Mechanism of drag reduction for circular cylinders with patterned surface

• Reduced drag of patterned cylinders over a wide range of Re numbers.
• Hexagonal patterns cannot be characterized as roughness structures.
• Hexagonal bumps affect the flow like spherical dimples of smaller k/d ratio do.
• Main separation is delayed caused by a partial separation.
• Angle of a separation line is not constant over the length of cylinder.

In this paper, the flow over cylinders with a patterned surface (k/d = 1.98 × 10−2) is investigated in a subsonic wind tunnel over Reynolds numbers ranging from 3.14 × 104 to 2.77 × 105 by measuring drag, flow visualization and measuring velocity profiles above the surface of the cylinders, to observe the effect of hexagonal patterns on the flow of air. These patterns can also be referred as hexagonal dimples or bumps depending on their configuration. The investigations revealed that a patterned cylinder with patterns pressed outwards has a drag coefficient of about 0.65 times of a smooth one. Flow visualization techniques including surface oil-film technique and velocity profile measurement were employed to elucidate this effect, and hence present the mechanism of drag reduction. The measurement of velocity profiles using hot-wire anemometry above the surface reveal that a hexagonal bump cause local separation generating large turbulence intensity along the separating shear layer. Due to this increased turbulence, the flow reattaches to the surface with higher momentum and become able to withstand the pressure gradient delaying the main separation significantly. Besides that, the separation does not appear to occur in a straight line along the length of the cylinder as in case of most passive drag control methods, but follow exactly the hexagonal patterns forming a wave with its crest at 115° and trough at 110°, in contrast to the laminar separation line at 85° for a smooth cylinder.