• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.9
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• pp.659-668
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• 2008

Combustion instability has been considered as very important issue for developing gas turbine and rocket engine. There is a need for fundamental understanding of combustion instability. In this study, combustion instability was numerically and experimentally investigated in a dump combustor with bluff body. The fuel and air mixture had overall equivalence ratio of 0.9 and was injected toward dump combustor. The pressure oscillation with approximately 256Hz was experimentally obtained. For numerical simulation, the standard k-$\varepsilon$ model was used for turbulence and the hybrid combustion model (eddy dissipation model and kinetically controlled model) was applied. After calculating steady solution, unsteady calculation was performed with forcing small perturbation on initial that solution. Pressure amplitude and frequency measured by pressure sensor is nearly the same as those predicted by numerical simulation. Furthermore, it is clear that a combustion instability involving vortex shedding is affected by acoustic-vortex-combustion interaction. The phase difference between the pressure and velocity is $\pi$/2, and that between the pressure and heat release rate is in excitation range described by Rayleigh, which is obvious that combustion instability for the bluff body combustor meets thermoacoustic instability criterion.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.2
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• pp.12-20
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• 1998

Exter ballistics of a typical high-speed projectile is studied through a flow-visualization experiment and an unstructured grid Navier-Srokes computation. Experiment produced a schlieren photograph that adequately shows the characteristic features of this complex flow, namely two kinds of oblique cone shocks and turbulent wake developing into the downstream. A hybrid scheme of finite volume-element method is used to simulate the compressible Reynolds-Averaged Navier-Stok- es solution on unstructured grids. Osher's approximate Riemann solver is used to discretize the cinvection term. Higher-order spatial accuracy is obtained by MUSCL extension and van Albada ty- pe flux limiter is used to stabilize the numerical oscillation near the solution discontinuity. Accurate Gakerkin method is used to discretize the viscous term. Explict fourth-order Runge-Kutta method is used for the time-stepping, which simplifies the application of MUSCL extension. A two-layer k-$\varepsilon$ turbulence model is used to simulate the turbulent wakes accurately. Axisymmetric folw and two-dimensional flow with an angle of attack have been computed. Grid-dependency is also checked by carrying out the computation with doubled meshes. 2-D calculation shows that effect of angle of attack on the flow field is negligible. Axi-symmetric results of the computation agrees well with the flow visualization. Primary oblique shock is represented within 2-3 meshes in numerical results, and the varicose mode of the vortex shedding is clearly captured in the turbulent wake region.

• Journal of Mechanical Science and Technology
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• v.18 no.4
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• pp.597-605
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• 2004

Turbulent temperature field in a channel subject to strong wall injection has been investigated via direct numerical simulation technique. These flows are pertinent to internal flows inside hybrid rocket motors. A simplified model problem where a regression process at the propellant surface is idealized by wall injection has been investigated to understand how the temperature field is modified. The effect of strong wall injection displaces thermal boundary layer away from the wall and this causes a sharp drop of friction temperature. Turbulent diffusivity and dissipation time scale for temperature field are found to show large variations in the streamwise direction under application of wall blowing. It is, thus, expected that more sophisticated turbulence models would be required to predict the disturbed temperature field accurately.

• International Journal of Aeronautical and Space Sciences
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• v.8 no.2
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• pp.1-10
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• 2007

A two-dimensional backward facing step flow was comptuted using a Detached Eddy simulation(DES) based on the SST turbulence model. The expansion ratio(ER) was 1.125 and the Reynolds number based on the step height and the mean velocity in the upstream channel was 37,500. The flow condition was the same as with the experimental research[1]. The reattachment length, oscillatory characteristics of the flow and the coherent structures of the present simulation were compared to demonstrate the improtance of spanwise grid spacing.

