• Journal of Ship and Ocean Technology
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• v.7 no.3
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• pp.34-55
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• 2003

Explicit Kutta condition approximation, proved useful in existing time-domain solver of the unsteady propeller problem, requires a specified functional behavior of the vorticity in space near the trailing edge. In this paper, the strength of the discrete vortices is controlled to have a specified behavior in space in the frequency domain approach. A new formulation is introduced and is implemented for analysis of a lifting surface of a rectangular planform. Sample computations carried out according to the new formulation compares well with that of existing unsteady lifting problem in the time domain.

• The KSFM Journal of Fluid Machinery
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• v.13 no.5
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• pp.43-53
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• 2010

This study investigates a design optimization of a rotating two-pass rectangular cooling channel with staggered arrays of pin-fins. The radial basis neural network method is used as an optimization technique with Reynolds-averaged Navier-Stokes analysis of fluid flow and heat transfer with shear stress transport turbulent model. The ratio of the diameter to height of the pin-fins and the ratio of the streamwise spacing between the pin-fins to height of the pin-fin are selected as design variables. The optimization problem has been defined as a minimization of the objective function, which is defined as a linear combination of heat transfer related term and friction loss related term with a weighting factor. Results are presented for streamlines, velocity vector fields, and contours of Nusselt numbers, friction coefficients, and turbulent kinetic energy. These results show how fluid flow in a two-pass square cooling channel evolves a converted secondary flows due to Coriolis force, staggered arrays of pin-fins, and a $180^{\circ}$ turn region. These results describe how the fluid flow affects surface heat transfer. The Coriolis force induces heat transfer discrepancy between leading and trailing surfaces, having higher Nusselt number on the leading surface in the second pass while having lower Nusselt number on the trailing surface. Dean vortices generated in $180^{\circ}$ turn region augment heat transfer in the turning region and in the upstream region of the second pass. As the result of optimization, in comparison with the reference geometry, thermal performance of the optimum geometry shows the improvement by 30.5%. Through the optimization, the diameter of pin-fin increased by 14.9% and the streamwise distance between pin-fins increased by 32.1%. And, the value of objective function decreased by 18.1%.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.32 no.8
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• pp.37-46
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• 2004

Several notes on ground effects drawn from Navier-Stokes analyses and their aerodynamic interpretations were addressed here; For two-dimensional ground effect, the change of surface pressure due to image vortex, the venturi effect due to thickness and the primary inviscid flow phenomena of ground effect, and for three-dimensional ground effect, strengthened wing tip vortices, increased effective span and the outward drift of trailing vortices. Irodov's criteria were evaluated to investigate the static longitudinal stability of conventional NACA 6409 and DHMTU 8-30 airfoils. The analysis results demonstrated superior static longitudinal stability of DHMTU 8-30 airfoil. The DHMTU airfoil has quite lower value of lrodov's criterion than the conventional NACA airfoil, which require much smaller tail volume to stabilize the whole WIG-craft at its design stage.

• 한국가시화정보학회:학술대회논문집
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• 2002.11a
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• pp.23-26
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• 2002

The phase-averaged velocity fields of 3 dimensional turbulent wake behind a marine propeller measured by 2D PIV and stereoscopic PIV(SPIV) were compared directly. In-plane velocity fields obtained from the consecutive particle images captured by one camera in 2D PIV have perspective errors due to out-of-plane motion. However, the perspective errors can be removed by measuring three component velocity fields using SPIV method with two cameras. It is also necessary to measure three components velocity fields for the investigation of complicated near-wake behind the propeller for the suitable propeller design. 400 instantaneous velocity fields were measured for each of four different blade phases of $0^{\circ},\;18^{\circ},\;36^{\circ}C\;and\;54^{\circ}$. They were ensemble averaged to investigate the spatial evolution of the propeller wake in the downstream region. The phase-averaged velocity fields show the viscous wake developed along the blade surfaces and tip vortices were formed periodically. The perspective errors caused by the out-of-plane motion was estimated by the comparison of 2D PIV and SPIV results. The difference in the axial mean velocity fields measured by both techniques are nearly proportional to the mean out-of-plane velocity component which has large values in the regions of the tip and trailing vortices. The axial turbulence intensity measured by 2D PIV was overestimated since the out-of-plane velocity fluctuations influence the in-plane velocity vectors and increase the in-plane turbulence intensities.

• Wind and Structures
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• v.35 no.3
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• pp.177-191
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• 2022

Trains emerging on a streamlined bridge-girder may have salient interference effects on the aerodynamic properties of the bridge. The present paper aims at investigating these interferences by wind tunnel measurements, covering surface pressure distributions, near wake profiles, and flow visualizations. Experimental results show that the above interferences can be categorized into two primary effects, i.e., an additional angle of attack (AoA) and an enhancement in flow separation. The additional AoA effect is demonstrated by the upward-moved stagnation point of the oncoming flow, the up-shifted global symmetrical axis of flow around the bridge-girder, and the clockwise-deflected orientation of flow approaching the bridge-girder. Due to this additional AoA effect, the two critical AoAs, where flow around the bridge-girder transits from trailing-edge vortex shedding (TEVS) to impinging leading-edge vortices (ILEV) and from ILEV to leading-edge vortex shedding (LEVS) of the bridge-girder are increased by 4° with respect to the same bridge-girder without trains. On the other hand, the underlying flow physics of the enhancement in flow separation is the large-scale vortices shedding from trains instead of TEVS, ILEV, and LEVS governed the upper half bridge-girder without trains in different ranges of AoA. Because of this enhancement, the mean lift and moment force coefficients, all the three fluctuating force coefficients (drag, lift, and moment), and the aerodynamic span-wise correlation of the bridge-girder are more significant than those without trains.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.8 s.227
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• pp.921-928
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• 2004

