| Vortex induced vibration(VIV)is a common seen phenomenon in nature and practical engineering,and it is caused by the interaction between the structure and alternatively shedding vortices in the wake when the fluid flow around a bluff body.VIV may cause fatigue damages of engineering structures,especially when “lock-in”occurs,the vibration of structures will be more intense,which can even lead to accidents.Though a variety of measures for suppressing VIV of structures have been developed by scholars now,there are still many deficiencies,which can't fully meet the requirements of practical engineering.Therefore,a new flow control method,synthetic jets,is selected to control the two-degree-of-freedom vortex induced vibrations of the circular cylinder,which is the most common seen structure in engineering.The influence of several relevant parameters of synthetic jets on the amplitude response,frequency response,force characters and wake structures of the circular cylinder are studied in detail,and the acting mechanisms of synthetic jets are also revealed.This work not only enriches the control methods of vortex induced vibration,but also can provide guidance for actual engineering.Both numerical simulation and experiment are conducted in this work.In numerical simulation,the pressure-based coupled algorithm is used to solve the two-dimensional unsteady Naiver-Stokes equations,and the user-defined functions(UDFs)is adopted to discrete and solve the movement equations of the circular cylinder.To model the two-degree-of-freedom VIVs of the circular cylinder,the overset mesh is applied in numerical simulation.After obtaining the numerical results,the effects of the momentum coefficient,position angle,orientation angle,excitation frequency and phase difference on the control of two-degree-of-freedom VIVs of the circular cylinder are studied in detail.In experiment,two synthetic jet actuators driven by the voice coil motors are designed and fabricated based on the CFD results,and the actuators are embedded at both ends of the cylinder model to generate synthetic jets on its surface.The flow structures in the wake of the circular cylinder before and after synthetic jets control are analyzed through the flow visualization test in the wind tunnel,and the actual control effect of synthetic jets are verified.The following conclusions can be drawn from the studies in this work.Firstly,the influence of the momentum coefficient Cu and position angle ? of synthetic jets issued horizontally on the vibration characters,force characters and wake structures of the circular cylinder in two-degree-of-freedom VIVs are studied by numerical simulation,and the influence rules of above two parameters are obtained.The ability of synthetic jets on the control of two-degree-of-freedom VIVs of the circular cylinder will increase with the momentum coefficient increasing.When the synthetic jets issued horizontally are positioned at the cylinder's leeward side,both in-flow and cross-flow oscillations of the circular cylinder can be suppressed,the drag forces of the cylinder will decrease significantly,and the wake structures tend to be symmetric.In the studied Reynold number,the best suppression of VIV can be obtained at around ? =45°,and the reduction rate of the cross-flow oscillation amplitude is even larger than99% when Cu = 4.0.Secondly,for synthetic jets issued horizontally and have different phase differences(?? = 0,?/2,and ?),the position angle that has the best suppression effect on VIV of the circular cylinder and sufficient momentum coefficient are selected to study the influence of non-dimensional synthetic jet excitation frequencies in the range of 0.2and 7.0(0.2 ? ? 7.0)on two-degree-of-freedom VIVs of the circular cylinder by numerical simulation.On the whole,the in-phase synthetic jets have the best suppression effect on two-degree-of-freedom VIVs of the circular cylinder.When ?5.0,synthetic jets with three different phase differences can suppress two-degree-of-freedom VIVs of the circular cylinder effectively,and the vortices in the wake are stretched and distributed symmetrically.However,when the phase differences between two synthetic jets exist,the cross-flow oscillation frequencies of the controlled case are consisted with the synthetic jet excitation frequencies.When ? 4.0,various“lock-in” phenomena can be observed from both in-flow and cross-flow oscillations of the circular cylinder,including primary lock-in,subharmonic lock-in and superharmonic lock-in.When the oscillation frequencies of the circular cylinder are close to the natural frequencies caused by the “lock-in” phenomena,synthetic jets have no good performance on VIV suppression,and even have opposite effect.Furthermore,the influence of synthetic jets issued horizontally,vertically and in the normal direction on two-degree-of-freedom VIVs of the circular cylinder under the condition of three different phase differences(?? = 0,?/2,and ?)are analyzed by numerical simulation,the action mechanism of synthetic jets and the robustness of the control effect are also studied.The results indicate that synthetic jets with different phase differences can effectively control two-degree-of-freedom VIVs of the circular cylinder in a wide range of reduced velocities.The phase difference can influence the control effects of synthetic jets on two-degree-of-freedom VIVs of the circular cylinder,but the orientation angle plays a decisive role.Synthetic jets issued horizontally can suppress two-degree-of-freedom VIVs of the circular cylinder,but when synthetic jets are issued vertically,the vibration of the circular cylinder is enhanced.However,when synthetic jets are issued in the normal direction,the control effects are influenced by the position angle,and depends on how close it is to the horizontal direction or vertical direction.The synthetic jets issued in horizontal direction can suppress the vortex induced vibration of the circular cylinder by delaying the boundary layer separations in both blowing and suction phases,and synthetic jets issued in vertical direction can enhance vortex induced vibration of the circular cylinder by promoting the generation of vortices in the wake.Finally,the synthetic jet actuator and circular cylinder model are designed and fabricated,and the actuators are embedded at both ends of the circular cylinder to generate synthetic jets on its surface.The hot wire probe is used to measure the flow speeds at the slots on the surface of the circular cylinder,and the wind tunnel test is conducted to study the control effect of synthetic jets on the flow field in the wake of a fixed circular cylinder.The results show that the scheme used in experiment can generate synthetic jets on the surface of the circular cylinder in the excitation frequency range of 20 Hz and 40 Hz,and the distribution of momentum coefficients along the length of the circular cylinder shows the characters of higher at both ends and lower in the middle.The wind tunnel tests show that synthetic jets issued horizontally can suppress the boundary layer separation in the wake of the circular cylinder,and the vortex shedding in the wake of the circular cylinder completely disappears when Cu ?4.67. |
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