• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.1000-1002
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• 2017

Here, the unsteady Navier-Stokes solver has been developed using implicit dual time stepping method. The implicit dual time stepping method introduced the pseudo time step for solving the new residual including the steady state residual and real time derivative. For the validation of code, Stokes 2nd problem, the laminar flow on the oscillating flat plate was selected and compare the calculating results with analytic solutions. The calculating velocity profile and skin friction has a good agreement with analytic solutions.

• Journal of computational fluids engineering
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• v.16 no.1
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• pp.53-59
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• 2011

In this study, steady and unsteady aerodynamic analyses of a huge rocket configuration have been conducted in a transonic flow region. The launch vehicle structural response are coupled with the transonic flow state transitions at the nose of the payload fairing. Before performing the coupled fluid-structure transonic aeroealstic simulations transonic aerodynamic characteristics are investigated for the pitching motions of the rocket at finite angle-of-attack. An unsteady CFD analysis method with a moving grid technique based on the Reynolds-averaged Navier-Stokes equations with the k-w SST transition turbulence model is applied to accurately predict the transonic loads of the rocket at pitching motion. It is shown that the fluctuating amplitude of the lateral aerodynamic loads imposed on the rocket due to the pitching motion can be significantly increased in the transonic flow region.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.12
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• pp.1624-1634
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• 1997

A numerical study on two-dimensional unsteady transonic cascade flow has been performed by adopting dual time stepping and the k-.omega. turbulence model. An explicit 4 stage Runge-Kutta scheme for the compressible Navier-Stokes equations and an implicit Gauss-Seidel iteration scheme for the k-.omega. turbulence model are proposed for fictitious time stepping. This mixed time stepping scheme ensures the stability of numerical computation and exhibits a good convergence property with less computation time. Typical steady-state convergence accelerating schemes such as local time stepping, residual smoothing and multigrid combined with dual time stepping shows good convergence properties. Numerical results are presented for unsteady laminar flow past a cylinder and turbulent shock buffeting problem for bicircular arc cascade flow is discussed.

• Proceedings of the Korea Water Resources Association Conference
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• 2005.09a
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• pp.138-140
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• 2005

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.6
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• pp.1163-1170
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• 1992

A computer program was developed to analyze the two-dimensional unsteady incompressible viscous flow behind a rectangular bluff body between two parallel plates. The Peaceman-Rachford alternating direction implicit numerical method and Wachspress parameter were adopted to solve the governing equations in vorticity-transport and stream function formulation. The steady state flow and the vortex flow behind a rectangular bluff body in a chemical were investigated for Reynolds numbers of 200 and 500. The vortex shedding was generated by a physical pertubation numerically imposed at the center of the flow field for a short time. It was observed that the perturbed flow became periodic after a transient period.

• The KSFM Journal of Fluid Machinery
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• v.5 no.2 s.15
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• pp.29-35
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• 2002

The present study is concerned with the flow patterns induced by various impellers in a rectangular tank. Impellers are FBT (Flat blade turbine), PBT (Pitched blade turbine), Shroud turbine, Rushton turbine, and Helical ribbon turbine types. The solutions of flows in moving reference frames require the use of 'moving' cell zone. The moving zone approaches are based on MRF (Multiple reference frame), which is a steady-state approximation and sliding method, which is an unsteady-state approximation. Numerical results using two moving zone approaches we compared with experiments by Ranade & Joshi, which have done extensive LDA measurements of the flow generated by a standard six-bladed Rushton turbine in a cylindrical baffled vessel. In this paper, we simulated the flow patterns with above-mentioned moving zone approaches and impellers. Turbulence model used is RNG $k-{\epsilon}$ model. Sliding-mesh method is more effective than MRF for simulating the rectangular tank with inlet and outlet. RNG $k-{\epsilon}$ model strongly underestimates the velocity of experimental data and velocity by Chen & Kim's model, but it seems to be correctly predicted in overall distribution.

• Journal of the Korean Society of Combustion
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• v.7 no.1
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• pp.31-36
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• 2002

Vortical structures are investigated numerically for both cold and combusting flows from a two-dimensional bluff-body burner in the transitional flow regime from steady to unsteady state. The Reynolds number of the central fuel flow is varied from 10 to 230 at a fixed air Reynolds number of 400. The flame sheet model of infinite chemical reaction and unit Lewis number are assumed in the simulation. The temperature dependence of the viscosity and diffusivity of the gas mixture is also considered. The vortex shedding is observed depending on the fuel flow. For cold flow, four different types of vortical structure are identified. However, for combusting flow of methane-air system the vortical structures change significantly due to a large amount of heat release during the combustion process, in contract to cold flow.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.3
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• pp.247-255
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• 2016

The purpose of this study is to investigate unsteady-state characteristics of a pintle thruster with various pintle speed. Based on steady state experimental results, non-linear pintle stroke equation is obtained and applied to the unsteady state experimental system. For the unsteady state experiments, three different pintle speeds are used: 3.10 mm/s, 5.65 mm/s, 10.83 mm/s, respectively. Results show that backward pintle stroke results in faster convergence time because of high chamber pressure during backward pintle stroke sequence. During the forward and backward process, thrust curve shows singular points. These phenomenons is caused by variation of mass flow rate, which is mainly due to changes of both chamber pressures and nozzle throat area. This behavior becomes distinctive for a faster pintle speed case.

• Journal of Mechanical Science and Technology
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• v.20 no.12
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• pp.2250-2264
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• 2006

In micro- and nanoflows, the Boltzmann distribution is valid only when the electric double layers (EDL's) are not overlapped and the ionic distributions establish an equilibrium state. The present study has numerically investigated unsteady two-dimensional fully-developed electroosmotic flows between two parallel flat plates in the nonoverlapped and overlapped EDL cases, without any assumption of the Boltzmann distribution. For the study, two kinds of unsteady flows are considered: one is the impulsive application of a constant electric field and the other is the application of a sinusoidally oscillating electric field. For the numerical simulations, the ionic-species and electric-field equations as well as the continuity and momentum ones are solved. Numerical simulations are successful in accurately predicting unsteady electroosmotic flows and ionic distributions. Results show that the nonoverlapped and overlapped cases are totally different in their basic characteristics. This study would contribute to further understanding unsteady electroosmotic flows in micro- and nanofluidic devices.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1997.04a
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• pp.43-51
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• 1997

Steady and unsteady numerical simulations are conducted for the experiments performed to investigate the ram accelerator flow field by using the expansion tube facility in Stanford University. Navier-Stokes equations for chemically reacting flows are analyzed by fully implicit and time accurate numerical methods with Jachimowski's detailed chemistry model for hydrogen-air combustion involving 9 species and 19 reaction steps. Although the steady state assumption shows a good agreement with the experimental schlieren and OH PLIF images for the case of $2H_2$+$O_2$+$17N_2$, it fails in reproducing the combustion region behind the shock intersection point shown in the case of $2H_2$+$O_2$+$12N_2$, mixture. Therefore, an unsteady numerical simulation is conducted for this case and the result shows all the detailed flow stabilization process. The experimental result is revealed to be an instantaneous result during the flow stabilization process. The combustion behind the shock intersection point is the result of a normal detonation formed by the intersection of strong oblique shocks that exist at early stage of the stabilization process. At final stage, the combustion region behind the shock intersection point disappears and the steady state result is retained. The time required for stabilization of the reacting flow in the model ram accelerator is found to be very long in comparison with the experimental test time.