• The Journal of the Acoustical Society of Korea
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• v.43 no.1
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• pp.103-111
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• 2024

Axial-flow fans are used to transport fluids in relatively low-pressure flow regimes, and a variety of design variables are employed. The tip geometry of an axial fan plays a dominant role in its flow and noise performance, and two of the most prominent flow phenomena are the tip vortex and the tip leakage vortex that occur at the tip of the blade. Various studies have been conducted to control these three-dimensional flow structures, and winglet geometries have been developed in the aircraft field to suppress wingtip vortices and increase efficiency. In this study, a numerical and experimental study was conducted to analyze the effect of winglet geometry applied to an axial fan blade for an air conditioner outdoor unit. The unsteady Reynolds-Averaged Navier-Stokes (RANS) equation and the FfocwsWilliams and Hawkings (FW-H) equation were numerically solved based on computational fluid dynamics techniques to analyze the three-dimensional flow structure and flow noise numerically, and the validity of the numerical method was verified by comparison with experimental results. The differences in the formation of tip vortex and tip leakage vortex depending on the winglet geometry were compared through a three-dimensional flow field, and the resulting aerodynamic performance was quantitatively compared. In addition, the effect of winglet geometry on flow noise was evaluated by numerically simulating noise based on the predicted flow field. A prototype of the target fan model was built, and flow and noise experiments were conducted to evaluate the actual performance quantitatively.

• Journal of the Korean GEO-environmental Society
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• v.16 no.1
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• pp.45-53
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• 2015

This study has obtained Soil Water Retention Curve (SWRC) of the unsaturated soil from the volumetric pressure plate extractor test and the triaxial compression tests was also conducted. By using the rainfall data measured in the site the seepage analysis of unsteady flow was performed with the program of SEEP/W in Geostudio 2007 and stability of the slope was analyzed with SLOPE/W program. Results of analyses show that shear strength of the unsaturated soil increases with the increase of matric suction. And it was also found that the net volumetric stress and the apparent cohesion increased with the matric suction. The seepage analysis of rainfall represents that the increasing rate of negative pore pressure at the zone of large negative pore pressure is appeared to be high even though lower rainfall intensity, but this tendency declines with ground depth. The stability analysis of slope was carried out for the actual plane of failure with the data representing the field condition. The factor of safety thus calculated was about unity (1.0) or just below, which means that the adopted method of analysis is in good agreement with the field condition.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.8
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• pp.1067-1074
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• 2010

In this study, a loosely coupled fluid-structure interaction (FSI) analysis was conducted for a low-pressure (LP) final-stage rotor blade. Preliminary FSI analyses of a $15^{\circ}$ sweptback wing and a NASA Rotor 37 compressor blade were performed for verifying the boundary conditions. The results were compared with the established literatures for each model. The FSI analysis of the $15^{\circ}$ sweptback wing was carried out under both stable and unstable conditions. The excessive deformation of the wing was observed within 0.05 s under the unstable condition which is higher than the divergence speed of a wing compared with the stable condition. On the basis of the results of a steady-state study, an unsteady state FSI analysis was conducted for a NASA Rotor 37. Different deformations were observed at trailing edge of the blade in the static FSI and dynamic FSI analysis. A 3D FE model of a LP rotor was generated from the span-wise section data. In order to develop a reasonable model, an impact test was performed and compared to the FE model. Using this FE model, the steady-state FSI analysis was performed successfully.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2003.05a
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• pp.91-93
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• 2003

