• International Journal of Aeronautical and Space Sciences
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• v.14 no.3
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• pp.201-209
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• 2013

Much numerical and experimental research has been done for the flow around an oscillating airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, MAV's lift and thrust generation. Until now, researches mainly have been concentrated on analyzing the wake flow for the variation of frequency and amplitude at a low angle of attack. In this study, wake structures and acoustic wave propagation characteristics were studied for a plunging airfoil at high angle of attack. The governing equations are the Navier-Stokes equation with LES turbulence model. OHOC (Optimized High-Order Compact) scheme and 4th order Runge-Kutta method were used. The Mach number is 0.3, the Reynolds number is, and the angle of attack is from $20^{\circ}$ to $50^{\circ}$. The plunging frequency and the amplitude are from 0.05 to 0.15, and from 0.1 to 0.2, respectively. Due to the high resolution numerical method, wake vortex shedding and pressure wave propagation process, as well as the propagation characteristics of acoustic waves can be simulated. The results of frequency analysis show that the flow has the mixed characteristics of the forced plunging frequency and the vortex shedding frequency at high angle of attack.

• Journal of Korea Water Resources Association
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• v.55 no.1
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• pp.33-42
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• 2022

In this study, flow patterns in the ice-harbor fishway were analyzed according to fluctuations of the upstream water level, an increase of weir interval, and the presence or absence of orifices using a three-dimensional commercial numerical model, Flow-3D. In order to prove the suitability of the numerical simulation results, the flow velocity and flow rate at the exit of the fishway were observed using a 3D ultrasonic velocimetry on an actual ice-harbor fishway installed downstream of the Daegok bridge in Gyeongan-Cheon. Four types of turbulence modules can be selected for the Flow-3D model. As a result of verification with observation data, the RNG model best described the flow characteristics in the ice-harbor fishway. The velocity structure in the fishway according to fluctuations of the upstream water level was simulated. The results showed that the plunging flow and the streaming flow were mixed at the lowest water level. When the water level increased about 10 cm or more from the lowest water level, the plunging flow disappeared in all pools and only the streaming flow occurred. Contrary to expectations, even when the water level is rose a little, the flow simply occurred mainly on the streaming flow. If the interval between the weirs is increased, both the plunging flow and the streaming flow are showed continued even if the water level rises. In addition, compared to the case where there are no orifices at the bottom of the weirs, the plunging flow tends to be generated in several pools. It is necessary to prevent blocking orifices through active management so that various flow patterns in the fishway can be generated in multiple pools.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.3
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• pp.6-13
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• 2006

Aerodynamic characteristics for two-dimensional cyclic motion profile of flapping airfoil in low Reynolds number flows are investigated. Plunging motion and lead-lag motion in the two dimensional space with different plunging and lead-lag amplitudes are combined to cyclic motion profile and the flow around the airfoil is simulated. Present result shows that the improved aerodynamic efficiencies for a given flapping airfoil by adding periodic lead-lag motion of airfoil rather than the pure plunging case. The thrust coefficient and lift coefficient are compared with each cycle during the flapping period and aerodynamic characteristics are obtained on upstroke motion and downstroke motion.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.4
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• pp.44-52
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• 2002

Aerodynamic characteristics of a flapping airfoil in low Reynolds number flows are numerically studied using the unsteady, incompressible Navier-Stokes flow solver with a two-equation turbulence model. For more efficient computation of unsteady flows over flapping airfoil, the flow solver is parallel-implemented by MPI programming method Unsteady computations are performed for low Reynolds number flows over a NACA four-digit series airfoils. Effects of pitching, plunging, and flapping motion with different reduced frequency, amplitude, thickness and camber on aerodynamic characteristics are investigated. Present computational results yield a better agreement in thrust at various reduced frequency with experimental data.

• Journal of the Institute of Convergence Signal Processing
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• v.22 no.3
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• pp.122-127
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• 2021

Flow features near the ship bow such as wave breaking, small scale phenomena have been studied using numerical methods. In this study, the bow shaped wedge was adopted which is from previous paper [1, 2] and the conditions of simulation were Re = 1.64 × 10⁵) and Fr = 2.93. Star CCM+, one of the commercial CFD programs has been used for the simulations. Simulation results such as wave profiles near the ship bow, shape of plunging jet, air entrainment, and wave breaking process have been compared with previous experimental and numerical studies. Overall results showed good agreements with previous studies. Profiles of bow waves showed that overturning jet has been created and broken along the wedge. Plunging wave breaking has been observed along the wedge and four components of plunging wave breaking process were shown. It is confirmed that velocity near the overturing jet significantly increased during plunging wave breaking.