Measurements of local heat/mass transfer coefficients in rotating two-pass ducts are presented. Ducts of three different aspect ratios (W/H), 0.5, 1.0 and 2.0, are employed with a fixed hydraulic diameter ($D_h$) of 26.7 nm. $90^{\circ}$-rib turbulators are attached on the leading and trailing walls symmetrically. The rib height-to-hydraulic diameter ratio ($e/D_h$) is 0.056, and the rib pitch-to-rib height ratio (p/e) is 10. The experimental conditions are the same as those of the previous part of the study. As the rib height-to-duct height ratio (e/H) increases, the core flow is more disturbed and accelerated in the midsections of ribs. Therefore, the obtained data show higher heat/mass transfer in the higher aspect ratio duct. Dean vortices also augment heat/mass transfer in the turn and in the upstream region of the second pass. However, the effect becomes less significant for the higher aspect ratio because the surface area increases in the present geometric condition. The effect of rotation produces heat/mass transfer discrepancy.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.4
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• pp.456-466
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• 2004

An experimental study has been conducted to investigate the flow and heat/mass transfer characteristics within a rectangular film cooling hole of normal injection angle for various blowing ratios and Reynolds numbers. The results are compared with those for the square hole. The experiments have been performed using a naphthalene sublimation method and the flow field has been analyzed by numerical calculation using a commercial code (FLUENT). The heat/mass transfer around the hole entrance region is enhanced considerably due to the reattachment of separated flow and the vortices generated within the hole. At the hole exit region, the heat/mass transfer increases because the main flow induces a secondary vortex. It is observed that the overall heat/mass transfer characteristics are similar to those for the square hole. However, the different heat/mass transfer patterns come out due to increased aspect ratio. Unlike the square hole, the heat/mass transfer on the trailing edge side of hole entrance region has two peak regions due to split flow reattachment, and heat/mass transfer on the hole exit region is less sensitive to the blowing ratios than the square hole.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.7
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• pp.842-851
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• 2007

Oscillating foil propulsion, the engineering application of fish-like movement of a hydrofoil, has received in recent decades as a possible competitor for propellers. The oscillating foil produces an effective angle of attack, resulting in a normal force vector with thrust and lift components, and it can be expected to be a new highly effective propulsion system. We have explored propulsion hydrodynamics as a concept in wake flow pattern. The present study has been examined various conditions such as oscillating frequencies and amplitudes in NACA0010 profile. Flow visualizations showed that high thrust was associated with the generation of moderately strong vortices, which subsequently combine with trailing-edge vorticity leading to the formation of a reverse $K\acute{a}rm\acute{a}n$ vortex street. Vortex generation was inherent to jet production and playeda fundamental role in the wake dynamics. And it was shown that the strong thrust coefficient obtained as the Strouhal number was larger.

• 한국전산유체공학회:학술대회논문집
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• 2007.10a
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• pp.187-193
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• 2007

The flight vehicles have cavities such as wheel wells and bomb bays. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves. Resonance phenomena can damage the structures around the cavity and negatively affect aerodynamic performance and stability. In the present study, numerical analysis was performed for cavity flows by the unsteady compressible three dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with Wilcox's ${\kappa}\;-\;{\omega}$ turbulence model. The cavity has the aspect ratios of 2.5, 3.5 and 4.5 for two-dimensional case, same aspect ratios with the W/D ratio of 2 for three-dimensional case. The Mach and Reynolds numbers are 0.53 and 1,600,000 respectively. The flow field is observed to oscillate in the "shear layer mode" with a feedback mechanism. Based on the SPL(Sound Pressure Level) analysis of the pressure variation at the cavity trailing edge, the dominant frequency was analyzed and compared with the results of Rossiter's formula. The MPI(Message Passing Interface) parallelized code was used for calculations by PC-cluster.

• 한국전산유체공학회:학술대회논문집
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• 2008.03a
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• pp.297-301
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• 2008

The flight vehicles have cavities such as wheel wells and bomb bays. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves. Resonance phenomena can damage the structures around the cavity and negatively affect aerodynamic performance and stability. The flow field is observed to oscillate in the "shear layer mode" with low aspect ratio. In the present study, numerical analysis was performed for cavity flows by the unsteady compressible three dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with Wilcox's $\kappa$-$\omega$ turbulence model. The flow field is observed to oscillate in the shear layer mode" with large aspect ratio. Based on the SPL(Sound Pressure Level) analysis of the pressure variation at the cavity trailing edge, the dominant frequency was analyzed and compared with the results of Rossiter's formul. The aero-acoustic wave analyzed with CPD(Correlation of Pressure Distribution).