Drag Reduction using particles as Shear Free Surfaces in a turbulent pipe flow

A novel passive drag reduction technique using solid additives termed "Shear Free Surfaces"(SFS) is studied experimentally. It works by modifying the large scales of the turbulent flow. The lateral transport of momentum is attenuated due to kinematic wall blocking effect bringing about the effect of drag reduction at the global scale. In the present study, solid particles that move with the local flow velocity, and hence, essentially have no shear on the mean, are considered as Shear Free Surfaces. The SFSes for the present study are solid spheroids and organic seeds of different sizes and densities procured locally (ABS, Brassica Nigra and Brassica Juncea). Experiments are performed in a horizontal gravity driven pipe flow at low volume fractions to determine the drag reducing characteristics of the SFSes under consideration. A consistent 2% drag reduction, measured from the "carrier only" discharge which discounts the particle volume fraction, is observed. It is seen that the scaling of the particle dimension with the inner wall units essentially determines if the particle is able to reduce drag in the flow. When the particle size is less than the distance of region of peak production from the wall, ideally half, the particles are able to bring about significant drag reduction. It is also observed that when the particles' settling velocity is more than the turbulence velocity scale which is the friction velocity, the settling effects dominate and the particles cause additional drag.