Warn-Gyu Park

Numerical study of flow characteristics of the high speed train entering into a tunnel

Abstract When a high speed train enters into a tunnel, the aerodynamic forces severely change and, consequently, the stability and performance significantly deviate from the value of design point which is usually set at cruising speed on the plain ground. The compression wave is also generated ahead of the train due to the piston-like action of tunnel entry motion. The present work is to understand the flow field such as variation of aerodynamic forces and generation of compression wave during tunnel entry motion by applying three-dimensional unsteady Navier–Stokes equation solver. To account for the relative motion of stationary tunnel and moving train, the sliding multi-block method has been implemented.

Numerical study of impact force and ricochet behavior of high speed water-entry bodies

Abstract Numerical method for computing impact forces and ricochet behavior of the water-entry body was developed. The present method assumes the impact occurs within very short time interval and the fluid viscosity effect does not have enough time to play a significant role during impact. That is, the flow around a water-entry body is assumed as inviscid potential flow and is solved by the source panel method. As the body enters into the water, the elements can be classified into three types of elements: (i) fully submerged into water, (ii) intersected by water surface and, (iii) fully outside of water. Since only submerged portion of the body is considered in the present impact analysis, the elements that are outside of water is excluded. The panels fully submerged into the water are routinely treated and the panels intersected with the water free surface are redefined and reorganized to be amenable to the source panel method. To validate the capability of computing impact forces of the present method, a disk cylinder and two ogives were tested and compared with experimental and other numerical data. Good agreements with those results were obtained. After the impact forces were determined, the ricochet off the free surface was simulated by coupling with the dynamic equations of two degrees of freedom, in case of the changes of the water-entry angle and the weight of the body.

Numerical viscous flow analysis around a high-speed train with crosswind effects

Recently, a high-speed train having the maximum Mach number of 0.25-0.3 has been developed in several countries to satisfy demands for new systems of fast and comfortable mass transportation. To predict unsteady aerodynamic loads, performances, and acoustic noise level at this high speed, the accurate simulation of the flowfield is needed. The objective of the present work is to apply the iterative time-marching scheme to the incompressible turbulent flow around a high-speed train with crosswing effects. Even though the crosswind may significantly affect the longitudinal and lateral stabilities of the train, the details of flow properties of the crosswind are not well understood.

Pollution dispersion analysis using the puff model with numerical flow field data

The computations of the flowfield and pollutant dispersion over a flat plate and the Russian hills of various slopes are described. The Gaussian plume and the puff model have been used to calculate concentration of pollutant. The Reynolds-averaged unsteady incompressible Navier–Stokes equation with low Reynolds k–e model has been used to calculate the flowfield. The flow data of a flat plate and the Russian hills from Navier–Stokes equation solutions has been used as the input data for the puff model. The computational results of flowfield agree well with experimental results of both a flat plate and Russian hills. The concentration prediction by the Gaussian plume model and the Gaussian puff model also agrees fairly well with experiments.

MULTIPHASE FLOW SIMULATION OF WATER-ENTRY AND -EXIT OF AXISYMMETRIC BODIES

A fully-compressible, multiphase, homogeneous mixture model, based on unsteady Reynolds-averaged Navier-Stokes equations is presented in this study. Dual-time preconditioning method was employed to improve the computational efficiency of the solution. The multiphase flow solver has been applied to computations of: (1) cavitating flows over underwater projectiles; (2) transonic flow past an underwater projectile; (3) water impact of a circular cylinder entering the water; (4) water-entry of a hemisphere with one degree of freedom; and (5) supercavitating flows over an axisymmetric projectile during water-entry and water-exit. The surface pressure coefficients, water impact forces, vertical accelerations, and impact velocities are compared with available experiments and other published results. Good agreements with those results are obtained. Aspects of water-entry and water-exit flow physics of a projectile with and without gaseous exhaust plume including cavity shape, phase topography and drag coefficients are presented.Copyright

