• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.41 no.5
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• pp.335-341
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• 2013

The variation of systematic bias errors in the wind tunnel testing has been studied. A Design of Experiments(DOE) approach to an experimental study of fuselage drag and stability characteristics of a helicopter configuration was applied. When forces and moments measured in one time block differ significantly from measurements made in another time block under assumption that sample observations can be expected to yield same results within permissible measuring errors. The practical implication of this paper is that the systematic error can not be assumed not to exist. The those error reduction could be achieved through the process of randomization, blocking, and replication of the data points.

• Journal of computational fluids engineering
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• v.18 no.1
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• pp.91-97
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• 2013

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified ${\kappa}-{\omega}$ SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

• Journal of Power System Engineering
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• v.9 no.4
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• pp.209-213
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• 2005

The effects of density change of ambient gas on mixture formation process have been investigated in high temperature and pressure field. To analyze the mixture formation process of evaporating diesel spray is important for emissions reduction in actual engines. Ambient gas density was selected as experimental parameter. The ambient gas density was changed from $r_a=5.0kg/m^3\;to\;r_a=12.3kg/m^3$ with a high pressure injection system(ECD-U2). For visualization of the experiment phenomenon, a CVC(Constant Volume Chamber) was used in this study. The ambient temperature and injection pressure are kept as 700K and 72MPa, respectively. The images of liquid and vapor phase in the evaporating free spray were simultaneously taken by exciplex fluorescence method. As experimental results, with increasing ambient gas density, the tip penetration of the evaporating free spray decreases due to the increase in the drag force from ambient gas.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.6
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• pp.410-420
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• 2016

The geometric dimension and shape of microchannels that are formed during surface texturing are widely studied for applications in flow control, and drag and friction reduction. In this research, a new method for controlling the deformation of U channels during micro-rolling-based surface texturing was developed. Since the width of the U channels is almost constant, controlling the depth is essential. A calibration procedure of initial rolling gap, and proportional-integral PI controllers and a linear interpolation have been applied simultaneously to control the depth. The PI controllers drive the position of the pre-U grooved roll as well as the rolling gap. The relationship between the channel depth and rolling gap is linearized to create a feedback signal in the depth control system. The depth of micro channels is studied on A2021 aluminum lamina surfaces. Overall, the experimental results demonstrated the feasibility of the method for controlling the depth of microchannels.

• Journal of the Korean Society of Visualization
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• v.4 no.2
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• pp.69-75
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• 2006

The microbubbles were used in various fields, such as turbulent control, drag reduction, material science and life science. The X-ray PTV using X-ray micro-imaging technique was employed to mea-sure the size and velocity of micro-bubbles moving in an opaque tube simultaneously. Micro-bubbles of $10{\sim}60{\mu}m$ diameter moving upward in an opaque tube (${\phi}$=2.7mm) were tested. Due to the different refractive indices of water and air, phase contrast X-ray images clearly show the exact size and shape of over-lapped microbubbles. In all of the working fluids tested (deionized water, tap water, 0.01 and 0.10M NaCl solutions), the measured terminal velocity of the microbubbles rising through the solution was proportional to the square of the bubble diameter. The rising velocity was increased with increasing mole concentration. The microbubble can be useful as contrast agent or tracer in life science and biology. The X-ray PTV technique should be able to extract useful information on the behavior of various bio/microscale fluid flows that are not amenable to analysis using conventional methods.

• Wind and Structures
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• v.22 no.6
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• pp.703-721
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• 2016

This paper presents a wind tunnel study of wind loads of the large billboard structures with two-plate and three-plate configurations. Synchronous dynamic pressures on the surfaces of plates are measured, and the characteristics of local pressures, integrated forces on each individual plate and on the overall structures are investigated. The influences of wind direction and plate configuration on wind load characteristics, and the contributions of overall crosswind load and torque to the stress responses are examined. The results showed that the wind load characteristics of windward plate in both two- and three-plate configurations are very similar. The contribution of overall crosswind load makes the total resultant force from both alongwind and crosswind loads less sensitive to wind direction in the case of three-plate configuration. The overall torque is lower than the value specified in current codes and standards, and its contribution is less significant in both two-plate and three-plate configurations.

