Drag Reduction with Riblets on Rowing Shells

Abstract

An experiment is described in which riblets, which are longitudinal surface groves that have been shown by NASA to reduce turbulent flow skin friction on flat plates by much as 8 percent, are used to attempt to reduce the overall vehicle drag of rowing shells. Two single rowing shells, borrowed from the USNA rowing team and ballasted (with the outriggers removed) to represent a 200 lbf rower, were attached to the drag balance of the carriage of the USNA tow tank and towed at steady velocities from 4 to 20 ft/sec in 2 ft/sec increments. The attachment mechanism allowed the shells to move freely in heave and pitch (sinkage and trim) which were measured with a sonic height probe and angleometer, respectively. Various surface conditions tested include bare hull, flat (non-ribleted) tape, and riblet tape (cleaned and uncleaned) in combination with two sizes of trip wire and three trip locations. The particular riblet tape used provided a surface geometry of saw-tooth pattern transverse to the flow with a height and spacing of 0.003 in. This provided an optimum non-dimensional height (s+) recommended by Walsh and Lindemann (1984) of NASA of 12 to 15 over most of the shell length for the higher velocities. The overall vehicle drag results are compared with and without riblets and with and without trip wire. Riblet drag reductions of between 3 and 5 percent are observed. The trip wire results showed an increase in drag due to their parasite drag at. the lower velocities but a drag reduction at the higher velocities. Predictions of the viscous component of the drag difference between configurations are developed including the effects of sinkage and trim. The predictions compare qualitatively with the experimental results except for the apparent drag reduction with a trip wire at the higher velocities.