• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.5
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• pp.646-652
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• 1999

Detailed characteristics of the mean flow field inside the rotor of a multiblade fan with scroll are presented in this paper by measurements and visualizations. The measurements were taken with a five-hole probe and conformed by smoke test. How field is distinguished clearly in 3 regions with respect to the flow directions. The first region is near the exit of scroll where the fluid flows the opposite direction to the rotation of rotor. The second is opposite side of the scroll exit where the fluid flows the same direction to the rotation of rotor. The third is the region where the fluid flows toward the blades directly with the largest values comparatively. The strongest recirculation is happened in the second region, and the weakest one is in the third region. This complex configuration makes the flow field highly non-uniform and may cause to generate a noise and ineffective flow efficiency.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.1
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• pp.82-93
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• 2002

The effects of the interaction between the flow and temperature field and a boundary layer due to a variety of the height of a vortex generator are experimentally investigated. The test facility consists of a boundary-layer wind tunnel with the vortex generator protruding from the bottom surface. In order to control the strength of the longitudinal vortices, the angle of attack and the spacing distance of the vortex generator are 20 degree and 40 mm, respectively. The height of the vortex generator (H) is 15 mm, 20 mm and 30 mm and the cord length of it is 50 mm. Three-component mean velocity measurements are made using a 5-hole probe system and the surface temperature distribution is measured by the hue capturing method using thermochromatic liquid crystals. By using the method mentioned above, the following conclusions are obtained from the present experiment. The boundary layer is thinned in the downwash region where the strong downflow and the lateral outflow of the boundary layer fluid occur and thickened in the upwash re,3ion where the longitudinal vortex sweeps low momentum fluid away from the bottom surface. In case that the height of the vortex generator increases, the averaged circulation and the maximum vorticity of the vortex pair decrease. The contours of the non-dimensional temperature show the similar trends fur all the cases (H=15 mm, 20 mm and 30 mm). The peak augmentation of the distribution of the local non-dimensional temperature occurs in the downwash region near the point of minimum boundary-layer thickness.

• Journal of Advanced Marine Engineering and Technology
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• v.25 no.5
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• pp.1076-1085
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• 2001

Experimental investigations of the longitudinal vortices, which are produced by wing type vortex generators set up behind a circular cylinder in a rectangular channel, are presented. When the circular cylinder is set up in the rectangular channel, a horseshoe vortex is formed just upsteam of the circular cylinder. It generates a turbulent wake region behind the circular cylinder. Therefore, the region of the pressure loss behind the circular cylinder in increased and the size of the wake is small. These problems can be achieved by longitudinal vortices which are generated by wing-type vortex generator. In order to control the strength of longitudinal vortices, the angle of attack of the vortex generators is varied from 20 degree to 45, but the spacing between the vortex generators is fixed 6cm. The 3-dimensional mean velocity measurements are made using a five-hole probe. The vorticity field and streamwise velocity contour are obtained from the velocity field. The following results are obtained. Circulation strength is the maximum value when the angle of attack($\beta$) is $30^{\circ}$, and the vorticity field and streamwise velocity contour in case of $\beta$=$20^{\circ}$ show the trend similar to these in case of $\beta$=$30^{\circ}$, but do not in case of $\beta$=$45^{\circ}$.

• Journal of the Society of Naval Architects of Korea
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• v.34 no.2
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• pp.11-19
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• 1997

The new definition of calibration coefficients is proposed for a five-hole Pitot tube. Two-angle chart calibration other than one-angle variation is considered to improve the accuracy in the measurement of the three-dimensional velocity fields. Several sets of correlation coefficients are introduced for different shapes of the probe tip. The calibration method with one-angle variation is compared with the new two-angle chart calibration method and the improvement of the present method is clearly shown.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.288-297
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• 2000

This paper describes the results of an experimental investigation of the flow characteristics and the heat transfer rate on a surface by interaction of a pair of vortices. The test facility consists of a boundary-layer wind tunnel with a vortex introduced into the flow by half-delta wings(vortex generators) protruding from the surface. In order to control the strength of the two longitudinal vortices, the angles of attack of the vortex generators are varied from 20 degree to 45 degree, but spacings between the vortex generators are fixed to 4 cm. The 3-dimensional mean velocity downstream of the vortex generators is measured by a five-hole pressure probe, and the hue-capturing method using the thermochromatic liquid crystals has been used to provide the local distribution of the heat transfer coefficient. By using the method mentioned above, the following conclusions are obtained from the present experiment. The boundary layer is thinned in the regions where the secondary flow is directed toward the wall and thickened where it is directed away from the wall. The peak augmentation of the local heat transfer coefficient occurs in the downwash region near the point of minimum boundary-layer thickness. Streamwise distributions of averaged Stanton number on the measurement planes show very similar trends for all the cases(${\beta}=20^{circ},\;30^{\circ}\;and\;45^{\circ}$).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.298-305
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• 2000

