Hoon-ki Choi

USE OF THE MOMENTUM INTERPOLATION METHOD FOR FLOWS WITH A LARGE BODY FORCE

A numerical study of the use of the momentum interpolation method for flows with a large body force is presented. The inherent problems of the momentum interpolation method are discussed first. The origins of problems of the momentum interpolation method are the validity of linear assumptions employed for the evaluation of the cell-face velocities, the enforcement of mass conservation for the cell-centered velocities, and the specification of pressure and pressure correction at the boundary. Numerical experiments are performed for a typical flow involving a large body force. The numerical results are compared with those by the staggered grid method. The fact that the momentum interpolation method may result in physically unrealistic solutions is demonstrated. Numerical experiments changing the numerical grid have shown that a simple way of removing the physically unrealistic solution is a proper grid refinement where there is a large pressure gradient. An effective way of specifying the pressure and pressure correction at the boundary by a local mass conservation near the boundary is proposed, and it is shown that this method can effectively remove the inherent problem of the specification of pressure and pressure correction at the boundary when one uses the momentum interpolation method.

The Role of Turbulence Models for Predicting Thermal Stratification

A numerical study of evaluation of turbulence models for predicting the thermal stratification phenomenon is presented. The tested models are the elliptic blending model (EBM), the two-layer model, the shear stress transport model (SST) and the elliptic relaxation model (V2-f). These four turbulence models are applied to the prediction of a thermal stratification in an upper plenum of a liquid metal reactor experimented at the Japan Nuclear Cooperation (JNC). The algebraic flux model is used for treating the turbulent heat fluxes for all the models. The EBM and V2-f models predict properly the steep gradient of the temperature at the interface of the cold and hot regions which is observed in the experimental data, and the EBM and V2-f models have the capability of predicting the temporal oscillation of the temperature. The two-layer and SST models predict the diffusive temperature gradient at the interface of a thermal stratification and fail to predict a temporal oscillation of the temperature. In general the EBM predicts best the thermal stratification phenomenon in the upper plenum of the liquid metal reactor.Copyright

New shape design method of an epicycloidal gear for an epicycloid drive

This paper proposed a new approach to the shape design of an epicycloidal gear, which is a key part of an epicycloidal drive with a fixed outer ring. Applying the concept of instant center of velocity of a contact mechanism to the drive, this paper derived the analytical equation fot designing the epicycloidal plate gear, including the generation of curve conjugated to the pin-tooth of an outer ring and the calculation of transmission angles and determination of the contact points. This method has been completed by three steps: the first is to find the instant center of velocity for the epicycloidal drive; the second is to determinate the transmission angles and the contact points in the special coordinate systems; and the last is to generate the full shape of the epicycloidal gear by the coordinate transformation technique. A program for design automation of shape of the cycloidal gear has been developed with C++ language. Finally, an example was given to demonstrate the feasibility of the application.

Evaluation of Turbulence Models for Thermal Striping in a Triple-Jet

A computational study for the evaluation of the current turbulence models for the prediction of a thermal striping in a triple-jet is performed. The tested turbulence models are the two-layer model, the shear stress transport model and the elliptic relaxation model. These three turbulence models are applied to the prediction of the thermal striping in a triple-jet in which detailed experimental data are available. The performances of the tested turbulence models are evaluated through comparisons with the experimental data. The predicted mean and root-mean-square values of the temperature are compared with the experimental data, and the capability of predicting the oscillatory behavior of the ensemble-averaged temperature is investigated. From these works it is shown that only the elliptic relaxation model is capable of predicting the oscillatory behavior of the ensemble-averaged temperature. It is also shown that the elliptic relaxation model predicts best the time-averaged and root-mean-square of the temperature fluctuation. However, this model predicts a slower mixing at the far downstream of the jet.Copyright

Development of a new sealing-type abutment for improving contact state between the abutment and the fixture

When repeating chewing force is applied to the dental implant, gap is often generated at the interface surface between the abutment and the fixture, and results in some deteriorations such as loosening of fastening screw, dental retraction and fixture fracture. To cope with such problems, this study focuses on the design of the interface surface shape of the abutment, and proposes a new sealing-type abutment so as to have a number of grooves along the conical-surface circumference. To show the effectiveness of the proposed method, a number of simulations are carried out by a finite element method in consideration of external chewing force and pretention between the abutment and the fixture. The simulation result shows that the proposed sealing-type abutment shows a better sealing effect than a conventional one.

Study on stabilization of small overflow induced free surface

Stable water supply to turbine system is essential requirement for small hydro power system using waste cooling water. Complex flow pattern in water channel is expected to be stable with overflow discharge. For reliable hydro power system, detailed analysis for flow characteristics is required to identify proper design parameters. In this study, small overflow over sluice is proposed to have stable free surface flow which eventually makes entire flow stable. Three-dimensional numerical analysis with this overflow is performed with standard k-e model and Volume of fluid (VOF) model. Three different design parameters of height of sluice, shape of crest and height of inflow hole are selected and evaluated for their effects through analysis. The overflow with low height of sluice stabilizes free surface flow and triangular crest shape enhances flow stabilization. Water supply through the inflow hole to the turbine system is stable with low height one.