The effect of surface protrusions on the near wake of a circular cylinder

Wind tunnel tests of the near wake of a circular cylinder with surface protrusions were carried out to investigate the flow characteristics of the controlled wake. Two circular cylinders of diameter D = 40 mm were wrapped helically by three small wires (d = 0.075D) with pitches of 5D and 10D, respectively and tested at Reynolds numbers ranging from 5000 to 50 000. Wake velocity was measured using hot-wire anemometry. The surface protrusions make the near wake to have periodic structure along the spanwise direction. It is also observed that the surface protrusions elongate the vortex formation region, decrease the vortex shedding frequency and shrink the wake width. This is based primarily on the suppression of large scale vortices into slender elliptic shape. The suppression of vortices and consequent shrinkage of the wake increase the velocity deficit in the near wake.