Numerical simulation and analysis of condensation shocks in cavitating flow

2018 ◽  
Vol 838 ◽  
pp. 759-813 ◽  
Author(s):  
Bernd Budich ◽  
S. J. Schmidt ◽  
N. A. Adams
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Volume Fraction ◽  
Cavity Growth ◽  
Pressure Rise ◽  
Shock Propagation ◽  
Two Phase ◽  
Cloud Cavitation ◽  
Cavity Collapse ◽  
Partial Cavitation

We analyse unsteady cavity dynamics, cavitation patterns and instability mechanisms governing partial cavitation in the flow past a sharp convergent–divergent wedge. Reproducing a recent reference experiment by numerical simulation, the investigated flow regime is characterised by large-scale cloud cavitation. In agreement with the experiments, we find that cloud shedding is dominated by the periodic occurrence of condensation shocks, propagating through the two-phase medium. The physical model is based on the homogeneous mixture approach, the assumption of thermodynamic equilibrium, and a closed-form barotropic equation of state. Compressibility of water and water vapour is taken into account. We deliberately suppress effects of molecular viscosity, in order to demonstrate that inertial effects dominate the flow evolution. We qualify the flow predictions, and validate the numerical approach by comparison with experiments. In agreement with the experiments, the vapour volume fraction within the partial cavity reaches values ${>}80\,\%$ for its spanwise average. Very good agreement is further obtained for the shedding Strouhal number, the cavity growth and collapse velocities, and for typical coherent flow structures. In accordance with the experiments, the simulations reproduce a condensation shock forming at the trailing part of the partial cavity. It is demonstrated that it satisfies locally Rankine–Hugoniot jump relations. Estimation of the shock propagation Mach number shows that the flow is supersonic. With a magnitude of only a few kPa, the pressure rise across the shock is much lower than for typical cavity collapse events. It is thus far too weak to cause cavitation erosion directly. However, by affecting the dynamics of the cavity, the flow aggressiveness can be significantly altered. Our results indicate that, in addition to classically observed re-entrant jets, condensation shocks feed an intrinsic instability mechanism of partial cavitation.

scholarly journals Spectral content of cloud cavitation about a sphere

2016 ◽  
Vol 812 ◽  
Author(s):  
K. L. de Graaf ◽  
P. A. Brandner ◽  
B. W. Pearce
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Cavity Growth ◽  
Leading Edge ◽  
Transition To Turbulence ◽  
Variable Pressure ◽  
Dynamic Surface ◽  
Spectral Content ◽  
Cloud Cavitation ◽  
Cavity Collapse

The physics and spectral content of cloud cavitation about a sphere are investigated in a variable-pressure water tunnel using dynamic surface pressure measurement and high-speed imaging. Experiments are conducted using a polyvinyl chloride sphere at a Reynolds number of $1.5\times 10^{6}$ with cavitation numbers, $\unicode[STIX]{x1D70E}$, ranging from inception to supercavitation. Three distinct shedding regimes are identified: a uni-modal regime for $\unicode[STIX]{x1D70E}>0.9$ and two bi-modal regimes for $0.9>\unicode[STIX]{x1D70E}>0.675$ and $0.675>\unicode[STIX]{x1D70E}>0.3$. For small cavity lengths ($\unicode[STIX]{x1D70E}>0.9$), Kelvin–Helmholtz instability and transition to turbulence in the overlying separated boundary layer form the basis for cavity breakup and coherent vortex formation. At greater lengths ($\unicode[STIX]{x1D70E}<0.9$), larger-scale shedding ensues, driven by coupled re-entrant jet formation and shockwave propagation. Strong adverse pressure gradients about the sphere lead to accumulation and radial growth of re-entrant flow, initiating breakup, from which, in every case, a condensation shockwave propagates upstream causing cavity collapse. When the shedding is most energetic, shockwave propagation upstream may cause large-scale leading edge extinction. The bi-modal response is due to cavity shedding being either axisymmetric or asymmetric. The two bi-modal regimes correspond to $\unicode[STIX]{x1D70E}$ ranges where the cavity and re-entrant jet either remain attached or become detached from the sphere. There is a distinct frequency offset at transition between regimes in both shedding modes. Despite the greater cavity lengths at lower $\unicode[STIX]{x1D70E}$ values, the second bi-modal regime initially exhibits shorter shedding periods due to increased cavity growth rates. The second regime persists until supercavitation develops for $\unicode[STIX]{x1D70E}<0.3$.

