Wind tunnel study reveals outward hexagonal surface bumps reduce circular cylinder drag by 35%, highlighting an effective passive flow control technique.
Flow over circular cylinders with patterned surfaces is investigated and discussed taking into consideration the well known characteristics of flows over rough and dimpled cylinders in this paper. Investigations were performed in a subsonic wind tunnel to observe the effect of hexagonal patterns on the flow of air at Reynolds numbers ranging from 3.14E+04 to 2.77E+05. The investigations revealed that a patterned cylinder with patterns pressed outwards (can be referred as hexagonal bumps) has a drag coefficient equal to 65% of the smooth one. Various flow visualization techniques including measurement of velocity profiles in the wake region and smoke flow visualization were employed to elucidate the effect and hence comprehend the reason of drag reduction. Besides that the investigation of vortex shedding frequencies determined by using hot wire anemometry suggested that they do not change significantly with the decrease in drag coefficient in contrast to the dimpled cylinders.
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Butt et al. (2013) studied this question.
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