• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2002.04a
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• pp.140-145
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• 2002

At present, wind tunnel test or CFD is used for predicting aerodynamic drag coefficient in motor company. But, wind tunnel test requires much cost and time, and CFD has about 30% error. In this study a predicting program of the aerodynamic drag coefficient based on empirical techniques was developed. Also a mathematical optimization method using GRG method was added to the program. The program was applied to six cars. Aerodynamic drag coefficient values of six cars were Predicted with 4.857% average error. The optimization method was also applied to six cars. Three parameters selected from sensitivity analysis were determined to reduce the afterbody drag coefficient to the value established by a designer and when some parameters were changed for a developing automotive, optimal modifiable parameters were determined to preserve the same drag coefficient as the original automotive. It was verified that this program could predict the aerodynamic drag coefficient effectively and accurately, and this program with GRG method could determine optimal values of parameters.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.5
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• pp.223-227
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• 2002

Wind tunnel test or CFD is used for predicting aerodynamic drag coefficient in domestic motor companies. But, wind tunnel test requires much cost and time, and CFD has a relatively large error. In this study a predicting program of the aerodynamic drag coefficient based on empirical techniques was developed. Also GRG method was added to the program in order to decide optimal values of some parameters. The program was applied to 24 cars and the aerodynamic drag coefficients were predicted with 4.82% average error. Optimization was also accomplished to 6 cars. Some parameters to be modified were determined (1) to reduce the afterbody drag coefficient to the value established by a designer and (2) to preserve the same drag coefficient as the original automotive when some parameters have to be changed in the viewpoint of design. It was verified that the developed program can predict the aerodynamic drag coefficient appropriately and determine optimal values of some parameters.

• Journal of the Korean Society of Propulsion Engineers
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• v.21 no.1
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• pp.98-103
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• 2017

In this paper, determination method for drag force and drag coefficient from results of firing test is described. The drag force and drag coefficient are determined through inverse operation of 2-dimensional projectile equation of motion. Determination method was verified by comparing analytical drag coefficient with data from flight test. Analysis of drag coefficient and drag reduction was performed with the data of flight test using artillery projectiles with base bleed unit.

• Wind and Structures
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• v.16 no.2
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• pp.193-211
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• 2013

This paper presents results from a wind tunnel study that examined the drag coefficient and wind flow over an asymmetric wave train immersed in turbulent boundary layer flow. The modeled wavy surface consisted of eight replicas of a statistically-valid hurricane-generated wave, located near the coast in the shoaling wave region. For an aerodynamically rough model surface, the air flow remained attached and a pronounced speed-up region was evident over the wave crest. A wavelength-averaged drag coefficient was determined using the wind profile method, common to both field and laboratory settings. It was found that the drag coefficient was approximately 50% higher than values obtained in deep water hurricane conditions. This study suggests that nearshore wave drag is markedly higher than over deep water waves of similar size, and provides the groundwork for assessing the impact of nearshore wave conditions on storm surge modeling and coastal wind engineering.

• Journal of Korea Water Resources Association
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• v.50 no.6
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• pp.419-427
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• 2017

In mountainous rivers, the drag force acting on cobbles abundant in the riverbed surface is important in predicting behavior and response of the river. However there is little research for the drag coefficients of cobbles. This paper is to carry out the experiments for drag force of cobble and analyze the relation between the cobble shape and the drag coefficient. The effects of the shape factor on the drag coefficients $C_D$ when the long axis or the short axis of the cobbles are parallel to the direction of flow velocity were analyzed. The coefficient of drag force increased with the nominal diameter Reynolds number $R_{ep}$. The drag coefficients are greater in short axis than long axis. The coefficient of determination of the relation between $C_D$ and $R_{ep}$ is greater in long axis than short axis. This means that the drag forces acting on the irregularly-shaped cobbles depend on the axis. A change of the drag force distribution has brought about the alternative swing of cobbles. For $R_{ep}$ > 12,000, the amplitude of the swing has been increased sharply and especially was greater in short axis than long axis.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.5
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• pp.86-96
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• 1998

