• Ocean Systems Engineering
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• v.4 no.3
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• pp.185-200
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• 2014

The objective of the present simulations is to evaluate the applicability of the standard $k-{\varepsilon}$ turbulence model in engineering practice in the subcritical to supercritical flow regimes. Two-dimensional numerical simulations of flow around a circular cylinder at $Re=1{\times}10^5$, $5{\times}10^5$ and $1{\times}10^6$, had been performed using Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with the standard $k-{\varepsilon}$ turbulence model. Solution verification had been studied by evaluating grid and time step size convergence. For each Reynolds number, several meshes with different grid and time step size resolutions were chosen to calculate the hydrodynamic quantities such as the time-averaged drag coefficient, root-mean square value of lift coefficient, Strouhal number, the coefficient of pressure on the downstream point of the cylinder, the separation angle. By comparing the values of these quantities of adjacent grid or time step size resolutions, convergence study has been performed. Solution validation is obtained by comparing the converged results with published numerical and experimental data. The deviations of the values of present simulated quantities from those corresponding experimental data become smaller as Reynolds numbers increases from $1{\times}10^5$ to $1{\times}10^6$. This may show that the standard $k-{\varepsilon}$ model with enhanced wall treatment appears to be applicable for higher Reynolds number turbulence flow.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.11
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• pp.933-940
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• 2016

A comparative study between a wind tunnel test and an XFOIL simulation looking at the aerodynamic performance of the airfoil for MW-class wind turbine was conducted for validation in the design stage. Tests are carried out for 21% and 30% thickness-ratio airfoils developed for 5 ~ 10 MW offshore wind turbine and the results are compared with the output from the XFOIL simulation at Reynolds number $1.0{\times}10^7$. The test is performed at a free-stream velocity of 50 m/s, corresponding to a Reynolds number of $2.2{\times}10^6$ based on the chord. Surface roughness is simulated using a zig-zag tape. Discrepancies between the results of the test and the XFOIL analysis are found, however, meaningful data for surface pressure distribution, basic performance and surface roughness effect are obtained from the tests, while useful lift-to-drag ratio data is found by the XFOIL simulation.

• Wind and Structures
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• v.3 no.3
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• pp.209-220
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• 2000

The Strouhal number is an important nondimensional number which is explanatory of aerodynamic instability phenomena. It takes on the different characteristic constant value depending upon the cross-sectional shape of the body being enveloped by the flow. A number of investigations into this subject, especially on the drag test, surface pressure test and hot-wire test, have been carried out under the fixed state of the body in the past. However, almost no investigations concerning the determination of the St on wind-induced vibration of the body have been reported in the past even though the aerodynamic behavior of the body is very important because the construction of wind-sensitive structures is recently on the sharp increase. Based on a series of wind tunnel tests, this paper addresses a new method to determine the Strouhal number of rectangular cylinder in the uniform flow. The central idea of the proposed method is that the Strouhal number can be obtained directly by the aerodynamic behaviors of the body through wind-induced vibration test. The validity of proposed method is evaluated by comparing with the results obtained by previous studies in three B/Ds at attack angle $0^{\circ}$ and a square cylinder with various attack angles. The values and trends of the proposed Strouhal numbers are in good agreements with values of previous studies. And also, the Strouhal numbers of B/D=1.5 and 2.0 with various attack angles are obtained by the proposed method and verified by other method. This proposed method is as good as any other previous methods to obtain the Strouhal number.

• Journal of the Society of Naval Architects of Korea
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• v.36 no.3
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• pp.1-7
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• 1999

This paper describes the modification of hull bottom for the air lubrication technique to the passenger boat in service at the Chung-Ju lake, which has a large beam-draft ratio. From numerical analysis of 2-D cavity problem by potential theory, the cavity shape, length and the pressure in cavity are estimated for the simplified geometry of hull bottom, and the non-dimensional parameters affecting air cavity phenomena are investigated. Extensive resistance tests for the model ship which has variation of step height and side strip have been performed to investigate the formation of air cavity and the drag reduction effectiveness. And also, the development of attached cavity to the bottom were observed from the flat bottom made by transparent acrylic plate. From this survey on the modification of bottom shape and the air lubrication technique, the total resistance of model ship could be reduced by about 25% at the design speed compared to the proto type hull form.

• Journal of the Korean Society of Propulsion Engineers
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• v.12 no.2
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• pp.24-32
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• 2008

A non-reacting experimental study on a normal injection into a Mach 1.92 crossflow which flows over various geometries(flat plate, small cavity, large cavity) was carried out to investigate the effect of the momentum flux ratio(J). The aft ramp of the cavity advances the increase of the penetration height and the strong two-dimensional shock from recompression region mainly affects the shock structure and mixing layer at the downstream flow. As flow runs downward, the transverse penetration height increases with increasing J(J = 0.9, 1.7, 3.4). However, above some critical ratio, jet penetration height growth with increasing J is not appeared in flow-field. Large scale cavity has a good mixing efficiency but it increases the drag loss in the combustor.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.45 no.3
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• pp.241-251
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• 2017

