• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.17 no.12
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• pp.1223-1229
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• 2007

Flow traveling over a cavity opening forms a vortex due to unstable shear layer and induces an aerodynamic pressure excitation from the diffusion of the vortex convecting out of the trailing edge of the opening. The interaction between the excitation force and the cavity response sustains resonance in the resonator(cavity) and locked-in vortex shedding at the leading edge of the opening. The aerodynamic excitation force can be described from the diffusion of the vortex over the trailing edge and the level of its diffusivity is related to the strength of vorticity seeded at the leading edge. In this study, the control scheme of the internal pressure oscillation was proposed from regulating the vorticity at the leading edge by use of an oscillating spoiler. It was found that the relative motion between the spoiler and the air mass at the cavity opening influenced vorticity strength and the control was achieved by direct feedback of the cavity pressure fluctuation to the actuator.

• Journal of Power System Engineering
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• v.20 no.6
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• pp.64-70
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• 2016

The unsteady-state, incompressible and three-dimensional large-eddy simulation(LES) was carried out to evaluate the vorticity distribution of a small-size axial fan(SSAF). The X-component vorticity profiles developed around blade tips turn from axial to radial, and diminish the density of distribution according to the increase of static pressure. Otherwise, the Z-component vorticity profiles evenly develop at the region larger than the half radial distance of blade at the operating points of A and B, partly at the trailing-edge region of blade and radially over bellmouth according to the increase of static pressure.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.196-199
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• 2005

Flow traveling over a cavity opening forms a vortex due to unstable shear layer and induces an aerodynamic pressure excitation from the diffusion of the vortex convecting out of the trailing edge of the opening. The interaction between the excitation force and the cavity response sustains resonance in the resonator(cavity) and locked-in vortex shedding at the leading edge of the opening. The aerodynamic excitation force can be described from the diffusion of the vortex over the trailing edge and the level of its diffusivity is related to the strength of vorticity seeded at the loading edge. In this study, the control scheme of the internal pressure oscillation was proposed from regulating the vorticity at the leading edge by use of an oscillating spoiler. It was found that the relative motion between the spoiler and the air mass at the cavity opening influenced vorticity strength and the control was achieved by direct feedback of the cavity pressure fluctuation to the actuator.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.3
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• pp.233-237
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• 2011

In the present experiments, vortical structures behind the hydrofoil trailing edge are visualized and analyzed as an elementary study for propeller singing phenomena. Two sorts of hydrofoil are selected for the measurement of shedding vortices. One was KH45 hydrofoil section and the other is KH45 with the truncated trailing edge that is positioned at X/C = 0.9523(C=chord length). Assuming the Strouhal number of 0.23, the shedding frequencies of vortices are extracted by analyzing the boundary layer thickness and the flow speed. The frequency distribution of shedding vortices is obtained with the variation of angle-of-attack while the flow speed is fixed to 8m/s. The truncation of the trailing edge makes the frequency of shedding vortices about 120Hz lower than that of original trailing edge and makes the vorticity value higher than the original trailing edge.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.259-264
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• 2001

High-resolution simulations using vortex methods have been performed for simulating unsteady viscous flows around an impulsively started square cylinder. In order to investigate the phenomenon from laminar to transition flow, simulations are performed for Reynolds numbers 25, 50, 150 and 250. At extremely low Reynolds number, flow around a square cylinder is known to separate at the trailing edges rather than the leading edges. With an increase of Reynolds number, the flow separation at the leading edges will be developed. The main flow characteristics of developing recirculation region and separations from leading and trailing edges are studied with the unsteady behavior of the wake after the cylinder starts impulsively. A notable change in the flow evolution is found at Re=150, that is, it is shown that the flow separations begin at both leading and trailing edges of the square cylinder. On the other hand, when Re=250, the strong secondary vorticity from the rear surfaces of the square cylinder increases the drag coefficient as the primary vortex layer is pushed outwards. The comparisons between results of the present study and experimental data show a good consistency.

