Numerical Study On Characteristics Of Vortex Induced Vibration Of Cylinders In Lock-in Regions

Vortex induced vibration(VIV)is a common phenomenon in engineering,which is essentially an intricate fluid-structure interaction problem.,It not only has scientific significance,but also provide the basis for calculating hydrodynamic loads on engineering structures.Researchers have carried out lots of experiments and numerical studies on VIV,and have obtained a gradual and in-depth understanding.Flow separation,hydrodynamic response,cylinder's motion response and vortex shedding modes are the four important problems widely concerned.In addition,the vortex shedding modes are closely related to the response branches of cylinder's motion.Along with increasing on reduced velocity,there is a lock-in region in the motion response,which is an important phenomenon in VIV.Due to the complexity and diversity of VIV,further study is still necessary.In this thesis,a numerical model of VIV was established,and the model was verified using experimental data.Velocity circulation around the cylinder and the hydrodynamic response were studied,and in the study both single-frequency vibration and dual-frequency vibration were taken into account.The main conclusions about VIV(single frequency)are as follows:1)the velocity circulation around the cylinder tends to present regular strips on spatial-temporal distribution.Corresponding to different response branches,the extremum of the circulation around a vibration cylinder is slightly different;2)With the increasing on the inflow velocity,the velocity of the circulation spreading outwards increases,with which the fixed cylinder has the same characteristic.And the frequency of the circulation also increases;3)In the results of the EOF decomposition on non-dimensional velocity circulation,the total contribution rate of the first two modes is as high as 97%,and both modes are the first main frequency of temporal distribution.Besides,the spatial distribution corresponding to each mode is highly correlated with that at the certain location in the magnitude and phase;4)Vortex shedding mode has a significant effect on the spatial-temporal distribution of the velocity circulation,and the two modes(2S and 2P modes)result in different circulation distribution.The main conclusions on the double frequency forcing vibration in lock-in regions are as follows:1)When there's a stable beat phenomenon in the cylinder's motion,the lift response shows three segments(stabilization,recession,burst);2)The hydrodynamic response during stabilization segment is the same as results of VIV in amplitude,frequency and phase.The length and amplitude of the burst segment are all changed with the frequency differences between two frequencies.And the frequency of the vortex force is about 1.5 times the potential force,which is decomposed from lift force;3)The velocity circulation around the cylinder still distribute in alternate-strips ways on space-time dimensions,and the strip length is synchronized with the magnitude of the lift;4)In EOF decomposition result,the contribution rate of the first two modes of time distribution frequency is more than 88%.In spatial modes corresponding to temporal modes,the 1st mode reflects the overall changing trend of spatial distribution,the 2^nd mode reflects the phase changing trend;5)The hydrodynamics and the amplitude of the velocity circulation distribution is similar in cases,which have the same absolute value of the frequency difference;6)With the decreases of frequency difference,there is no significant change in lift and velocity circulation distribution of stabilization segment.While the changes in that is same during burst segment,the amplitude of that increases,and the burst segment becomes wider.