• Proceedings of the KSME Conference
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• 2000.11b
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• pp.544-549
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• 2000

The effects of nonequilibrium condensation on the shock boundary layer interaction over a transonic bump model were investigated experimentally and numerically. An experiment was conducted using a supersonic indraft wind tunnel. A droplet growth equation was incorporated into two-dimensional Navier-Stokes equation systems. Computations were carried out using a third-order MUSCL type TVD finite-difference scheme with a second-order fractional time step. Computations compared with the experimental results. Nonequilibirum condensation suppressed the boundary layer separation and the pressure fluctuations due to the shock boundary layer interaction. Especially the nonequilibrium condensation was helpful to suppress the high frequency components of the pressure fluctuations.

• 한국전산유체공학회:학술대회논문집
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• 2008.10a
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• pp.92-96
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• 2008

For the Mach reflection of symmetric shock waves, only the wave configuration of an oMR(DiMR+DiMR) is theoretically admissible. For asymmetric shock waves, an oMR(DiMR+InMR) will be possible if the two slip layers assemble a convergent-divergent stream tube while an oMR(InMR+InMR) is absolutely impossible. In this paper, an overall Mach reflection configuration with double inverse MR patterns is confirmed using the CFD technique. Classical two- and three-shock theories are also applied for the theoretical analysis. In addition, oscillations of shock wave patterns are computed for the interaction of a hypersonic flow and double-wedge-like geometries.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1229-1237
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• 1998

Experiments of shock-wave/turbulent boundary layer interaction were conducted by using a supersonic wind tunnel. Nominal Mach number was varied in the range of 1.6 to 3.0 by means of different nozzles. The objective of the present study is to investigate the effects of boundary layer suction on normal shock-wave oscillations caused by shock wave/boundary layer interaction in a straight duct. Two-dimensional slits were installed on the top and bottom walls of the duct to bleed turbulent boundary layer flows. The bleed flows were measured by an orifice. The ratio of the bleed mass flow to main mass flow was controlled below the range of 11 per cent. Time-mean and fluctuating wall pressures were measured, and Schlieren optical observations were made to investigate time-mean flow field. Time variations in the shock wave displacement were obtained by a high-speed camera system. The results show that boundary layer suction by slits considerably reduce shock-wave oscillations. For the design Mach number of 2.3, the maximum amplitude of the oscillating shock-wave reduces by about 75% compared with the case of no slit for boundary layer suction.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.12
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• pp.3980-3990
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• 1996

When a moist air is rapidly expanded in a supersonic nozzle, nonequilibrium condensation occurs at a supersaturation state. Condensation shock wave appears in the nozzle flow if the releasing latent heat due to condensation goes beyond a critical value. It has been known that self-excited oscillations of the condensation shock wave generate in an air or a steam nozzle flow with a large humidity. In the present study, the passive control technique using porous wall with a cavity underneath was applied to the condensation shock wave. The effects of the passive control on the steady and self-excited condensation shock waves were experimentally investigated by Schlieren visualization and static pressure measurements. The result shows that the present passive control is a useful technique to suppress the self-excited oscillations of condensation shock wave.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.3
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• pp.31-40
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• 2020

The structure and dynamics of multiple shock waves are studied numerically using a finite volume solver for a model with nozzle exit Mach number of 1.75. At first, the shock variation based on images were analyzed using a Matlab program then later to the wall static pressure variation. The amplitude and frequency variation for multiple shock waves are analyzed. The cross-correlation between the shock location suggests that the first and the second shocks are well correlated while the other shocks show a phase lag in the oscillation characteristics. The rms values of pressure fluctuations are maximum at the shock locations while the other parts in the flow exhibit a lower value os standard deviation.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.79.4-80
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• 2021

