Hyung-Woon Roh

Application of computational techniques for studies of wind pressure coefficients around an odd-geometrical building

Abstract Most studies on wind flows around buildings are focused on the flow fields of buildings with prismatic shapes. Only a few works on wind flows around odd-geometrical buildings are reported. Comprehensive studies for the flow phenomena in odd-geometrical buildings are of significance for practical building construction and heating-ventilating-air conditioning applications. Characteristics of approaching wind are determined by the climate data based on the history of typhoons that have passed through Korea. The wind characteristics obtained by the statistical data are used for numerical simulations. Three-dimensional wind flows around the model of odd-geometrical building are simulated using the finite volume method. Velocity vectors and average-wind-pressure coefficients for equivalent static load are calculated by numerical results. The magnitudes of the wind pressure coefficients for the model building are relatively small compared with those of prismatic-shaped buildings. The largest average-wind-pressure coefficient for southwestern wind of 1.04 can be found near the top on the south wall for an odd-geometrical building with wind openings, the average-wind-pressure coefficients are decreased by approximately 19–27% on the windward surfaces and 32–53% on the leeward surfaces compared with those for buildings with prismatic shapes.

CFD Prediction on Vortex in Sump Intake at Pump Station

In large pump station, vortex generation such as free-surface vortex and submerged vortex occurring around pump intake, or at bell-mouth inlet has been an important flow characteristics which should be considered always to keep away the suction of air-entrained or cavitated flow. In this study, a commercial CFD code was used to predict accurately the vortex generation for the specified intake design. These result shows the preliminary result of submerged vortex prediction for the Turbo-machinery Society of Japan Sump Test CFD standard model. At bottom wall, air volume fraction (red color) was found in a large scale to explain the submerged vortex generation at particular operation and configuration condition. And these indicate the free surface formation behind the bell mouth. Particularly, non-uniform approaching flow is a major parameter to govern the occurrence of the free-surface vortex. Futhermore the comparison between turbulence ( & model) mode were executed in this study.

A Study on the Formation of Fouling in a Heat Exchanging System for River Water

When the water flowing inside of the heat transfer equipments such as heat exchangers, condensers, and boilers is heated, calcium, magnesium sulfate, and other minerals in the water are deposited and built up for scales on the heat transfer surfaces. When those scales accumulate on the heat transfer surfaces, their performance of the heat transfer become progressively reduced due to the increase of the heat transfer resistance. The mechanism of this reduced heat transfer is called fouling. This study investigated the formation of the fouling in a heat exchanger with river and tap water flowed inside of it as a coolant. In order to visualize the formation of the fouling and to measure the fouling coefficients, a lab-scale heat exchanging system was used. Based on the experimental results, it was found that the formation of fouling for river water was quite different with the formation for tap water.

Development of a High Performance Francis Turbine for Runner Replacement Using a CFD-Based Design System

Recently, Korean Government is encouraging to propagate the small hydro-electric power in renewable energy development policy as the “Low CO2 Green Growth” policy. However, turbines in the Korea Water Resources Corporation (below K-water) operated at an average of 10% less than well-designed turbines due to the design weakness or the inferior manufacturing techniques for the small hydropower facilities. Thus, maintenance fees increased because the cavitations had excessively occurred on the main parts of turbines such as the runner, guide vane and Stator, the life cycles of turbines were reduced and the frequency breakdowns were increased. In order to improve the efficiency the CFD-based design system is applied to the Francis turbine replacement project with Korea Fluid Machinery Association (below KFMA) and K-water. Therefore, the inversed design technique and the fully turbulent 3-dimensional flow simulations are performed for both the existing and new turbines at design and off design conditions. As a result, the runner is optimized to the greatest extent with a possible minimum cost under the geometrical constraints of the existing machine. The performances of the new design are verified by extensive model tests and the guarantees have all been successfully met.© 2010 ASME

Effects of the velocity waveform of the physiological flow on the hemodynamics in the bifurcated tube