Computational fluid dynamics analysis was performed for the case 3 of the EFD-CFD workshop. Solvers were used for three commercial CFD codes(Star-CCM+, Fluent and CFX) and an open source CFD code(SU2). The grid were generated four types depending on the total cells using commercial grid generation code(Pointwise). Mach number of 0.4 and 0.8, 2 degree angle of attack and Mach number of 0.9, 1 degree angle of attack were calculated. Similar pressure coefficient curve and normal force coefficient were showed from the coarse grid to fine grid of four codes. But there is a difference in the drag coefficient. The position of the shock wave was predicted forward as the discretization order increased in calculations using Star-CCM+ and Fluent. The computation time to converge, Fluent, Star-CCM +, CFX are in order, and SU2 takes much time to converge.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.45 no.3
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• pp.259-267
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• 2017

Flow fields around a KARI-11-180 airfoil, SDM and transonic body are numerically simulated by using an unstructured meshes based compressible flow solver developed at KAIST. RANS equations are solved to analyse the flow fields and Roe's FDS method is adopted to evaluate convective fluxes. Turbulence effect of the flow fields is modeled by a SA model, SST model and ${\gamma}-{\widetilde{Re}}_{{\theta}t}$ model. It is found that smaller drag coefficients are predicted for the KARI-11-180 airfoil when a transition phenomenon is considered and small deviations exist between CFD and EFD results. For the SDM, flow separation is observed at a leading edge and calculated aerodynamic properties show similar tendencies to experimental results. A shock wave on main wings of the transonic body is successfully captured by the present flow solver at a Mach number 0.9. Estimated pressure profiles by means of the present CFD method also agree well with those of wind tunnel results.

• Journal of Advanced Marine Engineering and Technology
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• v.32 no.6
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• pp.869-876
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• 2008

A thermocompressor is the equipment which compresses a vapor to a desired discharge pressure. Since it was first used as the evacuation pump for a surface condenser, it has been widely adopted for energy saving systems due to its high working confidence. In the present study, the geometrical analysis of the shape between the jet nozzle and the diffuser inlet, the drag force was calculated by means of the integrated equation of motion and the computational fluid dynamic (CFD) package called FLUENT. The computer simulations were performed to investigate the effects by the various suction flow rates, the distance from jet nozzle outlet to the diffuser inlet and the dimensions of the diffuser inlet section through the iterative calculation. In addition, the results from the CFD analysis on the thermocompressor and the experiments were compared for the verification of the CFD results. In the case of a jet nozzle, the results from the CFD analysis showed a good agreement with the experimental results. Furthermore, in this study, a special attention was paid on the performance of the thermocompressor by varying the diffuser convergence angle of $0.0^{\circ}$, $0.5^{\circ}$, $1.0^{\circ}$, $2.0^{\circ}$, $3.5^{\circ}$ and $4.5^{\circ}$. With the increase of the diffuser convergence angle. the suction capacity was improved up to the degree of $1.0^{\circ}$ while it was decreased over the degree of $1.0^{\circ}$.

• Selected Papers of The Society of Naval Architects of Korea
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• v.1 no.1
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• pp.76-90
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• 1993

A new propeller series is developed using the newly developed blade section (KH 18 section) which has better cavitation characteristics and higher lift-drag ratio at wade angle-of-attack range than a conventional section. The radial patch distribution of the new series propellers is variable stance they were designed adaptively to a typical wake distribution. Basic geometric particulars of the series propellers. such as chord length, thickness, skew and rake distributions, are determined on the basis of recent full scale propeller geometric data. The series is developed for propellers having 4 blades, and blade area ratios of 0.3, 0.45, 0.6 and 0.75. Mean pitch ratios are varied as 0.5, 0.6, 0.7, 0.95 and 1.1 for each blade area ratio. The new propeller series consists of 20 propellers and is named as the KD(KRISO-DAEWOO)-propeller series. Propeller open-water tests are performed at the towing tank, and cavitation observation tests and fluctuating pressure tests are carried out at the cavitation tunnel of KRISO. $B_{p}-\delta$ curves, which can be used to select the optimum propeller diameter at the preliminary design stage, are derived from a regression analysis of the propeller open-water test results. The KD-cavitation chart is derived from the cavitation observation test results by choosing the local maximum lift coefficient and the local cavitation number as parameters. The cavity extent predicted by the KD-cavitation chart would be more accurate compared to that by an existing cavitation charts, such as the Burrll's cavitation chart, since the former is derived from the cavitation observation test results in a typical ship's wake, while the lather is derived from the test results in a uniform flow.

• Proceedings of KOSOMES biannual meeting
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• 2007.05a
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• pp.169-174
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• 2007

The purpose in having a control rod an a buoy system is to control the motion of it. The system may be composed entirely af a single circular cylinder finder and a lang mooring anchor cable. A control rod has one function to perform in meeting its purpose, and that is to develop a control force in consequence of its orientation and movement relative to the water. The forces and moments generated as a result of the effects of mutual interference, then determine the stability characteristics of the body. In this paper, the study of control-rod-attached buoy's 2-dimensional section was accomplished. model tests and numerical simulations had been carried out with different diameters of control rods. and varying the Reynolds number $Re=5,000{\sim}25,000$ based an the cylinder diameter(D=50mm) to. predict the performance af the body and the 2 frame particle tracking method had been used to obtain the velocity distribution in the flaw field 50mm circular cylinder had been used during the whale experiments and measured results had been compared with each other.