A numerical study has been carried out of three-dimensional turbulent flows around a MIRA reference vehicle model both with and without wheels in computation. Two convective difference schemes with two k-$\varepsilon$ turbulence models are evaluated for the performance such as drag coefficient, velocity and pressure fields. Pressure coefficients along the surfaces of the model are compared with experimental data. The drag coefficient, the velocity and pressure fields are found to change considerably with the adopted finite difference schemes. Drag forces computed in the various regions of the model indicate that design change decisions should not rely just on the total drag and that local flow structures are important. The results also indicate that the RNG model with the QUICK scheme predicts fairly well the tendency of velocity and pressure fields and gives more reliable drag coefficient rather than the other cases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.10
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• pp.797-805
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• 2014

The purpose of this study was to investigate the drag and lift characteristics of the cavitator of a supercavitating underwater vehicle and the pressure loss due to water intake. These investigations were performed by changing the diameter, velocity, radius of curvature of the intake, and angle of attack of the cavitator. With increasing ratio of the intake diameter to the cavitator diameter ratio($d/D_1$), the drag coefficient and the pressure loss coefficient of the water intake decreased. The greater the increase in the ratio of the intake velocity-to-free stream velocity ratio(S), the smaller was the decrease in the drag coefficient and the lift coefficient. When the intake had a radius of curvature(c), the pressure loss coefficient decreased. On the contrary, the effect of the radius of curvature on the drag coefficient was imperceptible. For angles of attack (${\alpha}$) of the caviatator in the range of $0^{\circ}$ to $10^{\circ}$, the drag coefficient and the pressure loss coefficient changed slightly, whereas the lift coefficient increased linearly with increasing angle of attack.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.7
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• pp.489-495
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• 2020

An accelerometer-based system was designed and constructed for drag measurement in a shock tunnel. Drag coefficient of a conical model was measured under a Mach 6 flow condition. A simple and intuitive calibration method was presented to compensate for the friction force of the drag measurement system, and the results of the measurement were compared with computational fluid dynamics in which the simple conical model was analyzed. The influence of drag measurement interference by supports of various shapes was identified and the design was presented to minimize. The drag coefficient measurement using the modified support showed that the error of the drag coefficient by the support was decreased.

• Journal of computational fluids engineering
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• v.4 no.1
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• pp.34-40
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• 1999

Drag reduction on vehicles are the main concern for the body shape designers in order to lower the fuel consumption rate and to aid the driving stability. The drag of bluff bodies like transportation vehicles is mostly pressure drag due to the flow separation, which can be minimized by controlling the location and size of the separation bubble. In the present study, the TURBO-3D code is incorporated with optimal algorithm based on analytical approximation method to obtain an optimal afterbody shape of the MIRA Model corresponding to the lowest drag coefficient. For this purpose three mutually independent afterbody angles are chosen as design variables, while the drag coefficient is chosen as an objective function. It is demonstrated in the present study that an optimal body shape having the lowest drag coefficient which is about 6% lower than that of the original shape has been successfully obtained within number of iterations of tile optimal design loop.

• 한국전산유체공학회:학술대회논문집
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• 1998.05a
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• pp.67-74
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• 1998

Reducing drag of vehicles are the main concern for the body shape designers in order to lower fuel consumption rate and to aid the driving stability. The drag of bluff bodies like transportation vehicles is mostly pressure drag due to the flow separation, which can minimized by controlling the location and size of the separation bubble. In the present study, the TURBO-3D code is incorporated with optimal algorithm based on analytical approximation method to obtain optimal afterbody shape of the MIRA Model corresponding to the lowest drag coefficient. For this purpose three mutually independent afterbody angles are chosen as design variables, while the drag coefficient is chosen as an objective function. It is demonstrated in the present study that an optimal body shape having lowest drag coefficient which is about $6\%$ lower than that of the original shape has been successfully obtained within number of iterations of the optimal design loop.