TRPIV Measurement Investigation On Drag-Reduction Mechanism Of Wall-bounded Turbulent Flow With Polymer Solution

In this paper, time-resolution particle image velocimetry (TRPIV) is used to carry outexperimental study about the drag-reduction mechanism of wall-bounded turbulentflow in a regurgitant channel flow with low concentration of polymer solution. At thesame velocity fields in turbulent boundary layer, which were0.125m/s,0.145m/s and0.180m/s, respectively, statistical analysis were employed to compare the physicalproperties between water and polymer solution, such as the mean velocity profile, theReynolds shear stress and the turbulent intensity. According to the new coherentstructure conditional sampling method and the concept of multi-scale spatial locallyaveraged velocity structure function along the longitudinal direction, extracting thespatial topology of physical quantities such as fluctuating velocity, mean velocitystrain rate, Reynolds stress, vorticity during the process of coherent structure burst inTRPIV dataset. The result shows that the skin friction coefficient of polymer solutionis lower than water at the same free-stream velocity. Besides，the drag reduction rateincreases with the increasing of flow velocity, added from13.56%to29.67%. Inaddition, we found out that the thickness of the buffer layer seems to increase and thelog-law in the mean velocity profile is translated upward in polymer solution. And atthe same normal position, the turbulent intensity and Reynolds shear stress is largelydecreased compared with water.Compared with the water, the polymer solution’s spatial topology of physicalquantities such as fluctuating velocity, mean velocity strain rate, spanwise vorticityduring the process of coherent structure burst has hardly changed, but the value ofphysical quantities is decreased. Adding high polymer to the solution has the mostsignificant impact on the sweeping events of coherent structures, when the influenceof the ejecting events is less. So the drag-reduction achieved by means of suppressingthe bursting events of coherent structure and weakening turbulent transportphenomena in the polymer solution.