Control of laminar vortex shedding behind a circular cylinder using tabs

윤지은,김정우,최해천 대한기계학회 2014 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.28 No.5

Small, thin flat plates (called tabs hereafter) are attached to the upper and lower surfaces of a circular cylinder to control vortex sheddingand reduce the mean drag and lift fluctuations at the Reynolds number of 100. We vary the location and size of the tabs and thedistance between the adjacent tabs. The maximum amount of drag reduction by the tabs is 17%. It is found that the tabs perturb twodimensionalvortex shedding and introduce spanwise mismatch of vortex shedding, which weakens the strength of vortex shedding oreven annihilates vortex shedding. The present result suggests that these tabs are an effective passive device for the control of vortex sheddingbehind two-dimensional bluff bodies.Small, thin flat plates (called tabs hereafter) are attached to the upper and lower surfaces of a circular cylinder to control vortex sheddingand reduce the mean drag and lift fluctuations at the Reynolds number of 100. We vary the location and size of the tabs and thedistance between the adjacent tabs. The maximum amount of drag reduction by the tabs is 17%. It is found that the tabs perturb twodimensionalvortex shedding and introduce spanwise mismatch of vortex shedding, which weakens the strength of vortex shedding oreven annihilates vortex shedding. The present result suggests that these tabs are an effective passive device for the control of vortex sheddingbehind two-dimensional bluff bodies.

반구 전방에 생성된 말굽와류 흡입제어에 의한 후류영역 마찰저항 감소에 관한 연구

구본국,강용덕 해양환경안전학회 2019 해양환경안전학회지 Vol.25 No.6

The aim of this study was to investigate the possibility of the skin-friction reduction by vortex control. A vortical system such as a horseshoe vortex, a hairpin vortex, and a wake region was induced around a hemisphere attached on a Perspex flat plate in the circulating water channel. Hairpin vortices were developed from the wake region and horseshoe vortices were formed by an adverse pressure gradient in front of the hemisphere. The horseshoe vortices located on the flank of the hemisphere induced a high momentum flow in the wake region by the direction of their vorticity. This process increased the frequency of the hairpin vortices as well as the frictional drag on the surface of the wake region. To reduce the skin-friction drag, suction control in front of the hemisphere was applied through a hole. Flow visualization was performed to optimize the free-stream velocity, size of the hemisphere, and size of the suction hole. Once the wall suction control mitigated the strength of the horseshoe vortex, the energy supplied to the wake region was reduced, causing the frequency of the hairpin vortex generation to decrease by 36.4%. In addition, the change in the skin-friction drag, which was measured with a dynamometer connected to a plate in the wake region, also decreased by 2.3%. 층류 경계층 내 반구에 의해 유기되는 말굽와류를 흡입 제어했을 때 후류영역에서의 마찰저항 변화를 측정하였다. 이를 위해 회류수조에서 유동가시화를 실시하여 최적의 자유유속, 반구 크기 및 흡입제어 구멍 크기를 결정하였고, 반구 후류영역에 설치된 평판과 연결된 동력계로 표면 마찰저항 감소를 측정하는 실험을 수행하였다. 평판에 설치된 반구 전방에는 유입 유동에 의해 반구를 감싸는 말굽와류가 생성되며 그 주위 와도 방향에 의해 후류영역으로는 빠른 유속의 유동이 유입되어 머리핀 와류 생성을 촉진시킨다. 따라서 반구 전방에 생성되는 말굽와류 세기를 흡입 제어에 의해 약화시킴으로써 반구 좌우측으로 길게 형성된 유속방향 와류가 후류영역으로 공급하는 에너지는 감소하게 된다. 즉, 반구 전방의 말굽와류를 제어함으로써 후류영역으로부터 생성되는 헤어핀 와류 발생 주파수가 줄어들게 된다. 염료 주입을 이용한 유동 가시화 영상을 해석한 결과로 머리핀 와류의 발생 빈도가 흡입제어에 의해 36.4% 감소되었고, 후류영역에서 측정된 표면 마찰저항은 2.3% 감소되는 것으로 나타났다.

Dynamics and instability of the Karman wake mode induced by periodic forcing

Mureithi, Njuki W. Techno-Press 2004 Wind and Structures, An International Journal (WAS Vol.7 No.4

