A Computational Study of a Circular Cylinder at Low Reynolds Number for Open Loop Control of Von Karman Vortex Shedding (original) (raw)
Related papers
2018
The suppression of vortex shedding of a larger circular cylinder by the interference effect of 8 individually rotating wake-control cylinders equally distributed about its center is investigated by means of three-dimensional numerical simulations at a Reynolds number of 10,000. Visualization of the wakes revealed a complex disruption of the vortex tubes for the higher rotation speeds. A consequent reduction in the mean drag of 84% and the fluctuating lift in 89% was obtained when compared with the reference values of a bare cylinder. The power lost to rotate the control cylinders at the highest rotation speed is lower than the power lost by the system due to drag. Configurations of control cylinder that can suppress vortex shedding may produce more efficient suppressors for flow-induced vibrations.
Control of vortex shedding behind circular cylinder for flows at low Reynolds numbers
International Journal for Numerical Methods in Fluids, 2001
It has been observed by researchers in the past that vortex shedding behind circular cylinders can be altered, and in some cases suppressed, over a limited range of Reynolds numbers by proper placement of a second, much smaller, 'control' cylinder in the near wake of the main cylinder. Results are presented for numerical computations of some such situations. A stabilized finite element method is employed to solve the incompressible Navier-Stokes equations in the primitive variables formulation. At low Reynolds numbers, for certain relative positions of the main and control cylinder, the vortex shedding from the main cylinder is completely suppressed. Excellent agreement is observed between the present computations and experimental findings of other researchers. In an effort to explain the mechanism of control of vortex shedding, the streamwise variation of the pressure coefficient close to the shear layer of the main cylinder is compared for various cases, with and without the control cylinder. In the cases where the vortex shedding is suppressed, it is observed that the control cylinder provides a local favorable pressure gradient in the wake region, thereby stabilizing the shear layer locally.
Low-dimensional feedback control of the von Karman vortex street at a Reynolds number of 100
2004
The effect of feedback flow control on the wake of a circular cylinder at a Reynolds number of 100 is investigated in direct numerical simulation. Our control approach uses a low dimensional model based on proper orthogonal decomposition (POD). The controller applies linear proportional and differential feedback to the estimate of the first POD mode. Actuation is implemented as displacement of the cylinder normal to the flow. The closed loop feedback simulations explore the effect of both fixed phase and variable phase feedback on the wake. While fixed phase feedback is effective in reducing drag and unsteady lift, it fails to stabilize this state once the low drag state has been reached. Variable phase feedback, however, achieves the same drag and unsteady lift reductions while being able to stabilize the flow in the low drag state. In the low drag state, the near wake is entirely steady, while the far wake exhibits vortex shedding at a reduced intensity. We achieved a drag reduction of close to 90% of the vortex-induced drag, and lowered the unsteady lift force by the same amount.
Control of Vortex Shedding from a Square Cylinder
Aiaa Journal, 2008
Small circular, square, and thin-strip cross-sectional elements are used to suppress vortex shedding from a square cylinder at Reynolds numbers in the range of 1:12 10 4 -1:02 10 5 . The axes of the element and cylinder are parallel. The element's size, position, and angle of attack are varied. Measurements of the fluctuating surface pressures and wake velocities, together with smoke flow visualization, show that vortex shedding from both sides of the cylinder is suppressed and the mean drag and fluctuating lift on the cylinder is reduced if the element is installed in an effective zone downstream of the cylinder. The effective zone of the circular element is shown to be much smaller than those of the other elements. The effects of Reynolds number and blockage ratio are investigated. A phenomenon of monoside vortex shedding is observed. The role of the element's bluffness is investigated and the suppression mechanism is discussed.
Journal of Fluid Mechanics, 2007
Vortex shedding behind a cylinder can be controlled by placing another small cylinder behind it, at low Reynolds numbers. This has been demonstrated experimentally by Strykowski & Sreenivasan (J. Fluid Mech. vol. 218, 1990, p. 74). These authors also provided preliminary numerical results, modelling the control cylinder by the innovative application of boundary conditions on some selective nodes. There are no other computational and theoretical studies that have explored the physical mechanism. In the present work, using an over-set grid method, we report and verify numerically the experimental results for flow past a pair of cylinders. Apart from providing an accurate solution of the Navier-Stokes equation, we also employ an energy-based receptivity analysis method to discuss some aspects of the physical mechanism behind vortex shedding and its control. These results are compared with the flow picture developed using a dynamical system approach based on the proper orthogonal decomposition (POD) technique.