Numerical study of defrosting phenomena of automotive windshield glass

ABSTRACT The present work was undertaken to analyze numerically the defrosting phenomena of automobile windshield glass. In order to analyze the phase change from frost to water on windshield glass by discharging hot air from a defroster nozzle, the flow and the temperature field of the cabin interior, the heat transfer through the windshield glass, and the phase change of frost should be solved simultaneously. In the present work, the flow field was obtained by solving three-dimensional incompressible Navier-Stokes Equations, and the temperature field was computed from the incompressible energy equation. The phase-change process was solved by the enthalpy method. For the code validation, the temperature and the phase change of the driven cavity were calculated. The calculation showed good agreement with other numerical results. Then, the present code was applied to the defrosting problem of a real automobile, and good agreement with the experimental data was also obtained.

Numerical investigation on cavitation flow of hydrofoil and its flow noise with emphasis on turbulence models

In this study, cavitation flow of hydrofoils is numerically investigated to characterize the effects of turbulence models on cavitation-flow patterns and the corresponding radiated sound waves. The two distinct flow conditions are considered by varying the mean flow velocity and angle of attack, which are categorized under the experimentally observed unstable or stable cavitation flows. To consider the phase interchanges between the vapor and the liquid, the flow fields around the hydrofoil are analyzed by solving the unsteady compressible Reynolds-averaged Navier–Stokes equations coupled with a mass-transfer model, also referred to as the cavitation model. In the numerical solver, a preconditioning algorithm with dual-time stepping techniques is employed in generalized curvilinear coordinates. The following three types of turbulence models are employed: the laminar-flow model, standard k − e turbulent model, and filter-based model. Hydro-acoustic field formed by the cavitation flow of the hydrofoil is pr...

Comparison of different two equation turbulence models for studying the effect of cold outlet diameter on cooling performance of vortex tube

This paper presents a comparison between different two equation turbulence models, in order to find appropriate turbulence model for studying the effect of cold outlet diameter on cooling performance of vortex tube. The standard k-ε, RNG k-ε, Realizable k-ε and standard k-ω model, have been used in this study. Comparison between numerical and experimental results indicates that standard k-ε model predicts energy separation phenomenon better than others. Distribution of static temperature, total temperature, static pressure, components of velocity and angular velocity have been obtained in order to understand the flow behavior inside the tube. Moreover, standard k-ε model has been used for studying the effect of cold outlet diameter on cooling performance of vortex tube.

Numerical Analysis of Partial Cavitaing Flow Past Axisymmetric Cylinders

Cavitating flow simulation is of practical importance for many hydraulic engineering systems, such as pump, turbine, nozzle, injector, etc. In the present work, a solver for cavitating flow has been developed and applied to simulate the flows past axisymmetric cylinders. Governing equations are the two-phase Navier-Stokes equations, comprised of continuity equation of liquid and vapor phase. The momentum equation is in the mixture phase. The solver employed an implicit, dual time, preconditioned algorithm in curvilinear coordinates. Computations were carried out for three axisymmetric cylinders: hemispherical, ogive, and caliber-0 forebody shape. Then, the present calculations were compared with experiments and other numerical results to validate the present solver. Also, the code has shown its capability to accurately simulate the re-entrant jet phenomena and ventilated cavitation. Hence, it has been found that the present numerical code has successfully accounted for cavitating flows past axisymmetric cylinders.

Automatic demisting control of automobile windscreen glass

Abstract Mist frequently forms on automobile windscreen glass in cold humid weather. The present paper describes the development of a control device for automatically demisting windscreen glass. Automatic demisting control is initiated when the surface temperature of the glass is lower than the dew point temperature at a given relative humidity and temperature of the cabin inside. It was decided to attach the sensors measuring surface temperature and humidity on the bottom right of the glass and on the back of the rear-view mirror respectively, based on the observation of experiments. The demisting control strategy was originally designed to increase blower speed, change mode, mix hot and cold air, and turn on/off the air conditioner as the mist formed increases. The automatic device was operated with the automatic temperature controller of a heating, ventilation, and air conditioning (HVAC) system and successfully demonstrated in a vehicle test in a cold chamber by following the designed control strategy.