This paper presents some fundamental results on the dynamics of the periodic Karman wake behind a circular cylinder. The wake is treated like a dynamical system. External forcing is then introduced and its effect investigated. The main result obtained is the following. Perturbation of the wake, by controlled cylinder oscillations in the flow direction at a frequency equal to the Karman vortex shedding frequency, leads to instability of the Karman vortex structure. The resulting wake structure oscillates at half the original Karman vortex shedding frequency. For higher frequency excitation the primary pattern involves symmetry breaking of the initially shed symmetric vortex pairs. The Karman shedding phenomenon can be modeled by a nonlinear oscillator. The symmetrical flow perturbations resulting from the periodic cylinder excitation can also be similarly represented by a nonlinear oscillator. The oscillators represent two flow modes. By considering these two nonlinear oscillators, one having inline shedding symmetry and the other having the Karman wake spatio-temporal symmetry, the possible symmetries of subsequent flow perturbations resulting from the modal interaction are determined. A theoretical analysis based on symmetry (group) theory is presented. The analysis confirms the occurrence of a period-doubling instability, which is responsible for the frequency halving phenomenon observed in the experiments. Finally it is remarked that the present findings have important implications for vortex shedding control. Perturbations in the inflow direction introduce 'control' of the Karman wake by inducing a bifurcation which forces the transfer of energy to a lower frequency which is far from the original Karman frequency.

Investigation of passive flow control on the bluff body with moving-belt experiment

Rho, Joo-Hyun,Lee, Dongho,Kim, Kyuhong The Korean Society for Aeronautical and Space Scie 2016 International Journal of Aeronautical and Space Sc Vol.17 No.2

The passive control methods such as horizontal and vertical fences on the lower surface of the bluff body were applied to suppress the vortex shedding and enhance the aerodynamic stability of flow. For investigating the effects of the passive control methods, wind tunnel experiments on the unsteady flow field around a bluff body near a moving ground were performed. The boundary layer and velocity profiles were measured by the Hot Wire Anemometer (HWA) system and the vortex shedding patterns and flow structures in a wake region were visualized via the Particle Image Velocimetry (PIV) system. Also, it is a measuring on moving ground condition that the experimental values of the critical gap distances, Strouhal numbers and aerodynamic force FFT analyses. Through the experiments, we found that the momentum supply due to moving ground caused the vortex shedding at the lower critical gap distance rather than that of fixed ground. The horizontal and vertical fences increase the critical gap distance and it can suppress the vortex shedding. Consequently, the stability characteristics of the bluff body near a moving ground could be effectively enhanced by the simple passive control such as the vertical fences.

Investigation of Passive Flow Control on the Bluff Body with Moving-Belt Experiment

노주현,이동호,김규홍 한국항공우주학회 2016 International Journal of Aeronautical and Space Sc Vol.17 No.2

The passive control methods such as horizontal and vertical fences on the lower surface of the bluff body were applied to suppress the vortex shedding and enhance the aerodynamic stability of flow. For investigating the effects of the passive control methods, wind tunnel experiments on the unsteady flow field around a bluff body near a moving ground were performed. The boundary layer and velocity profiles were measured by the Hot Wire Anemometer (HWA) system and the vortex shedding patterns and flow structures in a wake region were visualized via the Particle Image Velocimetry (PIV) system. Also, it is a measuring on moving ground condition that the experimental values of the critical gap distances, Strouhal numbers and aerodynamic force FFT analyses. Through the experiments, we found that the momentum supply due to moving ground caused the vortex shedding at the lower critical gap distance rather than that of fixed ground. The horizontal and vertical fences increase the critical gap distance and it can suppress the vortex shedding. Consequently, the stability characteristics of the bluff body near a moving ground could be effectively enhanced by the simple passive control such as the vertical fences

Investigation of passive flow control on the bluff body with moving-belt experiment

Joo-Hyun Rho,Dongho Lee,Kyuhong Kim 한국항공우주학회 2016 International Journal of Aeronautical and Space Sc Vol.17 No.2

The passive control methods such as horizontal and vertical fences on the lower surface of the bluff body were applied to suppress the vortex shedding and enhance the aerodynamic stability of flow. For investigating the effects of the passive control methods, wind tunnel experiments on the unsteady flow field around a bluff body near a moving ground were performed. The boundary layer and velocity profiles were measured by the Hot Wire Anemometer (HWA) system and the vortex shedding patterns and flow structures in a wake region were visualized via the Particle Image Velocimetry (PIV) system. Also, it is a measuring on moving ground condition that the experimental values of the critical gap distances, Strouhal numbers and aerodynamic force FFT analyses. Through the experiments, we found that the momentum supply due to moving ground caused the vortex shedding at the lower critical gap distance rather than that of fixed ground. The horizontal and vertical fences increase the critical gap distance and it can suppress the vortex shedding. Consequently, the stability characteristics of the bluff body near a moving ground could be effectively enhanced by the simple passive control such as the vertical fences

Proportional feedback control of laminar flow over a hemisphere

이정일,손동건 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.8

In the present study, we perform a proportional feedback control of laminar flow over a hemisphere at Re = 300 to reduce its lift fluctuations by attenuating the strength of the vortex shedding. As a control input, blowing/suction is distributed on the surface of hemisphere before the separation, and its strength is linearly proportional to the transverse velocity at a sensing location in the centerline of the wake. The sensing location is determined based on a correlation function between the lift force and the time derivative of sensing velocity. The optimal proportional gains for the proportional control are obtained for the sensing locations considered. The present control successfully attenuates the velocity fluctuations at the sensing location and three dimensional vortical structures in the wake, resulting in the reduction of lift fluctuations of hemisphere.

