A study with particle-image velocimetry of the influence of drag-reducing polymers on the structure of turbulence

Particle-image velocimetry has been used to study the effect of drag-reducing polymers on the structure of turbulence in a channel flow, under conditions of 41% and 55% drag reduction. The fluctuating velocity fields in the x-y plane and in one x-z plane were measured. The striking features of these results are the damping of small scales and the repression of fluctuations of the velocity component normal to the wall. The role of the wall in creating turbulence diminishes greatly at large drag reductions; Warholic et al. (1999) have shown that a turbulent flow with zero Reynolds stress exists at maximum drag reduction. Velocity fields presented for conditions approaching this critical behavior are of particular interest.