Normal forces on cylinders in near-axial flow

The transverse forces on submerged cylinders in nearly axial flow were studied experimentally in a towing tank. Two rigid cylinders with length to diameter ratios of 10 and 30 were towed with angles from 0 degrees to 20 degrees between their centerline and the incoming flow, and with a combination of forced harmonic oscillations in the normal direction and constant speeds along the cylinder axis. The most important parameters are found to be the angle of attack and a Reynolds number combining the longitudinal and normal components of the flow. The Reynolds number is important because the transition from laminar to turbulent boundary layer due to the axial component of the flow has a significant influence on the normal force. This seems to be true both for constant and oscillating angle of attack. The finite length of the cylinder and the Keulegan-Carpenter number associated with an oscillating cylinder in cross-flow are found to be less influential than expected. Time-domain simulation models for the normal force suitable for general motion of the body are discussed. For constant angles of attack their standard deviation is found to be comparable to the measurement accuracy, but in the case of forced transverse oscillations the estimated errors of the force models increase to a factor three or more of the measurement errors. (c) 2006 Published by Elsevier Ltd.