Combined effects of riblets and polymers on drag reduction in pipes

In the present Brief Communication, experiments are reported establishing a superposition of drag reduction due to riblets on drag reduction due to polymers, in fully developed turbulent flow of dilute aqueous solutions of polymers (2–50 ppm) through 25.4 mm (1 in.) diameter pipes, lined with a film of grooved isosceles triangles of equal height and base (S=0.11 mm). The range of S+ (which is the base and height of the riblets expressed in wall units), where drag reduction is superimposed, changes with polymer concentration. The higher the concentration, the narrower the range of S+. Also, the amount of superimposed drag reduction changes with the type of polymer used.