• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.413-418
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• 2000

The noise sources from a rotating cylinder were identified to describe the blunt trailing edge noise. Firstly, LES formulation was applied to a non-orthogonal grid system and was tested with three-dimensional cross-flow over a cylinder with a yaw angle. The computed far-field noise showed peaks at Strouhal numbers ranging from 0.135 to 0.165 for the yawed cylinder flow with end-plates placed at both extremes under the yaw angle of $30^{\circ}$ and Reynolds number of $1.15{\times}10^4$. It was observed that the slantwise shedding at angles other than the cylinder yaw angle is intrinsic to inclined cylinder, with the result of shedding angles between $15^{\circ}$ and $31^{\circ}$. To study the trailing-edge wake thickness and unsteady lift-coefficient distribution in the span-wise direction of a rotating fan blade, the flows around rotating cylinder with 1,000 rpm were simulated and the far-field noise was exactly computed using the Ffowcs-Williams and Hawkings equation with quadrupole source term. The incoming velocities and stagnant zones were continuously distributed along the cylinder, and their changes made the Strouhal sheddings to occur at different phases even at almost same Strouhal number.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.117-120
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• 2008

Numerical simulations and experiment of a hydrodynamic and thermally developed turbulent flow through square ducts (3.0 ${\times}$ 3.0 cm) with twisted tape inserts and with twisted tape inserts plus interrupted ribs are conducted to investigate regionally averaged heat transfer and friction factors. Turbulent swirl flows having Reynolds numbers ranging from 8,900 to 29,000, a rib height-to-channel hydraulic diameter(e/D$_h$) of 0.067, and a length-to-hydraulic diameter(L/D$_h$) of 30, are considered. The square ribs are arranged to follow the trace of the twisted tape and along the flow direction defined as axial interrupted ribs. The twisted tape has 0.1 mm thick carbon steel sheet with diameter of 2.8 cm, length of 90 cm, and 2.5 turns. Each wall is composed of isolated aluminum sections, and two cases of surface heating are set. The results show that uneven surface heating enhances the heat transfer coefficient over uniform heating conditions, and square ducts with twisted tape inserts plus interrupted ribs produces the best overall transfer performance.

• Journal of computational fluids engineering
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• v.22 no.1
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• pp.51-58
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• 2017

A penalized VIC method offers an efficient hybrid particle-mesh algorithm to simulate an incompressible viscous flow passing a solid body in an infinite domain. In this manner, the computational domain needs to be restricted to a relatively small region to reduce computational cost which would be very high in case of using a large domain. In this paper, we present how to dispose of far-field particles to avoid an unnecessarily large computational domain. The present approach constraints expansion of the domain and thus prevents the incremental computational cost. To validate the numerical approach, a flow around an impulsively started sphere was simulated for Reynolds numbers of 100 and 1000.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.7
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• pp.927-936
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• 1998

In the fully developed pipe flow, when jet is injected in cross to the flow there are complex transition flows caused by interaction of the cross flow and jet. These interactions are studied by means of the flow visualization methods and frequency analysis using a hot-wire anemometer. The velocity range of cross flow of the pipe is 0.3 m/s ~ 1.2 m/s and the corresponding Reynolds number, R$\sub$p/, based on the pipe diameter is 2.25 * 10$\^$3/ ~ 9.02 * 10$\^$3/. The velocity ratio (R), jet velocity/cross flow velocity, is chosen from 2 to 10. A circular cylinder is placed in the pipe instead of jet to observe the vortex shedding from the solid body. To compare the jet and circular cylinder flow, the vortical structure is analyzed in both cases and the structure of vortices and the origin of its formation are investigated, especially. The vortex shedding of the dominant coherent structure is compared between the jet flow and the circular cylinder flow. In the case of the jet flow, the Strouhal numbers are different depending on the existence of the upright vortex as well as the velocity ratio (R).

• Journal of Advanced Marine Engineering and Technology
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• v.38 no.10
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• pp.1257-1262
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• 2014

The variances of atomization characteristics with the misalignment of injectors defined as the fraction of skewness for various angles of impingement and pressure conditions were studied using the doublet impinging injectors with a like-on-like arrangement. Water was used as simulant and the spray characteristics along with the changes in the skewness were analyzed using the methods of spray image photography. Experiment was carried for the impinging nozzles of orifice diameter of 1.2 mm within Reynolds numbers ranging from $9{\times}10^3-4.5{\times}10^4$ and the fraction of skewness considered for the experiment ranges from 0.0 to 0.9 at ambient temperature condition. Flat sheet with a distinct rim produced perpendicular to the plane of impinging jets goes ondisappear and sheet appears comparatively shorterwith the increase in fraction of skewness resulting the atomization of fluid droplet very close to impingement point with decrease in breakup length and increase in spray angle up to certain extent. The maximum allowable skewness was found as the result. The skewness up to the certain extent can be considered as the parameter to control the atomization characteristics of simulant inside the combustion chamberproviding the high economic performance of fuel and time.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.8
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• pp.1094-1099
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• 2010

