• Proceedings of the KSME Conference
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• 2001.06e
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• pp.247-252
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• 2001

The flow around free end of a finite circular cylinder(FC) embedded in an atmospheric boundary layer has been investigated experimentally. The experiments were carried out in a closed-return type subsonic wind tunnel with varying aspect ratio of the finite cylinder mounted vertically on a flat plate. The wake structures behind a 2-D cylinder and a finite cylinder located in a uniform flow were also measured for comparison. Reynolds number based on the cylinder diameter was about Re=20,000. A hot-wire anemometer was employed to measure the wake velocity and the mean pressure distributions on the cylinder surface were also measured. The flow past the FC free end shows a complicated three-dimensional wake structure and flow phenomenon is quite different from that of 2-D cylinder. The three-dimensional flow structure was attributed to the downwashing counter rotating vortices separated from the FC free end. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation length is increased compared to that of 2-D cylinder. Due to the descending counter-rotating twin-vortex, in the region near the FC free end, regular vortex shedding from the cylinder is suppressed and the vortex formation region is hardly established. In the wake center region, the mean velocity for the FC located in atmospheric boundary layer has large velocity deficit, compared to that of uniform flow.

• Journal of the Society of Naval Architects of Korea
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• v.43 no.5 s.149
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• pp.538-549
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• 2006

A code is developed to simulate a viscous flow field around a deformable body using the hybrid Cartesian/immersed boundary method. In this method, the immersed boundary(IB) nodes are defined near the body boundary then velocities at the IB nodes are reconstructed based on the interpolation along the normal direction to the body surface. A new method is suggested to define the IB nodes so that a closed fluid domain is guaranteed by a set of IB nodes and the method is applicable to a zero-thickness body such as a sail. To validate the developed code, the vorticity fields are compared with other recent calculations where a cylinder orbits and moves into its own wake. It is shown the code can handle a sharp trailing edge at Reynolds number of $10^5$ under moderate requirements on girds. Finally the developed code is applied to simulate the vortex shedding behind a deforming foil with flapping tail like a fish. It is shown that the acceleration of fluids near the flapping tail contributes to the generation of the thrust for propulsion.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.4
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• pp.439-449
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• 2002

Experimental investigations of the flow structure and heat transfer enhancement in a channel with a built-in circular cylinder and a wing-let type vortex generator are presented. Without any vortex generators, relatively low heat transfer takes place in the downstream of the circular cylinder where is a recirculation region with low velocity fluid is formed. However with a wing-let type longitudinal vortex generator in the wake region behind the cylinder, heat transfer in the region can be enhanced. In order to control the strength of longitudinal vortices, the angle of attack of the vortex generators is varied from $20^{circ} to 45^{\circ}$, but spacings between the vortex generations are fixed to be 5 mm. The 3-dimensional mean velocity field downstream of the vortex generator is measured by a five-hole pressure probe, and the hue-capturing method using thermochromatic liquid crystals has been used to provide the local distribution of the heat transfer coefficient. The vorticity field and streamwise velocity contour are obtained from the velocity field. Streamwise distributions of averaged Stanton number on the measurement planes show very similar trends for all the experimental cases($\beta=20^{circ}, 30^{circ} and 45^{\circ}$). Circulation strength and heat transfer coefficient have the maximum values when the angle of attack($\beta$) is $30^{\circ}$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.8
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• pp.1191-1200
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• 2002

The measurements of velocity vectors are made in the near wake(X/d=5.0) of a circular cylinder with serrated fins. Velocity of fluid which flow through fins decreases as increasing fin height and freestream velocity and decreasing fin pitch. Therefore the velocity distribution at X/d=0.0 has lower gradient with increasing freestream velocity and fin height and decreasing fin pitch. The discontinuity of the streamwise velocity gradient is observed near the fin edge and causes significant changes in V-component velocity distribution in the near wake. This change attributes to the differences in Strouhal number and entraintment flow behavior. Increased turbulent intensity around a circular cylinder due to the serrated fins and entrainment flow are important factors for the recovery of velocity defect. The widths of velocity and turbulent intensity distribution of fin tubes are wider than those of a circular cylinder. The normalized velocity and turbulent intensity distributions with a hydraulic diameter which is proposed in this paper are in closer agreement with those of a circular cylinder.

• Journal of Energy Engineering
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• v.15 no.1 s.45
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• pp.28-34
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• 2006

The convective heat transfer on the outer surface of tube arrays perpendicular to the flow direction was experimentally investigated. The test sections which include the aligned and staggered arrangements were made and the local heat transfer coefficients on the outer surface of the tube were measured after the Hour has been fully developed. The results showed that the local heat transfer coefficients of the staggered arrangement, which has transverse pitch of 0.075 m and longitudinal pitch of 0.08 m, were about 15% greater than that of the aligned arrangement. Also, the overall mom Nusselt number of the former was greater thu that of the latter.

