, the water-ring vacuum pumps cavitation problems can be solved.

A meshing strategy for a quadratic iso-parametric FEM in cavitation computation in nonlinear elasticity

The approximation properties of a quadratic iso-parametric finite element method for a typical cavitation problem in nonlinear elasticity in two dimensions are analyzed. More precisely, (1) the finite element interpolation errors are established in terms of the mesh parameters; (2) a mesh distribution strategy based on an error equi-distribution principle is given; (3) the convergence of finite element cavity solutions is proved. Numerical experiments show that, in fact, the optimal convergence rate can be achieved by the numerical cavity solutions.

Communication: On the diffusion tensor in macroscopic theory of cavitation.

The classical description of nucleation of cavities in a stretched fluid relies on a one-dimensional Fokker-Planck equation (FPE) in the space of their sizes r, with the diffusion coefficient D(r) constructed for all r from macroscopic hydrodynamics and thermodynamics, as shown by Zeldovich. When additional variables (e.g., vapor pressure) are required to describe the state of a bubble, a similar approach to construct a diffusion tensor D^ generally works only in the direct vicinity of the thermodynamic saddle point corresponding to the critical nucleus. It is shown, nevertheless, that "proper" kinetic variables to describe a cavity can be selected, allowing to introduce D^ in the entire domain of parameters. In this way, for the first time, complete FPE's are constructed for viscous volatile and inertial fluids. In the former case, the FPE with symmetric D^ is solved numerically. Alternatively, in the case of an inertial fluid, an equivalent Langevin equation is considered; results are compared with analytics. The suggested approach is quite general and can be applied beyond the cavitation problem.

Analysis of lubricity and cavitation problem of oil and refrigerant mixture for hermetic compressors

The study is based on the results of experimental and theoretical research, the aim of which was to reveal the potential for the decrease of friction losses in the refrigerator's compressors. Lower viscosity oils as well as decreasing crankshaft diameters should lower the said losses. However, these measures can cause an adverse effect, if hydrodynamic lubrication regime transforms into boundary one, and the bearing eccentricity ratio reaches unity. The main objective of the study is to explain why the almost ideal surfaces of shaft and bush do wear when the carrying capacity of lubricating film theoretically exceeds real loads at eccentricity ratio 0.9 or even less. This disagreement, which also is presented in the study, can be explained by cavitation only partially. This study makes an assumption that the boundary value, i.e. oil film height, at which cavitation is starting, depends on a minimum height of the lubricating oil film, i.e. minimum clearance between shaft and bush.

Research on Polyurethane Elastomer Abrasion Protection for Impeller of Water Pump

The pumps used in the Guhai Pumping Irrigation Project were suffered from sediment abrasion in the long term and their flow passage components, including impellers and pump shells, were also seriously abraded, which led to common problems of poor reliability, low operation efficiency and high energy consumption in almost all the pump stations. Therefore, these problems had made these pumps unable to satisfy the requirements for construction of resource-saving society and development of hydraulic engineering modernization. The steel plate welded impeller technology was a new technique specially developed to solve the serious abrasion of pump impellers. However, the results of long-term application of this technology had demonstrated that the composite resin mortar coating had a limited capability to resist cavitation and thus the parts which are fragile to cavitation on the impellers will fall off easily, which further aggravated the impeller abrasion. After one irrigation season, there were multiple penetrated holes occurring on the impellers, which significantly affects the operation efficiency of the pumps. In order to solve the cavitation resistance issue of composite resin mortar. The polyurethane elastomer technology was developed for the first time, which the polyurethane material with the currently best performance in cavitation resistance was adopted, and the appropriate promoters were added into the polyurethane to improve its properties. In addition, high-strength binder and mechanical connection mode were developed in this technology, which greatly enhanced the binding strength between the polyurethane coating and the pump impellers. Besides, vacuum casting technique was adopted to obtain the pump impeller coating which had excellent performance in abrasion and cavitation resistance. This technology was firstly tested in the Guhai Dazhanchang Pump Station in March 2015. After one and half years operation, it is found that the coating had been improved obviously, and the pumps had long-term stable operation efficiency, which had reached the anticipated testing goal. It is provided reference for the pump stations in Yellow River Diversion Project and pump manufacturers to solve the pump impeller cavitation problem.

