EVALUATION OF A NEW CAVITATION MODEL

Cong-Tu Ha,Su-Il Park,Warn-Gyu Park,Charles L. Merkle 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

The cavitating flow simulation is of practical importance for many engineering systems, such as marine propellers, pump impellers, nozzles, injectors, torpedoes, etc. The present work is to test new cavitation model. The governing equation is the Navier-Stokes equation based on homogeneous mixture model. The solver employs an implicit preconditioning algorithm in curvilinear coordinates. The computations have been carried out for the cylinders with 0-, 1/2-, and 1-caliber forebody and, then, compared with experiments and other numerical results. Fairly good agreements with experiments and numerical results have been achieved.

Intelligent Phase Plane Switching Control of Pneumatic Artificial Muscle Manipulators with Magneto-Rheological Brake

Tu Diep Cong Thanh,Kyoung Kwan Ahn 제어로봇시스템학회 2005 제어로봇시스템학회 국제학술대회 논문집 Vol.2005 No.6

Some Numerical Results of Two-Fluid Flows Using Full Navier-Stokes Equations

Cong-Tu Ha,Warn-Gyu Park 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.11

Numerical simulations of two-fluid flow models based on the full Naviver-Stokes equations are presented. The models involve six and seven partial differential equations, namely seven- and six-equation models. The seven-equation model consists of a non-conservative equation for volume fraction evolution of one of the fluids and two sets of balance equations. Each set describes the motion of corresponding fluid which has its own pressure, velocity, temperature, etc. The closure is achieved by two stiffened-gas equations of state. Instantaneous relaxation towards equilibrium is achieved by velocity and pressure relaxation terms. The six-equation model is deduced from the seven-equation one by assuming zero velocity relaxation term. In this model, a single velocity is used for both fluids. The numerical solutions are obtained by applying of the Strang splitting technique. The numerical solutions are examined over a set of one-, two-, and three-dimensions for both six- and seven-equation models. The results indicate a very good agreement with the experimental ones. There is insignificant difference between the results of two models, but the six-equation model offers much more advantage economy over the the seven-equation one.

Numerical Simulations of Two-Phase Compressible Flows Using Full Navier-Stokes Equations Having Six and Seven Equations

Cong-Tu Ha,Warn-Gyu Park 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.10

This paper presents the numerical predictions obtained by solving two-fluid six- and seven- equation models. The seven-equation model has two mass, two momentum, and two energy equations for each phase, and a non-conservative equation for volume fraction evolution of one of the phases. In the six-equation model, only single momentum equation for the two-fluid mixture is used. The numerical solution is obtained by applying of the Strang splitting to take into account both parts of the model: the hyperbolic part and the non-differentiable source terms part. The non-differentiable part consists of the relaxation terms of the velocity and pressure, and a mass solver. For the solution of the hyperbolic part, a second-order HLL Riemann solver with MUSCL reconstruction is employed. To demonstrate the capability of the present numerical scheme, the numerical solutions are examined over several representative tests of one-, two-, and three-dimensions for both six- and seven-equation models. The numerical predictions from the seven-equation model are found to be similar to those from the six-equation model, but the six-equation model is far more efficient.

SENSITIVITY EVALUATION OF EMPIRICAL COEFFICIENTS IN THE CAVITATION MODELS

Cong-Tu Ha,Wam-Gyu Park 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.11

Cavitating flow is notoriously complex because of the issues of large density jump across the irregularly shaped phase boundary, interaction between phase change, turbulence, and the stiffness in the numerical solution. These issues can be modeled with the aid of the multiphase transport equations with appropriate source terms to regulate the mass transfer between phases. In this paper, a sensitivity analysis of empirical coefficients to access the predictive capability of the existing cavitation models is presented. Three cavitation models are considered. Two empirical coefficients used in each cavitation model are accessed, namely Cdest, and Cprod, that directly affect the evaporation and condensation of phases. The coefficients are non-dimensionalized with the freestream values to have correct dimensional form as the convective terms. The present compressible multiphase Reynolds averaged Navier-Stokes solver employs a preconditioning algorithm. The standard k-ε turbulence model with wall functions is employed as the turbulence closure. The governing equations are solved on multi-block structured curvilinear grids. The sensitivity of the computations for turbulent cavitating flows over the axisymmetric bodies to the empirical coefficients are performed. The empirical coefficients in three cavitation models have been successfully calibrated with the experimental data for different flow conditions. The sensitivity analysis indicates that the cavitation flow characteristic as well as the stability of the numerical method be quite sensitive to the model coefficients. To confirm the use of the calibrated model coefficients, the computations of cavitating flow over the Clark-Y hydrofoil is then carried out. The multiphase solver provides overall satisfactory results for the prediction of cavitating flow behavior.

