• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.8
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• pp.10-20
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• 2002

The present paper investigates methods to control dynamic stall using an optimal approach. An unsteady aerodynamic sensitivity analysis code is developed by a direct differentiation method from a two-dimensional unsteady compressible Navier-Stokes solver including a two-equation turbulence model. Dynamic stall control is conducted by minimizing an objective function defined at an instant instead of integrating for a period of time. Unsteady sensitivity derivatives of the objective function are calculated by the sensitivity code, and optimization is carried out using a linear line search method at every physical time step. Numerous examples of dynamic stall control using control parameters such as nose radius, maximum thickness of airfoil, or suction show satisfactory results.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.04a
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• pp.295-298
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• 2007

The effectiveness of passive control techniques for reducing the pressure oscillation generated in a supersonic cavity flow was investigated numerically and experimentally, respectively. The control device includes a sub-cavity installed in the upstream edge of a rectangular cavity. Time-dependent supersonic cavity flow characteristics with turbulent features were examined by using the three-dimensional, mass-averaged Navier-Stokes computation based on a finite volume scheme and large eddy simulation. The results show that the pressure oscillation near the trailing edge dominates overall time-dependent cavity pressure variations. Such an oscillation can be attenuated more significantly in the presence of the sub-cavity compared with the cavity without sub-cavity, and a larger sub-cavity leads to better control performance.

• Proceedings of KOSOMES biannual meeting
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• 2005.11a
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• pp.159-163
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• 2005

This experimental study was performed to investigate internal flow and unsteady flow characteristics using a model for actual shape of a plate heat exchanger and visualization of flow through the particle image velocimetry. Seven Reynolds numbers were selected by calculation with the height of grooved channel and sectional mean velocity of inlet flow in the experiment, and instantaneous velocity distributions and flow characteristics were experimently investigated. The triangular grooved channel had a compound flow consisting of the flow in lower channel and the groove flow receiving shear stress by the channel flow in the experiment. The sheared mixing layer, in the boundary between the triangular groove and the channel, affected main flow to raise turbulent in the channel.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.428-433
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• 2000

The characteristics of unsteady heat transfer and boundary layer flow in the SSME turbine rotor passage are investigated with LRN $k-{\varepsilon}$ turbulence model. The unsteady flow and heat transfer in a rotor blade passage as a result of wake/blade interaction is modeled by the inviscid/boundary-layer flow approach. The relevant governing equations are discretized to a system of finite different equations by means of a BTBCS implicit method. These equations have been solved numerically, for the velocity and temperature fields using TDMA method. Heat flux on the blade surface and flow parameters in the rotor passage are calculated with wake interaction. Numerical results show that velocity, pressure, turbulent kinetic energy and heat flux on the blade surface are varied periodically by wake passing.

• 한국전산유체공학회:학술대회논문집
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• 2006.10a
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• pp.120-125
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• 2006

Piezoelectric actuators have been investigated for flow control in the field of fluid dynamics. Numerical simulation for a single diaphragm piezoelectric actuator operating in quiescent air is performed to investigate the complex flow field around the slot exit. A periodic velocity transpiration condition is applied to simulate the effect of the moving diaphragm. The computational results for the flow field around the slot exit agree well with the experimental data. The results also show that low pressure regions due to the vortex pairing cause non-monotonic variations in the vertical velocity.

• Journal of computational fluids engineering
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• v.7 no.1
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• pp.20-27
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• 2002

Flow over a circular cylinder is studied numerically using a turbulence model. Based on the κ-ε-f/sub μ/ model of Park and Sung[6], a new damping function is used. The efficiency of the strain dependent damping function is addressed for vortex-shedding flows past a circular cylinder. The mean velocity and Reynolds stresses are compared with available experimental data at Re/sub D/= 3900. Also, the computational results for the Strouhal number are evaluated at several Reynolds number. The predictions by κ-ε-f/sub μ/ model are in good agreement with the experiments.

• Journal of the Korean Society of Propulsion Engineers
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• v.14 no.3
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• pp.9-15
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• 2010

Numerical investigation was carried out on axisymmetric over-expanded rocket nozzle to predict flow fields of transitional shock separation patterns. The unsteady, compressible N-S equations with k-$\omega$ SST for turbulence model closure were solved using a fully implicit finite volume scheme. Computed results were in good agreement with previous experimental works. It was found that strong side-loads were generated during the transition of RSS to FSS due to the development of a vortex ring in the inviscid jet core region. Hysteresis phenomenon exhibited by the shock-separation patterns was also found during the start-up and shut-down processes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.8
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• pp.21-29
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• 2002

In the present study, a passive control method, using a porous wall and cavity system, is applied to the shock wave/boundary layer interactions in transonic moist air flow. The two-dimensional, unsteady, compressible, Navier-Stokes equations, which are fully coupled with a droplet growth equation, are solved by the third-order MUSCL type TVD finite difference scheme. Baldwin-Lomax model is employed to close the governing equations. In order to investigate the effectiveness of the present control method, the total pressure loss of the flow and the time-dependent behaviour of shock motions are analyzed in detail. The computed results show that the present passive control method considerably reduces the total pressure losses due to the shock wave/boundary layer interaction in transonic moist air flow and suppresses the unsteady shock wave motions over the airfoil as well. It is also found that the location of the porous ventilation significantly affects the control effectiveness.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.10
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• pp.649-657
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• 2017

A rupture in a high-pressure pipe causes the fluid in the pipe to be discharged in the atmosphere at a high speed resulting in a supersonic jet that generates the compressible flow. This supersonic jet may display complicated and unsteady behavior in general. In this study, Computational Fluid Dynamics (CFD) analysis was performed to investigate the compressible flow generated by a supersonic jet ejected from a high-pressure pipe. A Shear Stress Transport (SST) turbulence model was selected to analyze the unsteady nature of the flow, which depends upon the various gases as well as the diameter of the pipe. In the CFD analysis, the basic boundary conditions were assumed to be as follows: pipe of diameter 10 cm, jet pressure ratio of 5, and an inlet gas temperature of 300 K. During the analysis, the behavior of the shockwave generated by a supersonic jet was observed and it was found that the blast wave was generated indirectly. The pressure wave characteristics of hydrogen gas, which possesses the smallest molecular mass, showed the shortest distance to the safety zone. There were no significant difference observed for nitrogen gas, air, and oxygen gas, which have similar molecular mass. In addition, an increase in the diameter of the pipe resulted in the ejected impact caused by the increased flow rate to become larger and the zone of jet influence to extend further.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.6
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• pp.28-44
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• 2020

In this paper, the scavenging process of various transfer ports was evaluated to improve the performance of a small two-stroke engine for unmanned aerial vehicles. Three-dimensional computational fluid dynamics simulations were performed to four transfer ports for the evaluation, and a three-phase scavenging model was developed and applied to the simulation results for the quantitative comparison of scavenging performance. the short-circuit of fresh charge was restrained and an in-cylinder turbulent kinetic energy was enhanced by changing the transfer port. Also, a difference in the scavenging for each port were confirmed by applying the three-phase model to the simulation results.