FLOW AROUND BLUFF BODIES WITH CORNER MODIFICATIONS ON CROSS-SECTIONS

Summer 2019

Thesis

Program or Major

Mechanical Engineering

Degree Name

Master of Science

Christopher White

Ivaylo Nedyalkov Nedyalkov

Abstract

This research aims to illustrate how the flow around a cylinder changes when the cylinder’s cross-section is systematically changed from square to circle by modifying the corner radius.

Numerous research studies have been performed on the flow around circular cylinders and square cylinders leading to a relatively complete understanding of them. In the early 20th century, von Kármán and Rubach described the theoretical basis and provided an analytical solution to the flow around circular cylinders at low Reynolds number. Later, experiments on the flow around square cylinders were conducted by Nakaguchi, Bearman, and other researchers. However, until now, only a few researchers have focused on how the flow structure evolves when the cylinder’s cross-section gradually changes from a square to a circle by increasing the radius of the corner edges.

In this study, five numerical simulations were conducted. Each simulation performed calculations on a cylinder model where the shape was changed systematically from a square to a circle. C1 is a square cylinder with a side length of D = 0.375 inches and r / D = 0 where r is the radius of the corner edge; C2 to C4 denote three rounded-corner square cylinders with the same side length D and r / D ratios of 0.167, 0.247, and 0.333, respectively. C5 is a circular cylinder with a diameter of 0.375 inches (r / D = 0.5). Simulations were performed in two dimensions at Reynolds number of 10 to 200, using the control volume technique and Gauss-Siedel iterative method in conjunction with the Algebraic Multigrid (AMG) solver in ANSYS FLUENT 18.2.

The simulated flow around the cylinders was illustrated by planar contours of various flow variables (e.g., velocity and pressure). The evolution of the flow behavior when transitioning from a square cylinder to a round cylinder are described by comparing the scaled reattachment length, Strouhal number, and drag coefficient between the different cylinders. It is shown that independent of the Reynolds number, the drag coefficient and scaled reattachment length decrease, while the Strouhal number increases when a cylinder changes from a square shape into a circular shape. However, under specific conditions, the drag coefficient of a rounded-corner square cylinder may be lower than that of a circular cylinder with the same dimension.

In addition, a series of experiments were performed to study the flow around the described set of cylinders above at higher Reynolds number ranging from approximately 4400 to 16000. The experiments were performed in an Engineering Laboratory Design (ELD) Model 404 wind tunnel located in Kingsbury Hall at the University of New Hampshire. The test-section dimensions are 18 inches x 18 inches cross-section and 36 inches in length with a maximum speed of 150 mph. The cylinders were centered in the test-section with the cylinder length perpendicular to the flow direction. The aerodynamic drag on a cylinder was measured as a function of wind speed in the tunnel using a TecQuipment AFA2 lift/drag force balance. The wind speed in the test section was measured using a Pitot-static tube connected to a differential pressure transducer. The trends of drag coefficient observed in the experiments are similar to those observed in the simulations.

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