scholarly journals Flow topologies in bubble-induced turbulence: a direct numerical simulation analysis

2018 ◽  
Vol 857 ◽  
pp. 270-290 ◽  
Author(s):  
Josef Hasslberger ◽  
Markus Klein ◽  
Nilanjan Chakraborty
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Turbulent Flows ◽  
Volume Fraction ◽  
Fluid Velocity ◽  
Small Scale ◽  
Flow Topology ◽  
Two Phase ◽  
Local Flow ◽  
Interface Curvature

This paper presents a detailed investigation of flow topologies in bubble-induced two-phase turbulence. Two freely moving and deforming air bubbles that have been suspended in liquid water under counterflow conditions have been considered for this analysis. The direct numerical simulation data considered here are based on the one-fluid formulation of the two-phase flow governing equations. To study the development of coherent structures, a local flow topology analysis is performed. Using the invariants of the velocity gradient tensor, all possible small-scale flow structures can be categorized into two nodal and two focal topologies for incompressible turbulent flows. The volume fraction of focal topologies in the gaseous phase is consistently higher than in the surrounding liquid phase. This observation has been argued to be linked to a strong vorticity production at the regions of simultaneous high fluid velocity and high interface curvature. Depending on the regime (steady/laminar or unsteady/turbulent), additional effects related to the density and viscosity jump at the interface influence the behaviour. The analysis also points to a specific term of the vorticity transport equation as being responsible for the induction of vortical motion at the interface. Besides the known mechanisms, this term, related to surface tension and gradients of interface curvature, represents another potential source of turbulence production that lends itself to further investigation.

scholarly journals ASHEE-1.0: a compressible, equilibrium–Eulerian model for volcanic ash plumes

2016 ◽  
Vol 9 (2) ◽  
pp. 697-730 ◽  
Author(s):  
M. Cerminara ◽  
T. Esposti Ongaro ◽  
L. C. Berselli
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Volume Fraction ◽  
Radial Profile ◽  
Air Entrainment ◽  
Compressible Turbulence ◽  
Eulerian Model ◽  
Volcanic Plumes ◽  
Large Eddy ◽  
Non Equilibrium

Abstract. A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydisperse gas–particle mixtures forming volcanic plumes. Starting from the three-dimensional N-phase Eulerian transport equations for a mixture of gases and solid dispersed particles, we adopt an asymptotic expansion strategy to derive a compressible version of the first-order non-equilibrium model, valid for low-concentration regimes (particle volume fraction less than 10−3) and particle Stokes number (St – i.e., the ratio between relaxation time and flow characteristic time) not exceeding about 0.2. The new model, which is called ASHEE (ASH Equilibrium Eulerian), is significantly faster than the N-phase Eulerian model while retaining the capability to describe gas–particle non-equilibrium effects. Direct Numerical Simulation accurately reproduces the dynamics of isotropic, compressible turbulence in subsonic regimes. For gas–particle mixtures, it describes the main features of density fluctuations and the preferential concentration and clustering of particles by turbulence, thus verifying the model reliability and suitability for the numerical simulation of high-Reynolds number and high-temperature regimes in the presence of a dispersed phase. On the other hand, Large-Eddy Numerical Simulations of forced plumes are able to reproduce the averaged and instantaneous flow properties. In particular, the self-similar Gaussian radial profile and the development of large-scale coherent structures are reproduced, including the rate of turbulent mixing and entrainment of atmospheric air. Application to the Large-Eddy Simulation of the injection of the eruptive mixture in a stratified atmosphere describes some of the important features of turbulent volcanic plumes, including air entrainment, buoyancy reversal and maximum plume height. For very fine particles (St → 0, when non-equilibrium effects are negligible) the model reduces to the so-called dusty-gas model. However, coarse particles partially decouple from the gas phase within eddies (thus modifying the turbulent structure) and preferentially concentrate at the eddy periphery, eventually being lost from the plume margins due to the concurrent effect of gravity. By these mechanisms, gas–particle non-equilibrium processes are able to influence the large-scale behavior of volcanic plumes.

