Laboratory Study of Lift Forces on Circular Piles

Ocean waves exert lift (i.e. transverse) forces on piles under some circumstances in addition to the longitudinal forces. The lift forces are related to the formation and shedding of the eddies. The lift forces were found to be as great as 60% of the longitudinal forces. The Keulegan-Carpenter number is found to be a reasonable parameter to determine the regime in which eddies form, and to be related to the ratio to the lift to longitudinal forces. Eddies form in the lee of a vertical cylindrical pile subject to water waves when the Keulegan-Carpenter number has a value of about 3. For higher Keulegan-Carpenter numbers two eddies are developed and shed and the lift force is more or less regular with a frequency twice the frequency of the waves. For further increase in the Keulegan-Carpenter number, turbulence develops and the lift forces show a random pattern. The ratio of the lift to the longitudinal forces appears to be at a maximum for a Keulegan-Carpenter number of about 15.