• The KSFM Journal of Fluid Machinery
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• v.14 no.5
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• pp.31-38
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• 2011

Optimization using a hybrid multi-objective evolutionary algorithm coupled with response surface approximation has been performed to improve the performance of a transonic axial compressor with circumferential casing grooves. In order to optimize the operating stability and peak adiabatic efficiency of the compressor with circumferential casing grooves, tip clearance, angle distribution at blade tip and the depth of the circumferential casing grooves are selected as design variables. Three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model are discretized by finite volume approximations. The trade-off between two objectives with the interaction of blade and casing treatment is determined and discussed with respect to the representative clusters in the Pareto-optimal solutions compared to the axial compressor without the casing treatment.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
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• 2006.11a
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• pp.416-419
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• 2006

Numerical analysis is carried out to identify the wall thinning effect inside the feed water valve. The finite volume method is applied to make analysis for the viscous flows. The commercial cock FLUENT is used for the simulation and the GAMBIT for the grid generation. The RNG $\kappa-\varepsilon$ model is used for the turbulence and the tet-hybrid grid is applied for the modeling. The velocity vector, the pressure contour, the change of residual along the iteration number, and the dynamic head are predicted for the hydrodynamic investigation.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.25-28
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• 2008

An aerodynamic calculation in hover of KUH main rotor blade is performed using a three-dimensional unstructured hybrid mesh viscous flow solver. The flow solver utilizes a vertex-centered finite-volume scheme that is based on the Roe's flux-difference splitting with an implicit Jacobi/Gauss-Seidel time integration. The eddy viscosity are estimated by the Spalart-Allmaras one-equation turbulence model. A solution-adaptive mesh refinement technique is used for efficient capturing of the tip vortex. Calculations are performed at several operating conditions with varying collective pitch setting for KUH main rotor blade in hover. Good agreements are obtained between the present and other results using HOST and CAMRAD II in overall rotor performance. It is demonstrated that the present vertex-centered flow solver is an efficient and accurate tool for the assessment of rotor performance in hover.

• 한국전산유체공학회:학술대회논문집
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• 1997.10a
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• pp.73-80
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• 1997

Objective of the present study is to examine the convergence characteristics of the various multi-stage Runge-Kutta methods in solving the incompressible Navier-Stokes equations of a time-marching from casted by the artificial compressibility method. Convergence characteristics are examined over 2-stage, 4-stage and hybrid type (using 4-, 3-, 2-stages sequentially) Runge-Kutta methods for a laminar lid-driven cavity flow, and also for a turbulent bump channel flow using Chien's low-Reynolds number turbulence model. Efforts are made to establish a stable and fast convergent multi-stage Runge-Kutta method with minimal artificial dissipations.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2003.11a
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• pp.78-83
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• 2003

General algorithm is developed for the prediction of internal flow-induced noise. This algorithm is based on the integral formula derived by using the General Green Function, Lighthills acoustic analogy and Curls extension of Lighthills. Novel approach of this algorithm is that the integral formula is so arranged as to predict frequency-domain acoustic signal at any location in a duct by using unsteady flow data in space and time, which can be provided by the Computational Fluid Dynamics Techniques. This semi-analytic model is applied to the prediction of internal aerodynamic noise from a throttle valve in an automotive engine. The predicted noise levels from the throttle valve are compared with actual measurements. This illustrative computation shows that the current method permits generalized predictions of flow noise generated by bluff bodies and turbulence in flow ducts.

• Journal of Industrial Technology
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• v.23 no.A
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• pp.9-13
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• 2003

Numerical experiments are done by a commercial code, PHOENICS to evaluate the backlayer phenomenon of smoke in case of the road tunnel fire. The independent and dependent variables are ventilation air velocity and the length of backlayer of smoke respectively. Hybrid scheme and ${\kappa}-{\varepsilon}$ turbulence model are adopted in the simulation process and mass residual is used as a convergence criterion. The experimental results say that the length of backlayer is reduced with the increase of ventilating air velocity and that there is a critical air velocity which prevents from the onset of backlayering phenomena. One finds that there is a fresh air region near the bottom of tunnel which could make the passenger escape safely from the polluted region by smoke. These phenomena come from the vertical stratification of the smoke air mixture in the tunnel.