• 한국가시화정보학회:학술대회논문집
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• 2004.12a
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• pp.169-175
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• 2004

The characteristics of flow around a rotating propeller were investigated using PIV technique. For each of four different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}\;and\;54^{\circ}$four hundred instantaneous velocity fields were ensemble averaged to investigate the spatial evolution of the flow around a propeller. The phase-averaged mean velocity fields show that the viscous wake formed by the boundary layers developed on the blade surfaces and the slipstream contraction in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. The boundary layer developed along the ship hull bottom surface of the ship stern provides a strong turbulent shear layer, affecting the vortex structure in the propeller near-wake. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. The turbulence intensity has large values around the tip and trailing vortices. As the wake moves downstream, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and adjacent wake flow.

• 유체기계공업학회:학술대회논문집
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• 2004.12a
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• pp.452-455
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• 2004

The influence of wake flow on a flat-plate blade having different shapes near the hailing edge has been investigated in this paper. Two different shapes near the trailing edge namely the pressure surface cutting (TP) and the suction surface cutting (TS) have been used. The calculation has been performed for two different angles of attack (a =10 and 15 degree) in both the cases. RANS equations have been solved using SST-model as a turbulence closure. Cp and CL values obtained for TS are higher compared to those obtained for TP. Also, the vorticity distribution obtained in case of TS is stronger as compared to that obtained in the case of TP The Karmann Vortex is observed in both the cases but it is more clear in TS case. It is found that in the case of TS, flow separation does not occur upto the trailing edge on both the suction and the pressure sides of the blade while in the case of TP, the flow separateson the pressure surface near the trailing edge of the flat-plate blade.

• Journal of computational fluids engineering
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• v.16 no.2
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• pp.24-29
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• 2011

Immersed boundary lattice Boltzmann method has been applied to analyze the characteristics of the self-propelled fish motion swimming robot. The airfoil NACA0012 with caudal fin stroke model was considered to examine the characteristics. The foil in steady forward motion and a combination of steady-state harmonic deformation produces thrust through the formation of a flow downstream from the trailing edge. The harmonic motion of the foil causes unsteady shedding of vorticity from the trailing edge, while forming the vortices at the leading edge as well. The resultant thrust is developed by the pressure difference formed on the upper and lower surface of the airfoil. and the time averaged thrust coefficient increases as Re increase in the region of $Re{\leqq}700$. The suggested numerical method is suitable to develop the fish-motion model to control the swimming robot, however It would need to extend in 3D analysis to examine the higher Re and to determine the more detail mechanism of thrust production.

• Journal of the Society of Naval Architects of Korea
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• v.45 no.6
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• pp.620-630
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• 2008

The characteristics of complicated propeller wake influenced by hull wake are investigated by using a two-frame PIV (Particle Image Velocimetry) technique. As the propeller is significantly affected by the hull wake in a real marine vessel, the measurements of propeller wake under the hull wake would be certainly necessary for more reliable validation and the prediction of numerical simulation with wake modeling. Velocity field measurements have been conducted in a medium-size cavitation tunnel with a hull wake. Generally, the hull wake generated by the boundary layer of ship's hull produces the different loading distribution on the propeller blade in both upper and lower propeller planes. The difference of the propeller wake behaviors caused by the hull wake is discussed in terms of axial velocity, vorticity and turbulence kinetic energy distribution in the present study.

• Journal of ILASS-Korea
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• v.14 no.1
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• pp.1-7
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• 2009

This study carried out numerical analysis of flow field of combustion gas and sorbent to test sorbent efficiency of DSI process. To provide rapid mixing for increase utilization rate of sorbent, streamwise vorticity can be introduced into the flowing streams by other means; for example, by installing vortex generators immediately downstream of the wavy trailing edge. Computing results show that the degree of sorbent dispersion depends strongly on duct structure. Highest dispersion efficiency received when vortex generator was installed inside of duct. The results presented in this study a optimum condition for the development of practical DSI process.