3D measurements of flow topology in a polymer drag-reduced turbulent boundary layer

Three-dimensional particle tracking velocimetry and the invariants of the velocity gradient tensor (VGT), rate of deformation tensor (RDT) and rate of rotation tensor (RRT) are used to characterize the fine scale flow motions in a Newtonian and polymer drag-reduced turbulent boundary layer (TBL). Relative to the Newtonian TBL, the polymer-laden flow has a similar momentum thickness Reynolds number Re_θ of 2300, but a 33% lower skin friction coefficient. Joint probability density functions (JPDFs) of the VGT invariants (Q and R) for the Newtonian TBL produce the familiar tear-drop pattern, commonly seen in direct numerical simulations of Newtonian turbulence. Compared to the Newtonian TBL, the polymer-laden flow has significantly attenuated values of R, implying a reduction in the tendency for the flow to exhibit extension. Invariants in RDT (Q_D and R_D) imply that straining motions within the polymeric flow are more two-dimensional compared to the Newtonian TBL. Moreover, JPDFs of Q_D and the invariant in the RRT (Q_W) suggest that the flow consists of fewer biaxial extensional events and more shear-dominate, sheet-like motions. Few, if any, experimental investigations have measured the fine scale motions in a Newtonian and polymer drag-reduced TBL. This investigation provides the first experimental evidence in support of the notion that an attenuation of biaxial straining motions is central to the mechanism of polymer drag reduction.