• Nuclear Engineering and Technology
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• v.54 no.6
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• pp.2276-2296
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• 2022

This study validates the applicability of the CUPID code for simulating subcooled wall boiling under high-pressure conditions against number of DEBORA tests. In addition, a new numerical technique in which the interfacial momentum non-drag forces are calculated at the cell faces rather than the center is presented. This method reduced the numerical instability often triggered by calculating these terms at the cell center. Simulation results showed good agreement against the experimental data except for the bubble sizes in the bulk. Thus, a new model to calculate the Sauter mean diameter is proposed. Next, the effect of the relationship between the bubble departure diameter (D_dep) and the nucleation site density (N) on the performance of the Wall Heat Flux Partitioning (WHFP) model is investigated. Three correlations for D_dep and two for N are grouped into six combinations. Results by the different combinations show that despite the significant difference in the calculated D_dep, most combinations reasonably predict vapor distribution and liquid temperature. Analysis of the axial propagations of wall boiling parameters shows that the N term stabilizes the inconsistences in D_dep values by following a behavior reflective of D_dep to keep the total energy balance. Moreover, ratio of the heat flux components vary widely along the flow depending on the combinations. These results suggest that separate validation of D_dep correlations may be insufficient since its performance relies on the accompanying N correlations.

• Journal of the Korean Society of Visualization
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• v.20 no.3
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• pp.94-101
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• 2022

The evaluation of the drag and lift as the aerodynamic performance of airfoils is essential. In addition, the analysis of the velocity and pressure fields is needed to support the physical mechanism of the force coefficients of the airfoil. Thus, the present study aims at establishing two different deep learning models to predict force coefficients and flow fields of the airfoil. One is the convolutional neural network (CNN) model to predict drag and lift coefficients of airfoil. Another is the Encoder-Decoder (ED) model to predict pressure distribution and velocity vector field. The images of airfoil section are applied as the input data of both models. Thus, the computational fluid dynamics (CFD) is adopted to form the dataset to training and test of both CNN models. The models are established by the convergence performance for the various hyperparameters. The prediction capability of the established CNN model and ED model is evaluated for the various NACA sections by comparing the true results obtained by the CFD, resulting in the high accurate prediction. It is noted that the predicted results near the leading edge, where the velocity has sharp gradient, reveal relatively lower accuracies. Therefore, the more and high resolved dataset are required to improve the highly nonlinear flow fields.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.3
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• pp.97-105
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• 1998

High pressure injection is recently used to reduce the emissions and increase the power of DI diesel engine. This high pressure injection makes the spray strike the cylinder wall. This spray/wall impingement is known to affect the emission and performance of DI diesel engine such that it is very important to know the spray/wall impingement process. In this study, multidimensional computer program KIVA-II was used to clarify the effect of spray wall impingement by different injection spray angle with the spray/wall impingement model consiedering rebound and slide motion and also the improved submodel for liquid breakup, drop distortion model.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.7
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• pp.769-778
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• 2005

The velocity and pressure fields of a ship's Weis-Fogh type propulsion mechanism are studied in this paper using an advanced vortex method. The wing (NACA0010 airfoil) and channel are approximated by source and vortex panels. and free vortices are introduced away from the body surfaces. The viscous diffusion of fluid is represented using the core-spreading model to the discrete vortices. The velocity is calculated on the basis of the generalized Biot-Savart law and the pressure field is calculated from an integral, based on the instantaneous velocity and vorticity distributions in the flow field. Two-dimensional unsteady viscous flow calculations of this propulsion mechanism are shown. and the calculated results agree qualitatively with the measured thrust and drag due to un-modeled large fluctuations in the measured data.