Spherical void expansion in rubber-like materials: The stabilizing effects of viscosity and inertia

Dynamic cavitation is known to be a typical failure mechanism in rubber-like solids. While the mechanical behaviour of these materials is generally rate-dependent, the number of theoretical and numerical works addressing the problem of cavitation using nonlinear viscoelastic constitutive models is scarce. It has been only in recent years when some authors have suggested that cavitation in rubber-like materials is a dynamic fracture process strongly affected by the rate-dependent behaviour of the material because of the large strains and strain rates that develop near the cavity. In the present work we further investigate previous idea and perform finite element simulations to model the dynamic expansion of a spherical cavity embedded into a rubber-like ball and subjected to internal pressure. To describe the mechanical behaviour of the rubber-like material we have used an experimentally calibrated constitutive model which includes rate-dependent effects and material failure. The numerical results demonstrate that inertia and viscosity play a fundamental role in the cavitation process since they stabilize the material behaviour and thus delay failure.

HYPERBOLE Symposium 2017 (HYdropower plants PERformance and flexiBle Operation towards Lean integration of new renewable Energies)

Preface to the book of proceedingsThe Conference was held in Porto on February 2 -3, 2017 within the framework of the research project, granted by the European Commission (ERC/FP7- ENERGY-2013-1-Grant 608532). This research project aimed to enhance plants and towards integration of new renewable . To achieve this, the following three fold objectives were defined: i) understanding the root causes for the unit operating range limitations of hydropower plants, ii) modeling and simulating the hydropower plants dynamics over their full range of operation and, then, iii) assessing the impact of the enhanced operation of hydropower plant on the development of so-called New Renewable Energy Resources. The project methodology was based on a system modeling relying on multidisciplinary approach considering hydraulic, mechanical, electrical and, finally, control engineering aspects. Both experimental and numerical simulations tools were used to investigate the aforementioned aspects for selected case studies: i.e. a Francis hydroelectric unit of 440 MW, a reference reversible pump-turbine and the Portuguese Transmission System as a whole.The conference brought together 123 energy specialists from hydraulic, power systems and energy markets to setup a forum to share research advances and technical achievements in the aforementioned topics. In addition, the conference was a unique opportunity to disseminate the results of EU-funded FP7 project.During the conference opening ceremony, Prof. François Avellan, EPFL, coordinator and scientific representative of the research project presented the general background of the project. Then, Prof. António Sá da Costa, chair of the Portuguese Renewable Energy Association (APREN) presented the status of renewable electricity in Portugal. Finally, Mr. Décio Silva, Elec. Eng. MSc., from EDP generation, presented the Venda Nova III Repowering Project, to introduce the technical tour organized for the afternoon following the conference.Then, during the plenary session, the leaders of the project work packages gave an overview of the research project. Prof. François Avellan introduced the project, while Dr. Thomas Aschenbrenner, VOITH Hydro, and Mr. Pierre-Yves Lowys, presented the hydraulic and the mechanical challenges, respectively. Then, Mr. Johan Hell, ANDRITZ, presented the concepts of high flexible hydropower generation for future grids and Prof. Carlos Moreira, INESTEC, the role of pumped storage power plants in electric power systems. Finally, Prof. Eduard Egusquiza, UPC, presented the on-site monitoring of the Francis hydroelectric unit case study.For the closing ceremony, Mr. Luciano dos Santos, GE Renewable Energy Hydro, gave an outlook about the digital disruption of energy. Then, Mr. Manuel Oliveira, EDP Generation, presented as project manager, the details of the Venda Nova III Repowering Project.Finally, Prof. François Avellan addresses the concluding remarks of the Conference and thanks the attendees, the local organizers, Dr. Bernardo Silva, Prof. Carlos Moreira and Prof. João Peças Lopes, the partners of the research project and the European Commission for their supports.It is worth to mention that, to increase further the public impact of the Conference among the scientific community, the 7th Meeting of the IAHR Working Group on Cavitation and Dynamic Problems was organized in Porto on February 1, 2017, 1 day just before the Conference. The authors wanted to thank Prof. Romeo Susan-Resiga, leader of the IAHR Working Group, and Dr. Sebastian Muntean both from the Romanian Academy Timişoara.Lausanne, February 13, 2017Porto, February 13, 2017Prof. François Avellan, Eng. Dr.Dr. Bernardo SilvaProf. Carlos Moreira