Three dimensional laminar flows are numerically simulated around the submerged spheres. The finite volume based Navier-Stokes equations with unstructured grids are solved to make clear the hydrodynamic phenomena near and far away from the body. Reviews are made on with the vorticity, velocity, dynamic pressure, residuals, drags, etc. The Reynolds numbers under consideration are 425, 300, 250 and 100. In case of small spacing between spheres, the flow is more stable than that past a single sphere. According to the analysis, the flow past two spheres is found to be unstable as the spacing becomes larger. The rear sphere shows the deduction of stagnation pressure, which results in the decrease of the resistance. The predicted drag coefficients verify that the present numerical calculation is reasonable.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.6
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• pp.766-772
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• 2003

The pseudospectral method for stability analysis was used to find the most influential disturbance mode for transition of plane channel flows and Blasius flow at their critical Reynolds numbers. A number of various oblique disturbance waves were investigated for their pseudospectra and resolvent norm contours in each flow, and an exhaustive search method was employed to find the disturbing waves to which the flows become most unstable. In plane Poiseuille flow an oblique disturbance with a wavelength of 3.59h (where h is the half channel width) at an angle $28.7^{\circ}$ was found to be the most influential for the flow transition to turbulence, and in plane Couette flow it is an oblique wave with a wavelength of 3.49h at an angle of $19.4^{\circ}$. But in Blasius flow it was found that the most influential mode is a normal wave with a wavelength of $3.44{\delta}_{999}$. These results imply that the most influential disturbance mode is closely related to the fundamental acoustic wave with a certain shear sheltering in the respective flow geometry.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.10
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• pp.876-883
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• 2007

Numerical simulations are conducted for the design of a micro thermal mass air flow sensor (MAFS), which consists of a microfabricated heater and thermopiles on the silicon-nitride ($Si_3N_4$) thin membrane structure. It is important to find the proper locations of these thermal elements in the design of MAFS with improved sensitivity. Three heating modes of the micro-heater are considered: constant temperature, constant power and heating pulses. The analyses are focused on the membrane temperature profile near the sensing section. Considered are the practical flow velocities, ranging from 3 m/s to 35 m/s, and the corresponding Reynolds numbers from 1000 to 10000. The results show that one of optimum sensing locations is about $100{\mu}m$ away from the microheater. It is concluded that the heating mode and configurations of thermal elements are the main factors for the MAFS with higher sensitivity.

• Journal of computational fluids engineering
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• v.10 no.2
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• pp.30-37
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• 2005

To evaluate LBM we performed the simulation of the unsteady two dimensional flow over a square cylinder in a channel in moderate Reynolds number range, $100\~500$ by using LBM and Fractional-Step method. Frist of all we compared LBM solution of Poiseuille flow applied Farout and periodic boundary conditions with the analytical solution to verify the applicability of the boundary conditions. For LBM simulation the calculation domain was formed by structured 500x100 grids. Prescribed maximum velocity and density inlet and Farout boundary conditions were imposed on the in-out boundaries. Bounceback boundary condition was applied to the channel and the cylinder waifs. The flow patterns and vortex shedding strouhal numbers were compared with previous research results. The flow patterns by LBM were in agreement with the flow pattern by fractional step method. Furthermore the strouhal number computed by LBM simulation result was more accurate than that of fractional step method through the comparison of the previous research results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.3
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• pp.439-447
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• 2002

The microchannel flow in miniature TCDs (thermal conductivity detectors) is investigated numerically. The solutions based on the boundary layer approximation are not very accurate in the region of the duct inlet for low Reynolds numbers. In this study, two-dimensional Navier-Stokes equations are considered to analyze the gas flow in a miniature TCD. Effects of channel size, inlet and boundary conditions on the heat transfer rate are examined. When the gas stream is not preheated, the distances for a miniature TCD to reach the conduction-dominant region for duct flow are found to be approximately two and three times the thermal entry length for duct flow with constant properties, respectively, leer constant wall temperature and constant wall heat flux boundary conditions. If the gas temperature at the channel inlet is close to the mean gas temperature in the conduction-dominant region, the entrance region is much shorter compared to other cases considered in this study.