• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.715-720
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• 2000

Stratified flow past a three-dimensional obstacle such as a sphere has been a long-lasting subject of geophysical, environmental and engineering fluid dynamics. In order to investigate the effect of the stratification on the near wake, in particular, the unsteady vortex formation behind a sphere, numerical simulations of stratified flows past a sphere are conducted. The time-dependent Navier-Stokes equations are solved using a three-dimensional finite element method and a modified explicit time integration scheme. Laminar flow regime is considered and linear stratification of density is assumed under Boussinesq approximation. The computed results include the characteristics of the near wake and the unsteady vortex shedding. With a strong stratification, the separation on the sphere is suppressed and the wake structure behind the sphere becomes planar, resembling that behind a vertical cylinder.

• 대한기계학회논문집B
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• 제28권7호
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• pp.771-779
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• 2004

Unsteady behavior of the early wake in the viscous flow field past an impulsively started semicircular cylinder is studied numerically. In this paper, we propose the hybrid diffusion scheme to simulate dynamic characteristics of wake such as a fishtail-like flapping and an alternate vortex-shedding more accurately. This diffusion scheme based on particle strength exchange is mixed with the stochastic nature of random walk method. Also, the viscous splitting algorithm which calculates convective and diffusion terms successively is applied in order to handle random walk method effectively. Consequently, the early behavior of wake due to the breakdown of symmetrical vortici balance is more practically simulated with the vortex particle method.

• 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.

• 한국안전학회지
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• 제22권1호
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• pp.24-29
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• 2007

The effectiveness of a UV(ultra violet) disinfection system depends on the characteristics of the waste water, flow conditions, the intensity of UV radiation, the amount of time the microorganisms are exposed to the radiation, and the reactor configuration. The wast water flow conditions are important factors in the design of UV disinfection system from the point of enhancement view of UV disinfection. The turbulent energy intensity in the wake by the vortex shedding are effective for UV radiation. Therewith the effectiveness of vortex generator is considered as a enhancement of UV disinfection. The experimental results presented give important evidences and explain that it is possible to predict UV disinfection performance based on flow experiments. An experimental investigation of two types of the vortex generator is presented. The qualitative and quantitative evaluations of the wake are made by flow visualization using smoke wire method and the measurement of vortex frequencies in the wind tunnel. From the experiment, following results were obtained that the delta wing type vortex generator is more effective than circular type because of the higher vortex frequencies and the smaller drag.

• 한국추진공학회지
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• 제19권1호
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• pp.42-49
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• 2015

3차원 Large Eddy Simulation(LES)와 Proper Orthogonal Decomposition(POD) 기법을 이용하여 고체로켓의 인히비터에서 발생하는 연소실내 압력 진동 특성을 분석하였다. 인히비터 후방에서 발생한 와류는 Flow-acoustic coupling에 의해 주기적으로 반복하여 생성, 소멸이 이루어지는 것을 확인하였고, 이 와류가 내삽 노즐 입구 도출부에 충돌하면서 유동이 불균질하게 분해되고, 후방 돔으로 유입된 유동에 의한 압력 진동은 연소실 압력 진동 가진의 원인이 된다. 또한 인히비터에서 발생하는 와흘림(vortex shedding) 주기는 연소실내 와류 발생 주기와 일치하며, 실험에서 측정된 압력 진동 주파수와 비교 분석하였다.

• Wind and Structures
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• 제20권2호
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• pp.327-347
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• 2015

This paper investigates the effects of Reynolds number (Re) on the aerodynamic characteristics of a twin-deck bridge. A 1:36 scale sectional model of a twin girder bridge was tested using the Wall of Wind (WOW) open jet wind tunnel facility at Florida International University (FIU). Static tests were performed on the model, instrumented with pressure taps and load cells, at high wind speeds with Re ranging from $1.3{\times}10^6$ to $6.1{\times}10^6$ based on the section width. Results show that the section was almost insensitive to Re when pitched to negative angles of attack. However, mean and fluctuating pressure distributions changed noticeably for zero and positive wind angles of attack while testing at different Re regimes. The pressure results suggested that with the Re increase, a larger separation bubble formed on the bottom surface of the upstream girder accompanied with a narrower wake region. As a result, drag coefficient decreased mildly and negative lift coefficient increased. Flow modification due to the Re increase also helped in distributing forces more equally between the two girders. The bare deck section was found to be prone to vortex shedding with limited dependence on the Re. Based on the observations, vortex mitigation devices attached to the bottom surface were effective in inhibiting vortex shedding, particularly at lower Re regime.

