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.

Navier-Stokes 식을 이용한 회전 진동하는 2차원 원형 실린더 주위 유동 해석

이명국(M.K. Lee),김재수(J.S. Kim) 한국전산유체공학회 2011 한국전산유체공학회지 Vol.16 No.3

Although the geometiy of circular cylinder is simple, the flow is complicate because of the flow separation and vortex shedding. In spite of many numerical and experimental researches, the flow around a circular cylinder has not been clarified even now. It has been known that the unsteady vortex shedding from a circular cylinder can vibrate and damage a structure. Lock-on phenomenon is veiy important in the flow around an oscillating circular cylinder. The lock-on phenomenon is that when the oscillation frequency of the circular cylinder is at or near the frequency of vortex shedding from a stationary cylinder, the vortex shedding synchronizes with the cylinder motion. This phenomenon can be recognized by the spectral analysis of the lift coefficient history. At the lock-on region the vortex is shedding by the modulated frequency to the body frequency. However, the vortex is shedding by the mixed frequencies of natural shedding and forced body frequency in the region of non-lock-on. In this paper, it was analyzed the relation between the frequency of rotary oscillating circular cylinder and the vortex shedding frequency.

이중원형실린더에서 발생하는 와류방출 해석

김대근,김호경,허재철,김성만 한국풍공학회 2009 한국풍공학회지 Vol.13 No.3

This paper uses the commercial code of ADINA to analyze the vortex shedding for two staggered circular cylinders and changes in the wind load placed on the cylinders along with changes to the approach wind velocity and the incidence angles of wind. On this basis, this paper has reached the following conclusions: First, the Karman vortex takes place predominantly when the Reynolds number is 4,970, while the narrow turbulent wake prevails in the rear of the cylinders when the number stands at 331,344. This air current phenomenon as well as approach wind incidence angle results in the asymmetry of pneumonic force against the downwind cylinders. Second, the upwind and downwind cylinders discharge vortexes with almost identical frequencies under the simulated conditions of this paper, but the differences in phase between vortexes in the two cylinders do change by approach wind velocity. 본 연구에서는 상용코드인 ADINA를 이용하여, 유입풍속과 바람의 입사각을 변화시키며 이중원형실린더에서 발생하는 와류방출 현상과 실린더에 작용하는 공기력의 변동양상을 분석하였다. 본 연구결과 다음과 같은 결과를 얻을 수 있었다. 첫째, 레이놀즈수가 4,970인 경우에는 Karman vortex가 지배적으로 발생하며 레이놀즈수가 331,344인 경우에는 실린더 후면에 폭이 좁은 후류(turbulent wake)가 지배적으로 발생하는데, 이러한 기류현상은 바람의 입사각과 함께 풍하측 실린더에 작용하는 공기력의 비대칭성을 유발하는 원인이 된다. 둘째, 본 연구의 모의조건에서 풍상측과 풍하측 실린더는 거의 동일한 주파수의 와류를 방출하고 있으나, 풍상측과 풍하측 실린더에서 발생하는 와류 간의 위상차는 유입풍속에 따라 변동하는 특성을 보인다.

DNS of vortex-induced vibrations of a yawed flexible cylinder near a plane boundary

Zhimeng Zhang,Chunning Ji,Md. Mahbub Alam,Dong Xu 한국풍공학회 2020 Wind and Structures, An International Journal (WAS Vol.30 No.5

Vortex-induced vibrations of a yawed flexible cylinder near a plane boundary are numerically investigated at a Reynolds number Ren = 500 based on normal component of freestream velocity. Free to oscillate in the in-line and cross-flow directions, the cylinder with an aspect ratio of 25 is pinned-pinned at both ends at a fixed wall-cylinder gap ratio G/D = 0.8, where D is the cylinder diameter. The cylinder yaw angle () is varied from 0 to 60 with an increment of 15. The main focus is given on the influence of  on structural vibrations, flow patterns, hydrodynamic forces, and IP (Independence Principle) validity. The vortex shedding pattern, contingent on , is parallel at =0 , negatively-yawed at   15 and positively-yawed at   30. In the negatively- and positively-yawed vortex shedding patterns, the inclination direction of the spanwise vortex rows is in the opposite and same directions of , respectively. Both in-line and cross-flow vibration amplitudes are symmetric to the midspan, regardless of . The RMS lift coefficient CL,rms exhibits asymmetry along the span when   0, maximum CL,rms occurring on the lower and upper halves of the cylinder for negatively- and positively-yawed vortex shedding patterns, respectively. The IP is well followed in predicting the vibration amplitudes and drag forces for α ≤ 45° while invalid in predicting lift forces for α ≥ 30°. The vortex-shedding frequency and the vibration frequency are well predicted for  = 0 - 60 examined.

Numerical investigation of vortex shedding and vortex-induced vibration for flexible riser models

전정수,김우전 대한조선학회 2010 International Journal of Naval Architecture and Oc Vol.2 No.2

The numerical study about the vortex-induced vibration and vortex shedding in the wake has been presented. Prior to the numerical simulation of flexible riser systems concerning engineering conditions, efficiency validating of the proposed FSI solution method have been performed. The comparison between numerical simulation and published experimental data shows that the CFD method designed for FSI solution could give acceptable result for the VIV prediction of flexible riser/pipe system. As meaningful study on VIV and vortex shedding mode with the focus on flexible riser model systems, two kinds of typical simulation cases have been carried out. One was related to the simulation of vortex visualization in the wake for a riser model subject to forced oscillation, and another was related to the simulation of fluid-structure interaction between the pipes of coupled multi-assembled riser system. The result from forced oscillation simulation shows that the vortex-induced vibration with high response frequency but small instantaneous vibration amplitude contributes to vortex conformation as much as the forced oscillation with large normalized amplitude does, when the frequency of forced oscillation was relatively high. In the multi-assembled riser systems, it has been found that the external current velocity and the distance between two pipes are the critical factors to determine the vibration state and the steady vibration state emerging in quad-pipe system may be destroyed more easily than dual-pipe system.

