• Wind and Structures
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• 제4권3호
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• pp.227-246
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• 2001

The objective of this study is to understand the flow above the front edge of low-rise building roofs. The greatest suction on the building is known to occur at this location as a result of the formation of conical vortices in the separated flow zone. It is expected that the relationship between this suction and upstream flow conditions can be better understood through the analysis of the vortex flow mechanism. Experimental measurements were used, along with predictions from numerical simulations of delta wing vortex flows, to develop a model of the pressure field within and beneath the conical vortex. The model accounts for the change in vortex suction with wind angle, and includes a parameter indicating the strength of the vortex. The model can be applied to both mean and time dependent surface pressures, and is validated in a companion paper.

• 대한기계학회논문집B
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• 제23권9호
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• pp.1106-1112
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• 1999

Substantial losses behind axial flow rotor are generated by the wake, various vortices in the hub region and the tip leakage vortex in the tip region. Particularly, the leakage vortex formed near blade tip is one of the main causes of the reduction of performance, generation of noise and aerodynamic vibration in downstream. In this study, the three-dimensional flow fields in an axial flow rotor were calculated with varying tip clearance under various flow rates, and the numerical results were compared with experimental ones. The numerical technique was based on SIMPLE algorithm using standard $k-{\varepsilon}$ model(WFM) and Launder & Sharma's Low Reynolds Number $k-{\varepsilon}$ model(LRN). Through calculations, the effects of tip clearance and attack angle on the 3-dimensional flow fileds behind a rotor and leakage flow/vortex were investigated. The presence of tip leakage vortex, loci of vortex center and its behavior behind the rotor for various tip clearances and attack angles was described well by calculation.

• Wind and Structures
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• 제32권4호
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• pp.309-319
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• 2021

The vortex-drift pattern over a girder surface, actually demonstrating the complex fluid-structure interactions between the structure and surrounding flow, is strongly correlated with the VIVs but has still not been elucidated and may be useful for modeling VIVs. The complex fluid-structure interactions between the structure and surrounding flow are considerably simplified in constructing a vortex model to describe the vortex-drift pattern characterized by the ratio of the vortex-drift velocity to the oncoming flow velocity, considering the aerodynamic work. A spring-suspended sectional model (SSSM) is used to measure the pressure in wind tunnel tests, and the aerodynamic parameters for a typical streamlined closed-box girder are obtained from the spatial distribution of the phase lags between the distributed aerodynamic forces at each pressure point and the vortex-excited forces (VEFs). The results show that the ratio of the vortex-drift velocity to the oncoming flow velocity is inversely proportional to the vibration amplitude in the lock-in region and therefore attributed to the "lock-in" phenomena of the VIVs. Installing spoilers on handrails can destroy the regular vortex-drift pattern along the girder surface and thus suppress vertical VIVs.

• Wind and Structures
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• 제19권2호
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• pp.113-144
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• 2014

Separation bubble and conical vortices on a large-span flat roof were observed in this study through the use of flow visualization. The results indicated that separation bubble occurred when the flow was normal to the leading edge of the flat roof. Conical vortices that occur under the cornering flow were observed near the leading edge, and their appearance was influenced by the wind angle. When the wind changed from along the diagonal to deviating from the diagonal of the roof, the conical vortex close to the approaching flow changed from circular to be more oblong shaped. Based on the measured velocities in the conical vortices by flow visualization, a proposed two-dimensional vortex model was improved and validated by simplifying the velocity profile between the vortex and the potential flow region. Through measured velocities and parameters of vortices, the intensities of conical vortices and separation bubble on a large-span flat roof under different wind directions were provided. The quasi-steady theory was corrected by including the effect of vortices. With this improved two-dimensional vortex model and the corrected quasi-steady theory, the mean and peak suction beneath the cores of the conical vortices and separation bubble can be predicted, and these were verified by measured pressures on a larger-scale model of the flat roof.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제27권4호
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• pp.232-244
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• 2023

The unsteady dynamics of the starting flow of a flat plate is studied by using a vortex shedding model. The model describes the body and separated vortex from the trailing edge of the plate by vortex sheets, retaining a singularity at the leading edge. The model is applied to simulate the flow of an accelerated plate for small angles of attack. For numerical computations, we take two representative cases of the translational velocity of a plate: impulsive translation and uniform acceleration. The model successfully demonstrates the formation of wakes shed from the plate. The wake behind the plate is stronger for a larger angle of attack. Predictions for the lifting force from the model are in agreement with results of Navier-Stokes simulations.