The periodicity of the physiological flow has been the major interest of analytic research in this field up to now. Among the mechanical forces stimulating the biochemical reaction of endothelial cells on the wall, the wall shear stresses show the strongest effect to the biochemical product. The objective of present study is to find the effects of velocity waveform on the wall shear stresses and pressure distribution along the artery and to present some correlation of the velocity waveform with the clinical observations. In order to investigate the complex flow phenomena in the bifurcated tube, constitutive equations, which are suitable to describe the rheological properties of the non-Newtonian fluids, are determined, and pulsatile momemtum equations are solved by the finite volume prediction. The results show that pressure and wall shear stresses are related to the velocity waveform of the physiological flow and the blood viscosity. And the variational tendency of the wall shear stresses along the flow direction is very similar to the applied sinusoidal and physiological velocity waveforms, but the stress values are quite different depending on the local region. Under the sinusoidal velocity waveform, a Newtonian fluid and blood show big differences in velocity, pressure, and wall shear stress as a function of time, but the differences under the physiological velocity waveform are negligibly small.

Mathematical Modeling for Estimation of Heart Work

Evaluation of the heart work is starting to emerge as a new diagnostic tool for arterial diseases. The aim of this study is to develop a mathematical model for the estimation of heart work utilizing the pulse waves between two points of a vessel. In order to calculate heart work, medical data such as blood pressure waveforms (which are measured using a cuff) are utilized. The heart work is calculated by employing the modified Windkessel model together with the viscosity models of Casson or Herschel-Bulkely (H-B). The results indicate that the compliance values at the proximal and distal locations differ for the Casson and H-B models.

Evaluation of heart work as a prediagnostic tool using the modified Windkessel model and different whole blood viscosity models

The cardiovascular diseases, especially the atherosclerosis, rated among the top mortality reasons and its significance has been increasing progressively. Therefore, having a diagnostic tool to predict the cardiovascular diseases is prominent. The most common diagnostic tool for this type of disorders is using angiography; however, evaluating the Work of Heart is emerging as an easy to use and non-invasive tool recently. In this study, the modified Windkessel model and different whole blood viscosity models (Casson and Herschel-Bulkley) are employed to evaluate the heart work. The results revealed that both of the viscosity models can be used in conjunction with the modified Windkessel model.

Atherogenic Risk Stratification According to Changes in the Geometrical Shape of the Coronary Artery

A previous study showed that hemodynamics is correlated with stenosis in the coronary artery. The flow characteristics and the distributions of the hemodynamic wall parameters in the coronary artery are investigated under physiological flow condition. The present study also aims to establish the mechanism of the generation of atherosclerosis by analyzing the hemodynamic variables in the coronary artery where atherosclerosis frequently occurs. The stenosis phenomena due to atherosclerosis are related to not only the biochemical reaction between blood and blood vessels but also the hemodynamic factors sush as flow separation and oscillatory wall shear stress. As the bifurcated angle increases, the size of the recirculation area that appears in the cross section increases and disturbed flow is observed in this area. We speculate that this area is the starting point of atherosclerosis in the coronary artery.

Flow Analysis of the Rotating Sludge Suction Collector by Numerical Simulation

Sedimentation phenomenon of suspended solids occurs by the gravitational force. Pollution particles are separated from slowly flowing waste water in clarifier. Recently, the sludge suction collector is Preferred rather than the scraper type sludge collector due to the enhancement of the clarifier efficiency. The sludge suction collector is usually operated by the user`s experiences without any scientific and/or technical consideration. To evaluate the performance of sludge suction collector, the three dimensional numerical simulation was conducted by the finite volume method. To analyze the performance, the velocity vectors and the suction flow rates of the orifices were investigated. The result showed that each suction flow rate through out the collector was equivalent in the sludge suction collector and the efficiency of suction collector was good to remove high concentrated sludge in clarifier.

The Study of the Cavitation for the Urgency Released Valve in Hydraulic Dam

In general, the hollow jet valve, the fixed cone valve had been used for the urgency released or maintenance of the flow rate. Nowadays, the butterfly valve, the gate valve are applied in economic performance and operation maintenance more than the hollow jet valve, the fixed cone valve. However, in the case of butterfly valve, it should be required the strict application standard to the cavitation coefficient because the structural axis and disk were situated in pipe channel and the occurring the shock problem by Karman Vortex. Therefore, there were investigated the valve cavitation and accident investigation by field survey to establish the applicable extensibility of the urgency released valve as the preliminary study.