• 한국전산유체공학회:학술대회논문집
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• 2001.05a
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• pp.181-188
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• 2001

The flow past a circular cylinder forced to vibrate transversely is numerically simulated by solving the two-dimensional Wavier-Stokes equations modified by the vibration velocity of a circular cylinder at a Reynolds number of 164. The higher-order finite difference scheme is employed for the spatial discretization along with the second order Adams-Bashforth and the first order backward-Euler time integration. The calculated cylinder vibration frequency is between 0.60 and 1.30 times of the natural vortex-shedding frequency. The calculated oscillation amplitude extends to $25\%$ of the cylinder diameter and in the case of the lock-in region it is $60\%$. It is made clear that the cylinder oscillation has influence on the wake pattern, the time histories of the drag and lift forces, power spectral density and phase diagrams, etc. It is found that these results include both the periodic (lock-in) and the quasi-periodic (non-lock-in) state. The vortex shedding frequency equals the driving frequency in the lock-in region but is independent in the non-lock-in region. The mean drag and the maximum lift coefficient increase with the increase of the forcing amplitude in the lock-in state. The lock-in boundaries are also established from the present direct numerical simulation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.12
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• pp.3135-3147
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• 1993

A numerical method is presented which can solve the steady flow and heat transfer from a rotating and heated circular cylinder in a uniform flow for a range of Reynolds number form 5 to 100. The steady response of the flow and heat transfer is simulated for various spin parameter. The effects on the flow field and heat transfer characteristics known as lift, drag and heat transfer coefficient are analyzed and the streamlines, velocity vectors, vorticity, temperature distributions around it were scrutinized numerically. As spin parameter increases the region of separation vortex becomes smaller than upper one and the lower region will vanish. The lift force, a large part is due to the pressure force, increases as the Reynolds number and it increases linearly as spin parameter increases. The pressure coefficient changes rapidly with spin parameter on the lower surface of the cylinder and the vorticity is sensitive to the spin parameter near separation region. As spin parameter increases the maximum heat coefficient and the thin thermal layer on front region are moved to direction of rotation. However, with balance between the local increase and decrease, the overal heat transfer coefficient is almost unaffected by rotation.

• Journal of the Society of Naval Architects of Korea
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• v.36 no.1
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• pp.37-46
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• 1999

The influence of the tip clearance upon the cavitation inception were investigated by experiments for ducted rotors having different tip clearances The axial and tangential mean velocities around the ducted rotors were measured using an L.D.V. system to investigate the correlation between tip vertex cavitation inception and hub vortex cavitation. Observation results for tip vortex cavitation and hub vortex cavitation show good agreement in trend with the analysis results of velocity measurement. An optimum tip clearance for ducted rotor was selected to delay the tip vortex cavitation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1300-1307
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• 1995

The effect of thermal stratification on the turbulent dispersion from a fine cylindrical heat source was experimentally examined in a wind tunnel with and without a strong temperature gradient. A 0.5 mm dia. nichrome wire was used as a line heat source. Turbulent intensities, r.m.s. value of temperature and convective heat fluxes were measured by using a hot-wire and cold-wire combination probe. The results show that the peack value and the spread of the vertical turbulent intensity for the stratified case are far lower than those in the neutral case, which indicates that the stable temperature gradient suppresses the vertical velocity component. All of the third order moments including heat fluxes measured in the stable condition have very small values than those of the neutral case. This nature suggests that the decrease of scalar fluctuations in the stably stratified flow is mainly due to the suppression ofthe turbulent diffusion processes by the stable stratification. A simple gradient model with a composite timescale which has a simple weighted algebraic mean between dynamic and thermal time scale yields reasonably good numerical values in comparison with the experimental data.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.5
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• pp.691-697
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• 2008

Heat exchangers are commonly used in practice in a wide range of application, from heat and air-conditioning system in a household, to chemical processing and power production in large plant. An Experimental study was conducted to investigated the fluid flow and heat transfer around four circular cylinders of in-line in a cross flow of air. The local and average heat transfer characteristics for tube banks are investigated in the present study. Heat transfer in a heat exchanger usually involves convection in each fluid and conduction through the wall separating the two fluid. The in-line pitch ratio was in the range $1.5{\leq}L/d{\leq}4.0$, where L is the center distance and d the cylinder diameter, and in the Reynolds number $8,000{\leq}Re{\leq}50,000$. The local and mean Nusselt numbers were estimated. Subsequently, the heat transfer characteristics of four circular cylinders are found to exhibit a strong dependency upon the separation point of their upstream cylinders.