The aerodynamics of a set of two-dimensional, stationary elliptic cylinders with eccentricity of 0.44, 0.92 and 0.98 is studied experimentally during the organised wake condition (R=2x10⁴-10⁵). The results indicate the effect of eccentricity and angle of attack on mean and unsteady pressure coefficients, Strouhal number and wake geometry, the three important parameters in the aeroelastic instability study. In conjunction with the available literature, the information presented here attempts at providing better understanding of the bluff body aerodynamics in the region between the two extreme cases of circular cylinder (e=0) and flat plate (e=∞) . The cylinder eccentricity and attitude has considerable effect on vortex shedding frequency, with variation of the Strouhal number in the range 0.037-0.274. Although basing Strouhal number on projected dimension or wake width reduces its dependence on angle of attack, the use of transverse distance between the separation points provides, in addition, a smoother transition even at higher eccentricities and smaller angles of attack. The measurement of fluctuating pressure in the narrow band around the Strouhal frequency showed substantial dependence on the Reynolds number at zero angle of attack, particularly for the ellipses of high eccentricity Considerable phase difference may exist between the pressure signals, however, it has negligible effect on the unsteady lift. The thesis also presents analytical results on the location of shear layer separation as obtained using Görtler's series in conjunction with mean pressure profile. The analytical data compared favourably with the results of flow visualisation through Schlieren technique. The high speed movie of the near wake region gave preliminary information about the location of the first vortex and the oscillations of the separating shear layers. The rise and decay of the unsteady pressure in the vortex formation region appeared to substantiate the visual observations. In general, the wake geometry ratio varied around the Kármán stability value of 0.281. The correlation of spanwise results clearly emphasized the three dimensional character of the unsteady aerodynamics. In general, the spanwise variation of fluctuating pressure is enhanced with increase in angle of attack. The phase data suggest inclination of the vortex line at the model to be ≲11°. Increase in angle of attack results in improved alignment of the vortex line with the cylinder axis. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Note: / This thesis deals with flow-induced ovalling oscillation and the mechanical characteristics of cylindrical shells. The experiments were conducted with epoxy shells which were either clamped at both ends or clamped at one end and free at the other end. The response of the shells in cross flow was studied in a wind tunnel. A mini-shaker and a fotonic sensor were used to determine the mechanical characteristics (including the natural frequencies, modal damping, and the axial and circumferentia1 mode shapes) of the shells. An analytical method was developed, with the use of potential flow theory for the aerodynamic force, to calculate the oscillation frequencies as well as the negative aerodynamic damping values of clamped-clamped shells. Theory and experiments prove that flow-induced ovalling vibration of a cylindrical shell is not a three-dimensional effect, but rather an aeroelastic phenomenon. Vibration of the shell does not depend on flow periodicity in the wakes and an integral relationship (f/fvs) between the shell frequency (f) and the vortex shedding frequency (fvs) is not necessary at the onset of ovalling. / Cette these traite des oscillations induites par ecoulements, incluant les characteristiques mecaniques de coques cylindriques. Un mini-vibrateur et un capteur optique (Fotonic) ont ete utilises pour determiner les characteristiques mecaniques (inc1uant les frequences naturelles: l'amortissement, et les formes modales axiales et circonferentielles) des coques. Une methode analytique a ete developpee pour le calcul des frequences d'oscillations, ainsi que les valeurs negatives de l'amortissement aerodynamique des coques fixes, faisant usage de la theorie des ecoulements potentiels pour les forces aerodynamiques. La theorie et les essais sur ces coques demontrent que la vibration induite par ecoulements est un phenomene aeroelastique bi-dimensionnel, plutot que tri-dimensionnel. Les vibrations de la coque ne dependent pas de la periodicite de ecoulement dans le sillage, et un rapport integral (f/f ) vs entre la frequence de la coque et la frequence du detachement de tourbillons n'existe pas necessairement, au seuil des vibrations.

