• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.1
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• pp.9-16
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• 2002

The particles velocity in the instantaneous flow field and velocity change of particles along the jet centerline for various particle diameter in a circular turbulent impingement jet are investigated by using particle image velocimetry(PIV) and an equation of particle motion simplified by terms of inertia forces, drag and gravitational force. The jet Reynolds number was 3300 and 8700, and glass beads of 30,58 and 100$\mu$m in diameter were used. The PIV results show that the direction and size of velocity depends not only on the number density of particle but also on the particle momentum. The results obtained form calculation suggest that the particle velocity near the first impingement region deviated from local air velocity, which accords well with the PIV results. The rebound height of particle increase with the particle diameter. In the second-impingement, particle velocities increased sluggishly with Re=3300 but particle velocities uniformed with Re=8700 in stagnation region.

• Ocean Systems Engineering
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• v.3 no.3
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• pp.167-180
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• 2013

Hydrodynamic characteristics of a bluff cylinder oscillating along transverse direction in steady flow were experimentally investigated at Reynolds number of $2{\times}10^5$. The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phase angle are studied. Vortex shedding mechanics is applied to explain the experimental results. The results show that explicit similarities exist for hydrodynamic characteristics of an oscillating cylinder in high and low Reynolds number within subcritical regime. Consequently, it is reasonable to utilize the test data at low Reynolds number to predict vortex induced vibration of risers in real sea state when the Reynolds numbers are in the same regime.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.10a
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• pp.91-94
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• 2003

An experimental study has been carried out in a supersonic blow-down wind tunnel for examining the influence of streamwise vortices on normal shock-wave/boundary layer interaction. It has been reported by the earlier investigator the streamwise vortices generated by the blowing jets can significantly suppress the shock-induced separation and reduce the wave drag. The blowing jets generate the streamwise vortices with 45$^{\circ}$ angle in the spanwise direction. The shock waves are visualized by a Schlieren optical system. Appropriate measurement systems are provided for the characterization of shock wave/boundary layer interaction. The chamber pressure ratio and blowing pressure ratio are varied from 1.5 to 2.4 and 1.0 to 2.0 respectively.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.195-198
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• 2006

The possibility of skin friction reduction in laminar channel flow is investigated when the flow is subjected to stationary distributed surface blowing and suction. Blowing and suction provided at the channel walls is steady in time but varies as a sine function along the streamwise direction. The skin friction changes depending on the wavelength and amplitude of the actuation. Especially, the skin friction is reduced below that of fully developed laminar flow as the wavelength decreases and amplitude increases. The optimal wavelength of producing minimum skin friction is $\pi/2{\delta}$, where $\delta$ is the channel half-height It is observed that the distributed blowing and suction induces strong negative Reynolds shear stress in the near-wall region at the end of the suction part.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.444-445
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• 2006

Flying trajectories of fine particles within a size range of $10{\sim}60{\mu}m$ were studied during centrifugal atomizing processes. A FORTRAN program was written by using increment method. Calculation results revealed that the drag force might reach very high value of 522-7800 g for fine powder of $10{\sim}60{\mu}m$. Flying distance in horizontal direction could be shortened if the particles fly obliquely due to the huge drug force. On the other hand, very fine powder could be projected to far distances when the atmosphere flow velocity is much stronger. Fortunately such particles could be contracted within a cylinder closed to the atomizer when the atmosphere flow was weaken or retained in a limited diameter.

• Journal of Advanced Marine Engineering and Technology
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• v.36 no.1
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• pp.94-100
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• 2012

The passive control by vertical splitter plate of fluid force acting on a square prism near a plane wall was studied by measuring of fluid force on the prism and by visualization of the flow field using PIV. The hight of the splitter plate was 10% of the square width. The experimental parameters were the attaching position of vertical splitter plate and the space ratios G/B to the prism height. Time variation of vorticity was most remarkable at 3.0B(B: prism height) position toward wake direction from the center of the prism. The point of inflection of average lift coefficient and Strouhal number on the prism were represented at the space ratio G/B=0.4~0.6 for the prism having vertical splitter plate. The drag of the prism was reduced average 5.0% with the space ratios by attaching the vertical splitter plate at the upper and rear corner on the prism. In this case, the size of the separated region on the upside of the prism was smaller than that of prism having no the splitter plate.

