Study On The Mechanism Of System Rotation Affecting Flow Structures In Wall-Bounded Turbulent Flow

Wall-bounded turbulent flow in rotating reference system can be widely observed in natural and engineering situations.Coherent structures in the flow are key to the production,maintenance and development of turbulence.In present study,the effects of spanwise and streamwise rotation on channel flow are investigated,with emphases on the vortical structures in small or large scales.Turbulent flows in rotating square duct are also simulated,where the variation of secondary flow pattern with rotation number are studied.The main content and achievements are as follows.(1)The effects of Taylor-G?rtler vortices(TG vortices)and Coriolis force on nearwall quasi-streamwise vortices are investigated in spanwise rotating channel flow at three Reynolds numbers 2800,7000 and 20000.Turbulence is found to be enhanced in the upwash flow of the TG vortices,where the fluid is pumped away from the pressure wall,and suppressed in the downwash flow.It is revealed that this is due to the vertical stretching caused by the TG vortices and the streamwise torque generated by the Coriolis force.The effects of TG vortices and Coriolis force are reduced when increasing the Reynolds number.(2)Coherent structures in streamwise rotating channel flow are investigated.It is confirmed using conditional averaging method that of the near-wall quasi-streamwise vortices rotating in opposite directions,the rotation of the channel tends to promote the cyclones(vortices which rotate in the same direction as the channel)and suppress the anti-cyclones.Large-scale streamwise-elongated vortices are also extracted,of which the cyclones are found in the lower-or upper-half channel and tilt to spanwise direction,while the anti-cyclones prefer to emerge in the central region of the channel.When rotation is suddenly imposed,the streamwise integral length increases linearly with the growth rate proportional to the rotation number,which is consistent with the linear propagation of inertial wave.It is revealed that the preference of the large-scale vortices for different regions is not related to the near-wall structures.The mean shear may be responsible for the tilting of the large-scale vortices and further cause the preference.(3)Turbulent flows in rotating square duct are simulated at 0?Ro??40.The rotating axis is parallel to one pair of walls and normal to another pair of walls.Turbulence statistics at different rotation number are presented and compared to that of spanwise rotating channel flow.The complete variation process of secondary flow pattern with rotation number is obtained,with special concern on the additional vortices near the pressure wall.It is found that the position of its reattachment point and its size are respectively determined by the local maximum pressure and the spanwise pressure gradient on the pressure wall.Via budget analysis of the mean momentum equations,it is revealed that an oblique flow toward the corner and positive Coriolis force near the lateral walls lead to the local maximum pressure on the pressure wall.