• 유체기계공업학회:학술대회논문집
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• 2000.12a
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• pp.122-128
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• 2000

Aerodynamic optimization of an automotive air-conditioning blower is a hard task because of the highly complex flow phenomena related to three-dimensional flow separations and the unsteady nature caused by the interaction between primary and secondary air flows throughout the fan. In this paper, an aerodynamic study on a forward-curved centrifugal fan has been carried out Firstly we obtained the fan performance curves versus flow rates showing its unstable nature in the surging operation range. Secondly aerodynamic characterizations were carried out by investigating the velocity and pressure fields in the casing flow passage using a 5-hole pilot probe, at different operating conditions. Surface flow pattern near the cut-off area exhibits similar flow behavior above the best efficiency operating point, although the pressure level increases substantially with the Increase of flow rate. Vorticity in the casing passage flow occurs in all (low rates, downstream from the r-Z plane $\theta$=120 deg., where the position of its core changes with the circumferential location. Although complex, the general flow behavior were common, giving insight in its main aerodynamic features.

• The KSFM Journal of Fluid Machinery
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• v.14 no.5
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• pp.18-23
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• 2011

A CFD analysis method is developed and applied for investigating the gas flow and the byproduct particle trajectory in Roots type vacuum pump. The internal fluid flow and thermal fields between the rotors and the housing of vacuum pump are analyzed by using the dynamic mesh, the numerical methods for unsteady 2-D Navier-Stokes equation and the standard k-$\varepsilon$ turbulence model of the Fluent code. Coupled with the flow simulation results, the particle trajectory of the byproduct flowing into the pump with gas stream is analyzed by using discrete phase modeling technique. The CFD analysis results show the pressure, the velocity and the temperature distributions in pump change abruptly due to the rotation of rotors, and back flows are produced due to the strong reverse pressure gradients at rotor/rotor and rotor/housing clearances. The predicted byproduct particle trajectory results also show the particles impinge on the clearance surfaces between the housing and the rotor of pump and then may form the deposit layer causing the failure of pump.

• 한국전산유체공학회:학술대회논문집
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• 1995.10a
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• pp.55-63
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• 1995

The flow field around a high-speed train including cross-wind effects has been simulated. This study solves 3-D unsteady incompressible Navier-Stokes equations in the inertial frame using the iterative time marching scheme. The governing equations are differenced with 1st-order accurate backward difference scheme for the time derivatives, 3th-order accurate QUICK scheme for the convective terms and 2nd-order accurate central difference scheme for the viscous terms. The Marker-and-Cell concept was applied to efficiently solve continuity equation, which is differenced with 2nd-order accurate central difference scheme. The 4th-order artificial damping is added to the continuity equation for numerical stability. A C-H type of elliptic grid system is generated around a high-speed train including ground. The Baldwin-Lomax turbulent model was implemented to simulate the turbulent flows. To validate the present procedure, the flow around a high speed train at constant yaw angle of $45^{\circ}\;and\;90^{\circ}$ has been simulated. The simulation shows 3-D vortex generation in the lee corner. The flow separation is also observed around the rear of the train. It has concluded that the results of present study properly agree with physical flow phenomena.

• Journal of computational fluids engineering
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• v.11 no.4 s.35
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• pp.1-8
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• 2006

Clocking effects of a stator on the performance and internal flow in an UTRC 1.5 stage axial turbine are investigated using a three-dimensional unsteady flow simulation. Six relative positions of two rows of stator are investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% is obtained depending on the clocking positions. However, internal flows have some different characteristics from that in the previous study at the best and worst efficiency positions, since the first stator wake is mixed out with the rotor wake before arriving at the leading edge of the second stator. Instead of the first stator wake, it is found that the wake interaction of the first stator and rotor has a important role on a relative efficiency variation at each clocking position. The time-averaged local efficiency along the span at the maximum efficiency is more uniform than that at the minimum efficiency. That is, the spanwise efficiency distribution at the minimum efficiency has larger values in mid-span but smaller values near the hub and casing in comparison to those at the maximum efficiency. Moreover, the difference between maximum and minimum instantaneous efficiencies during one period is found to be smaller at the maximum efficiency than at the minimum efficiency.

• Journal of computational fluids engineering
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• v.17 no.1
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• pp.94-100
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• 2012

In this paper, the cavitating flows around a hydrofoil have been numerically investigated by using a 2-d multi-phase RANS flow solver based on pseudo-compressibility and a homogeneous mixture model on unstructured meshes. For this purpose, a vertex-centered finite-volume method was utilized in conjunction with 2nd-order Roe's FDS to discretize the inviscid fluxes. The viscous fluxes were computed based on central differencing. The Spalart-Allmaras one equation model was employed for the closure of turbulence. A dual-time stepping method and the Gauss-Seidel iteration were used for unsteady time integration. The phase change rate between the liquid and vapor phases was determined by Merkle's cavitation model based on the difference between local and vapor pressure. Steady state calculations were made for the modified NACA66 hydrofoil at several flow conditions. Good agreements were obtained between the present results and the experiment for the pressure coefficient on a hydrofoil surface. Additional calculation was made for cloud cavitation around the hydrofoil. The observation of the vapor structure, such as cavity size and shape, was made, and the flow characteristics around the cavity were analyzed. Good agreements were obtained between the present results and the experiment for the frequency and the Strouhal number of cavity oscillation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.5 s.236
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• pp.609-616
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• 2005