• Journal of the Korean Society of Marine Environment & Safety
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• v.5 no.2
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• pp.35-44
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• 1999

High negative pressure coefficient is formed in the corner of the bluff body structures. For many curtain wall designers this phenomena is of interest because this high negative pressure coefficient is adopted in structural calculation. The present study is aimed to investigate shedding vortex characteristics of two-dimensional rectangular prism flow. Unsteady calculation by finite difference method based upon SOLA is carried out for three aspect ratios(1:1, 1:2, 1:3) of Re=10$^4$ in viscous incompressible flow within infinite domain. Fluctuation of velocity components at various pick-up points and time variation of drag and lift coefficients are analysed by FFT method to reveal shedding vortex frequency patterns. At aspect ratio 1:1, one primary Strouhal number appears for about all pick-up points. At aspect ratio 1:2, two representative Strouhal numbers are classified by pick-up positions and their flows show two different reattachment patterns. For aspect ratio 1:3, frequency spectrum maintains multiple peaks.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.11a
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• pp.27-41
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• 1996

In this study the heat transfer and fluid flow of the molten pool in stationary gas tungsten arc welding using argon shielding gas were investigated. Transporting phenomena from the welding arc to the base material surface, such as current density, heat flux, arc pressure and shear stress acting on the weld pool surface, were taken from the simulation results of the corresponding welding arc. Various driving forces for the weld pool convection were considered, self-induced electromagnetic, surface tension, buoyancy, and impinging plasma arc forces. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Because fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation. The numerical model was applied to AISI 304 stainless steel and compared with the experimental results.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.10a
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• pp.91-94
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• 2003

An experimental study has been carried out in a supersonic blow-down wind tunnel for examining the influence of streamwise vortices on normal shock-wave/boundary layer interaction. It has been reported by the earlier investigator the streamwise vortices generated by the blowing jets can significantly suppress the shock-induced separation and reduce the wave drag. The blowing jets generate the streamwise vortices with 45$^{\circ}$ angle in the spanwise direction. The shock waves are visualized by a Schlieren optical system. Appropriate measurement systems are provided for the characterization of shock wave/boundary layer interaction. The chamber pressure ratio and blowing pressure ratio are varied from 1.5 to 2.4 and 1.0 to 2.0 respectively.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.3
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• pp.413-422
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• 2004

The flow characteristic in the wake around a circular cylinder with tripping wires, which was set in constant distance, was experimentally investigated in the uniform flow, Re=2.92$\times10^4$. The measurement of velocity vector and pressure distribution are carried out various angles of tripping wires in the range of $50^(\circ)$ to $80^(\circ)$ with $10^(\circ)$ interval. The results show that velocity profiles and pressure distributions are different with angles of tripping wires. The drag of the circular cylinder was decreased about 60% maximum when tripping wires' angle was $50^(\circ)$. The lowest reduction of the velocity and wake width was occurred by coanda effect when the angle was $60^(\circ)$, and the vortex shedding periodicity become rare at the same time.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.459-462
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• 2002

Many active and passive flow control methods have been studied since decades, but there are only few works about flow control methods using ion wind. This paper presents an experimental study on the wake control behind a circular cylinder using ion wind, a bulk motion of neutral molecules driven by locally ionized air of corona discharge. Experiments are done f3r different electrohydrodynamic numbers - the ratio of an electrical body farce to a fluid Inertial force - from 0 to 2 and for the Reynolds number ranging from $4{\times}10^3\;to\;8{\times}10^3$. Pressure distributions over a cylinder surface are measured and flow visualizations are carried out by smoke wire method. Flow visualizations confirm that ion wind affects significantly the wake structure behind a circular cylinder and pressure drag could be dramatically reduced by the superimposing ion wind.

• Coupled systems mechanics
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• v.7 no.4
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• pp.421-440
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• 2018

In this study, a computational method for wall shear stress combined with an implicit direct-forcing immersed boundary method is presented. Near the immersed boundaries, the sub-grid stress is determined by a wall model in which the wall shear stress is directly calculated from the Lagrangian force on the immersed boundary. A coupling mathematical model of the transition process for a model Francis turbine comprising turbulent flow and rotating rigid guide vanes is established. The spatiotemporal distributions of pressure, velocity, vorticity and turbulent quantity are gained with the transient process; the drag and lift coefficients as well as other forces (moments) are also obtained as functions of the attack angle. At the same time, analysis is conducted of the characteristics of pressure pulsation, velocity stripes and vortex structure at some key parts of flowing passage. The coupling relations among the turbulent flow, the dynamical force (moment) response of blade and the rotating of guide vane are also obtained.