Experimental study of drag loads on open and closed low-aspect-ratio circular cylinders in steady current

Abstract

We investigate the drag load on low-aspect ratio circular cylinders, submerged as in infinite fluid and subjected to steady current. A series of experiments are carried out in a circulating water tunnel with three different types of models; open and closed cylinders, and open with a splitter plate inside the middle of the cylinders. The length-to-draft ratio of the models, L/D, varies from 0.15 to 0.6. The Reynolds number, based on the cylinder diameter, varies from 40000 to 105000. The experiments show small Reynolds number dependency. The drag coefficient, CD, varies substantially for the open cylinder case; being around 2 for the lowest aspect ratios L/D≤0.2, while 0.65-0.8 for L/D>0.25. There is an abrupt change of flow pattern for L/D=0.25: for L/D≤0.2 the flow which is separated from the front half of the cylinder goes through the cylinder, producing a wide von Karman type vortex street, while for L/D≥0.25 the flow does not go through the models and, as a consequence, produces a significantly more narrow wake. The wide wake regime is suppressed when the splitter plate is added, with drag coefficients CD=0.6−0.8. The drag coefficients for the closed cylinders are lower than the ones with splitter plate, and also consistent with published data on low-aspect ratio closed cylinders. A rational drag load model is proposed for the high-drag regime, with reasonable results. Tests where the cylinder with splitter plate is sectioned into a front and a rear half are presented, revealing a very small contribution of the drag from the rear half. This is discussed in view of existing studies on two-dimensional flat plates in tandem.