Hyoung-Bum Kim

Laboratory measurements of velocity and turbulence field behind porous fences

Flow characteristics of turbulent wake behind porous fences have been investigated experimentally. The velocity fields were measured using the two-frame PTV method in a circulating water channel. The fence models used in this study have geometric porosity (e) of 0%, 20%, 40% and 65%, respectively. Each fence model was located in uniform flow whose boundary layer thickness (δ) at the fence location was about 0.1 of the fence height (H). Among the porous fences used in this study, the porous fence with porosity e=20% shows the maximum reduction of mean streamwise velocity, but it has the highest vertical mean velocity at about x/H=1 location and large turbulence intensity in the near wake region. However, the porous fence with e=40% has good flow characteristics for abating wind erosion with small turbulent fluctuations and a relatively large reduction in mean velocity. Except for the solid fence (e=0%), two shear layers develop from the porous fences. As the fence porosity (e) increases, the height of the shear layer and the streamline curvature decrease. When the porosity (e) is greater than 40%, there is no re-circulation flow behind the fence due to the strong bleed flow, the Reynolds shear stress is nearly negligible in the entire near-wake region and relatively small turbulent kinetic energies are concentrated in the region just behind the fence (x/H<0.5). When the fence porosity is less than 20%, the Reynolds shear stress and turbulent kinetic energy are strong over the fence and in the shear layer near the reattachment region.

The effect of surface protrusions on the near wake of a circular cylinder

Wind tunnel tests of the near wake of a circular cylinder with surface protrusions were carried out to investigate the flow characteristics of the controlled wake. Two circular cylinders of diameter D = 40 mm were wrapped helically by three small wires (d = 0.075D) with pitches of 5D and 10D, respectively and tested at Reynolds numbers ranging from 5000 to 50000. Wake velocity was measured using hot-wire anemometry. The surface protrusions make the near wake to have periodic structure along the spanwise direction. It is also observed that the surface protrusions elongate the vortex formation region, decrease the vortex shedding frequency and shrink the wake width. This is based primarily on the suppression of large scale vortices into slender elliptic shape. The suppression of vortices and consequent shrinkage of the wake increase the velocity deficit in the near wake.

Performance improvement of two-frame particle tracking velocimetry using a hybrid adaptive scheme

The performance of a two-frame particle tracking velocimetry (PTV) system was enhanced with an adaptive hybrid scheme. The original two-frame PTV method, based on the match probability concept, employs global match parameters for the entire flow field. This does not fully consider the detailed local velocity changes of the flow, and reduces the recovery rate of the velocity vectors, while increasing the number of erroneous vectors in regions of high velocity gradients. In the new hybrid PTV method, the preliminary particle image velocimetry (PIV) results are used to determine the local match parameters that are required for a two-frame particle-tracking algorithm. Both computer simulations and real flow measurements were performed to check the performance of the adaptive hybrid PTV. Compared with the original method, the new technique enhances the PTV performance by increasing the velocity vector recovery rate, while greatly reducing the number of erroneous vectors. In addition, the adaptive hybrid method provides better resolution near solid boundaries compared with the conventional cross-correlation PIV method.

Hole diameter effect on flow characteristics of wake behind porous fences having the same porosity

Abstract The hole diameter effect on the flow characteristics of wake behind porous fences has been investigated experimentally in a circulating water channel having a test section of 300 w ×200 h ×1200 l (mm) . Three porous fences having different hole diameters of d=1.4,2.1,2.8 mm were tested in this study, but they have the same e=38.5% geometric porosity. One thousand instantaneous velocity fields for each fence were measured consecutively by the hybrid PTV system employing a high-speed CCD camera. Free stream velocity was fixed at 10 cm / sec and the corresponding Reynolds number based on the fence height was Re=2,985. Consequently, the fence with the smallest hole diameter d=1.4 mm (d 1.4 ) decreases the streamwise velocity component and increases the vertical velocity component. Among the three hole diameters tested in this study, the d1.4 fence has the largest turbulence intensity in the shear layer developed from the fence top. Regardless of the hole diameter, however, all three fences having the same porosity reduce the reduction of turbulent intensity in the lower region below the fence height (y/H

Velocity field measurements of flow around a triangular prism behind a porous fence

The shelter effect of a porous wind fence on a triangular prism was experimentally simulated in a circulating water channel. A porous wind fence of porosity e=38.5% was installed in front of the prism model. The fence and prism model were embedded in a turbulent boundary layer at Reynolds number of about 3460. The instantaneous velocity field around the fence and prism model was measured using a two-frame Particle Tracking Velocimetry (PTV) technique. In addition to the well-known mean velocity reduction, the fence decreases the turbulent intensity and turbulent kinetic energy around the prism. Especially, at the top of the prism model, the turbulent kinetic energy is about half of that without the fence. By installing the fence in front of the prism, the length of the re-circulation region also decreases compared with that of the no fence case.

