Study On The Characteristics Of Double Lock-in Region For Vortex-induced Vibration Of π-shaped Beam

At present,the π-shaped cross-section is widely used in medium and long-span bridges.However,due to its bluff body characteristics,its cross-section has poor aerodynamic stability,which is easy to induce vortex-induced vibration(VIV),and often has a double lock-in region of VIV.Therefore,the research on the characteristics of double lock-in region of π-shaped beam is helpful to further understand its VIV mechanism,and can provide more targeted support for the selection of vibration suppression measures.Therefore,in this paper,the π-shaped beam with aspect ratio B/D = 10 is taken as the research object.Two research methods,wind tunnel test and numerical simulation,are adopted.By means of simultaneous vibration measurement and pressure measurement test and fluent dynamic mesh technology,the characteristics and vibration mechanism of double lock-in region of VIV are studied.The main research contents are as follows:Taking the π-shaped section of representative bridges in China as reference,a π-shaped beam with aspect ratio B/D = 10 is designed,and simultaneous vibration and pressure measurement tests are carried out.The VIV characteristics are studied from the aspects of VIV response,power spectrum analysis,pressure distribution and relationship between local aerodynamic force and vortex-induced force.The results show that there are two lock-in region for vertical bending and torsion VIV under three attack angles,and the length,peak amplitude and 3d B bandwidth of each lock-in region are affected differently.The wind pressure distribution on the surface is obviously regional when VIV occurs,the dominant region of vertical bending vortex vibration is the front,back of the upper surface(LL<0.2 和 LL>0.4)and back of the lower surface(0.6<LL<0.95),while the dominant region of torsional vortex vibration is the back half of the lower surface(0.6<LL<0.95).Vertical bending VIV has stronger vortex excitation power than torsional VIV.There are high-order harmonics in the spectrum of both vortex-induced forces,which reflects the nonlinear characteristics of vortex-induced forces.Four measures are set to explore the causes of double VIV lock-in region.In addition,the effects of different aspect ratio,mass ratio and damping ratio are mainly studied,and the VIV response is compared and analyzed.The results show that the wake is closely related to the formation of the first VIV region,and the complicated air flow at the bottom of the beam is the possible cause of the second VIV region of the π-shaped beam.The aspect ratio mainly affects the second lock-in region,with the increase of aspect ratio,its the length of region and the peak value of reduction amplitude increase gradually;with the increase of mass ratio,the amplitude peak of the two lock-in region almost decreases linearly,while the length of lock-in region increases first and then decreases;the damping ratio has the most significant influence on the vortex amplitude.When the mass ratio and damping ratio increase to a certain value,the first vertical bending VIV region with smaller amplitude can be completely suppressed,but the influence on the second VIV region is limited.The VIV of π-shaped beam is simulated by using fluent dynamic mesh technology,and the results are in good agreement with the experimental results.The VIV mechanism is analyzed from the aspects of vortex shedding evolution,flow field pressure and aerodynamic force.The results show that the "Karman vortex street" formed by the periodic shedding of vortices at the rear edge of the cross-section is the main cause of the first VIV region,while the shedding vortices at the lower edge of the front end and the secondary vortices at the tail end of the upper surface due to the structural motion are the main causes of the second VIV region.During the development of VIV,the phase difference between aerodynamic lift and displacement response increases gradually.In the rising region of VIV,aerodynamic lift promotes the displacement response,while in the falling region,aerodynamic lift suppresses the displacement response.