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A direct numerical simulation study has been conducted to examine the flow that develops on long circular cylinders that are aligned, or nearly aligned, with the freestream. Results are presented for turbulent boundary layers and vortex - shedding yawed flow. Although flows of these types occur in a range of engineering applications, they remain relatively unexplored compared with flat - plate flow. The numerical scheme employed for solution of the governing Navier - Stokes equations is similar to that used in some previously published simulations, but here rather different boundary conditions are adopted. At the outer edge of the cylindrical computational domain, the imposed boundary conditions confine the vorticity field within a finite radius while allowing the continuous velocity field to converge to the free - stream velocity at large distances from the cylinder. Axial flows are considered with radius Reynolds numbers in the range 311 to 20800, ratios of boundary layer thickness to cylinder radius in the range 0.15 to 27.5, and boundary layer thicknesses of between 160 and 800 viscous units ( v / u [subscript τ] ). The mean - flow and turbulence statistics for axisymmetric boundary layers are found to differ significantly from flat - plate results when the boundary layer is strongly curved, that is when the boundary layer is thick in relation to the cylinder radius. The effects of curvature are mainly observed in the outer flow except when the cylinder radius in viscous units is small. Particular attention is given to the assessment of similarity scaling relations for the mean velocity profile, velocity fluctuation statistics and temporal wall - pressure spectra. Structural features of axisymmetric turbulence are examined by inspection of instantaneous flow fields, correlation functions and conditionally - averaged flow structures. In very thick boundary layers on thin cylinders, the simulations reveal evidence of large - scale fluid motion across the cylinder, although the mechanisms of turbulence generation do not appear to be significantly different from those in flatplate flow. Simulations of turbulence in near - axial flow over cylinders are considered with radius Reynolds numbers up to 674 and yaw angles up to 0.5 degrees. No previous flow simulations of this kind are reported in the literature. The mean - flow and turbulence statistics are found to depart rapidly from axisymmetry as the yaw angle increases. The quality of the calculated results suggests that the computational procedure is suitable for use in a more comprehensive investigation of near - axial flow over cylinders. For cylinders inclined at sufficiently large yaw angles to the free - stream, turbulent boundary layer flow gives way to oblique vortex - shedding from the cylinder. Simulated flow fields corresponding to a radius Reynolds number of 311 and a yaw angle of 3 degrees are examined to reveal the three - dimensional structure of the flow. The results suggest that the oscillating flow fields in the cylinder wake have the character of a wave travelling in the axial direction at the same speed as the axial component of the free - stream. / Thesis (Ph.D.)--School of Mechanical Engineering, 2006.

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2006. / The aim of this thesis is to study the stabilising effect which vertical ribs may have on a tall cylindrical structure, with the focus on the proposed Solar Chimney. The report begins with a brief introduction to the Solar Chimney concept. A study is made of flow regimes which exist for flow around a circular cylinder and these flow regimes, together with their characteristics, are described. Various threats to the Solar Chimney are identified and will be investigated. The natural shape and strength of the Saguaro Cactus leads to the investigation of vertically ribbed cylinders. Experimental wind tunnel tests are performed in Tokyo, Japan to obtain external pressure distributions. A smooth cylindrical model, as well as two different configurations of vertically ribbed cylinders are tested. These external pressure distributions are numerically integrated in order to obtain calculated drag coefficient values for the cylinders. The drag coefficients for a smooth cylinder and cylinders with vertical ribs are obtained experimentally by means of total drag force measurements. These tests were performed in the wind tunnel at the University of Stellenbosch. The effect of aspect ratio of a smooth cylinder is also addressed. Computational Fluid Dynamics (CFD) analyses are carried out using the CFD software program, FLUENT. Both the drag coefficient and the external pressure distributions were investigated in this way. The differences between theoretical conditions and those of real life conditions of a smooth cylinder are discussed. The report ends by concluding the stabilising effect of vertical ribs on tall cylindrical structures and how these ribs may eliminate some of the present threats to the proposed Solar Chimney.