Visualization study on suppression of vortex shedding from a cylinder
Journal of Visualization, 2007
A narrow strip has been introduced as a control element to suppress vortex shedding from a cylinder. The strip is set parallel to the cylinder axis, and the key parameter of control in this study is the strip position, which is determined by the angle of attack of the strip and the distance between the strip and the cylinder axis. A circular cylinder and a square cylinder were tested respectively. Flow visualization and hot-wire measurement were performed in a low turbulence wind tunnel in the range of Reynolds numberRe=4.0×103≈2.0×104. Test results show that, vortex shedding from both sides of the cylinder can be effectively suppressed if the strip is located in a certain zone in the wake. The effective zones in circular cylinder wakes at different Reynolds numbers have been found out, and the mechanism of the suppression has been discussed.
Energies, 2020
A discrete vortex method is implemented with a hybrid control technique of vortex shedding to solve the problem of the two-dimensional flow past a slightly rough circular cylinder in the vicinity of a moving wall. In the present approach, the passive control technique is inspired on the fundamental principle of surface roughness, promoting modifications on the cylinder geometry to affect the vortex shedding formation. A relative roughness size of ε*/d* = 0.001 (ε* is the average roughness and d* is the outer cylinder diameter) is chosen for the test cases. On the other hand, the active control technique uses a wall plane, which runs at the same speed as the free stream velocity to contribute with external energy affecting the fluid flow. The gap-to-diameter varies in the range from h*/d* = 0.05 to 0.80 (h* is the gap between the moving wall and the cylinder bottom). A detailed account of the time history of pressure distributions, simultaneously investigated with the time evolution ...
Physically, when the Reynolds number exceeds some critical value, vortex shedding past a circular cylinder appears naturally and immediately. Numerically, if the domain geometry and the approaching flow conditions are symmetric, vortex triggering requires a long run-time, especially for low Reynolds numbers. The present study proposes to reduce this run-time by acting on the initial conditions instead of the classical approaches based on boundary conditions perturbation. In contrast to these, the proposed technique does not bring any energy to the flow. The vortex shedding is simply triggered by introducing a lateral gradient to the initial streamwise velocity. Simulations are performed using the ANSYS CFX 12 Ò finite-element-based finite volume code. A two-dimensional laminar flow at Re = 100 is first considered. Without perturbing the initial uniform flow, the obtained results are in good agreement with the experimental and numerical results available in the literature. With perturbed initial conditions, the main characteristics of the flow are properly found while the run-time required for triggering the ultimate regular periodic regime with vortex is considerably reduced. Simulation results show that the extent of the run-time reduction depends on the amplitude of the initial perturbation. The search of the optimal value corresponding to the largest run-time reduction led us to propose an analytical expression by assuming that the natural vortex shedding frequency is equal to the periodic lateral perturbation frequency. The validity of the expression was verified for Re = 100 and confirmed for Re = 60, 80 and 120. It was then used to perform many simulations in a reasonable time. All obtained results show that the proposed technique gives better results compared to the impulsive start technique and better mimics the physical reality, at least for low Reynolds numbers.
Passive control of circular cylinder wake in shallow flow
2013
The control of vortex shedding of a circular cylinder in shallow water using a splitter plate located in the downstream of the circular cylinder was studied by employing particle image velocimetry (PIV) technique. Experiments were carried out in a water channel having a test section of 8000 mm  1000 mm  750 mm dimensions at a Reynolds number of 6250. The length of the splitter plate (L) was varied within the range of 0.5 6 L/D 6 2 with an increment of 0.5. The plate was submerged into water at different height ratios (h p /h w) such as 0.25, 0.5, 0.75 and 1.0. Mean velocity vector field, corresponding vorticity contours, streamline topologies and turbulent quantities were calculated using 300 instantaneous velocity vector field measured by PIV. As the ratio of h p /h w increases, the effect of the splitter plate on the suppression of the vortex shedding increases. Flow characteristics and examination of spectra indicate that Karman vortex shedding is attenuated pronouncedly for the cases of L/D P 1 and h p /h w P 0.75. The transverse Reynolds normal stress is more effective on the attenuation of turbulent kinetic energy than the streamwise Reynolds normal stress. The value of peak transverse Reynolds normal stress is reduced to 90% of that of the bare cylinder at most.
Analysis of vortex formation around a circular cylinder at low Reynolds number
Vortex shedding is one of the most interesting phenomenon in turbulent flow. This phenomenon was first studied by Strouhal. In this paper, the analysis of vortex shedding around a 2 dimensional circular cylinder with Reynolds No of 200, 500, and 1000 with different angle of attack 0 , 5 , and 10 has been studied. In this simulation an implicit pressure-based finite volume method and second order implicit scheme is used. Flow has been studied with the help of Navier-Stokes and continuity equations. The pressure, drag coefficients and vortex shedding for different Reynolds numbers and different angle of attack were computed and compared with other numerical result that show good agreement.