The flow characteristics and the heat transfer rate on a surface by interaction of a pair of vortices were studied experimentally. The test facility consisted of a boundary-layer wind tunnel with a vortex introduced into the flow by half-delta wings protruding from the surface. In order to control the strength of the longitudinal vortices, the angles of attack of the vortex generators were varied from - 20 degree to - 45 degree, but spacings between the vortex generators were fixed to 4 cm. The 3-dimensional mean velocity measurements were made using a five-hole pressure probe. Heat transfer measurements were made using the thermochromatic liquid to provide the local distribution of the heat transfer coefficient. Unlike common flow down, common flow up vortices moved toward the centerline as they developed and interacted strongly with each other but not with the boundary layer. Spanwise profiles of Stanton number were similar for ${\beta}=-20^{\circ}\;and\;-35^{\circ}$, but not similar for ${\beta}=-45^{\circ}$. The case of ${\beta}=-20^{\circ}\;and\;-35^{\circ}$ showed the two peak Stanton number, but the case of ${\beta}=-45^{\circ}$ showed the only one peak Stanton number.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.10
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• pp.1281-1292
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• 2001

Experiments were done for the comparison of performance and flow characteristics between a two -stage axial flow fan and a counter-rotating axial flow fan. Each stage of the two -stage axial flow fan used fur the present study has an eight bladed rotor and thirteen slater blades. The front and the rear rotor of the counter - rotating axial flow fan have eight blades each and are driven by coaxial counter ro latins shafts through a gearbox located between the rear rotor and the electric motor. Both of the two axial fan configurations have identical rotor blades and the same operating condition fur the one -to-one comparison of the two. Performance curves of the two configurations were obtained and compared by varying the blade pitch angles and axial gaps between the blade rows. The fan characteristic curves were obtained following the Korean Standard Testing Methods for Turbo Fans and Blowers (KS B 6311). The fa n flow characteristics were measured using a five-hole probe by a non-nulling method. The velocity profiles between the hub and tip of the fans were measured and analyzed at the particular operating condition s of peak efficiency, minimum and maximum pressure coefficients. The peak efficiency of the counter-rotating axial fan was improved about 2% respectively, compared with the two stage axial fan. At the minimum pressure coefficient point of the two stage axial fan, the fan inlet flow patterns show that axial velocity highly decreased in the vicinity of the blade tip region. Also, the reverse flow took place at the blade tip.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.11 no.3
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• pp.379-387
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• 2000

In this paper, to transmit and receive the differently polarized wideband signals simultaneously with a single antenna system, the wideband, dual-frequency and dual-polarized(WDFDP) microstrip array antenna is proposed and designed. This antenna operates simultaneously at 14.25 GHz and 12.50 GHz. To extend to two dimensional array, microstrip feed line and coaxial probe through via-hole are used. The experimental results show that the proposed WDFDP microstrip antenna can be used in the compact portable terminals with a single antenna system and the characteristic of the resonators in this antenna produces a greatly enhanced bandwidth.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.59.2-59.2
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• 2019

Event Horizon Telescope (EHT) aims to resolve the innermost region to the super massive black hole (SMBH) with its extremely high angular resolution (~20-25 uas) and enhanced sensitivity (down to 1-10 mJy) in concert with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm wavelength. This has a great importance as the first observational probe of the black hole shadow which has been theoretically predicted as a ring-like emission affected by the general relativistic effect under a strong gravitational field of SMBH. During the 2017 April 5-11, four nights of EHT observing campaign were carried out towards its primary targets, M87 and $SgrA{\ast}$. To robustly ensure the data processing, independent pipelines for various radio data calibration softwares (e.g., AIPS, HOPS, CASA) have been developed and cross-compared each other. The EHT has also been developing newer interferometric imaging techniques (e.g., eht-imaging-library, SMILI, dynamical imaging), as well as using an established method (CLEAN). With these, the EHT has designed various strategies which will be adopted for convincing imaging results. In this talk, I review how the robustness of EHT data processing and imaging will be validated so that the results can be ensured against well known uncertainties or biases in the interferometric data calibration and imaging.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.501-507
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• 2004

The uniquely large dimensions of Giant radio galaxies (GRGs) make it possible to probe for stringent limits on total energy content, Faraday rotation, Alfven speeds, particle transport and radiation loss times. All of these quantities are more stringently limited or specified for GRG's than in more 'normal' FRII radio sources. I discuss how both global and detailed analyses of GRG's lead to constraints on the CR electron acceleration mechanisms in GRG's and by extension in all FRII radio sources. The properties of GRG's appear to rule out large scale Fermi-type shock acceleration. The plasma parameters in these systems set up conditions that are favorable for magnetic reconnection, or some other very efficient process of conversion of magnetic to particle energy. We conclude that whatever mechanism operates in GRG's is probably the primary extragalactic CR acceleration mechanism in the Universe.