• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2001년도 추계학술대회논문집 II
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• pp.1406-1412
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• 2001

Roof racks have become a very popular feature of vehicles as the market demand for SUV's and RV's has increased drastically over the years. Aeolian tone from the cross bars however. could be a source of severe discomfort for the passengers. Both experimental and numerical steps are taken to enhance the understanding of the generation mechanism of the wind noise. A successful reduction of the noise is achieved by imposing asymmetry in the section geometry, which reduces the strength of Karmann vortices shed downstream.

• 한국항공우주학회지
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• 제39권3호
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• pp.210-217
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• 2011

대칭익형 단면을 가지는 모형 깃 끝와류의 후류유동구조의 전개과정을 관찰하기 위하여 2차원 LDV 시스템을 이용하여 끝와류의 회전성분과 축성분을 상평균 기법으로 측정하였다. 비대칭익 로터 깃의 특징에 비하여 대칭익 로터깃의 끝와류는 $27^{\circ}$ 부근까지 성장 과정이 진행되어 다소 늦게 형성되었으며 그 이후부터 상대적으로 완만하게 확산모드가 진행되었다. 회전성분은 실험이 진행된 $360^{\circ}$ 까지 Vatistas의 n=2모델에 우수한 접근성을 보이면서 자기상사성을 갖는 것으로 관찰되었고 축방향성분은 비대칭익에 비하여 훨씬 강한 세기로 나타나면서 정규분포 성향을 나타내었다. 이들의 결과들은 대칭익의 경우 로터의 추력 발생면에서 끝단손실이 상대적으로 큰 것으로 확인이 되었다.

• 한국대기환경학회지
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• 제23권2호
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• pp.214-224
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• 2007

A three-dimensional computational fluid dynamics (CFD) model with the renormalization group (RNG) $k-{\varepsilon}$ turbulence model is used to examine the effects of difference in building height on flow and pollutant dispersion in asymmetric street canyons. Three numerical experiments with different street canyons formed by two isolated buildings are performed. In the experiment with equal building height, a portal vortex is formed in the street canyon and a typical recirculation zone is formed behind the downwind building. In the experiment with the downwind building being higher than the upwind building, the ambient flow comes into the street canyon at the front of the downwind building and incoming flow diverges strongly in the street canyon. Hence, pollutants released therein are strongly dispersed through the lateral sides of the street canyon. In the experiment with the upwind building being higher than the downwind building, a large recirculation zone is formed behind the upwind building, which is disturbed by the downwind building. Pollutants are weakly dispersed from the street canyon and the residue concentration ratio is largest among the three experiments. This study shows that the difference in upwind and downwind building height significantly influences flow and pollutant dispersion in and around the street canyon.

• 한국유체기계학회 논문집
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• 제10권2호
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• pp.32-40
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• 2007

The present study investigated local pressure drop in a rotating smooth square duct with turning region. The duct has a hydraulic diameter $(D_h)$ of 26.7mm and a divider wall of 6.0mm or $0.225D_h$. The distance between the tip of the divider and the outer wall of the duct is $1.0D_h$. The Reynolds number (Re) based on the hydraulic diameter is kept constant at 10,000, and the rotation number (Ro) is varied from 0.0 to 0.20. The pressure coefficient distribution $(C_p)$, the friction factor (f) and the thermal performance $({\eta})$ are presented on the leading, the trailing and the outer surfaces. It is found that the curvature of the $180^{\circ}-turn$ produces Dean vortices that cause the high pressure drop in the turning region. The duct rotation results in the pressure coefficient discrepancy between the leading and trailing surfaces. That is, the high pressure values appear on the trailing surface in the first-pass and on the leading and side surfaces in the second-pass. As the rotation number increases, the pressure discrepancy enlarges. In the fuming region, a pair of the Dean vortices in the stationary case transform into one large asymmetric vortex cell, and then the pressure drop characteristics also change.