Numerical Study on Flow and Heat and Mass Transfer in Pulsating Heat Pipe

2019 ◽  
Author(s):  
Jian-Hong Liu ◽  
Fu-Min Shang ◽  
Nikolay Efimov
Keyword(s):  
Numerical Simulation ◽  
Mass Transfer ◽  
Heat Pipe ◽  
Gas Phase ◽  
Heat And Mass Transfer ◽  
Mixture Model ◽  
Volume Fraction ◽  
Pulsating Heat Pipe ◽  
Two Phase ◽  
Pipe Model

Abstract Numerical simulation was performed to establishing a two-dimensional pulsating heat pipe model, to investigate the flow and heat transfer characteristics in the pulsating heat pipe by using the Mixture and Euler models, which were unsteady models of vapor-liquid two-phase, based on the control-volume numerical procedure utilizing the semi-implicit method. Through comparing and analyzing the volume fraction and velocity magnitude of gas phase to decide which model was more suitable for numerical simulation of the pulsating heat pipe in heat and mass transfer research. It was showed there had gas phase forming in stable circulation flow in the heating section, the adiabatic section using the Mixture and Euler models respectively, and they were all in a fluctuating state at 10s, besides, the pulsating heat pipe had been starting up at 1s and stabilizing at 5s, it was all found that small bubbles in the heat pipe coalescing into large bubbles and gradually forming into liquid plugs and gas columns from the contours of volume fraction of the gas phase; through comparing the contours of gas phase velocity, it could be seen that there had further stably oscillating flow and relatively stabler gas-liquid two-phase running speed in the pulsating heat pipe used the Mixture model, the result was consistent with the conclusion of the paper[11] extremely, from this it could conclude that the Mixture model could be better simulate the vaporization-condensation process in the pulsating heat pipe, which could provide an effective theoretical support for further understanding and studying the phase change heat and mass transfer mechanism of the pulsating heat pipe.

3D MС Simulation of Grain Growth Kinetics and the Zener Limit in Polycrystals

2014 ◽  
Vol 598 ◽  
pp. 8-12
Author(s):  
K.R. Phaneesh ◽  
Anirudh Bhat ◽  
Gautam Mukherjee ◽  
Kishore T. Kashyap
Keyword(s):  
Monte Carlo ◽  
Grain Growth ◽  
Growth Kinetics ◽  
Large Scale ◽  
Volume Fraction ◽  
Growth Exponent ◽  
Cube Root ◽  
Second Phase ◽  
Two Phase ◽  
Second Phase Particles

Large scale Potts model Monte Carlo simulation was carried on 3-dimensional square lattices of 1003 and 2003 sizes using the Metropolis algorithm to study grain growth behavior. Simulations were carried out to investigate both growth kinetics as well as the Zener limit in two-phase polycrystals inhibited in growth by second phase particles of single-voxel size. Initially the matrices were run to 10,000 Monte Carlo steps (MCS) to check the growth kinetics in both single phase and two-phase poly-crystals. Grain growth exponent values obtained as a result have shown to be highest (~ 0.4) for mono-phase materials while the value decreases with addition of second phase particles. Subsequently the matrices were run to stagnation in the presence of second phase particles of volume fractions ranging from 0.001to 0.1. Results obtained have shown a cube root dependence of the limiting grain size over the particle volume fraction thus reinforcing earlier 3D simulation efforts. It was observed that there was not much difference in the values of either growth kinetics or the Zener limit between 1003 and 2003 sized matrices, although the results improved mildly with size.

Numerical Simulation of Modified Trailing Edge of Francis Turbine Stay Vane Based on Solid-Liquid Two-Phase Flow

2014 ◽  
Vol 700 ◽  
pp. 643-646
Author(s):  
Dong Wang ◽  
Si Qing Zhang ◽  
Yun Long Zhang
Keyword(s):  
Numerical Simulation ◽  
Volume Fraction ◽  
Solid Volume Fraction ◽  
Francis Turbine ◽  
Phase Flow ◽  
Oblique Angle ◽  
Trailing Edge ◽  
Two Phase ◽  
High Solid ◽  
Solid Liquid