A comprehensive numerical study is carried out to investigate for the understanding of the flow evolution and flame development in a supersonic combustor with normal injection of ncumally injecting hydrogen in airsupersonic flows. The formulation treats the complete conservation equations of mass, momentum, energy, and species concentration for a multi-component chemically reacting system. For the numerical simulation of supersonic combustion, multi-species Navier-Stokes equations and detailed chemistry of H2-Air is considered. It also accommodates a finite-rate chemical kinetics mechanism of hydrogen-air combustion GRI-Mech. 2.11[1], which consists of nine species and twenty-five reaction steps. Turbulence closure is achieved by means of a k-two-equation model (2). The governing equations are spatially discretized using a finite-volume approach, and temporally integrated by means of a second-order accurate implicit scheme (3-5).The supersonic combustor consists of a flat channel of 10 cm height and a fuel-injection slit of 0.1 cm width located at 10 cm downstream of the inlet. A cavity of 5 cm height and 20 cm width is installed at 15 cm downstream of the injection slit. A total of 936160 grids are used for the main-combustor flow passage, and 159161 grids for the cavity. The grids are clustered in the flow direction near the fuel injector and cavity, as well as in the vertical direction near the bottom wall. The no-slip and adiabatic conditions are assumed throughout the entire wall boundary. As a specific example, the inflow Mach number is assumed to be 3, and the temperature and pressure are 600 K and 0.1 MPa, respectively. Gaseous hydrogen at a temperature of 151.5 K is injected normal to the wall from a choked injector.A series of calculations were carried out by varying the fuel injection pressure from 0.5 to 1.5MPa. This amounts to changing the fuel mass flow rate or the overall equivalence ratio for different operating regimes. Figure 1 shows the instantaneous temperature fields in the supersonic combustor at four different conditions. The dark blue region represents the hot burned gases. At the fuel injection pressure of 0.5 MPa, the flame is stably anchored, but the flow field exhibits a high-amplitude oscillation. At the fuel injection pressure of 1.0 MPa, the Mach reflection occurs ahead of the injector. The interaction between the incoming air and the injection flow becomes much more complex, and the fuel/air mixing is strongly enhanced. The Mach reflection oscillates and results in a strong fluctuation in the combustor wall pressure. At the fuel injection pressure of 1.5MPa, the flow inside the combustor becomes nearly choked and the Mach reflection is displaced forward. The leading shock wave moves slowly toward the inlet, and eventually causes the combustor-upstart due to the thermal choking. The cavity appears to play a secondary role in driving the flow unsteadiness, in spite of its influence on the fuel/air mixing and flame evolution. Further investigation is necessary on this issue. The present study features detailed resolution of the flow and flame dynamics in the combustor, which was not typically available in most of the previous works. In particular, the oscillatory flow characteristics are captured at a scale sufficient to identify the underlying physical mechanisms. Much of the flow unsteadiness is not related to the cavity, but rather to the intrinsic unsteadiness in the flowfield, as also shown experimentally by Ben-Yakar et al. [6], The interactions between the unsteady flow and flame evolution may cause a large excursion of flow oscillation. The work appears to be the first of its kind in the numerical study of combustion oscillations in a supersonic combustor, although a similar phenomenon was previously reported experimentally. A more comprehensive discussion will be given in the final paper presented at the colloquium.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.48 no.9
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• pp.711-716
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• 2020

Injection visualization of heated mixed simulant droplets based on hydrocarbon fuel was performed under supercritical state environment. Mixed simulant consisted of Decane and Methylcyclohexane with different critical pressure and critical temperature. Flows injected into the supercritical state environment created droplet by Rayleigh breakup mechanism, and the Oh number and Re number were determined to confirm the breakup area. The temperature of the mixed simulant varied from Tr=0.49 to Tr=1.34. The flow rate was maintained at 0.7 to 0.8 g/s. Droplet became shorter in breakup length as heated and into a lumped form. Second droplet was formed and when Tr=1.34, the phase was not visible in the supercritical state with local unsteady flow.

• Transactions of the Korean Society of Automotive Engineers
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• v.2 no.1
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• pp.9-25
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• 1994

In the present study, the Reynolds-averaged Navier-Stokes equations, together with the equations of the $k-{\varepsilon}$ model of turbulence, were solved numerically in a general body-fitted coordinate system for three-dimensional turbulent flows around the six basic shapes of the magnetically levitated train(MAGLEV). The numerical computations were conducted on the MAGLEV model configurations to provide information on shapes of this type very near the elevated track at a constant Reynolds number of $1.48{\times}10^{6}$ based on the body length. The coordinate system was generated by numerically solving a set of Poisson equations. The convective transport equations were discretized using the finite-analytic scheme which employed analytic solutions of the locally-linearized equations. A time marching algorithm was employed to enable future extensions to be made to handle unsteady and fully-elliptic problems. The pressure-velocity coupling was treated with the SIMPLER-algorithm. Of particular interests were wall effect by the elevated track on the aerodynamic forces and flow characteristics of the six models calculated. The results indicated that the half-circle configuration with extended sides and with smooth curvature of sides was desirable because of the low aerodynamic forces and pitching moment. And it was found that the separation bubble was occured at wake region in near the elevated track.

