Experimental Study On Vortex-induced Vibration Control Of Cylinders

The cylinders in a cross flow is one of most basic and revealing cases in the general subject of fluid-structure interaction problems. When the fluids flow around cylinders in the modern Reynolds number, the vortex streets shed from the cylinder. The shedding vortices would generate oscillating forces, which in turn cause the structures to vibrate. The resultant vibration can influence the flow field and the structures, especially when the shedding frequency is at or near the structural natural frequency. The vortex-induced vibration (VIV) is of practical importance because of its potential destructive effect to structures, such as bridges, stacks, towers, offshore pipelines, and heat exchangers.In order to reduce or eliminate the potential disaster by VIV, many methods have been developed to control the VIV. However, these methods mostly control the VIV by modifying a wide range flow filed or by exciting at high power compared to the saving power, which limit their application in realistic engineering. In this thesis, a new VIV control method was proposed to control the vibration of single cylinder and two side-by-side cylinders. The VIV was controlled by micro actuators through influencing the interaction between the cylinder and the fluids.As we known, flow control is most effective when the control is near the critical regions, because the flow instabilities could be amplified quickly in these regions. Therefore the micro actuators periodically excited near the separation regions during VIV control. It was also reported that little disturbance could perturb the flow separation. However, no researcher reported how wide the exciting range and how large the exciting amplitude are required to influence the vortex-induced vibration.In this thesis, in order to control the VIV of big-size structure by micro actuator, we firstly gave the dynamic model of the vibration system. Two typical supported systems were modeled with vibration differential equation presented. The actuator's acting size and amplitude in flow control were then analyzed.Acoustic excitation was initially used in the VIV control due to its convenience. The internal acoustic excitation emitted from a slit with size 0.2mm×10mm on the cylinder. When the exciting position is near one separation region and the exciting frequency is matching to the instability frequency and its harmonic frequency, the VIV could be reduced by acoustic excitation.With another identical cylinder presented beside this one, the control effects are different with the spacing ratio between the two side-by-side cylinders changed. At the spacing ratio of 1.2, the VIV was controlled more greatly than single cylinder when two acoustic excitations symmetrically perturbed near separation regions. With the increment of the spacing ratio, the control effect decreased.While the slit's length is only 2mm, acoustic excitation cannot reduce the VIV even using same controlling characteristics. It implies that enough exciting range is needed to be able to control the VIV.As a type of micro actuator, acoustic excitation successfully reduced the VIV. But acoustic excitation can induce additional noise during control, and the wasting energy is also considerable. Therefore a new micro actuator made up of Piezoelectric Ceramic (PZT) cantilever was used in the VIV control. New experiment system was designed according to the model calculation, and the cylinder is spring-supported.The carried out experiment demonstrated that the VIV of single cylinder could also be reduced by micro PZT actuator. The control is most effective when the excitation is near separation region and when the exciting frequency is matching to the instability frequency and its harmonic frequency. For the control of two side-by-side cylinders, the VIV decreased most when two actuators perturbed simultaneously near separation regions at spacing ratio of 1.2. Because at this ratio, the two cylinders acted as one big bluff body, and perturbations at both separation regions can reduce the VIV more. When the spacing ratio was 1.8, there were two bistable vortices and the control had no effect. While at spacing ratio of 3.0, the control effect was same as that of single cylinder.The measurements of the cylinder's vibration velocity and flow fluctuations showed PZT actuators changed the phase between these two signals, and the vorticity was also reduced by the actuators. It shows that the actuators controlled the VIV by influencing the interaction between the cylinder and the fluid. The critical amplitude of the actu- ator for reducing VIV was also explored, which is of the same order of magnitude as viscous length-scale, while the input energy is at least one order less than the saving energy.During the experiments, a novel passive control method was discovered to effectively suppress the vortex-induced vibration. When the two side-by-side cylinders were arranged slightly non-coplanar as small spacing ratio of 1.2, the single vortex street was significantly suppressed and therefore the vortex-induced vibration was controlled.