5,257 research outputs found
Cavitation Induction by Projectile Impacting on a Water Jet
The present paper focuses on the simulation of the high-velocity impact of a projectile impacting on a water-jet, causing the onset, development and collapse of cavitation. The simulation of the fluid motion is carried out using an explicit, compressible, density-based solver developed by the authors using the OpenFOAM library. It employs a barotropic two-phase flow model that simulates the phase-change due to cavitation and considers the co-existence of non-condensable and immiscible air. The projectile is considered to be rigid while its motion through the computational domain is modelled through a direct-forcing Immersed Boundary Method. Model validation is performed against the experiments of Field et al. [Field, J., Camus, J. J., Tinguely, M., Obreschkow, D., Farhat, M., 2012. Cavitation in impacted drops and jets and the effect on erosion damage thresholds. Wear 290â291, 154â160. doi:10.1016/j.wear.2012.03.006. URL http://www.sciencedirect.com/science/article/pii/S0043164812000968 ], who visualised cavity formation and shock propagation in liquid impacts at high velocities. Simulations unveil the shock structures and capture the high-speed jetting forming at the impact location, in addition to the subsequent cavitation induction and vapour formation due to refraction waves. Moreover, model predictions provide quantitative information and a better insight on the flow physics that has not been identified from the reported experimental data, such as shock-wave propagation, vapour formation quantity and induced pressures. Furthermore, evidence of the Richtmyer-Meshkov instability developing on the liquid-air interface are predicted when sufficient dense grid resolution is utilised
Numerical prediction and experimental verification of cavitation of globe type control valves.
Globe valves are one of the oldest types of valve used for throttling applications for all sizes due to better controllability and wider range. One of the major limitations associated with the use globe valves in liquid application is cavitation and it takes place both in part open and in fully open conditions due to varied reasons. There are different designs of globe valves available but for control valve applications, cage and plug designs are widely employed. Cage and plug design consists of body, valve cage, plug and an actuating mechanism. Actuating mechanism is connected to the valve plug which is a moving part, through valve shaft. There are many investigations reported about the flow visualization and numerical simulation of normal type globe valves. But study on valves with cage and plug design are not reported in detail. The objective of the present study is to provide a three dimensional analysis of flow through a globe valve with cage and plug design with emphasis on the inception and development of cavitation in detail. Cavitation reduction is achieved by breaking the flow in the form of more than one liquid jet, thereby increasing the turbulence inside the valve flow path. The numerical simulation was done using GAMBIT to set up geometry and grid and FLUENT to solve difference equation postulated from the conservation of mass and momentum of the fluid in motion. The k-epsilon model was used for turbulence. Results of five configurations of the cage with constant flow areas and valve stroke are presented in this paper. The numerical results were verified with an experimental program employing total flow measurement and pressure drop created by the valve at full opening. The study was conducted for different jet configurations to generalize the results of the study. Experimental validation was done in the water test facility with an operating pressure of 1.6 MPa and flow rate of 0.05 m3/s. In the study, total area of opening for the flow and the valve stroke were kept constant. Accelerometers and dynamic pressure sensors were employed to sense the severity of cavitation at different differential pressures across the test valve.http://deepblue.lib.umich.edu/bitstream/2027.42/84230/1/CAV2009-final22.pd
Assessment of RANS turbulence models and Zwart cavitation model empirical coefficients for the simulation of unsteady cloud cavitation
The numerical simulation of unsteady cavitation flows is sensitive to the selected models and associated parameters. Consequently, three Reynolds Average Navier-Stokes (RANS) turbulence models and the Zwart cavitation model were selected to assess their performance for the simulation of cloud cavitation on 2D hydrofoils. The experimental cavitation tests from a NACA65012 hydrofoil at different hydrodynamic conditions were used as a reference to tune the modeling parameters and the experimental tests from a NACA0015 were finally used to validate them. The effects of near wall grid refinement, time step, iterations and mesh elements were also investigated. The results indicate that the Shear Stress Transport (SST) model is sensitive to near wall grid resolution which should be fine enough. Moreover, the cavitation morphology and dynamic behavior are sensitive to the selection of the Zwart empirical vaporization, Fv, and condensation, Fc, coefficients. Therefore, a multiple linear regression approach with the single objective of predicting the shedding frequency was carried out that permitted to find the range of coefficient values giving the most accurate results. In addition, it was observed that they provided a better prediction of the vapor volume fraction and of the instantaneous pressure pulse generated by the main cloud cavity collapse.Postprint (published version
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Numerical investigations on bubble-induced jetting and shock wave focusing: application on a needle-free injection
The formation of a liquid jet into air induced by the growth of a laser-generated bubble inside a needle-free device is numerically investigated by employing the compressible NavierâStokes equations. The three co-existing phases (liquid, vapour and air) are assumed to be in thermal equilibrium. A transport equation for the gas mass fraction is solved in order to simulate the non-condensable gas. The homogeneous equilibrium model is used in order to account for the phase change process between liquid and vapour. Thermodynamic closure for all three phases is achieved by a barotropic Equation of State. Two-dimensional axisymmetric simulations are performed for a needle-free device for which experimental data are available and used for the validation of the developed model. The influence of the initial bubble pressure and the meniscus geometry on the jet velocity is examined by two different sets of studies. Based on the latter, a new meniscus design similar to shaped-charge jets is proposed, which offers a more focused and higher velocity jet compared to the conventional shape of the hemispherical gasâliquid interface. Preliminary calculations show that the developed jet can penetrate the skin and thus, such configurations can contribute towards a new needle-free design
Interaction of particles with a cavitation bubble near a solid wall
Hard particle erosion and cavitation damage are two main wear problems that
can affect the internal components of hydraulic machinery such as hydraulic
turbines or pumps. If both problems synergistically act together, the damage
can be more severe and result in high maintenance costs. In this work, a study
of the interaction of hard particles and cavitation bubbles is developed to
understand their interactive behavior. Experimental tests and numerical
simulations using computational fluid dynamics (CFD) were performed.