CAVITATION WEAR OF PUMP IMPELLERS

Cavitation is a common phenomenon in pump systems, negatively influencing their operating parameters and components such as impellers and, thus, causing considerable financial losses. This paper explains the problem of cavitation and cavitation erosion. The causes of cavitation in pump systems are analyzed. A selection of centrifugal pump impellers damaged by cavitation erosion are presented and examined. The authors also discuss ways of preventing cavitation and cavitation erosion in pump systems. Finally, relevant conclusions are drawn.

Gerotor pump cavitation monitoring and fault diagnosis using vibration analysis through the employment of auto-regressive-moving-average technique

Gerotor pumps, as well known, are widely used in lubrication circuits of internal combustion engines for their simplicity, high efficiency and low costs.In this paper an experimental characterization of a Gerotor pump is shown. The research is a collaboration between the Hydraulic Power Research Group (HPRG) of the University of Naples “Federico II” and the Istituto Motori of CNR.The aim of this research is to investigate the possibility of using simple data to acquire, as vibrational data, on the pump under investigation to detect possible cavitation problem by implementing a proper mathematical procedure to this aim. As been demonstrated that these pumps are particularly subject to cavitation.The gerotor pumps are much subject to cavitate than expected, especially under particular operating conditions of the engine. Cavitation is also correlate to vehicle dynamics particularly with the recent tendency to reduce the mass of oil in the sump. Therefore, the research to avoid cavitation is crucial nowadays. Consequently this study has been firstly performed with an experimentation on the pump by monitoring the delivered oil flow-rate and the adsorbed torque on the pump shaft. An accurate analysis of the pressure oscillations in cavitation conditions, has been, also, conducted. Then, an accelerometer sensor has been properly located to study the cavitation with a fault diagnosis system based on vibration detection.The experimental tests have been performed on a test bench of the Hydraulic Laboratory of the University of Naples “Federico II” in Italy. The bench allows testing the pump working by varying the shaft speed, the oil temperature, the suction and the delivery pressure. The main measured parameters are shaft torque, oil flow rate, mean suction and delivery pressure and the instantaneous suction and delivery pressure.As expected, tests revealed the high influence of the suction pressure on the delivered oil flow rate, while no significant influence has been noted on the adsorbed torque. Furthermore, the pressure oscillations in the pump delivery are highly influenced by the suction pressure.A non intrusive accelerometer has been installed during the experimentation on the oil pump. Since the vibration due to cavitation is the main concern of this study, the accelerometer has been mounted at the suction port in the radial direction.More precisely, the paper presents a fault diagnosis system based on vibration detection. Firstly, a Fast Fourier Transform of the vibration signal has been computed. The investigation has been made with and without the presence of cavitation varying the pump rotation speeds. Limitations due to the detecting of the on line cavitation problems by monitoring the FFT vibration spectra have been overcame by implementing an alternative method based on stochastic approach.This diagnosis method of accelerometer time series analysis based on an Auto Regressive and Moving Average (ARMA) method has been used to determine the pump failure. The diagnosis results have demonstrated the ability of the proposed mathematical technique in the identification of cavitation phenomena proving that the proposed approach is a useful methodology to detect the presence of fault. The approach can predict, with good accuracy, pump failure in real time operation. In addition, a threshold vibration level in decibel scale is also fixed.