Quantitative analysis of flow inside the accumulator of a rotary compressor

Abstract The flow structure inside the accumulator of a rotary compressor was investigated experimentally using a hybrid particle tracking velocimetry (PTV) velocity field measurement technique. A proto-type accumulator was studied while operating under real working conditions. The flow inside the accumulator was qualitatively visualized using a high-speed camera. The period of one cycle of the rotary compressor was divided into 4 phases and the velocity fields inside the accumulator were measured using a phase-averaging technique. The flow inside the accumulator, especially in the region between the screen holder and tube holder, showed a pulsating periodic flow structure according to the rotational phase of rotary compressor.

Performance Improvement of 2-Frame PTV Method Using an Adaptive Hybrid Scheme

The performance of 2-frame PTV(particle tracking velocimetry) system was improved using an adaptive hybrid scheme. The original 2-frame PTV method based on the match probability concept employs global match parameters for the entire flow field. Since this does not reflect fully the detailed local velocity change, it sometimes reduces the recovery rate of velocity vectors and increases the number of erroneous vectors in the region where an extraordinary flow structure exists. In this study, the preliminary FFT-based PIV results are used as an input parameter to determine the local match parameters needed for the 2-frame particle tracking algorithm. A computer simulation using synthetic particle images was carried out to study the performance of the adaptive 2-frame PTV technique. The adaptive hybrid method shows the better performance with increasing the velocity vector recovery rate and decreasing the computation time, compared to the original 2-frame PTV method.

Study of Wake Control by Blowing and Suction in Front of the Vertical Fence

The effect of periodic blowing and suction of upstream flow on the separated shear flow behind the vertical fence was experimentally investigated. The fence was submerged in the turbulent shear flow and DPIV method was used to measure the instantaneous velocity fields around the fence. Periodic blowing and suction flow was precisely generated by the syringe pump. Spanwise nozzle made 2D planar periodic jet flow in front of the fence and the effect of frequency and maximum jet velocity was studied. From the results, the reattachment length can be reduced by 60% of uncontrolled fence case under the control.

Quantitative Visualization of Inlet Flow of the Centrifugal Blower

The inlet flow of centrifugal blower were quantitatively visualized using particle image velocimetry. Because the centrifugal blower system is one of the key parts of EV battery cooling system, the quantitative information of flow field of centrifugal blower is important to design and optimize the cooling system. Two types of inlet parts were used in this study. One is the straight inlet and the other is a bended one. The results showed the flow asymmetry exists in the straight model due to the pressure difference in the blower. In case of the bended one, the separation bubble and the increase of head loss appeared compared with the straight model.

Development of Wall Flow Sensor Using Micro Imaging Device

Key Words: Micro Imaging Sensor(미세영상장치), PIV(입자화상속도계), Defocusing Imaging (초점이탈)초록: 능동유동제어의되먹임신호및벽면전단응력측정등을위해벽면유동센서가사용되고있다. 본연구에서는광학마우스에사용되는미세영상장치를이용하여벽면근처에서2차원및3차원유체속도를측정할수있는센서를개발하였다. 미세영상장치에서나오는영상신호획득시스템을구축하고획득한영상에입자화상속도기법과초점이탈영상기법을적용하여측정영역에서의산란입자의위치를측정하였다. 모사유동실험을통해, 개발된벽면유동센서의공간해상도및측정정확도를검증하였고기존미세영상장치의quadrature 신호결과와비교하여입자화상속도기법을적용할경우, 측정정확도및측정범위가확대되는것을확인하였다.Abstract: A wall flow sensor has been used for feedback flow control and wall shear stress measurement. Inthis study, we developed a new wall flow sensor by combining the PIV algorithm and the micro imagesensor used in an optical mouse. The feasibility of the wall flow sensor was investigated by using simulatedfluid flow experiments. Compared with the quadrature signal from imaging devices, the accuracy of the wallflow velocity measurement was improved and the dynamic range increased. In addition, the depth informationof particles was also measured by using the defocusing imaging technique.