• 대한조선학회논문집
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• 제28권2호
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• pp.118-131
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• 1991

자유수면 근처에서의 박리유동 현상을 수치 시뮬레이션 방법과 유선 가시화 시험 방법으로 연구하였다. 수치 시뮬레이션은 물체표면에서 유기되고 확산(diffusion) 및 대류(convection)에 의해 유동중으로 박리되어 방출되는 보오티시티(vorticity)를 다수의 보오텍스로 치환하여 유동의 변화와 유체력을 구하는 보오텍스블럽 법을 사용하여 수행하였다. 이 방법으로 대규모의 와류의 생성 및 변화를 추정할 수 있으며 또한 자유수면과 와류유동의 상호간섭현상 그리고 동 유체력 등을 추정할 수 있었다. 유선 가시화 시험은 해사기술연구소의 공동수조에서 수행하였으며, 직류 전원에 의해 얻어진 수소기포를 가시화 입자로 이용하였다. 조명장치는 할로겐 램프를 이용하였고, 연속사진을 가시화의 결과로 사용하였다. 또한 자유수면의 시간적 공간적 변화를 파고계를 이용하여 계측하고 이로부터 와류유동의 변화 주기를 추정하였다.

• Journal of Advanced Marine Engineering and Technology
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• 제24권4호
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• pp.486-493
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• 2000

Flow mechanism around air flow sensor of electronic control engine, especially Karman vortex type, was investigated experimentally. The two-dimensional flow characteristics in the intermediate wake region behind a triangular vortex-generator respectively apex forward facing, apex backward facing and vertical flat plates following after apex forward facing(i.e vortex-flowmeter) were investigated at Reynolods number of $ReH=1.4\times10^4$; H is the width of a triangular vortex-generator. The vortex shedding frequency for wide Reynolds number from $7\times10^3$ to $2.1\times10^4$ was also surveyed. The velocity component was measured by X-type hot wire anemometer at 8H downstream from the bluff body. The coherent structure of the intermediate wake behind a bluff body was obtained by conditional phase average technique. As a result, it was verified that the vertical flat plates following after apex forward triangular vortex-generator make not only more linear relation between free stream velocity and vortex shedding frequency but also more periodic vortex in the vicinity of the center of wake.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2011년도 제36회 춘계학술대회논문집
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• pp.253-257
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• 2011

고체로켓에서 발생하는 vortex shedding 현상 중 인히비터로 인해 발생하는 연소실내 와류(vortex)의 특성을 조사하기 위해 Large Eddy Simulation을 수행하였다. 해석의 결과는 기존 연구자들의 결과와 잘 일치하며 정략적 및 정성적 분석을 수행하였다. 인히비터 후방에서 발생하는 vortex는 Flow-acoustic coupling 에 의해 주기적으로 반복되며 생성, 소멸이 이루어지는 것을 확인 할 수 있었으며, 발생 주기는 연소실내 mode 2의 주파수와 일치하는 것을 확인하였다. 3차원 해석결과 인히비터 후방에서 Roll 발생은 비균일한 노즐 유동을 발생시킨다.

• 한국전산유체공학회지
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• 제13권4호
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• pp.39-44
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• 2008

Numerical simulations of laminar flow over a sphere are conducted to investigate the effect of the Reynolds number on the characteristics of vortex shedding. The Reynolds numbers considered are between 300 and 475, covering unsteady planar-symmetric and asymmetric flows. Results show that the unsteady planar-symmetric flow can be categorized into two different regimes: single-frequency regime and multiple-frequency regime. The former has a single frequency component due to periodic shedding of the vortices with the same strength in every shedding cycle, while the latter has multiple frequency components due to cycle-to-cycle variation in the strength of shed vortices with the shedding angle fixed. The multiple-frequency planar-symmetric flow, which is newly found in the present study, occurs at Re=330${\sim}$360 between the single-frequency planar-symmetric flow and the asymmetric flow. On the other hand, the asymmetric flow occurs at Re${\geq}$365, where the vortices shed from the sphere show variation both in strength and shedding angle unlike the planar-symmetric flow. Also, it is shown that the breaking of planar symmetry is closely related to the imbalance of vortical strength between a pair of streamwise vortices.