In order to investigate the silt abrasion of modified trailing edge of stay vane in Francis turbine, the numerical simulation of trailing edge with different geometries were carried out based on the solid-liquid two-phase flow by means of Computation Fluid Dynamics. The results show that low solid volume fraction distributes on the chamfered surface of trailing edge, and high solid volume fraction distributes on the end of oblique surface. The smaller the modified angle is, the larger the distribution area of high solid volume fraction is, which show the trailing edge with smaller oblique angle may suffer from silt abrasion. Therefore, in order to solve the vibration caused by Karman vortex the trailing edge has to be sharpened, the oblique angle of trailing edge should not be too small. At end of trailing edge needs to ensure a certain thickness, especially the trailing edge near the lower ring can be thicker, which can meet the anti-abrasion requirements.

Tracking the Rheological Behavior Change of a Fresh Granular-Cement Paste Material to a Granular Material Based on DEM

2012 ◽  
Vol 446-449 ◽  
pp. 3803-3809
Author(s):  
Hooman Hoornahad ◽  
Eduard A. B. Koenders
Keyword(s):  
Numerical Simulation ◽  
Rheological Behavior ◽  
Volume Fraction ◽  
Analytical Techniques ◽  
Fluid Models ◽  
Continuum Approach ◽  
Two Phase ◽  
The Common ◽  
Phase Models ◽  
Multi Phase

The common approach to describe the rheological behavior of a granular-paste material relies on a description of the motion within the frame of continuum mechanics. However, since a granular-paste system cannot be considered as a homogeneous continuous fluid its behavior should not be estimated by common fluid models, such as Bingham or Herschell Bulkley models. Therefore, a continuum approach is not considered the best option to study the phase effects of a multi-phase material and its corresponding rheological behavior. In this particular case analytical techniques based on the multi-phase models are required. A more appropriate approach is to consider a granular-paste material as a two phase model that accounts for the effect of the gradually decreasing the volume fraction of the pasty phase until getting to zero value on the rheological behavior of the material. In this investigation, a cone test is used to evaluate the rheological behavior of a granular mix where a discrete element method (DEM) is considered as a basis of the numerical simulation.

Numerical simulation of in-pipe particle transportation in CO2 foam

2017 ◽  
Vol 5 (2) ◽  
pp. SF243-SF249
Author(s):  
Cheng Huang ◽  
Xiao Sun ◽  
Jinqiao Wu
Keyword(s):  
Numerical Simulation ◽  
Critical Point ◽  
Cross Section ◽  
Particle Distribution ◽  
Settling Velocity ◽  
Volume Fraction ◽  
Two Phase ◽  
Multiple Phase ◽  
Full Development ◽  
Fracturing Fluids

Proppant-carrying foam fracturing fluids have complex rheological and transportation properties. Current studies on these fluids often focus on experimental phenomena. However, due to the limitation of experimental research, only macroscopic properties, such as the critical settling velocity, can be obtained. Transportation mechanisms and volume fraction distributions are poorly understood as well. In our study, the liquid-solid drag coefficient is corrected, and the mathematical physical model of non-Newtonian fluids of the particle-foam multiple phase is established by using a two-phase model. Proppant settling and transport properties in foam fracturing fluids are numerically studied, particle distribution on pipe cross section is obtained at various conditions, and a criterion for full development of fluid flow in pipe is set. We also find that when the Reynolds number (Re) is less than 190, the critical point of full development of flow increases with Re, whereas when Re is greater than 190, the critical point of full development decreases exponentially with the increasing of Re before stabilizing at approximately 45.

scholarly journals Investigation of Water-Vapor Two-Phase Flow Characteristics in a Tight-Lattice Core by Large-Scale Numerical Simulation, (IV)

2005 ◽  
Vol 4 (2) ◽  
pp. 106-114 ◽  
Author(s):  
Hiroyuki YOSHIDA ◽  
Yasuo OSE ◽  
Masatoshi KURETA ◽  
Takuji NAGAYOSHI ◽  
Kazuyuki TAKASE ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Water Vapor ◽  
Large Scale ◽  
Two Phase Flow ◽  
Flow Characteristics ◽  
Phase Flow ◽  
Two Phase ◽  
Lattice Core ◽  
Tight Lattice
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