Experimental studies of unrestrained cylinders on an impermeable bottom under currents

Abstract

Experimental studies were conducted to measure the motion of unrestrained cylinders with different diameters (0.048 m-0. 1 15 m), specific gravities (1.57-3.17) and aspect ratios (3.56-8.67), rolling on an impermeable, flat bottom under currents of velocities ranging from 0.3 m/s to 1.0 m/s. The currents in the flume of a uniform cross section (0.47 m x 0.52 m) were steady and uniform in velocity except near the bottom and the side walls. A video processing method was developed to document the linear and angular positions of the cylinders as a function of time. The experimental error encountered due to video resolution was studied and minimized through the use of a proper data sampling rate The terminal velocities of the cylinders are found to increase with an increase in current velocity. When the current velocity is above a superthreshold current velocity, the ratio of the terminal velocity to the current velocity (relative terminal velocity) seems to be around 0.70. The relative terminal velocities of the cylinders are found to be independent of diameter, aspect ratio and blockage ratio when the current velocity is greater than the superthreshold velocity. When the current velocity is greater than the superthreshold velocity, the relative terminal velocities of the cylinders are almost independent of specific gravity. The measurements show that the relative terminal velocities of cylinders with small specific gravities (1.57-1.71) range from 0.70-0.72, slightly above 0.70, while those of cylinders with large specific gravities (2.77-3.17) range from 0.64-0.66, slightly below 0.70. However, the threshold and superthreshold velocities are smaller for a cylinder with smaller specific gravity. Analysis of angular and linear motion of the cylinders shows that the cylinders do not roll with pure rotation. Both steel and PVC cylinders experience back-slipping, where average angular velocity multiplied by the radius of the cylinder is greater than average linear center velocity in the x-direction.