A compressive interface-capturing scheme for computation of compressible multi-fluid flows

Ha, Cong-Tu,Kim, Dong-Hyun,Park, Warn-Gyu,Jung, Chul-Min Elsevier 2017 Computers & fluids Vol.152 No.-

<P><B>Abstract</B></P> <P>In this paper, a compressive high-resolution interface-capturing scheme is presented for the computation of compressible multi-fluid flows with high-density ratios and strong shocks. The proposed scheme is coupled with a preconditioned dual-time compressible mixture solver for robust and accurate computations over a wide range of Mach numbers. The scheme is simple and relatively easy to implement. It does not require any calculations for the interface curvature and the normal vector. The numerical approximations were implemented on general, structured grids using an implicit MUSCL upwind approach. Validation tests were conducted for a single reversible vortex, advection of an air-water interface, dam-break flow, and air shock-helium bubble interaction. Finally, a three-dimensional gas-lift flow is presented to demonstrate the capability of the present scheme for handling an interface with large jumps in pressure, temperature, and density.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A coupled dual-time stepping and compressive interface capturing scheme is presented. </LI> <LI> Interface flows are solved on curvilinear grids. </LI> <LI> Accuracy, efficiency, and robustness are assessed for various interface flows. </LI> </UL> </P>

NUMERICAL SIMULATION OF VERTICAL WATER-EXIT AND WATER-ENTRY OF CAVITATING PROJECTILES

Van-Tu Nguyen,Cong-Tu Ha,Wam-Gyu Park 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.11

The present study focuses on the simulation of unsteady ventilated-cavitating flow during the vertical water-exit and water-entry of projectiles with and without exhaust plume. The propulsion exhaust plume gas embedded within the surrounding liquid strongly affects on the wake of flow. For simulation of water-exit and water-entry behavior of the cavitating projectiles which presents aspects of the flow physics such as cavity shape and phase topography of the flow during the processes, a numerical multi-phase homogeneous mixture model is utilized. The model is developed based on the unsteady incompressible Reynolds-averaged Navier-Stokes equations. Preconditioning pseudo-time stepping method is employed to improve the computational efficiency of the model. The results indicate that characteristics of cavity flow induced during water-exit and water-entry primarily depend on the geometry of projectiles and the velocities of flow. In addition, the gas exhaust delays the surface closure of the water-entry cavity that is one of the most important events in later cavity growth.

Axisymmetric simulation of bubble condensation of pure steam and steam–air mixture

Ha, Cong-Tu,Park, Warn-Gyu Elsevier 2018 Nuclear engineering and design Vol.337 No.-

<P><B>Abstract</B></P> <P>A coupled compressive interface capturing scheme and dual-time preconditioned approach was developed for the two-dimensional axisymmetric computation of compressible interface flows with mass transfers. The fully-compressible three-phase homogeneous mixture flow model was implicitly solved using the dual-time preconditioned technique on generalized curvilinear grids. The interfaces between the three phases were captured by the solution of two interface advection equations using a compressive high resolution interface capturing method. The predictive capabilities of the numerical scheme were examined for a series of bubble condensations of pure steam and steam–air mixtures in different thermal and hydrodynamic subcooled boiling flows. Reasonably good agreement with the experimental data was obtained. Subsequently, several test cases on the condensation of single steam–air mixture bubbles were performed to investigate the effects of non-condensable gases on the characteristics of a condensing bubble. The numerical results revealed a nearly linear decrease of the condensation rate with an increase of the non-condensable gas void fraction in the mixture bubble.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Fully compressible three-phase flow model with mass transfers is presented. </LI> <LI> Interfaces are captured with a compressive interface solution of two interface advection equations. </LI> <LI> Numerical scheme is examined for bubble condensation of pure steam and steam–air mixture. </LI> <LI> The effects of air void fraction on steam condensation are investigated. </LI> </UL> </P>

NUMERICAL SIMULATIONS OF COMPRESSIBLE INTERFACE FLOWS USING A DUAL-TIME IMPLICIT PRECONDITIONED APPROACH COUPLED WITH A STANDARD VOF SCHEME

Ha Cong tu,D.H. Kim(김동현),W.G. Park(박원규) 한국전산유체공학회 2016 한국전산유체공학회 학술대회논문집 Vol.2016 No.11

APPLICATION OF VOLUME OF FLUID INTERFACE TRACKING METHOD TO SIMULATION OFWATER IMPACT

Van-Tu Nguyen,Duc-Thanh Vu,Cong-Tu Ha,Warn-Gyu Park 한국전산유체공학회 2013 한국전산유체공학회 학술대회논문집 Vol.2013 No.5

In this paper, numerical simulations of water-entry of bodies are presented. Homogeneous flow model based on unsteady incompressible Reynolds-averaged Navier-Stokes equations was used to solve the flow field. Dual-time pseudo-compressibility method was employed to improve the computational efficiency of the solver. To investigate behavior of the water impact phenomenon, a VOF interface tracking algorithm known as the least squares volume-of-fluid interface reconstruction algorithm (LVIRA) is utilized to track the water-air interface. This approach uses the geometric information from the reconstructed interface and the velocity fields in each physical time step to update the volume fractions by solving the advection equation for water phase. The application to the computations of water impact of a hemisphere, two cones, and a wedge through free fall in one degree of freedom are performed. The free surface deformation, pressure coefficients, impact velocities and vertical accelerations during the impact are compared with available experiments and theoretical results. Good agreements with those results are obtained.