• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2868-2872
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• 2007

In this study, a passive control using a boat-tail device is conducted for a three-dimensional car model in ground proximity. We consider various boat-tails and investigate the mechanism of drag reduction by them. By varying the length and slant angle of boat-tail, we obtain drag reductions up to 40%. From the oil-surface flow visualization and hot-wire measurement, the drag reduction by the boat-tail is characterized by the shear-layer instability and reattachment on the boat-tail, forming a small separation bubble at the upstream part of boat-tail surface, resulting in the delay of main separation and drag reduction. At high slant angles, the flow fully separates and drag is nearly same as that of no control.

• Biotechnology and Bioprocess Engineering:BBE
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• v.11 no.3
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• pp.179-186
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• 2006

End-Labeled Free-Solution Electrophoresis (ELFSE) is a new technique that is a promising bioconjugate method for DNA sequencing (or separation) and genotyping by both capillary and microfluidic device electrophoresis. Because ELFSE enables high-resolution electrophoretic separation in aqueous buffer alone (i.e., without a polymer matrix), it eliminates the need to load viscous polymer networks into electrophoresis microchannels. To achieve microchannel DNA separations with high performance, ELFSE requires monodisperse perturbing entities (i.e., drag-tags), which create a large amount of frictional drag when pulled behind DNA during free-solution electrophoresis, and which have other properties suitable for microchannel electrophoresis. In this article, the theoretical concepts of ELFSE and the required characteristics of the drag-tag molecules for the ultimate performance of ELFSE are reviewed. Additionally, the merits and limitations of current drag-tags are also discussed in the context of recent experimental data of ELFSE separation (or sequencing).

• Journal of Aerospace System Engineering
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• v.14 no.4
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• pp.18-24
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• 2020

The counter-flow jet from a supersonic/hypersonic vehicle causes a structural change in the shock wave generated around the aircraft, which can lead to reduced drag and heat loads. Since the idea is to mount a counter-flow jet device for drag reduction in the aircraft, it is necessary to understand the effect of such a device on the entire aircraft. In this study, the effect of drag reduction due to counter-flow jet on a conventional rocket configuration was analyzed through CFD analysis. The results showed that the drag reduction effect was the largest in the blunt region and that the counter-flow jet also affected the downstream of the aircraft. The analysis indicated that the drag reduction effect by the counter-flow jet was about 10 to 25 % when targeting the entire rocket-shaped area, while the effect was as high as 50% when targeting only blunt objects.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2000.04a
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• pp.18-18
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• 2000

In the high lift devices specifications for surface smoothness requirements, as manufacturing tolerances, arise out of aerodynamic consideration to minimize drag. True optimization of tolerances is a multi-disciplinary problem involving fluid mechanics, device performance, manufacturing philosophy and life cycle costing. One of the reasons for degradation of wetted surface is discrete roughness as a consequence of manufacturing defects, collectively termed as one of the excrescences effect. Usually, excrescence drag arising out of discrete roughness is of considerable lower order of magnitude as compared to the total drag of the flight bodies. Nor was there adequate predicting tool to account for the extent of drag degradation. Estimation of excrescence drag remained as a state-of-the art based on experimental results.

• 한국전산유체공학회:학술대회논문집
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• 2005.04a
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• pp.114-118
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• 2005

CFD simulation was peformed for 2D and 3D flying and rotating frisbees. Multiple reference method(MRF) was utilized to consider the rotation of 3D model. Geometry change of 2D model shows dramatic increase of lift, but 3D simulation results for geometry change show decrease of lift and drag. Ground effect increases the lift of the frisbee being close to ground.

• Journal of Aerospace System Engineering
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• v.7 no.3
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• pp.1-6
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• 2013

Numerical optimization of two dimensional high lift configuration was performed with flow solver and optimization method based on RSM(Response Surface Model). Navier-Stokes solver with Spalart-Allmaras turbulence model was selected for the simulation of highly complex and separated flows on the flap. For the simultaneous optimization of both flap shape and setting (gap/overlap), 10 design variables (eight variables for flap shape variation and two variables for flap setting) were chosen. In order to generate the response surface model, 128 experimental points were selected for 10 design variables. The objective function considering maximum lift coefficient, lift to drag ratio and lift coefficient at specific angle of attack was selected to reduce flow separation on the flap surface. The present method was applied to two dimensional fowler flap in landing configuration. After applying the present method, it was shown that the optimized high lift configuration had less flow separation on the flap surface and lift to drag ratio was suppressed over entire angle of attack range.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.217-220
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• 2006

