TRPIV Experimental Research Of Drag Reduction Mechanism By A Riblet Surface

Coherent structure is closely related to the high frictional resistance in wall turbulence,and it has a significant influence on the production,maintenance and evolution of the turbulence.We took coherent structure as research object,and studied the influence rule of vertical micro-grooves on the topological structure,inclination angle and scale of the coherent structure by particle image velocimetry.The conclusion we obtained is helpful to understanding the drag reduction mechanism by riblet surface,and provides theoretical guidance to optimize drag reduction method.To reveal spanwise vortices,swirling strength was derived by using a vortex identification technique,and we extracted the cores of the spanwise vortices by finding swirling strength's local extremum in whole field.Firstly,the proportion of the spanwise vortex,the average swirl strength,the average diameter of the vortex and the proportion of each scale vortex were analyzed in flow field over a riblet surface.All experimental results were compared with those over a smooth surface.As a result,the number of prograde spanwise vortex is decreased by riblet in inner boundary,while retrograde spanwise vortex is increased.The average strength of the vortex is decreased,whether prograde vortex or retrograde vortex.The proportion of small scale prograde vortex and mesoscale retrograde vortex are increased in inner boundary,while mesoscale scale prograde vortex and large retrograde vortex are decreased.The difference in the stream-normal scale of prograde spanwise vortex becomes smaller in inner boundary,but retrograde spanwise vortex's scale difference without change.Secondly,we used autocorrelation function of the streamwise fluctuation velocity to extract coherent structure,and compared the scale and inclination angle of coherent structure.We find that the streamwise and normal scale is reduced in inner boundary,while increased in logarithmic layer.In addition,coherent structure's inclination angle become more stable compared with smooth surface.Finally,we got the topology structure by linear stochastic estimation.Under the two conditions,the flow field is similar.The topology structure has a saddle point and a focal point in inner boundary,while has two saddle points and two focal points in logarithmic layer.The distance and inclination angle between saddle point and focal point over riblet surface are less than that over smooth surface in inner boundary.The distance between two focal point and inclination angle of the shear layer are more stable according normal position compared with smooth surface.