• Journal of Ship and Ocean Technology
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• v.6 no.2
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• pp.12-22
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• 2002

Offshore pipelines play an important role in the transportation of gas, oil, water and oil products. It is common to have a group of pipelines in the oil and gas field. To reduce the installation cost and time, dual pipelines are designed. There are great advantages in the installation of dual pipelines over two separate single lines. It can greatly reduce the cost for trench, back-filling and installation. However the installation of dual pipelines often requires technical challenges. Pipelines should be placed to be stable against external loadings during installation and design life period. Dual pipelines in trench can reduce the influence of external forces. To investigate the flow patterns and forces as trench depth and slope changes, number of experiments are conducted with PIV(Particle Image Velocimetry) equipment in a Circulating Water Channel. Numerical approaches to simulate experimental conditions are also made to compare with experimental results. The velocity fields around dual pipelines in trench are investigated and analysed. Comparison of both results show similar patterns of flow around pipelines. It is proved that the trench depth contributes significantly on hydrodynamic stability. The trench slope also affects the pipeline stability. The results can be applied in the stability design of dual pipelines in trench section. The complex flow patterns can be effectively linked in the understanding of fluid motions around multi-circular bodies in trench.

• Journal of the Society of Naval Architects of Korea
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• v.51 no.1
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• pp.88-98
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• 2014

A supercavitation is modern technology that can be used to reduce the frictional resistance of the underwater vehicle. In the process of reaching the supercavity condition which cavity envelops whole vehicle body, a vehicle passes through transition phase from fully-wetted to supercaviting operation. During this phase of flight, unsteady hydrodynamic forces and moments are created by partial cavity. In this paper, analytical and numerical investigations into the dynamics of supercavitating vehicle in transition phase are presented. The ventilated cavity model is used to lead rapid supercavity condition, when the cavitation number is relatively high. Immersion depth of fins and body, which is decided by the cavity profile, is calculated to determine hydrodynamical effects on the body. Additionally, the frictional drag reduction associated by the downstream flow is considered. Numerical simulation for depth tracking control is performed to verify modeling quality using PID controller. Depth command is transformed to attitude control using double loop control structure.

• Journal of Ocean Engineering and Technology
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• v.31 no.2
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• pp.121-129
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• 2017

The present study numerically investigated the influence of the waveform distribution on the tsunami-vegetation interaction using a non-reflected wave generation system for various tsunami waveforms in a two-dimensional numerical wave tank. First, it was possible to determine the wave attenuation mechanism due to the tsunami-vegetation interaction from the spatial waveform, flow field, vorticity field, and wave height distribution. The combination of fluid resistance in the vegetation and a large gap and creates a vortex according to the flow velocity difference in and out of the vegetation zone. Thus, the energy of a tsunami was increasingly reduced, resulting in a gradual reduction in wave height. Compared to existing approximation theories, the double volumetric ratio of the waveform increased the reflection coefficient of the tsunami-vegetation interaction by 34%, while decreasing the transfer coefficient and energy attenuation coefficient by 25% and 13%, respectively. Therefore, the hydraulic characteristics of a tsunami is highly likely to be underestimated if the solitary wave of the approximation theory is applied for the tsunami.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.460-468
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• 2008

Recent activities on the scramjet and rocket-ramjet combined-cycle engine of Japan Aerospace Exploration Agency(JAXA) are herein presented. The scramjet engines and combined-cycle engines have been studied in the world and JAXA has also studied such the engines experimentally, numerically and conceptually. Based on the studies, 2 to 3 m long, hydrogen-fueled engine models were designed and tested at the Ramjet Engine Test Facility(RJTF) and the High Enthalpy Shock Tunnel(HIEST). A scramjet engine model was tested in Mach 10 to 14 flight condition at HIEST. A 3 m long scramjet engine model was designed to reduce a dissociation energy loss in a high temperature condition. Drag reduction by a tangential injection and two ways of a transverse fuel injection were examined. Combustor model tests at three operating modes of the combined-cycle engine were conducted, demonstrating the combustor operation and producing data for the engine design at each mode. Aerodynamic engine model tests were conducted in a transonic wind tunnel, demonstrating the engine operation in the ejector-jet mode. A 3 m long combined-cycle engine model has been tested in the ejector-jet mode and the ramjet mode since March 2007. Carbon composite material was examined for application to the engines. Production of the cooling channel on a nickel alloy plate succeeded by the electro-chemical etching.