Effect Of Turbulence Intensity And Length Scale On Axial-flow-induced Vibration Of An Elastic Cylinder

The axial flow incduces small amplitude vibration of nuclear reactor fuel rods and pipeline,such vibration may cause the rods to wear and failure because they are closely packed.On the other hand,in order to achieve high heat-transfer efficiency,the fixture of the fuel rods are offten attached to turbulence generaters to increase the disturbance and strengthen the heat transfer,which further increases the instability of the structure.Different from traditional thermal power plants,nuclear reactors have extremely strict safety requirements,so it is of engineering significance to study axial-flow-induced vibration under different turbulence characteristic parameters.In order to control the turbulence parameters,several active jet grid are designed and calibrated using CTA（constant temperature anemometer）.Experiments are performed to test different grid parameter combination（change the grid bar arrangement,mesh size M,jet hole size and configuration,jet angle and so on;grids can be divided into A,B,C three series according to the mesh characteristics）and grid C1 is found to have the best control effect,the tubulence intensity Tu is found to be varied from 3.1%to 14.5%at x=10 M,U=0.4m/s.The influence of the following parameters on axial flow induced vibration of elastic cylinder is experimentally studied through the control of grid C1,the parameters are:non-dimensional flow velocity u^*（1.37～3.47）,Tu（3.4%～8.4%）,integral length scale L_x（0.008 m～0.015 m）,and Taylor Reynolds number Re_λ（15～150）.The one-elastic-one-rigid cylinder arrangement is also studied,the spacing ratio P^*=1.36 or 1.57.The experimental results show that for 1 single elastic cylinder,at u^*=1.37,Tu=6%,the A^*_rms（RMS amplitude）of the elastic cylinder increases from 0.005 to0.008 with increasing L_x from 0.009 m to 0.014 m,and the low-frequency vibration energy（f<1 Hz）of the elastic cyliner also increases with the increase of L_x.The low frequency vibration is induced by the large scale vortex on the cylinder wall.In addition,A^*_rms at u^*=1.37,2.04,2.75 are normalized,the normalized amplitude has the same linear positive correlation trend with Re_λ.For one-elastic-one-rigid cylinder arrangement,it is found that the A^*_rms of the elastic cylinder is larger than that of the single cylinder arrangement（P^*=∞）at u^*=1.37,Tu=3.4%and P^*=1.36;while Tu increases to 6%,the A^*_rms of P^*=1.36 is smaller than that of P^*=∞,and the low-frequency vibration energy（f<1Hz）decreases.It is proposed that large scale vortices dissipate in the gap between two cylinders.