Finsler geometry of nonlinear elastic solids with internal structure

Concepts from Finsler differential geometry are applied towards a theory of deformable continua with internal structure. The general theory accounts for finite deformation, nonlinear elasticity, and various kinds of structural features in a solid body. The general kinematic structure of the theory includes macroscopic and microscopic displacement fields–i.e., a multiscale representation–whereby the latter are represented mathematically by the director vector of pseudo-Finsler space, not necessarily of unit magnitude. A physically appropriate fundamental (metric) tensor is introduced, leading to affine and nonlinear connections. A deformation gradient tensor is defined via differentiation of the macroscopic motion field, and another metric indicative of strain in the body is a function of this gradient. A total energy functional of strain, referential microscopic coordinates, and horizontal covariant derivatives of the latter is introduced. Variational methods are applied to derive Euler–Lagrange equations and Neumann boundary conditions. The theory is shown to encompass existing continuum physics models such as micromorphic, micropolar, strain gradient, phase field, and conventional nonlinear elasticity models, and it can reduce to such models when certain assumptions on geometry, kinematics, and energy functionals are imposed. The theory is applied to analyze two physical problems in crystalline solids: shear localization/fracture in a two-dimensional body and cavitation in a spherical body. In these examples, a conformal or Weyl-type transformation of the fundamental tensor enables a description of dilatation associated, respectively, with cleavage surface roughness and nucleation of voids or vacancies. For the shear localization problem, the Finsler theory is able to accurately reproduce the surface energy of Griffith’s fracture mechanics, and it predicts dilatation-induced toughening as observed in experiments on brittle crystals. For the cavitation problem, the Finsler theory is able to accurately reproduce the vacancy formation energy at a nanoscale resolution, and various solutions describe localized cavitation at the core of the body and/or distributed dilatation and softening associated with amorphization as observed in atomic simulations, with relative stability of solutions depending on the regularization length.

A Robust Modification to the Universal Cavitation Algorithm in Journal Bearings

In the current study, a modified fast converging, mass-conserving, and robust algorithm is proposed for calculation of the pressure distribution of a cavitated axially grooved journal bearing based on the finite volume discretization of the Adams/Elrod cavitation model. The solution of cavitation problem is shown to strongly depend on the specific values chosen for the lubricant bulk modulus. It is shown how the new proposed scheme is capable of handling the stiff discrete numerical system for any chosen value of the lubricant bulk modulus (β) and hence a significant improvement in the robustness is achieved compared to traditionally implemented schemes in the literature. Enhanced robustness is shown not to affect the accuracy of the obtained results, and the convergence speed is also shown to be considerably faster than the widely used techniques in the literature. Effects of bulk modulus, static load, and mesh size are studied on numerical stability of the system by means of eigenvalue analysis of the coefficient matrix of the discrete numerical system. It is shown that the impact of static load and mesh size is negligible on numerical stability compared to dominant significance of varying bulk modulus values. Common softening techniques of artificial bulk modulus reduction is found to be tolerable with maximum two order of magnitudes reduction of β to avoid large errors of more than 3–40% in calculated results.

Numerical and Experimental Test on an Innovative Device Based on Multiple Rotors in Line to Tap Clean Energy from Tidal and River Current

The main object of this paper is to present the design and the results of an experimental test campaign on an innovative device for the exploitation of marine and river currents. The basic idea is to lay together one or more series of horizontal axis turbines in rows, each one connected to a generator placed on board a ship or a float. The whole system is sustained by a series of buoys, in this way is possible to avoid expensive submarine installations. All the work has been carried out at the Department of Industrial Engineering of the University of Naples "Federico II" and the test campaign has been first performed in the naval towing tank belonging to the same Department and then in real conditions in Messina Strait between Sicily island and mainland. This location is the only one in Italy suitable as test site thanks to a highly regular tidal current reaching a maximum speed of 3 m/s. The turbine's blade was first carefully designed with particular care for the cavitation problems. Two different series of experimental tests on a reduced scale model in the naval tank aimed to optimize the general layout of the device and the tests performed in Messina Strait mainly aimed to confirm the feasibility of the system and to evaluate its response in real operating conditions. The tests have confirmed the good dynamic behavior of the whole system and its feasibility to be scaled up to real scale.

