Circular cylinder wakes and their control under the influence of oscillatory flows: A numerical study
Understanding and control of wake vortices past a circular cylinder is a cardinal problem of interest to ocean engineering. The wake formation and vortex shedding behind a variety of ocean structures such as spars, are subjected to fatigue failure limiting their life span. The additional influences due to ocean waves and currents further exacerbate these effects. In the present study, flow past an isolated circular cylindrical structure subjected to an oscillatory upstream are numerically investigated. These studies involve high resolution simulations over the low Reynolds number range (100–200). Although the practical range of interest is in high Reynolds number range of 103 - 105, the flow physics and a number of qualitative and quantitative aspects are similar to the low Reynolds number flows. In the high Reynolds number range, statistical averaging tools in conjunction with suitable closure models would be necessary. The control of wake vortices is achieved with the aid of two small rotors located in the aft of the main cylinder. A control algorithm was coupled to determine the quantum of actuation to the rotating elements. Although control of wake vortices was observed for harmonic in-let forcing, residual vortical structures were found to persist at higher amplitudes of oscillation. To study the efficacy of this control, numerical simulations were further extended, when the circular cylinder was flexibly mounted. The control of flow induced vibrations was observed to be reasonably effective in controlling the wake generated behind the main cylinder due to oscillatory upstream.
An active flow control strategy for the suppression of vortex structures behind a circular cylinder
An algorithm is proposed to model, predict and control vortex shedding behind a circular cylindrical configuration. The main ingredients of the algorithm include multiple-feedback sensors, actuators (with zero net mass injection) and a control strategy. Along with the mass and momentum conservation equations, a control equation is implemented to enable the desired flow control goals. A number of sensors are chosen in the downstream of the body to report the state of the flow. The role of externally controllable actuators on the fluid flow patterns past a circular configuration is assessed. To enable, zero net mass injection, two simple rotary type mechanical actuators are located at 120°, right behind the main cylinder. The popular finite volume based SIMPLE scheme is employed for the numerical calculations. As a precursor, the scheme simulates flow past an isolated cylinder, which is validated over a moderate range of Reynolds numbers. The design parameters of interest such as Strouhal number, drag and lift coefficients etc are used for the purpose of validation. The simulated flow fields are compared against the flow visualization study, which clearly demonstrates the efficacy of the actuators at discrete levels of rotation. The basic character of the flow is completely modified at Uc/U∞ = 2.0 and Re = 100, where a complete suppression of vortex shedding is observed. This is tantamount to complete control of all the global instability modes. Fictitious tracer particles are released to visualize the vortex structures in the form of streaklines. The results clearly demonstrate the effectiveness of a rather simple active control algorithm in suppressing the vortex structures. All the relevant fluid flow features of the bluff-body fluid mechanics under the influence of actuators are studied in the sub-critical Reynolds number range of Re = 100-300. © 2009 Elsevier Masson SAS. All rights reserved.
Active flow control of vortex induced vibrations of a circular cylinder subjected to non-harmonic forcing
A wide variety of waves and currents are abound in wind and ocean engineering practice. These wave forms could be harmonic as well as non-harmonic and may lead to the formation of wake vortices, behind a circular cylinder. The alternating lift forces on such structures could in turn result in damaging flow induced vibrations. In the present study, we propose a simple momentum injection based active flow control strategy to suppress such vortex induced oscillations at low Reynolds numbers. Two small control cylinders located at 120°, behind the main cylinder play the role of actuators, that enforce the desired momentum injection. Detailed Computational Fluid Dynamics (CFD) simulations are carried out, by solving mass, momentum conservation equations in conjunction with a control equation, and a dynamical evolution equation for the structural motion. Non-harmonic inlet forcing on a flexibly mounted circular cylinder generates vortex induced vibrations, which is numerically simulated. Then by controlling the wake vortices, vortex induced vibrations are completely controlled. Analysis of the leeward region behind the main cylinder reveals a different wake signature, with blobs of residual vorticity along the wake centreline. This is attributed to the phase asynchrony between the inlet forcing and the vortex induced vibrations.
Numerical simulation of vortex induced vibrations and its control by suction and blowing
The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the Kármán vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle α=100°. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations. © 2012 Elsevier Inc.