• 한국연소학회:학술대회논문집
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• 2005.10a
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• pp.261-267
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• 2005

A comprehensive DES quality numerical analysis has been carried out for reacting flows in constant-area and divergent scramjet combustor configuration with and without a cavity. Transverse injection of hydrogen is considered over a broad range of injection pressure. The corresponding equivalence ratio of the overall fuel-air mixture ranges from 0.167 to 0.50. The work features detailed resolution of the flow and flame dynamics in the combustor, which was not typically available in most of the pervious studies. Much of the flow unsteadiness is related not only the cavity, but also to the intrinsic unsteadiness in the flowfield. The interactions between the unsteady flow and flame evolution may cause a large excursion of flow oscillation. The roles of the cavity, injection pressure, and heat release in determining the flow dynamics are examined systematically.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.6
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• pp.813-821
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• 1997

Three-dimensional turbulent incompressible flow through the tip clearance of a linear turbine rotor cascade with high turning angle has been analyzed numerically. As a preliminary study to predict the tip clearance loss realistically, a generalized k-.epsilon. model derived by RNG (renormalized group) method is used for the modeling of Reynolds stresses to account for the strain rate of turbulent flow. The effects of the tip clearance flow on the passage vortex, the total pressure loss are considered qualitatively. The existences of vena contract and tip clearance vortex have been confirmed and it has been shown that as the size of the tip clearance increases, the accumulated flow through the tip clearance and the total pressure loss downstream of the cascade increase.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2012.05a
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• pp.239-243
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• 2012

This paper is concerned with turbulent flow computations using Large Eddy Simulation (LES) and the flow interaction of vortex shedding in a cylindrical duct flow driven by mass blowing through the wall. The purpose is to analyze non-linear combustion characteristics in the presence of vortex shedding generated in a hybrid rocket motor. Experimental studies have shown sudden changes in pressure (referred as a DC-shift), which depend on the strength of vortex strength of incoming flow. The combustion instability because of a sudden change in pressure fluctuations is mainly related with the interaction between vortex shedding. Therefore LES computation on a duct with injected normal blowing was performed to simulate the turbulent flow interactions with the behaviors of vortices and vortex structures along the injected wall.

• Journal of Mechanical Science and Technology
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• v.20 no.2
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• pp.286-294
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• 2006

Flows in a ramjet inlet is simulated for the study of the rocket-ramjet transition. The flow is unsteady, two-dimensional axisymmetric, compressible and turbulent. Double time marching method is used for the unsteady calculation and HLLC method is used as a higher order MUSCL method. As for turbulent calculation, $\kappa-\omega$ SST model is used for more accurate viscous calculations. Sinusoidal pressure perturbation is given at the exit and the flow fields at the inlet is studied. The cruise condition as well as the ground test condition are considered. The pressure level for the ground test condition is relatively low and the effect of the pressure perturbation at the combustion chamber is small. The normal shock at the cruise condition is very sensitive to the pressure perturbation and can be easily detached from the cowl when the exit pressure is relatively high. The sudden decrease in the mass flux is observed when the inlet flow becomes subcritical, which can make the inlet incapable. The amplitude of travelling pressure waves becomes larger as the downstream pressure increases, and the wavelength becomes shorter as Mach number increases. The phase difference of the travelling perturbed pressure wave in space is 180 degree.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.12
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• pp.1056-1062
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• 2002

This paper reports the characteristics of the three dimensional turbulent flow in the 180 degree bends with decreasing cross-sectional area by numerical method. Calculated pressure and velocity, Reynolds stress distributions are compared to the experimental data. Turbulence model employed are low Reynolds number k-epsilon model and algebraic stress model. The results show that the main vortex generated from the inlet part of the bend maintained to outlet of the bend because of the contraction of cross-sectional area. The rate of increase of turbulent kinetic energy through the bend are lower than that of mean flow. Secondary flow strength of the flow is lower about 60% than that of square duct flow.