지면 운동에 따른 정사각주 후류의 와류 유동장 수치 해석

김태윤(Tae-yoon Kim),이보성(Bo-sung Lee),이동호(Dong-ho Lee),Y. Kohama 한국항공우주학회 2005 韓國航空宇宙學會誌 Vol.33 No.6

지면 근처에 존재하는 뭉뚝한 물체의 유동장 이해는 자동차 및 항공 업계에 매우 중요한 분야이다. 이를 위해 비압축성 평균 Navier-Stokes 방정식에 ε-SST 난류 모델을 적용하여 정사각주와 이동 지면의 간극 유동을 해석하였다. 비정상 진동을 억제하기 위하여 사각주 하부에 수직/수평의 펜스 설치 효과를 연구하였다. 지면이 운동할 경우에는 지면의 박리 전단층의 강도가 약화되어 사각주 상/하부의 박리 전단층 상호 작용을 촉진시키므로 고정 지면에 비하여 더 낮은 간극에서도 와류 배출이 발생한다. Understanding of the flow past a bluff body close to a moving ground is very important in automobile and aeronautical fields because of aerodynamic characteristic and instability induced by unsteady vortex shedding. The passive control method that mounted the vertical and horizontal plates at the lower surface of the cylinder is studied to suppress the unsteady oscillation motion. When the grounds moves, the diminish of the shear layer on the ground promotes the interaction between the lower and the upper separated shear layers of the cylinder, hence vortex shedding occurs at the lower gap height than the stationary ground.

오픈폼(OpenFOAM)을 활용한 본 카르만 와류의 오픈 루프 및 피드백 제어를 위한 원형 실린더 주위의 전산해석 연구

김정현,김규홍,C.F. Lange,C.R. Koch 한국항공우주학회 2013 한국항공우주학회 학술발표회 논문집 Vol.2013 No.11

본 연구에서는 저 레이놀즈 수에서 원형실린더 주위에 발생하는 유동현상에 대한 전산해석 연구를 수행하였다. 저 레이놀즈 수 영역에서는 본 카르만 와류의 흘림 현상이 발생하게 되는데, 이러한 후류 영역의 감소를 위해 Injection 과 Suction 이 가능한 두 개의 엑츄에이터를 원형실린더 표면에 설치하였다. 오픈 소스 기반의 전산유체역학 프로그램인 오픈폼을 이용하여 2 차원 Navier-Stokes 방정식을 수치적으로 접근하였고, 먼저 엑츄에이터가 없는 원형실린더 상태에서의 저 레이놀즈 수 영역에서 발생하는 본 카르만 와류의 흘림 현상을 묘사하는 수치적인 방법에 대하여 검증을 수행하였다. 검증이 완료된 해석 모델을 기반으로 후류 영역의 제어와 항력을 줄이기 위해 개방 루프 제어 시스템을 원형실린더 표면에 설치하여 전산해석을 수행하였다. 그 결과 항력의 감소뿐만 아니라 원형 실린더 뒤에서 발생하는 후류 영역의 주기성의 감소를 확인할 수 있었다. 이에 따라 오픈 소스 기반의 오픈폼의 활용가능성을 확인하였고 이를 바탕으로 향후 수행할 연구 방향을 제시하였다. A computational study of the flow past a circular cylinder at low Reynolds is performed. At this Reynolds number Von Karman vortex shedding wake is observed. To attenuate this wake, two actuators that inject fluid or remove fluid from the surface of the cylinder are used. The investigation is performed numerically by solving the Navier-Stokes equation in two-dimensions using OpenFOAM which is an open source code. The code is first validated without actuation on results from the literature for Von Karman vortex shedding at low Reynolds number and then used to study active(Open loop) control of vortex shedding to reduce drag and control the vortex strength.

Numerical simulation of fluid flow and forced convection heat transfer around a circular cylinder with control rods located in equilateral triangular arrangement

Somayeh Harimi,Azam Marjani,Sadegh Moradi 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.9

Numerical study of forced convection heat transfer and fluid flow in laminar flow regime for a circular cylinder attached by three control rods is performed using the overset grid method. The aim of this work is evaluation of the control rods performance placed in equilateral triangular arrangements in suppressing vortex induced vibration of a primary cylinder. The influence of the dimensionless parameters including attach angle α, spacing ratio G/D, and Reynolds number on the hydrodynamic forces of the primary cylinder is also investigated. The unsteady flow at Reynolds number of 200 and prandtl numbers of 0.7 and 7.0 is considered. In order to discretize the governing equations, a finite volume code based on the SIMPLEC algorithm is employed. Moreover, the local and mean Nusselt numbers are presented to illustrate the heat transfer characteristics of the primary cylinder and surrounding rods.