# THE EFFECT OF ASPECT RATIO ON THE AERODYNAMIC FORCES AND FREE END PRESSURE DISTRIBUTION FOR A SURFACE-MOUNTED FINITE HEIGHT CYLINDER

## Date

2017-08-23

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## Type

Thesis

## Degree Level

Masters

## Abstract

The surface-mounted finite-height cylinder is a fundamental engineering shape and can be found in a multitude of industrial applications. As a result, the local flow field is of great importance in the design of cylindrical components such as heat exchangers or buildings. While two-dimensional (2-D or “infinite”) cylinders are well-understood, the effects of the ground plane and the cylinder free end are significant and require further study. Of particular interest in this thesis is the pressure distribution on the free end of the cylinder, and a mean normal force that develops from it. A vast majority of studies on this topic have focused on short cylinders with a small aspect ratio (AR = height/diameter). The work in this thesis is an attempt to characterize how the pressure distribution and mean aerodynamic forces are influenced by the aspect ratio of the cylinder and the boundary layer thickness of the flow. The little-researched mean normal force, the mean drag force and its resultant mean bending moment, and the associated vortex shedding in the wake are investigated, along with the mean surface pressures and pressure fluctuations for the cylinder free end. A cylinder was designed for use in measuring these parameters for 22 evenly spaced aspect ratios in a range from 0.5 ≤ AR ≤ 11, and an additional cylinder and boundary layer were used to generate data for four different values of relative boundary layer thickness in the range 0.60 ≤ δ/D ≤ 2.86.
The results of this research fit in well with published data, and reveal that the flow regimes appear to be marked by two critical aspect ratios, located approximately at AR = 2.5 and AR = 6. Below the lower critical AR, the boundary layer and ground plane effects are dominant, and the Strouhal number and the mean drag and mean normal force coefficients are drastically reduced. The mean bending moment coefficient is high at low AR, possibly owing to the high point of action of the drag force caused by the velocity distribution in the boundary layer. Between the two critical aspect ratios, the mean force coefficients and Strouhal number are relatively insensitive to AR. Above the upper critical AR, the mean drag coefficient increases towards the value for a 2-D cylinder, while the mean normal force coefficient reduces, and is expected to approach a small, constant value. A vertical wall shear force that acts in the opposite direction of the free end pressures may account for the difference between the mean normal force results obtained from integration and those obtained from direct measurements. For high AR, the bending moment coefficient and point of action are relatively unchanged. The free end pressure distributions reveal similar features to previously published data, including “eye-like” enclosed regions of minimum pressure on the upstream half of the cylinder face, and an enclosed region of maximum pressure on the downstream half of the cylinder face. The eye-like structures disappear above the upper critical AR, and are replaced with a band of minimum pressure, upon which the pressure distribution is no longer influenced by aspect ratio. These two critical AR, along with the free end surface pressures and aerodynamic forces, are influenced by the boundary layer thickness, such that a thicker boundary layer creates higher critical aspect ratios.
This work is among the first to use consistent flow conditions in showing the effect of two critical aspect ratios on multiple fluid forces and flow structures over a large range of AR. This includes the mean normal force and bending moment. The range of values for the critical aspect ratios is narrowed by the use of small incremental changes in the cylinder aspect ratio. The pressure distributions, and the pressure fluctuations, on the cylinder free end were established in greater detail than earlier published studies as well, and the effects of a change in aspect ratio and boundary layer thickness can be clearly seen. It is hoped that the work contained herein will be an aid to the design and optimization of finite cylinders in future engineering applications.

## Description

## Keywords

Fluids, bluff body, cylinder, drag, normal force, pressure distribution, free end

## Citation

## Degree

Master of Science (M.Sc.)

## Department

Mechanical Engineering

## Program

Mechanical Engineering