• Journal of Ocean Engineering and Technology
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• v.23 no.4
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• pp.12-18
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• 2009

The present study focused on the compressibility of partially filled fluids in a sloshing tank. Filling ratios ranging from 18% to 26% were used to find compressible impact on a vertical wall. The model test was for 1/25 scale of a 138 K LNGC cargo tank. To investigate the two dimensional phenomenon of sloshing, a longitudinal slice model was tested. A high speed camera was used to capture the flow field, as well as the air pocket deformation. The pressure time history synchronized with the video images revealed the entire compressible process. Three typical impact phenomena were observed: hydraulic jump, flip through, and plunging breaker. In particular, the pressure time history and flow pattern details for flip through and plunging breaker are presented.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.21 no.1
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• pp.54-62
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• 2009

The present study investigates the behaviour of overtopping flows falling on the leeside of a caisson breakwater with dissipating blocks through laboratory measurements. The falling overtopping flows in the leeside are expected to directly affect the leeside stability of the breakwater. This study focuses on not the resultant stability but the characteristic pattern of the overtopping flows depending on wave conditions through examining front velocity and plunging distance in the leeside. Regular waves were used to investigate the dependence of the overtopping flow pattern on wave conditions and a modified image velocimetry combining the shadowgraphy and cross-correlation method was employed for measurements of image and velocity. From the measurements, it is shown that the plunging distance and front velocity of the overtopping flow in the breakwater leeside increase as the wave period or height increases. From non-dimensional relationships between the variables, empirical formula for the velocity and overtopping distance are suggested.

• Journal of Korea Water Resources Association
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• v.56 no.spc1
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• pp.1037-1048
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• 2023

Fishways are constructed to ensure the fish migration because river-crossing structures such as dams and weirs cut off the stream longitudinal connectivity and influence on aquatic ecosystems. However, the passage efficiency of fishes varies depending on flow characteristics in the fishway and fish species. In this study, three-dimensional numerical simulations are carried out using a RANS model and the volume of fluid method for resolving free surface fluctuations to calculate the turbulent flow in the pool and weir type fishway. The Flow velocity and turbulent kinetic energy in the pool of fishway are analyzed according to variation of the upstream water level and the length of pool. The present numerical simulations reasonably well reproduce the stream flow and plunging flow characteristics in the pool. The simulation results show that the stream flow changes to the plunging flow as the length of the pool increases. When the upstream level increases, the stream flow becomes more evident. Key parameters related to the fish migration within the fishway such as the flow velocity and the turbulent kinetic energy are examined to assess the ascending possibility of fishes.

• Journal of the Korean Society for Heat Treatment
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• v.32 no.3
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• pp.124-130
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• 2019

Friction stir welding is one of the interesting welding methods for titanium and its alloy which proceeds with plastic flow due to thermo-mechanical stirring and friction heat. Solid-state welding can solve severe problems such as high-temperature oxidation, interstitial oxygen diffusion and grain coarsening by liquid-state welding. Dynamic recrystallization and grain refinement can vary significantly with the plunging load and rotational speed of tool during friction stir welding, and suitable process conditions must be optimized to obtain microstructure and better mechanical characteristics. Suitable FSW conditions were 1000 kg of plunging load and 200 rpm of rotational speed and it showed YS 270 MPa, UTS 332.1 MPa, and El 17.3%, which were very similar to those of wrought titanium sheet.

• 한국전산유체공학회:학술대회논문집
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• 2003.10a
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• pp.291-293
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• 2003

This paper deals with a thrust generation of flapping-airfoil by dynamic stall. From many other previous research results, phase angle $ between pitching and plunging mode of flapping motion must be 90 deg. to satisfy maximum propulsive efficiency. In this case, leading edge vortex is relatively small. This phenomenon is related dynamic stall. So preventing leading edge vortex induced by dynamic stall guarantees maximum propulsive efficiency. But, in this paper we insist the leading edge vortex yields quite a positive influence on thrust generation and propulsive efficiency. In order to certify our opinion, pitching and plunging motions were calculated with the parameter of amplitude and frequency by using the unsteady, incompressible Navier-Stokes flow solver with a two-equation turbulence model. For more efficient computation, it is parallelized by MPI programming method.