• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.9
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• pp.1770-1779
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• 1992

The present study investigated dynamically and thermally interacting droplets in a closely spaced tandem array. By measuring the velocity and diameter of the droplet traveling along the isothermal vertical plate drag coefficients and vaporization rates of droplets at certain location were obtained. During the experiment initial droplet spacings were less than 5, and initial droplet diameters were ranged between 280 .mu.m and 700 .mu.m Drag coefficients on closely spaced droplets were placed far below the standard drag coefficient, for which it was caused turbulence induced from aforelocating droplets also narrow spaces among droplets restricted heat transfer to droplets from hot gas flow. In addition evaporated vapor entrapted between droplets was major factor in delaying droplet vaporization. With the experimental results the drag coefficient was correlated with respect to Reynolds number for the droplet as follows : $c_{D}$ =2.4/Red.$^{0.37}$

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.1
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• pp.369-376
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• 1996

Variations of the drag and lift forces of a circular cylinder in a planar turbulent jet were experimentally investigated. The force was directly measured using the load cell and estimated by integrating the pressure distribution on the cylinder. As the cylinder moves outward from the center of the jet, the direction of lift force changes and the drag force decreases. Reynolds number, the ratio of cylinder's diameter to half width of jet had effect on maximum drag coefficient and the location where the direction of lift changes.

• Transactions of the Korean Society of Mechanical Engineers
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• v.3 no.4
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• pp.151-157
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• 1979

An experomental study on drg reduction in the rough tubes is presunted using the drrective drag reducing proymer solutions. The friction factors of the rough tubes follow the maximum drag reduction asymptote for the lower Reynolds numbers in the turbulent flow. However, as the Reynols number is increased the rougher tube results deviate from the maximum drag rduction asymptote sooner than the less rough tube results. There appears a systematic deviation from the maximum drag reduction asymptote depending on the relative roughness just as friction factors for the Newtonian hluid inthe rough tubes exhibit in the turbulent region. The minor loss results inthe various fittings such as elbows, tees, and gate valves are presunted The fittings show higher values of the loss coefficient in the drag reducing polymer solutions than in the Newtonian fluid, which is quite contrary to the drag reduction phenomenon in the straight tubes. The eqivalent length of the fittings for the drag reducing polymer solutions is many times longer than that for Newtonian fluids due to the increase of the loss coefficient and the decrease of the friction factor. It is speculated that the solid-like behavior of the polymer solutions in the abruptly changing folw passage plays a significant role in increasing the loss coefficient.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.7
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• pp.616-621
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• 2014

Consumption of the fuel on the satellite operating in low earth orbit, is increased due to the air resistance and the amount of increase makes the satellite lifetime decrease or the satellite mass risen. Therefore the prediction of drag force of the satellite is important. In the paper, drag force and drag coefficient analysis of the parabolic antenna satellite in low earth orbit using direct simulation monte carlo method (DSMC) is conducted according to the mission altitude and angle of attack. To verify the DSMC simulated rarefied air movement, Starshine satellite drag coefficient according to the altitude and gas-surface interaction are compared with the flight data. Finally, from the analysis results, it leads to appropriate satellite drag coefficient for orbit lifetime calculation.

• Journal of Industrial Technology
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• v.22 no.B
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• pp.27-34
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• 2002

3-D numerical studies are performed to investigate the effect of the air dam height and approaching air velocities on the pressure distribution of notchback road vehicle. For this purpose, the models of test vehicle with four different air dam heights are introduced and PHOENICS, a commercial CFD code, is used to simulate the flow phenomena and to estimate the values of pressure coefficients along the surface of vehicle. The standard $k-{\varepsilon}$ model is adopted for the simulation of turbulence. The numerical results show that the height variation of air dam makes almost no influence on the distribution of the value of pressure coefficient along upper and rear surface but makes strong effects on the bottom surface. That is, the value of pressure coefficient becomes smaller as the height is increased along the bottom surface. Approaching air velocity makes no differences on pressure coefficients. Through the analysis of pressure coefficient on the vehicle surface, one tries to assess aerodynamic drag and lift of vehicle. The pressure distribution on the bottom surface affects more on lift than the pressure distribution on the upper surface of the vehicle does. The increase of air dam height makes positive effects on the lift decrease but no effects on drag reduction.

• Journal of Korean Tunnelling and Underground Space Association
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• v.7 no.4
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• pp.313-321
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• 2005

Drag coefficient is one of the critical design factors to quantify the piston effect in vehicle tunnels. Several problems are raised on the drag coefficient currently applied for the ventilation system design; unverified adoption of the projected frontal area of the vehicle from the foreign study in the past, and lack of consideration for the slip-streaming effect. This study aims at better estimation of the traffic-induced ventilation force in the local tunnels. Values for the projected frontal area of the vehicles running on the local roads at present are proposed and results of an extensive CFD study are studied on the slip-streaming effects in various traffic conditions to quantify $K_{blockage}$ and the drag coefficient in the domestic tunnels.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.11
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• pp.3589-3597
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• 1996

The Reynolds-averaged Navier-Stokes equations with the equations of the k-.epsilon. turbulence model are solved numerically in a general curvilinear system for a three-dimensional turbulent flow around an Ahmed body. The simulation is especially aimed at the evaluation of three finite differencing schemes for the convection term, which include the upwind differencing scheme(UDS), the second order upwind differencing scheme(SOU scheme) and the QUICK scheme. The drag coefficient, the velocity and pressure fields are found to be changed considerably with the adopted finite differencing schemes. It is clearly demonstrated that the large difference between computation and experiment in the drag coefficient is due to relatively high predicted values of pressure drag from both front part and vertical rear end base. The results also show that the simulation with the QUICK or SOU scheme predicts fairly well the flow field and gives more accurate drag coefficient than other finite differencing scheme.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.11
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• pp.1632-1639
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• 2001

Control of drag farce on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

• Journal of the Korean Society of Visualization
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• v.17 no.1
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• pp.34-42
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• 2019

Aerodynamic characteristics for a launch vehicle are numerically analyzed with various conditions. The local drag coefficients are high at the nose of the launch vehicle in subsonic region and on the main body in supersonic region because of the induced drag and the wave drag, respectively. The drag coefficients show the similar trend with the angle of attack except zero degree. However, the more the angle of attack increases, the more dependent on the Mach number the lift coefficient is. The body rotation for the flight stability destroys the vortex pair formed above the body opposite to the flight direction, so the flow fields are more or less complicated. The drag coefficient of the launch vehicle at sea level is about three times larger than that at altitude 7.2 km. And the thrust jet at the nozzle causes to reduce the drag coefficient compared with the jetless transonic flight.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.53 no.3
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• pp.228-239
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• 2017

This study investigated the drag coefficient and lift coefficient of thirteen kinds of knotless nettings used for large purse seine gear. By comparing the hydrodynamic characteristics with nets of the previous study, the characteristics of this study were derived as a purse seine gear. Thirteen kinds of nettings with different length of bar (l) and diameter (d) were used in the experiment, out of which six kinds used the 30 mm in mesh size and three kinds with 40 mm. The drag coefficient ($C_d$) also increased with increasing d/l. It can be expressed as $C_d=3.71499(d/l)+0.76595$ at a current speed 0.4 m/s and $C_d=4.30324(d/l)+0.69056$ at a current speed 0.5 m/s. Compared with previous studies, drag coefficient values were similar to knotless net of similar d/l and smaller than drag coefficient of knot net. Therefore, using knotless net in a purse seine has the advantage of reducing the resistance acting on the purse seine gear.