Experimentally, a cavitation bubble was generated with an electric spark near a
solid surface, and its interaction with hard particles of different sizes and
materials was observed using a high-speed camera. A simplified analytical
approach was developed to model the behavior of the particles near the bubble
interface during its collapse. Computationally, we simulated an air bubble that
grew and collapsed near a solid wall while interacting with one particle near
the bubble interface. Several simulations with different conditions were made
and validated with the experimental data. The experimental data obtained from
particles above the bubble were consistent with the numerical results and
analytical study. The particle size, density and position of the particle with
respect to the bubble interface strongly affected the maximum velocity of the
particles
Cavitation Number as a Function of Disk Cavitator Radius: a Numerical Analysis of Natural Supercavitation
Due to the greater viscosity and density of water compared to air, the maximum speed of underwater travel is severely limited compared to other methods of transportation. However, a technology called supercavitation â which uses a disk-shaped cavitator to envelop a vehicle in a bubble of steam â promises to greatly decrease skin friction drag. While a large cavitator enables the occurrence of supercavitation at low velocities, it adds substantial unnecessary drag at higher speeds. Based on CFD results, a relationship between cavitator diameter and cavitation number is developed, and it is substituted into an existing equation relating drag coefficient to cavitation number. The final relationship predicts drag from cavitator radius fairly well, with an absolute error less than 5.4% at a cavitator radius above 14.14mm and as low as 1.3% at the maximum tested radius of 22.5mm
Scaling laws for jets of single cavitation bubbles
Fast liquid jets, called micro-jets, are produced within cavitation bubbles
experiencing an aspherical collapse. Here we review micro-jets of different
origins, scales and appearances, and propose a unified framework to describe
their dynamics by using an anisotropy parameter , representing a
dimensionless measure of the liquid momentum at the collapse point (Kelvin
impulse). This parameter is rigorously defined for various jet drivers,
including gravity and nearby boundaries. Combining theoretical considerations
with hundreds of high-speed visualisations of bubbles collapsing near a rigid
surface, near a free surface or in variable gravity, we classify the jets into
three distinct regimes: weak, intermediate and strong. Weak jets
() hardly pierce the bubble, but remain within it throughout the
collapse and rebound. Intermediate jets () pierce the
opposite bubble wall close to the last collapse phase and clearly emerge during
the rebound. Strong jets () pierce the bubble early during the
collapse. The dynamics of the jets is analysed through key observables, such as
the jet impact time, jet speed, bubble displacement, bubble volume at jet
impact and vapour-jet volume. We find that, upon normalising these observables
to dimensionless jet parameters, they all reduce to straightforward functions
of , which we can reproduce numerically using potential flow theory. An
interesting consequence of this result is that a measurement of a single
observable, such as the bubble displacement, suffices to estimate any other
parameter, such as the jet speed. Remarkably, the dimensionless parameters of
intermediate and weak jets only depend on , not on the jet driver. In
the same regime, the jet parameters are found to be well approximated by
power-laws of , which we explain through analytical arguments
A Generalized Compressible Cavitation Model
A new multi-phase model for low speed gas/liquid mixtures is presented; it does not require ad-hoc closure models for the variation of mixture density with pressure and yields thermodynamically correct acoustic propagation for multi-phase mixtures. The solution procedure has an interface-capturing scheme that incorporates an additional scalar transport equation for the gas void fraction. Cavitation is modeled via a finite rate source term that initiates phase change when liquid pressure drops below its saturation value. The numerical procedure has been implemented within a multi-element unstructured framework CRUNCH that permits the grid to be locally refined in the interface region. The solution technique incorporates a parallel, domain decomposition strategy for efficient 3D computations. Detailed results are presented for sheet cavitation over a cylindrical headform and a NACA 66 hydrofoil
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