• 대한기계학회논문집B
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• 제27권7호
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• pp.883-892
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• 2003

The experiment using three-dimensional laser Dopperr velocimetery (LDV) measurements and the computation using the Reynolds stress model of the commercial code, FLUENT, were conducted to give a clear understanding on the structure of tip leakage flow in a forward-swept axial-flow fan operating at the maximum efficiency condition. The tip leakage vortex was generated near the position of the minimum wall static pressure, which was located at approximately 12% chord downstream from the leading edge of blade suction side, and developed along the centerline of the pressure trough within the blade passages. A reverse flow between the blade tip region and the casing, induced by tip leakage vortex, acted as a blockage on the through-flow. As a result, high momentum flux was observed below the tip leakage vortex. As the tip leakage vortex proceeded to the aft part of the blade passage, the strength of tip leakage vortex decreased due to the strong interaction with the through-flow and casing boundary layer, and the diffusion of tip leakage vortex caused by high turbulence. In comparison with LDV measurement data, the computed results predicted the complex viscous flow patterns inside the tip region, including the locus of tip leakage vortex center, in a reliable level.

• 한국유체기계학회 논문집
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• 제12권4호
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• pp.14-22
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• 2009

The head-capacity curves for pumps developed by the pump manufacturer are based on tests of a single pump operating in a semi-infinite basin with no close walls or floors and with no stray currents. Therefore, flow into the pump intake is with no vortices or swirling. However, pump station designers relying on these curves to define the operating conditions for the pump selected sometimes meet the reductions of capacity and efficiency, as well as the increase of vibration and additional noise, which were caused by air-entered flow in the pump station. From this background, the authors are carrying out a systematic study on the flow characteristics of intakes within a sump of pump station model. Multi-intake sump model with anti-submerged vortex device basin is designed and the characteristics of submerged vortex is investigated in the flow field by numerical simulation. In this study, a commercial CFD code is used to predict the vortex generation in the pump station accurately. The analysed results by CFD show that the vortex structure and effect of anti-submerged vortex device are different at each pump intake channel.

• Wind and Structures
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• 제4권4호
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• pp.279-298
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• 2001

The greatest suction on the cladding of flat roof low-rise buildings is known to occur beneath the conical vortices that form along the roof edges for cornering winds. In a companion paper, a model of the vortex flow mechanism has been developed which can be used to connect the surface pressure beneath the vortex to adjacent flow conditions. The flow model is experimentally validated in this paper using simultaneous velocity and surface pressure measurement on a 1 : 50 model of the Texas Tech University experimental building in a wind tunnel simulated atmospheric boundary layer. Flow visualization gives further insight into the nature of peak suction events. The flow model is shown to account for the increase in suction towards the roof corner as well as the presence of the highest suction at wind angles of $60^{\circ}$. It includes a parameter describing vortex suction strength, which is shown to be related to the nature of the reattachment, and also suggests how different components of upstream turbulence could influence the surface pressure.

• 한국전산유체공학회지
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• 제14권3호
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• pp.76-85
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• 2009

The shock wave is deformed and the vortex is elongated simultaneously during the shock-vortex interaction. More precisely, the shock wave is deformed to a S-shape, consisting of a leading shock and a lagging shock by which the corresponding local vortex flows are accelerated and decelerated, respectively: the vortex flow swept by the leading shock is locally expanded and the one behind the lagging shock is locally compressed. As the leading shock escapes the vortex in the order of microseconds, the expanded flow region is quickly changed to a compression region due to the implosion effect. An induced shock is developed here and propagated against the vortex flow. This happens for a strong vortex because the tangential flow velocity of the vortex core is high enough to make the induced-shock wave speed supersonic relative to the vortex flow. For a weak shock, the vortex is basically subsonic and the induced shock wave is absent. For a vortex of intermediate strength, an induced shock wave is developed in the supersonic region but dissipated prematurely in the subsonic region. We have expounded these three shock-vortex interaction patterns that depend on the vortex flow regime using a third-order ENO method and numerical shadowgraphs.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.371-379
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• 2000

A noise prediction method of axial flow fan is developed by incorporating through-flow method and vortex shedding noise model. Fan noise is assumed to be generated due to the pressure fluctuation induced by wake vortices of fan blades and radiate as diploe distribution. The wake vortices are analyzed by combining Karman vortex street model and through-flow analysis results, and the vortex-induced fluctuating pressure on blade surface is calculated by thin airfoil theory. The predicted sound pressure levels and directivity patterns of fan noise by the present method are favorably compared with fan noise test data. Furthermore, the present method is shown to be very useful for predicting the aero-acoustic performance map of the fan operated at off-design point.