Ho Seong Ji

Flow structure around a 3-D rectangular prism in a turbulent boundary layer

Abstract Flow around a three-dimensional (3-D) rectangular prism has been investigated by using a particle image velocimetry technique. The prism was immersed in a thick turbulent boundary layer. The ratio of the boundary layer thickness and the model height was about 0.06. Measurements were made at Reynolds number of 7.9×10 3 which is based on free stream velocity and model height. Detailed flow structures and characteristics including three circulation zones were obtained by averaging over a large number of instantaneous velocity maps. The 3-D structure in a wake zone is clearly seen from mean flow streamline topology. Turbulent kinetic energy distribution is also obtained approximately. Maximum turbulent kinetic energy was found at the separated layer in the upper boundary of the separation bubble near the leading edge of a roof. The magnitude of the maximum energy is about 2.5 times that in the wake region.

IN VITRO HEMODYNAMIC STUDY ON THE STENOTIC RIGHT CORONARY ARTERY USING EXPERIMENTAL AND NUMERICAL ANALYSIS

To investigate the hemodynamic and hemorheological features related to circulatory diseases, in vitro experiments are carried out using a micro-particle image velocimetry (μ-PIV) technique. Numerical simulations using a commercial computational fluid dynamics (CFD) code are also performed to compare with the experimental results. Five different non-Newtonian blood models and a Newtonian water model are employed to investigate the blood flow characteristics through a stenotic right coronary artery (RCA). The in vitro model is made of two-dimensional (2D) polydimethylsiloxane (PDMS) channel based on the clinical angiogram of the RCA with stenotic lesion. The hemodynamic and hemorheologic behaviors in the control volume near the stenotic lesion are evaluated by velocity profiles. The predicted and measured velocity profiles at the center of the channel have a reasonable agreement.

Effect of the wind direction on the near wake structures of an Archimedes spiral wind turbine blade

For urban usage of an Archimedes spiral horizontal axis wind turbine, the effect of wind direction was examined for the confront measurement planes from 0° to 15°. Aerodynamic characteristics in the near wake of wind turbine blade were investigated using the particle image velocimetry method. Ensemble and phase-averaged velocity fields were obtained in both front and lee sides of the blade with various angles of attack. The streamlines are parallel with the incoming flow for the range of attack angles; however, wake velocity is decreasing with increasing angle of attack. It has been found that the trajectory of tip vortices varies significantly in the front side plane compared to that of lee side plane when the angle of attack is higher than 10°. Distance between two consecutive vortices in the front plane decreases monotonically with increasing attack angle, while that in the lee side shows an opporsite manner.Graphical Abstract

Flow Characteristics around Archimedes Wind Turbine according to the Change of Angle of Attack

Abstract. This paper describes aerodynamic characteristics of an Archimedes spiral wind turbine with variousangles of attack. The range of angles was controlled from - 30 o (clockwise) to + 30 (clockwise). The rotatingspeed of wind turbine at the same angle of attack in both directions was different. The reason why the-maximum rotational speed was observed at 15 o in clockwise direction can be explained based on angularmomentum conservation. Quantitative flow visualization around Archimedes wind turbine blade was carriedout between -15 o (clockwise) and +15 (counter clockwise) using high resolution PIV method. The relation-ship between drag force and rotating speeds was discussed. From these results, optimum design on yawingsystem of Archimedes spiral wind turbine may provide high efficiency on small wind power system.Key Words:Spiral Wind Power Generation(스파이럴형풍력발전), Archimedes Spiral(아르키메데스나선 ),Aerodynamics Characteristics(공력특성), PIV measurement(PIV측정), Angle of Attack(받음각) 기호설명 θ: Angle of Attack [

Visualization of Gas/liquid Ejector Flow and Void Fraction Measurement using Fiber Optic Probe