• Ocean Systems Engineering
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• v.4 no.3
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• pp.185-200
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• 2014

The objective of the present simulations is to evaluate the applicability of the standard $k-{\varepsilon}$ turbulence model in engineering practice in the subcritical to supercritical flow regimes. Two-dimensional numerical simulations of flow around a circular cylinder at $Re=1{\times}10^5$, $5{\times}10^5$ and $1{\times}10^6$, had been performed using Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with the standard $k-{\varepsilon}$ turbulence model. Solution verification had been studied by evaluating grid and time step size convergence. For each Reynolds number, several meshes with different grid and time step size resolutions were chosen to calculate the hydrodynamic quantities such as the time-averaged drag coefficient, root-mean square value of lift coefficient, Strouhal number, the coefficient of pressure on the downstream point of the cylinder, the separation angle. By comparing the values of these quantities of adjacent grid or time step size resolutions, convergence study has been performed. Solution validation is obtained by comparing the converged results with published numerical and experimental data. The deviations of the values of present simulated quantities from those corresponding experimental data become smaller as Reynolds numbers increases from $1{\times}10^5$ to $1{\times}10^6$. This may show that the standard $k-{\varepsilon}$ model with enhanced wall treatment appears to be applicable for higher Reynolds number turbulence flow.

• Wind and Structures
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• v.26 no.4
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• pp.215-229
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• 2018

The appearance of a rivulet at the upper surface of a stay cable is responsible for rain-wind-induced vibration (RWIV) of cables of cable-stayed bridges. However, the formation mechanism of the upper rivulet and its aerodynamic effects on the stay cable has not been fully understood. Large eddy simulation (LES) method is used to investigate flow around and aerodynamics of a circular cylinder with an upper rivulet at a Reynolds number of 140,000. Results show that the mean lift coefficients of the circular cylinder experience three distinct stages, zero-lift stage, positive-lift stage and negative-lift stage as the rivulet located at various positions. Both pressure-induced and friction-induced aerodynamic forces on the upper rivulet are helpful for its appearance on the upside of the stay cable. The friction-induced aerodynamic forces, which have not been considered in the previous theoretical models, may not be neglected in modeling the RWIV. In positive-lift stage, the shear layer separated from the upper rivulet can reattach on the surface of the cylinder and form separation bubbles, which result in a high non-zero mean lift of the cylinder and potentially induces the occurrence of RWIV. The separation bubbles are intrinsically unsteady flow phenomena. A serial of small eddies first appears in the laminar shear layer separated from the upper rivulet, which then coalesces and reattaches on the side surface of the cylinder and eventually sheds into the wake.

• Proceedings of KOSOMES biannual meeting
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• 2006.11a
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• pp.265-269
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• 2006

Anti-Roll Tanks, also called Sloshing Tanks, is a rather common and sometimes an efficient method of limiting the roll angles. The important parameters, when considering using anti-roll tanks, are positioning, size, duct area, flow control device etc. Measurement by the PIV(Particle Image Velocimetry) was conducted to investigate the flow characteristics around control damper and inlet area of duct for three kind of inclined angle $(\alpha=0^*,\;10^*\;and\;20^*)$. Flow behaviors such as instantaneous and time-mean velocity vectors are investigated. Furthermore, to reveal boundaries between flowing and stagnant zones and to extract velocity profiles at any selected sections of the lower duct for passive anti-rolling tanks system.

• Journal of the Society of Naval Architects of Korea
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• v.50 no.6
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• pp.421-428
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• 2013

This study aims at numerically investigating the angle of attack characteristics of the rudder behind a rotating propeller. The rotating propeller of 5 blades and the full spade rudder are placed in the numerical water tunnel with a uniform flow condition to consider propeller-rudder interaction. The turbulence closure model is employed to simulate the three-dimensional unsteady incompressible viscous turbulent flow around the propeller and the rudder. The present numerical method are well verified by comparing with the experimental results. In order to identify the dependence of the angle of attack of the rudder on the rudder angle, a wide range of rudder angles is considered. The present study carried out the quantitative and qualitative analysis of the angle of attack in terms of the pressure distribution, streamlines and the evaluation of the flow incidence, resulting in that the angle of attack increases as we move from the root and the tip to the center of the rudder, regardless of the rudder angle. The distribution of the angle-of-attack along the span is strongly affected by rotating propeller flow and rudder angle. Consequently, the distribution of the angle-of-attack of the oncoming flow against the rudder leading edge plays a role in determination of rudder performance.