The lift and drag forces produced by a wing of a given cross-sectional profile are dependent on the wing planform and the angle of attack. Aspect ratio is the ratio of the wing span to the average chord. For conventional fixed wing aircrafts, high aspect ratio wings produce a higher lift to drag ratio than low ones for flight at subsonic speeds. Therefore, high aspect ratio wings are used on aircraft intended for long endurance. However, birds and insects flap their wings to fly in the air and they can change their wing motions. Their wing motions are made up of translation and rotation. Therefore, we tested flapping motions with parameters which affect rotational motion such as the angle of attack and the wing beat frequency. The half elliptic shaped wings were designed with the variation of aspect ratio from 4 to 11. The flapping device was operated in the water to reduce the wing beat frequency according to Reynolds similarity. In this study, the aerodynamic forces, the time-averaged force coefficients and the lift to drag ratio were measured at Reynolds number 15,000 to explore the aerodynamic characteristics with the variation of aspect ratio. The maximum lift coefficient was turned up at AR=8. The mean drag coefficients were almost same values at angle of attack from $10^{\circ}$ to $40^{\circ}$ regardless of aspect ratio, and the mean drag coefficients above angle of attack $50^{\circ}$ were decreased according to the increase of aspect ratio. For flapping motion the maximum mean lift to drag ratio appeared at AR=8.

• International Journal of Aeronautical and Space Sciences
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• v.17 no.3
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• pp.285-295
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• 2016

A contact strip shape of a high speed train pantograph system was optimized with CFD to increase the aerodynamic performance and stability of contact force, and the results were validated by a wind tunnel test. For design of the optimal contact strip shape, a Kriging model and genetic algorithm were used to ensure the global search of the optimal point and reduce the computational cost. To enhance the performance and robustness of the contact strip for high speed pantograph, the drag coefficient and the fluctuation of the lift coefficient along the angle of attack were selected as design objectives. Aerodynamic forces were measured by a load cell and HWA (Hot Wire Anemometer) was used to measure the Strouhal number of wake flow. PIV (Particle Image Velocimetry) was adopted to visualize the flow fields. The optimized contact strip shape was shown a lower drag with smaller fluctuation of vertical lift force than the general shaped contact strip. And the acoustic noise source strength of the optimized contact strip was also reduced. Finally, the reduction amount of drag and noise was assessed when the optimized contact strip was applied to three dimensional pantograph system.

• Journal of Sensor Science and Technology
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• v.32 no.1
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• pp.44-50
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• 2023

In this study, a polydimethylsiloxane (PDMS)-based drag force-type flowmeter was fabricated using a graphite-carbon nanotube (CNT) composite as a piezoresistive material and evaluated. The device was in the form of a cantilever, which was composed of the soft material, PDMS, and fabricated using a mold manufactured by a three-dimensional printer. The cost-effective graphite was mixed with CNTs to serve as a piezoresistive material. The optimal mixing ratio was investigated, and the piezoresistive material formed using a graphite:PDMS:CNT ratio of 1.5:1:0.01 was adopted, which showed a stable output and a high sensitivity. Various forward and backward air flows in the range of 0-10 m/s were measured using the fabricated flowmeter, and both tensile and compression characteristics were evaluated. The measured results showed a stable output, with the resistance change gradually increasing with the air flow rate. Repeatability characteristics were also tested at a repeated air flow of 10 m/s, and the flowmeter responded to the applied air flow well. Consequently, the fabricated device has a high sensitivity and can be used as a flowmeter.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.6 s.237
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• pp.755-762
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• 2005

To evaluate the aerodynamic efficiency of TR-S2 configuration designed by SUDC, wind tunnel tests of $40\%$ scaled model were done in KARI LSWT. The aerodynamic characteristics of plain and Semi-Slotted Flaperon were compared, and vortex generators were installed to improve flow pattern along the wing surface. Effects of the control surface such as elevator, rudder, aileron, and incidence angle of horizontal tail are measured for various testing conditions. Test results showed that Semi-Slotted Flaperon produced more favorable lift, lift/drag, and stall margins and application of vortex generator would be best choice to enhance wing performance. Longitudinal, lateral and directional characteristics of TR-S2 were found to be stable for the pitch and yaw motions.