While the SIMPLE algorithm is most widely used for the simulations of flow phenomena that take place in the industrial equipments or the manufacturing processes, it is less adopted for the simulations of the free surface flow. Though the SIMPLE algorithm is free from the limitation of time step, the free surface behavior imposes the restriction on the time step. As a result, the explicit schemes are faster than the implicit scheme in terms of computation time when the same time step is applied to, since the implicit scheme includes the numerical method to solve the simultaneous equations in its procedure. If the computation time of SIMPLE algorithm can be reduced when it is applied to the unsteady free surface flow problems, the calculation can be carried out in the more stable way and, in the design process, the process variables can be controlled based on the more accurate data base. In this study, a modified SIMPLE algorithm is presented fur the free surface flow. The broken water column problem is adopted for the validation of the modified algorithm (MoSIMPLE) and for comparison to the conventional SIMPLE algorithm.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.11
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• pp.1538-1551
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• 1997

Kinetic energy conservation for fixed and moving grids is examined in time-accurate finite element computation of fully unsteady inviscid flows. As numerical algorithms, fractional step method (FSM) and modified SIMPLE are used. To simulate the flow in moving grid system, arbitrary Lagrangian-Eulerian (ALE) method is adopted. In the present study, the energy conserving time integration rule for finite element algorithm is proposed and discussed schematically. It is shown that the discretization by Crank-Nicolson in time and Galerkin (central difference) in space must be used to ensure energy conservation. The developed code has been tested for a standing vortex in fixed or moving grid system, sloshing in a tank and propagation of a solitary wave, and has been shown to be a completely energy conserving algorithm.

• Journal of Advanced Marine Engineering and Technology
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• v.23 no.6
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• pp.770-777
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• 1999

In this study a phase diagram has been used to investigate the unsteadiness of two-dimensional lid-driven closed flows within a square cavity for twelve Reynolds numbers; $7.5{\times}10^3,\; 8{\times}10^3,\; 8.5{\times}10^3,\; 9{\times}10^3,\; 9.5{\times}10^3,\; 10^4,\;1.5{\times}10^4,\;2{\times}10^4,\; 3{\times}10^4,\; 7.5{\times}10^4$ and $10^5$. The results indicate that the first critical Reynolds number at which the flow unsteadiness of sinusoidal fluctuation appears from the temporal variation of total kinetic energy curves is assumed of sinusoidal fluctuation appears form the temporal variation of total kinetic energy curves is assumed to be in the neigh-bourhood of $Re=8.5{\times}10^3$ The second critical Reynolds number where the periodic amplitude and frequency collapse to random disturbance being existed around $Re=1.5{\times}10^4$ The exponentially decreasing vortices formed at the lower two corners are found commonly at the time-mean flow pattern of $Re=3{\times}10^4$.

• Journal of ILASS-Korea
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• v.5 no.1
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• pp.13-22
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• 2000

The present study has numerically analyzed the vaporization characteristics of fuel droplets in the high temperature convective flow field. The axisymmetric governing equations for mass, momentum, energy, and species are solved by an iterative and implicite unstructured finite-volume method. The moving boundary due to vaporization is handled by the deformable unstructured grid technique. The pressure-velocity coupling in the density-variable flows is treated by the SIMPLEC algorithm. In terms of the matrix solver, Bi-CGSTAB is employed for the numerically efficient and stable convergence. The n-decane is used as a liquid fuel and the initial droplet temperature is 300K. Computations are performed for the nonevaporating and evaporating droplets with the relative interphase velocity(25m/s). The unsteady vaporization process has been simulated up to the nondimensional time, 25. Numerical results indicate that the mathematical model developed in this study succesfully simulates the main features of the droplet vaporization process in the convective environment.

• Journal of the Society of Naval Architects of Korea
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• v.53 no.4
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• pp.266-274
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• 2016

Many researchers have been trying to improve the propulsion efficiency of a propeller. In this study, the numerical analysis is carried out for the POW(Propeller Open Water test) performance of a propeller equipped with an energy saving device called PHVC(Propeller Hub Vortex Control). PHVC is aimed to control the propeller hub vortex behind the propeller so that the rotational kinetic energy loss can be reduced. The unsteady Reynolds Averaged Navier-Stokes(URANS) equations are assumed as the governing flow equations and are solved by using a commercial CFD(Computational Fluid Dynamics) software, where SST k-ω model is selected for turbulence closure. The computed characteristic values, thrust, torque and propulsion efficiency coefficients for the target propeller with and without PHVC and the local flows in the propeller wake region are validated by the model test results of KRISO LCT(Large Cavitation Tunnel). It is concluded from the present numerical results that CFD can be a good promising method in the assessment of the hydrodynamic performance of PHVC in the design stage.