• Journal of Ocean Engineering and Technology
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• v.35 no.1
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• pp.24-37
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• 2021

As one of the development directions of high-performance ships to reduce greenhouse gas emissions, there is research on high-performance propellers. However, in the case of conventional screw propellers, as they have been studied for a long time, there is a limit to improving efficiency only by depending on the conventional design and analysis methods. In this study, we tried to solve the problems using the Coanda effect by spraying a jet on the surface of the hydrofoil. The Coanda hydrofoil consists of a tunnel and jet slit to make jet flow. The computation was performed for each tunnel and slit position, and the efficiency according to the geometry of the hydrofoil was analyzed. In addition, a study on the 3D geometry change was conducted to analyze the performance according to the span direction spraying range and hydrofoil shape. As the height of the slit and the diameter of the tip were lower, when the slit is located in the center of the hydrofoil, the lift force increased and the drag force decreased. The increase rate of lift-to-drag ratio was different according to the shape of the hydrofoil, and the efficiency of the spraying condition of 0.1S-0.5S, which had the least effect on the vortex at the tip of the blade, was high for all 3D hydrofoils. When the geometry of the slit was optimized, and also the shape and spray range of the hydrofoil in 3D was considered, the efficiency of the jet sprayed hydrofoil was increased.

• Journal of Ocean Engineering and Technology
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• v.36 no.2
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• pp.91-100
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• 2022

In general, the effect of roll motion is not considered in the study on maneuverability in calm water. However, for high-speed twin-screw ships such as the DTMB 5415, the coupling effects of roll and other motions should be considered. Therefore, in this study, the estimation of maneuverability using a 4-degree-of-freedom (DOF; surge, sway, roll, yaw) maneuvering mathematical group (MMG) model was conducted for the DTMB 5415, to improve the estimation accuracy of its maneuverability. Furthermore, a study on the change in turning performance according to the fin angle was conducted. To accurately calculate the lift and drag forces generated by the fins, it is necessary to consider the three-dimensional shape of the wing, submerged depth, and effect of interference with the hull. First, a maneuvering simulation model was developed based on the 4-DOF MMG mathematical model, and the lift force and moment generated by the side fins were considered as external force terms. By employing the CFD model, the lift and drag forces generated from the side fins during ship operation were calculated, and the results were adopted as the external force terms of the 4-DOF MMG mathematical model. A 35° turning simulation was conducted by altering the ship's speed and the angle of the side fins. Accordingly, it was confirmed that the MMG simulation model constructed with the lift force of the fins calculated through CFD can sufficiently estimate maneuverability. It was confirmed that the heel angle changes according to the fin angle during steady turning, and the turning performance changes accordingly. In addition, it was verified that the turning performance could be improved by increasing the heel angle in the outward turning direction using the side fin, and that the sway speed of the ship during turning can affect the turning performance. Hence, it is considered necessary to study the effect of the sway speed on the turning performance of a ship during turning.

• Wind and Structures
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• v.34 no.5
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• pp.407-419
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• 2022

Many factors should be considered by architects and designers for designing a tall building. Wind load is one of these important factors that govern the design of tall building structures and can become a serious challenge when buildings tend to be built very tall and slender. On the other hand, through the initial stages of a design process, choosing the design geometry greatly affects the wind-induced forces on a tall building. With this respect, geometric shapes with 3-fold rotational symmetry are one of the applied plan shapes in tall buildings. This study, therefore, aims to investigate the aerodynamic characteristics of 8 different geometrical shapes using Computational Fluid Dynamics (CFD) by measuring the drag and lift forces. A case study approach was conducted in which different building shape models have the same total gross area and the same height of 300 meters. The simulation was an incompressible transient flow that ran 1700 timesteps (85 seconds on the real-time scale). The results show a great difference between wind-induced force performance of buildings with different plan shapes. Generally, it is stated that the shapes with the same area, but with smaller perimeters, are better choices for reducing the drag force on buildings. Applying the lift force, the results show that the buildings with plan shapes that have rounded corners act better in crosswind flow while, those with sharp corners induce larger forces in the same direction. This study delivers more analytical understanding of building shapes and their behavior against the wind force through the parametric modelling.

• Nuclear Engineering and Technology
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• v.54 no.3
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• pp.860-866
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• 2022

An accurate prediction of the pressure drop along the flow paths is crucial in the design of advanced passive systems cooled by heavy liquid metal coolants. To date, a generic pressure drop correlation over spacer grids by Rehme has been applied extensively, which was obtained from substantial experimental data with multiple types of components. However, a few experimental studies have reported that the correlation may give large discrepancies. To provide a more reliable correlation for ring-type spacer grids, the current numerical study aims at figuring out the most critical factor among four hypothetical parameters, namely the flow area blockage ratio, number of fuel rods, type of fluid, and thickness of the spacer grid in the flow direction. Through a set of computational fluid dynamics simulations, we observed that the flow area blockage ratio dominantly influences the pressure loss characteristics, and thus its dependence should be more emphasized, whereas the other parameters have little impact. Hence, we suggest a new correlation for the drag coefficient as C_B = C_ν,m/ε^2.7, where C_ν,m is formulated by a nonlinear fit of simulation data such that C_ν,m = -11.33 ln(0.02 ln(Re_b)).