• Transactions of the Korean Society of Automotive Engineers
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• v.4 no.3
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• pp.176-186
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• 1996

The phenomena of drag reduction using small quantities of a liner macromolecules has attracted the attention of many experimental investigations. On the other hand drag reduction in two phase flow can be applied to the transport of crude oil, phase change system such as chemical reactor, pool and boiling flow, and to flow with cavitation which occurs pump impellers. But the research on dragreduction in two phase flow is not sufficient. The purpose of the present work is to evaluate the drag reduction by measuring pressure drop, void fraction, mean liquid velocity and turbulent intensity whether polymer additives a horizontal single and two phase system or not. Flow pattern of air-water two phase flow was classified by electrical conductivity probe signal. Velocities and turbulent intensities of signal were measured simultaneously with a Hot-film anemometer.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.6
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• pp.722-733
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• 1999

The wall pressure fluctuations of a turbulent boundary layer over a flat plate have been investigated in an anechoic wind tunnel facility. The anechoic wind tunnel consists of acoustically-lined duct, muffler, and splitter-type silencer for noise suppression and vanes for reducing head losses involved. To improve spectra characteristics in high frequency range, a 1/8" pressure-type microphone sensor, which has a pin-holed cap of various diameters, was employed in this experiment. It was shown that the pin-holed microphone sensor with a dimensionless diameter $d^+$ of 7.1 resolved the high frequency pressure fluctuations most effectively among ones with various pin-hole diameters. The measured wall pressure spectra in terms of three types of scaling parameters were in good agreement with other experimental and numerical results. The pressure events of high amplitude were found to contribute to total fluctuating pressure energies in the turbulent boundary layer significantly and supposed to radiate to the far-field effectively.

• Tribology and Lubricants
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• v.35 no.5
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• pp.300-309
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• 2019

Gas foil thrust bearings (GFTBs) support axial loads in oil-free, high speed rotating machinery using air or gas as a lubricant. Due to the inherent low viscosity of the lubricant, GFTBs often have super-laminar flows in the film region at operating conditions with high Reynolds numbers. This paper develops a mathematical model of a GFTB with turbulent flows and validates the model predictions against those from the literature. The pressure distribution, film thickness distribution, load carrying capacity, and power loss are predicted for both laminar and turbulent flow models and compared with each other. Predictions for an air lubricant show that the GFTB has high Reynolds numbers at the leading edge where the film thickness is large and relatively low Reynolds numbers at the trailing edge. The predicted load capacity and power loss for the turbulent flow model show little difference from those for the laminar flow model even at the highest speed of 100 krpm, because the Reynolds numbers are smaller than the critical Reynolds number. On the other hand, refrigerant (R-134a) lubricant, which has a higher density than air, had significant differences due to high Reynolds numbers in the film region, in particular, near the leading and outer edges. The predicted load capacity and power loss for the turbulent flow model are 2.1 and 2.3 times larger, respectively, than those for the laminar flow model, thus implying that the turbulent flow greatly affects the performance of the GFTB.

• Journal of Power System Engineering
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• v.10 no.4
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• pp.17-24
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• 2006

The turbulent flow field characteristics of a gun-type gas burner with and without a duct were investigated under the isothermal condition of non-combustion. Vectors and mean velocities were measured by hot-wire anemometer system with an X-type hot-wire probe in this paper. The turbulent flow field with a duct seems to cause a counter-clockwise recirculation flow from downstream to upstream due to the unbalance of static pressure between a main jet flow and a duct wall. Moreover, the recirculation flow seems to expand the main jet flow to the radial and to shorten it to the axial. Therefore, the turbulent flow field with a duct increases a radial momentum but decreases a axial momentum. As a result, an axial mean velocity component with a duct above the downstream range of about X/R=1.5 forms a smaller magnitude than that without a duct in the inner part of a burner, but it shows the opposite trend in the outer part.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.6
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• pp.340-346
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• 2009

This paper numerically investigates thermo-flow characteristics of offset strip fins to obtain a correlation between heat transfer and pressure drop. The flow was divided into three regimes, i.e. laminar, transition and turbulent. The predicted j and f values from the SST k-$\omega$ turbulence model agreed with previous correlations with the error less than 20% in transition and turbulent regimes. Heat transfer and pressure drop were predicted by varying Prandtl number from 0.5 to 40. The Prandtl number showed little effects on pressure drop but had great effect on the heat transfer characteristics. An overall correlation to predict j was suggested by incorporating the effect of Pr and a new j correlation was suggested for each Pr.