Channels, Fall 2020

Channels • 2 020 • Volume 5 • Number 1 Page 2 numerical analysis of drag reduction on circular and elliptical cylinders. In a previous investigation of grooved circular cylinders, Song [1] used two- dimensional computational fluid dynamics (CFD) to analyze the flow patterns around the surface of a cylinder and how groove parameters could predictably affect such patterns. They proposed a mechanism of drag reduction to be changes in the flow separation point. The location of this detachment was determined by three groove parameters: the depth of the grooves, the angle between the upstream stagnation point and the rearmost groove, and the width of the grooves. Song suggested a quadratic relationship between these groove parameters, highlighting an optimal set of parameters for greatest drag reduction. Rodriguez [2] and Buresti [3] studied numerically and experimentally, respectively, the effects of surface roughness on drag reduction. Their research focused on how surface roughness in varying flow regimes impacts drag and vortex shedding in the wake region. Other drag reduction research has been conducted branching from naturally formed flow profiles. Analyzing cactus surface roughness, Seong-Ho [4] used U-shaped grooves perpendicular to the flow to reduce drag over bluff bodies. They found that using a few grooves at an angle of approximately 90° from the upstream stagnation point yielded reductions up to 28% compared to smooth circular cylinders. Yunqing [5] researched drag reduction from the textured skin in some aquatic creatures. The desire to reduce drag is pervasive across the world of fluid mechanics. This paper presents an experimental and numerical analysis of circular and elliptical cylinders at a Reynolds number of 6 ∗ 10 4 and suggests an explanation regarding the specific mechanisms of drag reduction. Terminology and Methodology For clarity and succinctness in referencing groove parameter styles, some main terms adapted from [1] will be used to generally describe a cylinder’s cross section. The term “grooved cylinder” describes, in general, a right cylinder with lengthwise grooves which are parallel to the central axis of the cylinder such that a cross section through the central axis is the same at any point on the cylinder. “Smooth Model” describes a cylinder with no grooves around its perimeter. “Model A” is a cylinder with evenly spaced grooves surrounding its cross section’s entire perimeter with the groove pattern centered at the upstream stagnation point. “Model B” is also grooved and centered at the upstream stagnation point, but it is only grooved (nominally) on the upstream side of the cylinder with the rearmost groove located at an angle α from the stagnation point. Unless otherwise specified, α is assumed to be 90° for Model B. Fig. 1 shows cross sections of Models A and B.