Numerical investigation of vortex shedding and vortex-induced vibration for flexible riser models

Chen, Zheng-Shou,Kim, Wu-Joan The Society of Naval Architects of Korea 2010 International Journal of Naval Architecture and Oc Vol.2 No.2

The numerical study about the vortex-induced vibration and vortex shedding in the wake has been presented. Prior to the numerical simulation of flexible riser systems concerning engineering conditions, efficiency validating of the proposed FSI solution method have been performed. The comparison between numerical simulation and published experimental data shows that the CFD method designed for FSI solution could give acceptable result for the VIV prediction of flexible riser/pipe system. As meaningful study on VIV and vortex shedding mode with the focus on flexible riser model systems, two kinds of typical simulation cases have been carried out. One was related to the simulation of vortex visualization in the wake for a riser model subject to forced oscillation, and another was related to the simulation of fluid-structure interaction between the pipes of coupled multi-assembled riser system. The result from forced oscillation simulation shows that the vortex-induced vibration with high response frequency but small instantaneous vibration amplitude contributes to vortex conformation as much as the forced oscillation with large normalized amplitude does, when the frequency of forced oscillation was relatively high. In the multi-assembled riser systems, it has been found that the external current velocity and the distance between two pipes are the critical factors to determine the vibration state and the steady vibration state emerging in quad-pipe system may be destroyed more easily than dual-pipe system.

구 주위의 비정상 면대칭 및 비대칭 유동의 특성

김동주(Dongjoo Kim) 한국전산유체공학회 2008 한국전산유체공학회지 Vol.13 No.4

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=330~360 between the single-frequency planar-symmetric flow and the asymmetric flow. On the other hand, the asymmetric flow occurs at Re≥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.

Flow Interaction of Shedding Vortex with Injected Normal Blowing

Khin Oo Mon,Changjin Lee,Heeseok Koo 한국추진공학회 2012 한국추진공학회 학술대회논문집 Vol.2012 No.5

This paper is concerned with turbulent flow computations using Large Eddy Simulation (LES) and the flow interaction of vortex shedding in a cylindrical duct flow driven by mass blowing through the wall. The purpose is to analyze non-linear combustion characteristics in the presence of vortex shedding generated in a hybrid rocket motor. Experimental studies have shown sudden changes in pressure (referred as a DC-shift), which depend on the strength of vortex strength of incoming flow. The combustion instability because of a sudden change in pressure fluctuations is mainly related with the interaction between vortex shedding. Therefore LES computation on a duct with injected normal blowing was performed to simulate the turbulent flow interactions with the behaviors of vortices and vortex structures along the injected wall.

북대구 집광형 태양열 발전소 헬리오스탯 필드의 풍하중 분석

김현구(Kim Hyun-Goo),강용혁(Kang Yong-Heack),전완호(Jeon Wan-Ho) 한국태양에너지학회 2020 한국태양에너지학회 논문집 Vol.40 No.6

The concentration performance of a heliostat field consisting of hundreds to thousands of reflectors allows for a deformation-based focus deviation of only 3 mrad or less. The most problematic aspect is the wind load; hence, in this study, the extreme wind speed condition was analyzed. Preceding studies such as field experiments, wind tunnel tests, or Reynolds-averaged Navier–Stokes (RANS) simulations were limited to a single heliostat or a single row, such that the characteristics of vortex shedding from the heliostat field could not be practically understood. In this study, the wind flow over the heliostat field in northern Daegu for Concentrating Solar Power (CSP), consisting of 450 reflectors, was analyzed using large eddy simulations with 33 million cells, in order to identify the characteristics of vortex shedding. In the heliostat field, the windward front and the left and right sides received the peak wind load, and the characteristics of vortex shedding were different depending on the position in the field. A fast Fourier transform analysis of the time-series wind pressure confirmed that the vortex shedding frequency ranged from 0.09 to 0.32 s<SUP>-1</SUP>; it was determined that the vortex shedding from the upper and lower sides dominated that of the left and right sides of the heliostat. After predicting the wind load by Large Eddy Simulations (LES), structural analysis was performed to calculate the maximum displacement of the reflector. It was confirmed that the maximum displacement angle of the heliostat at the windward front, which receives the largest wind load at extreme wind events, satisfies the performance requirement of 1 mrad or less.

Further validation of the hybrid particle-mesh method for vortex shedding flow simulations

Lee, Seung-Jae,Lee, Jun-Hyeok,Suh, Jung-Chun The Society of Naval Architects of Korea 2015 International Journal of Naval Architecture and Oc Vol.7 No.6

This is the continuation of a numerical study on vortex shedding from a blunt trailing-edge of a hydrofoil. In our previous work (Lee et al., 2015), numerical schemes for efficient computations were successfully implemented; i.e. multiple domains, the approximation of domain boundary conditions using cubic spline functions, and particle-based domain decomposition for better load balancing. In this study, numerical results through a hybrid particle-mesh method which adopts the Vortex-In-Cell (VIC) method and the Brinkman penalization model are further rigorously validated through comparison to experimental data at the Reynolds number of $2{\times}10^6$. The effects of changes in numerical parameters are also explored herein. We find that the present numerical method enables us to reasonably simulate vortex shedding phenomenon, as well as turbulent wakes of a hydrofoil.