Three-Dimensional Flow Structures Behind a Wall-Mounted Short Cylinder with a Rear Inclined Hole

Abstract

Three-dimensional (3-D) flow structures around a wall-mounted short circular cylinder, into which was drilled a rear inclined hole (RIH) going from the rear surface to the top surface of the cylinder, were instantaneously measured at Reynolds number 10,720 in a water tunnel by high-resolution tomographic particle image velocimetry (Tomo-PIV). Based on the measured instantaneous 3-D velocity distribution, the 3-D vorticity field, the $Q$ criterion, and characteristics of arch-type and tip vortices were compared between the RIH cylinders and a standard cylinder. A 3-D $W$ -type arch vortex appeared behind the standard and RIH cylinders, and the height of the RIH cylinders was higher than that of the standard cylinder. Compared to the standard cylinder, the 3-D $W$ -type arch vortex and large-scale vortices broke down slowly in the wake of the RIH cylinders. A 3-D orthogonal wavelet multiresolution technique was used to decompose the 3-D velocity fields from the Tomo-PIV data. The time-averaged large-scale structures of the RIH cylinders exhibited a stronger M-shape arch vortex, and a strong $W$ -type-shape arch structure was extracted from the time-averaged intermediate-scale structures. Meanwhile, the tip vortices simultaneously existed in the large- and intermediate-scale structures. The heights of instantaneous large-scale streamwise structures were proportional to the hole height of the RIH cylinders. Large-scale spanwise vorticity components increase and the strength of vortex shedding decreases when using an RIH cylinder.