Mechanics of cutaneous wound rupture

A cutaneous wound may rupture during healing as a result of stretching in the skin and incompatibility at the wound–skin interface, among other factors. By treating both wound and skin as hyperelastic membranes, and using a biomechanical framework of interfacial growth, we study rupturing as a problem of cavitation in nonlinear elastic materials. We obtain analytical solutions for deformation and residual stress field in the skin–wound configuration while emphasizing the coupling between wound rupture and wrinkling in the skin. The solutions are analyzed in detail for variations in stretching environment, healing condition, and membrane stiffness.

Mixed lubrication problems in the presence of textures: An efficient solution to the cavitation problem with consideration of roughness effects

In order to consider the effect of the local surface properties, such as textures and roughness, a mass-conserving model is specifically developed based on the Elrod–Adams model. The classic flow factor methodology is incorporated in this model to deal with the roughness scale. Meanwhile, by reformulating the cavitation conditions according the Fischer–Burmeisster equations, the system of discretized equations for this model is changed to be unconstrained. The computational efficiency of the developed model is similar to that when cavitation is not considered. Finally, this model is evaluated in the predicting the performance of the textured ring/liner system under the start-up process.

On Olsson's Interphase Condition in Cavitation Analysis

Olsson's interphase condition (OIC) is carefully examined and scrutinized with respect to both physical and mathematical implications. It is a genuine initial value problem statement so that its full compliance is mandatory for analyzing time-dependent cavitation problems in the journal bearing. Implementation of OIC must include: (1) an unambiguous description of the initial state of the entire fluid film of the bearing, (2) a realistic description of fluid supply configuration, (3) accurate determination of the locations of the void boundaries together with the corresponding pressure gradients, and (4) a suitable morphology model for the cavitated fluid. A new computation algorithm is proposed for cavitation studies that are governed by cross-boundary interface continuity (CBIC), which is a modified statement of OIC.

Numerical simulation on the cavitation of waterjet propulsion pump

Waterjet propulsion system is widely used in high speed vessels with advantages of simple transmission mechanism, low noise underwater and good manoeuvrability. Compared with the propeller, waterjet propulsion can be used flow stamping to increasing cavitation resistance at high speed. But under certain conditions, such as low ship speed or high ship speed, cavitation problem still exists. If water-jet propulsion pump is run in cavitation condition for a long time, then the cavitation will cause a great deal of noise CFD is applied to analysis and predict the process of production and development of cavitation in waterjet propulsion pump.Based on the cavitation model of Zwart-Gerber-Belamri and a mixture of homogeneous flow model, commercial CFD software CFX was taken for characteristics of cavitation under the three operating conditions. Commercial software ANSYS 14.0 is used to build entity model, mesh and numerical simulation. The grid independence analysis determine the grid number of mixed flow pump model is about 1.6 million and the grid number of water-jet pump system unit is about 2.7 million.The cavitation characteristics of waterjet pump under three operating conditions are studied. The results show that the cavitation development trend is similar design and small rate of flow condition .Under the design conditions Cavitation bubbles are mainly gathered in suction surface of blade near the inlet side of the hub under the primary stage, and gradually extended to the water side in the direction of the rim with the loss of the inlet total pressure. Cavitation appears in hub before the blade rim, but the maximum value of gas content in blade rim is bigger than that in hub. Under large flow conditions, bubble along the direction of wheel hub extends to the rim gradually. Cavitation is found in the pressure surface of blade near the hub region under the critical point of cavitation nearby. When NPSHa is lower than critical point, the area covering by bubbles is about 40% in the suction surface of blade. It means that the critical point of cavitation of pump system is not the accrue point of install cavitation but cavitation has been developed to a certain stage.