• 대한기계학회논문집B
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• 제25권7호
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• pp.933-943
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• 2001

Using a semi-circled blunt body model, the geometrical effects of injection hole on the turbine blade leading edge film cooling are investigated. The film cooling characteristics of two shaped holes (laterally- and streamwise-diffused holes) and three cylindrical holes with different lateral injection angles, 30°, 45°, 60°, respectively, are compared with those of cylindrical hole with no lateral injection angle experimentally and numerically. Kidney vortices, which decrease the adiabatic film cooling effectiveness, appear on downstream of the cylindrical hole with no lateral injection angle. At downstream of the two shaped holes have better film cooling characteristics than the cylindrical one. Instead of kidney vortices, single vortex appears on downstream of injection holes with lateral injection angle. The adiabatic film cooling effectiveness is symmetrically distributed along the lateral direction downstream of the cylindrical hole with no lateral injection angle. But, at downstream of the cylindrical holes with lateral injection angle, the distribution of adiabatic film cooling effectiveness in the lateral direction shows asymmetric nature and high adiabatic film cooling effectiveness regions are more widely distributed than those of the cylindrical hole with no lateral injection angle. As the blowing ratio increases, also, the effects of hole shapes and injection angles increase.

• 대한조선학회논문집
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• 제41권4호
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• pp.30-37
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• 2004

Flow characteristics of the inflow ahead of a rotating propeller attached to a container ship model were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases. The mean velocity fields show the acceleration of inflow due to the rotating propeller and the velocity deficit in the near-wake region. The axial velocity distribution of inflow in the upper plane of propeller is quite different from that in the lower plane due to the thick hull boundary layer. The propeller inflow also shows asymmetric axial velocity distribution in the port and starboard side. As the inflow moves toward the propeller, the effect of phase angle variation of propeller blade on the inflow becomes dominant. In the upper plane above the propeller axis the inflow has very low axial velocity and large turbulent kinetic energy, compared with the lower plane. The boundary layer developed along the bottom surface of stern hull forms a strong shear layer affecting vortex structure of the propeller near-wake.

• International Journal of Fluid Machinery and Systems
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• 제3권2호
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• pp.137-149
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• 2010

The purpose of the present research is to suppress cavitation instabilities by using a circumferential groove. The circumferential groove was designed based on CFD so that the tip leakage vortex is trapped by the groove and does not interact with the next blade. Experimental results show that the groove can suppress rotating cavitation, asymmetric cavitation and cavitation surge. However, weak instabilities with higher frequency could not be suppressed by the groove. From the analysis of pressure pattern similar to that for rotor-stator interaction, it was found that the higher frequency components are caused by the interaction of backflow vortices with the inducer blades.

• 한국전산유체공학회지
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• 제11권3호
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• pp.64-70
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• 2006

A Computational study was carried out in order to investigate the moving wall effect of a circular cylinder at a Reynolds number of $2.0{\times}10^4$. The viscous-incompressible Navier-Stokes equations and Spalart-Almaras turbulent model of the commercial CFD code were adopted for this numerical analysis. The moving wall was set parallel with the freestream, and moving speed was equal to the freestream velocity. The gap ratio is defined as the distance ratio between the circular cylinder diameter and the height from the moving wall. The results show that there is vortex shedding over the critical gap ratio and aerodynamic loads including amplitude and the Strouhal number change according to the gap ratio.

• Journal of Advanced Marine Engineering and Technology
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• 제32권3호
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• pp.404-412
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• 2008

As a suitable volute configuration in the range of low specific speed, circular casing is suggested in this study. The internal flows in a centrifugal pump with the circular and spiral casings are measured by PIV and analyzed by CFD. The results show that the head and efficiency of the pump by a circular casing of very small radius are almost same as those by the spiral casing. Even at the best efficiency point, the internal flow of the pump by circular casing is asymmetric, and vortex and strong secondary flow occurs in the impeller passage. The radial velocity becomes higher remarkably only near the region of the discharge throat. The flow in the impeller outlet is strongly controlled by the circular casing because the velocity distribution almost does not affected by the position of the impeller blades.

• 대한기계학회논문집B
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• 제25권3호
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• pp.339-346
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• 2001

The impulsive spin-up flow in a shallow rectangular container is analyzed numerically by quasi 3-D unsteady laminar flow. In the non-inertia coordinates, the flow is generated by the virtual forces as Coriolis force, etc.. After the boundary layers grow up near sidewalls, primary vortexes separate from the sidewalls. As the Reynolds number increases, the subsidiary vortexes take place in the boundary layer. The rigid body rotation is started from the inner region and propagated to the outer region, finally all the fluid reaches the rigid body rotation. According to the Reynolds number and the aspect ratio, the development of vortex pattern is symmetric or asymmetric.