| dc.description.abstract | Understanding of the flow behind the bluff body poses a great challenge and remains almost entirely in the empirical and descriptive realm of knowledge. Flow in the wake of bluff bodies is characterized by the phenomenon of vortex shedding above a certain critical value of Reynolds number. Vortex shedding results in unsteady loading which can lead to total collapse of the structure if vortex-shedding frequency matches with the natural frequency of the body. Since most of the bluff bodies of practical importance are three-dimensional, it is important from an applied perspective to understand vortex shedding from three-dimensional bodies.
In the present study we have done a detailed investigation of three-dimensional vortex shedding brought about by the spanwise variation of body geometry or the oncoming velocity in a unified framework. For this purpose we will be resorting to the vorticity equation and especially concern ourselves with the mechanism of creation of streamwise vorticity by the vorticity tilting. The work in this thesis is primarily designed towards evaluating this premise and thereby understanding the underlying physics.
Experiments on tapered cylinders of taper ratio (defined as ratio of length of tapered cylinder to change in diameter) 35, 54, 100, 295 and 665 were conducted at different speeds viz. u = 0.195, 0.272, 0.393, 0.459. For smaller values of taper angle, the end condition at the tip side seems to have a strong effect on the initial phase of the vortices, which in turn is responsible for the obliqueness of the shed vortices. For large values of taper angle however, the end conditions seem to become irrelevant and there is a tendency for the vortex to be tilted away from the tip side when compared to the base side of the span of the tapered cylinder. Experimental results show a transient nature of vortex dislocations in the tapered cylinder wake; it was seen that vortex dislocation which gets formed is a dynamically evolving pattern which changes with time such that the tilting of vortex lines is seen to go through very large changes in their orientations.
A linear shear flow was generated using curved gauze. Experiments were performed on uniform cylinder for two speeds, u = 0.178, 0.458 which correspond to shear parameter (defined as its average dudz\frac{du}{dz}dzdu?) viz. p = 0.042, 0.014 respectively.
Qualitatively, the cell formation for the shear flow looks similar to that for the uniform flow over a cone. However, there is one major difference. In the case of shear flow, the inclination of the vortex line seems to be decided by the sign of the upstream velocity gradient — with the end of the shed vortex at the top of span (where the upstream velocity is higher) than the section of the shed vortex at the bottom of span (where the upstream velocity is lower). Whereas the cone in uniform flow has a proclivity to get chaotic readily, the shear flow was found relatively laminar and quiet.
An attempt was made to check equivalence between the cone-in-uniform flow and uniform cylinder in a shear flow, solely by relating the taper ratio and shear parameter. It was found that there are distinct differences in terms of the ‘quality’ of these two flows.
It appears that the steady conditions such as the upstream shear or the taper could not have yielded a time-varying pattern of dislocations. It is suggested that the induced velocity of vortices across the street at any spanwise location (as governed by Biot-Savart law) gives rise to a spanwise velocity gradient (in an unsteady fashion) and this results in the creation of the required vortex connections and dislocations to satisfy Helmholtz theorem. In order to test this proposal we designed an experiment with shear flow. For a given value of upstream linear shear, we designed a corresponding linearly tapered cylinder whose diameter also varies in the same fashion across the span such that /?=/ \sim =/?= constant. A remarkably steady wake pattern without any dislocations was obtained.
Control of three-dimensional vortex shedding phenomenon by using a two-dimensional control rod is studied by extending methodology of Sreenivasan & Strykowski to flow over cones and shear flows. Presence of a smaller 2D cylinder in the three-dimensional wake (due to a cone or upstream shear) resulted in the suppression of vortex shedding pattern for some low Reynolds number range. It is suggested that suppression of vortex shedding was possible because of the weakened global mode. | |