Abstract. Gas/liquid two-phase ejector is a device without moving parts, in which liquid is used to drivegas of a low-pressure source. In this paper, the hydrodynamic characteristics of a vertical down type two-phase ejector were studied using an air-water loop system. Entrained air flow rates were measured withinlet and outlet pressures of the ejector with varying water flow rate. Homogeneous bubbly flows in thedischarge pipe were confirmed by the high speed flow visualization method. Quantitative measurementsof void fraction were made using a newly developed fiber optic probe system. Key Words:Two-Phase Flow (이상유동), Ejector (이젝터), Flow Visualization (유동가시화), Fiber opticprobe(광섬유 탐침), Void fraction(기포분율) 1. 서론 기체-액체 이상유동 이젝터는 설치가 간편하고, 안정적인 작동 특성으로 인하여 여러 가지 경우의 화학공정및 플랜트에서 중요하게 사용되는 장치중 하나이다. 이상유동 이젝터는 액체를 주유동으로 분사시키고, 이젝터의 목 부분에서 형성되는 음압으로 기체를 흡입하여혼합과정과 이송과정을 동시에 실현시킨다 (1, 2) . 이젝터에 관한 선행연구로 Witte(1962)는 다중 오리피스와 상당히 긴 혼합튜브를 이용하여 이젝터의 등온압축효율이 40%까지 개선될 수 있음을 보고하였다. 이젝터의 성능을 평가하기 위한 최소한의 측정값은 1차유체 및 2차 유체의 체적 유량과 세 지점의 압력 즉 이젝터 전단 압력 P

Numerical Study on the Performance Assessment for Defrost and De-Icing Modes

The heating, ventilating, air conditioning (HVAC) system is a very important part of an automotive vehicle: it controls the microclimate inside the passengers compartment and removes the frost or mist that is produced in cold/rainy weather. In this study, the numerical analysis of the defrost duct in an HVAC system and the de-icing pattern is carried out using commercial CFX-code. The mass flow distribution and flow structure at the outlet of the defrost duct satisfied the duct design specification. For analyzing the de-icing pattern, additional grid generation of solid domain of ice and glass is pre-defined for conductive heat transfer. The flow structure near the windshield, streakline, and temperature fields clearly indicate that the de-icing capacity of the given defrost duct configuration is excellent and that it can be operated in a stable manner. In this paper, the unsteady changes in temperature, water volume fraction, and static enthalpy at four monitoring points are discussed.

Characteristics of pulsatile flows in curved stenosed channels

Spatial and temporal variations of the hemodynamic features occur under pulsatile conditions in complex vessel geometry. Wall shear stress affected by the disturbed flow can result in endothelial cell dysfunction, which leads to atherogenesis and thrombosis. Therefore, detailed understanding of the hemodynamic characteristics in a curved stenosed channel is highly important when examining the pathological effects of hemodynamic phenomena on the progression of atherosclerosis. The present study measures the velocity fields of pulsatile flows with three different Reynolds numbers in 3D curved vessel models with stenosis using time-resolved particle image velocimetry (PIV). Three different models were cast in PDMS polymer using models made by a 3D printer with different bend angles of 0°, 10°, and 20° between the longitudinal axes at the upstream and downstream of the stenosis. To investigate the 3D flow structures, a stack of 2D velocity fields was obtained by adjusting the position of the laser sheet along the Z-direction. The structures of flow fields in the stenosed models were analyzed using the distribution of the shearing strain as well as the skewness and full width at half maximum of the velocity profile. To support experiment results, distributions of pressure and 3D vortex in the curved stenosed channels were estimated by conducting the numerical simulation. These results indicate that the curvature of the tube considerably influences the skewness of the flow, and the shear stress is intensified near the outer curvature wall due to centrifugal force. The results would be helpful in understanding the effects of geometrical factors on plaque rupture and severe cardiovascular diseases.

Aerodynamic Characteristics of Horizontal Axis Wind Turbine With Archimedes Spiral Blade

To investigate the aerodynamic characteristics of an Archimedes spiral wind turbine for urban-usage, both experimental and numerical studies were carried out. The Archimedes spiral blade was designed to produce wind power using drag and lift forces on the blade together. Instantaneous velocity fields were measured by two-dimensional PIV method in the near field of the blade. Mean velocity profiles were compared to those predicted by the steady state and unsteady state CFD simulation. It was found that the interaction between the wake flow at the rotor downstream and the induced velocity due to the tip vortices were strongly affected by the wind speed and resulting rotational speed of the blade. PIV measurements revealed the presence of dominant vertical structures at downstream the hub and near the blade tip. Unsteady CFD simulation results agreed well with those of PIV experiments than the steady state analysis. The power coefficient (Cp) obtained by CFD simulation demonstrated that the new type of wind turbine produced about 0.25, relatively high value compared to other types of urban-usage wind turbine.Copyright