City Research Online

Abstract

The dependence of the drag coefficient of a sphere on the free-stream turbulence characteristics and Reynolds number has been acknowledged by researchers for many years now. Over the same period, a correlation between the drag coefficient and a combination of the various characteristics has also been sought. This project analyses the effects of the characteristics of importance in this respect and provides a correlation between the drag coefficient and a variation of characteristics.

The effects on the drag coefficient of varying the macroscale of turbulence keeping all other characteristics constant have been presented. These show that the magnitude of variation of the drag coefficient caused by this characteristic is such that it should be included in any correlation between the drag coefficients and characteristics that may be sought.

The effect on the pressure distribution of the various characteristics has also been presented, from which it has been shown that the macroscale of turbulence introduces a dynamic boundary layer which conforms with the physical model built in the earlier chapters. The mechanisms by which the dynamic boundary layer is formed and is affected by the various characteristics have been analysed together with the resulting variations of the drag coefficient. It has also been shown that because of the dynamic boundary layers introduced, the skin friction drag increases and raises its percentage contribution to the total drag to values much greater than those quoted by previous researchers who investigated flows of low macroscale of turbulence. Additionally, a resonance position of the drag coefficient values has been shown to exist at a fixed frequency ratio with an amplitude dependent on the Reynolds number.

A series of correlations have been shown to exist between the drag coefficient and the characteristics, with the scale ratio being identified as the characteristic upon which the correlations are made unique. Because of the nature of the correlation, a vibrations equation for a similar phenomenon has been used to define the curves. Within the determined limits this equation has been found to be an accurate method of predicting the drag coefficient from the characteristics.

In determining the macroscale of turbulence from the experiments conducted, a method of predicting the position of the peak of a Gaussian curve when this has been cut-off has been developed. This has been derived from the von Karman equation which defines the spectral density of turbulence.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QC Physics T Technology > T Technology (General) T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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