• Journal of the Korean Society for Aviation and Aeronautics
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• v.14 no.4
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• pp.1-10
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• 2006

A computational study was carried out in order to investigate the ground effect of a circular cylinder in the presence of a moving wall at a Reynolds number of 2.0${\times}$104. The viscous-incompressible Navier-Stokes equations and Spalart-Allmaras turbulent model of the commercial CFD code were adopted for this numerical analysis. The moving wall was set parallel with the freestream, and the speed of motion was equal to the freestream velocity. The gap ratio is defined as the distance ratio between the circular cylinder diameter and the height from the moving wall. The numerical results show that there are the differences among the each of the stages in evidence of the vorticity contours and the polar diagrams of $C_l$ vs. $C_d$. The 4 stages of the gap ratio are defined according to the flow features, whose stages are divided into small, intermediate, large and convergence gap ratios, respectively.

• Journal of the Korean Institute of Telematics and Electronics B
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• v.33B no.2
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• pp.56-64
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• 1996

In the previous paper of this study, an incoherent intensity pattern of a conducting cylinder is obtained by averaging out the multi-frequency intensities of the time-harmonic fields scattered from this cylinder. in this paper, an incoherent imaging of a conducting cylinder is obtained via the back-projection of the incoherent intensity patterns calculated from the circular rotational measurement configuration. This imaging method is vlaidated by imaging a conducting circular and a conducting elliptic cylinder and the measurement conditions to get better image are investigated.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.11
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• pp.717-726
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• 2016

This study evaluated the effect of aspect ratio of an enclosure with a heated inner circular cylinder on three-dimensional natural convection. The immersed boundary method was used to model the inner circular cylinder based on the finite volume method. The Rayleigh number was varied between $10^5$ and $10^6$, and the Prandtl number was maintained at 0.7. The aspect ratio of the three-dimensional enclosure was changed in steps of 1 within a range of 1-4 by increasing the width of the enclosure. In this study, the flow and thermal fields in the enclosure reached the steady state, and showed a mirror-symmetric pattern with respect to the center plane (x=0). In addition, the surface-averaged Nusselt number of the inner circular cylinder increased, while the total surface-averaged Nusselt number of the enclosure walls decreased with increase in the aspect ratio of the enclosure.

• International Journal of Naval Architecture and Ocean Engineering
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• v.6 no.3
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• pp.529-561
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• 2014

The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.3
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• pp.346-359
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• 1998

The aerodynamic forces and wake structure of the non-rotating downstream circular cylinder, of which the uniform freestream flow is interfered with another spinning upstream cylinder having the same diameter that is located upstream in a line have been investigated experimentally. When the spin rate of the downstream cylinder defined as the ratio of tangential surface velocity of the spinning cylinder to the freestream velocity increases gradually from zero to 1.4, the change of surface pressure distribution, aerodynamic forces of the non-rotating downstream cylinder were measured in case of several distance ratios of 1.5, 3.0, and 4.5 defined as the ratio of distance between the centers of two cylinders to the diameter. The wake flow patterns behind the cylinder were also investigated in each case. From the present experiments, it has been found that the spin rate significantly influences the aerodynamic forces and near-wake flow phenomena of the downstream cylinder in such a way that the drag increases as the spin rate and distance ratio increase and the wake width increases as the distance ratio increases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.4
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• pp.325-333
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• 2015

This study was devoted to elucidating the change in the flow characteristics of a laminar flow past a circular cylinder by modifying the cylinder shape with O-grooves. A numerical analysis was performed in a two-dimensional framework. The cylinder was represented using an immersed boundary method and marker particles on a Cartesian grid system. The number and locations of the O-grooves were the key parameters. An analysis of the flow pattern and flow-induced forces was performed at Re = 40 and 50. In addition, we calculated the critical Reynolds number depending on the number of O-grooves and their locations.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.8
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• pp.2601-2610
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• 1996

Surface protrusions have been attached on a cylinder surface to reduce the flow-induced structural vibration by controlling the wake flow. Wind tunnel tests on the near wake of a circular cylinder with surface protrusions were carried out to investigate the flow characteristics of the controlled wake. Three experimental models were used in this experiment; one plain cylinder of diameter D and two cylinders wrapped helically by three small wires of diameter d=0.075D with pitches of 5D and 10D, respectively. Free stream velocity was ranged to have Reynolds number from 5000 to 50,000. Streamwise and vertical velocity components of the wake were measured by a hot-wire anemometry. The spanwise velocity component measured by a one-component fiber optic LDV revealed that time-averaged wake field has a nearly two-dimensional structure. It was found that the surface protrusions elongate the vortex formation region, which decrease the vortex shedding frequency. The suppression of vortices caused by the surface protrusions increases the velocity deficit in the center of wake region.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.7
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• pp.915-922
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• 2001

A new phase-average technique using instantaneous velocity fields obtained by a PIV method has been developed. The technique tracks vorticity centers and estimates the value of circulation for a chosen domain. The locations of vortex-centers and the magnitudes of circulation are matched together then showing a sine wave feature due to the periodic vortex shedding from the circular cylinder. Ensemble averaged and phase averaged velocity fields are successfully measured for the circular cylinder wake where Reynolds number is 3900 based on free stream velocity and cylinder diameter. The convection velocities of the vortices center and the vortex shedding frequency were measured by a single hot-wire probe.

• Transactions of the Korean Society of Mechanical Engineers
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• v.9 no.5
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• pp.555-563
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• 1985

Two-dimensional turbulent wake flow behind a circular cylinder is investigated experimentally by suing linearized constant temperature hot wire anemometer. Turbulent fluctuations and mean velocity defects are measured in the rage of 5 dia.- 500 dia. downstream from the cylinder and for the Reynolds numbers of 2000-4000. Results with statistical treatment and digital data processing techniques are as follows: (1) The transition region from near wake to far wake is 30 dia. - 50 dia. downstream from the cylinder. (2) In the near wake, it is found that strong periodic ( f=845Hz) coherent structure exists. (3) It shows that the inertial subrange is 180Hz-2000Hz in self preservation region.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.804-809
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• 2003

In this study, 2D computations of the Aeolian tones for some obstacles (circular cylinder, square cylinder and NACA0012 airfoil) are simulated. First of all, we calculate the flow noise generated by a uniform flow around a two-dimensional circular cylinder at Re=150 are simulated by applying the finite difference lattice Boltzmann method (FDLBM). The third-order-accurate up-wind scheme (UTOPIA) is used for the spatial derivatives, and the second-order-accurate Runge-Kutta scheme is applied for the time marching. The results show that we successively capture very small acoustic pressure fluctuation with the same frequency of the Karman vortex street compared with the pressure fluctuation around a circular cylinder. The propagation velocity of the acoustic waves shows that the points of peak pressure are biased upstream due to the Doppler effect in the uniform flow. For the downstream, on the other hand, it is faster. To investigate the effect of the lattice dependence, furthermore, simulations of the Aeolian tones at the low Reynolds number radiated by a square cylinder and a NACA0012 airfoil with a blunt trailing edge at high incidence are also investigated.