Modification of Drag-reduced Flow by dosing water from the wall - Investigation of Turbulent Structure by PIV-

Abstract

Experimental investigation of modified drag-reduced channel flow by covering the near-wall region with pure water was carried out. To modify the near-wall turbulent structure of drag-reduced channel flow of surfactant solution, the wall-dosing apparatus, in which pure water is supplied from the porous surface of the channel wall into the viscoelastic bulk flow, was used. By covering the near-wall region with water, drag reduction rate is slightly enhanced, compared with the viscoelastic channel flow without water dosing. We examined the mechanism of this enhancement of drag reduction and the change of the turbulent structure by water dosing from instantaneous velocity fields captured by PIV. The decomposition of Cf shows that viscoelastic stress decreases largely while Reynolds shear stress increases, which causes drag reduction to be enhanced. Moreover, characteristic turbulent structures were observed in the instantaneous velocity fields. In the near-wall region of this flow, the feature as usually reported in Newtonian fluid wall turbulence was observed and dosed water acts as a promoter of turbulence, which destabilize the flow of the viscoelastic fluid. On the other hand, away from the wall the feature is the one usually observed in drag-reduced flow and dosed water acts as a passive scalar because the mean concentration of dosed water is too small to destabilize the viscoelastic flow.