The Free End Blow Effects On The Aerodynamic Forces Of Finite-length Square Cylinder

In order to reduce the interaction between high-rise building and surrounding air flow,and enhance wind resistance of buildings.An active flow control method applied to the top of buildings is proposed in this thesis.A blow slot is set on both sides of the free-end of the finite-length square cylinder,and the flow separation characteristics of the model are changed by blowing control to restrain aerodynamic force.Firstly,the effects of blowing control on aerodynamic force and flow structure of finite-length square cylinder are studied based on wind tunnel experiments.Then,combined with numerical simulation method,the control effect of blowing control on square cylinder separation flow is further explored and its control mechanism is revealed.The main research contents are as follows:(1)Based on wind tunnel experiments,this thesis studies the influence of free-end blowing control on aerodynamic force and flow characteristics of a finite-length square cylinder with aspect ratio H/d=5 under the uniform flow at a Reynolds numberRe=2.67×10~4.By changing the blow ratio,the overall aerodynamic force,local aerodynamic force,surface wind pressure distribution and lift coefficient time-frequency of the square cylinder under different blow conditions are analyzed in detail.It is found that the free-end blow has a significant inhibitory effect on the overall aerodynamic coefficient of the finite-length square cylinder.Free-end blow not only inhibits mean drag coefficient,fluctuating drag coefficient and fluctuating lift coefficient near the top of the square cylinder,but also affects the square cylinder along the high.Under blowing,the fluctuating wind pressure caused by spanwise vortex shedding is obviously weakened,and the overall fluctuating lift of square cylinder decreases greatly,which effectively inhibits the fluctuating characteristics of the surrounding flow field.(2)In this thesis,the influence of free-end blowing on wake structure of finite rectangular cylinder under typical working conditions is studied by flow visualization experiment and particle image velocimetry.The results show that flow at the leading edge of the finite-length square cylinder is accelerated by the free-end blow,and the shear flow forms reattachment at the top,which weakens the separation of the shear flow at the top and the influence of the downwash flow near the wake.The square cylinder wake becomes narrower and the backflow area decreases under blow.The fluctuating characteristics of the wake near the square cylinder is weakened,the alternate spanwise antisymmetric vortex is suppressed significantly,and the complexity of the wake structure is weakened.As a result,the fluctuating lift and the fluctuating drag of the square cylinder decrease,which changes the aerodynamic characteristics of the square cylinder.(3)Large Eddy Simulation is used to calculate the uncontrolled condition and blowing control condition.The flow field is carried out by using the advantage of 3d flow visualization,and the control mechanism of free-end blow around square cylinder is further revealed.The results show that under free-end blow,the shear flow forms reattachment,which changes the spiral flow of the wake and suppresses the flow structure upward on the side.In addition,the blow suppresses the development of the downwash to the downstream,the top of the vortex lines continually sag inward,presenting a"M-type"distribution,and the inclination of the vortex lines becomes steeper.Furthermore,the reverse flow zone appears in sunken range,vortices on both sides of wake are dispersed,and the fluctuating characteristics of the wake is obviously weakened.Chapters 67,Tables 0,References 117.