Sunspots' subsurface structure is an important subject to explain their stability and energy transport. Previous studies suggested two models for the subsurface structure of sunspots: monolithic model and cluster model. However, it is not revealed which model is more plausible so far. We obtain clues about the subsurface structure of sunspots by analyzing the motion of umbral flashes observed by the IRIS Mg II 2796Å slit-jaw images (SJI). The umbral flashes are believed as shock phenomena developed from upward propagating slow magnetohydrodynamic (MHD) waves. If the MHD waves are generated by convective motion below sunspots, the apparent origin of the umbral flashes known as oscillation center will indicate the horizontal position of convection cells. Thus, the distribution of the oscillation centers is useful to investigate the subsurface structure of sunspots. We analyze the spatial distribution of oscillation centers in the merged sunspot. As a result, we found that the oscillation centers distributed over the whole umbra regardless of the convergent interface between two merged sunspots. It implies that the subsurface structure of the sunspot is not much different from the convergent interface, and supports that many field-free gaps may exist below the umbra as the cluster model expected. For more concrete results, we should confirm that the oscillation centers determined by the umbral flashes accurately reflect the position of wave sources.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.11a
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• pp.107-116
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• 1996

Numerical simulations have been conducted to characterize unsteady flow structures in an axisymmetric supersonic inlet diffuser with sinusoidal pressure oscillations at the diffuser exit. The formulation is based on the unsteady Navier-Stokes equations and turbulence closure is achieved using a two-layer model with a too-Reynolds-number scheme for the near-wall treatment. The governing equations are formulated in an integral form, and are discretized by the four-stage Runge-Kutta scheme for temporal terms and the Harten-Yee upwind TVD scheme for convective terms. Results indicated that the inlet shock characteristics are significantly modified by acoustic oscillations originating from the combustor. The characteristics of shock/boundarv-layer interactions (such as the size of separation bubble, terminal shock shape, and vorticity intensity) are also greatly iufluenced by the shock oscillation due to acoustic waves.

• Journal of the Korean Society of Combustion
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• v.1 no.2
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• pp.41-49
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• 1996

A numerical study is conducted to investigate the periodically unstable shock induced combustion around blunt bodies in stoichiometric hydrogen-air mixtures. Euler equations are spatially discretized by upwind-biased third order scheme and temporally integrated by Runge-Kutta method. Chemistry model used in this study involves 8 elementary kinetics steps and 7 species. At a constant Mach number, the effects of projectile size, inflow pressure and inflow temperature are examined with Lehr#s experimental condition as a reference. In addition to oscillation frequency, characteristic distances and time averaged values are found from the result to find an relation with dimensionless parameters. As a result, it is found that the effects of inflow pressure and body size are very similar and $Damk{\ddot{o}}hler$ number plays an important role in determining the instability characteristics.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.11a
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• pp.341-344
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• 2008

Numerical simulations have been carried out for a supersonic two-dimensional flow over open, rectangular cavities (length-to-depth ratios are L/D = 1.0) in order to investigate the effect of non-equilibrium condensation of moist air on supersonic flows around the cavity for the flow Mach number 1.83 at the cavity entrance. In the present computational investigation, a condensing flow was produced by an expansion of moist air in a Laval nozzle. The results obtained showed that in the case with non-equilibrium condensation for L/D = 1.0, amplitudes of oscillation in the cavity became smaller than those without the non-equilibrium condensation. Furthermore, the occurrence of the non-equilibrium condensation reduced the peaks of power spectrum density and the frequency of the flow field oscillation increased in comparison with the case of $S_0$ = 0.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1617-1622
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• 2004

Two-phase flow of liquid and gas through pipe lines are frequently encountered in nuclear power plant or industrial facility. Pressure waves which can be generated by a valve operation or any other cause in pipe lines propagate through the two-phase flow, often leading to severe noise and vibration problems or fatigue failure of pipe line system. It is of practical importance to predict the propagation characteristics of the pressure waves for the safety design for the pipe line. In the present study, a theoretical analysis is performed to understand the propagation characteristics of a weak shock wave in a bubbly flow. A wave equation is developed using a small perturbation method to analyze the weak shock wave through a bubbly flow with comparably low void fractions. It is known that the elasticity of pipe and void fraction significantly affect the propagation speed of shock wave, but the frequency of relaxation oscillation which is generated behind the shock wave is not strongly influenced by the elasticity of pipe. The present analytical results are in close agreement with existing experimental data.