Passive control of vortex shedding in the wakes of bluff bodies

The results of an experimental investigation into the control of vortex shedding in the wakes of bluff bodies are presented. The main aim of this investigation was to gain further insight into how the wake structure of nominally 2D bodies such as circular cylinders could be modified by the introduction of some spanwise geometric disturbances. Types of models studied were those with a fixed separation point, such as square sections and thin plates and also those with moving separation points such as circular cylinders. Experiments were conducted in both air and water over a large range of Reynolds numbers (101 — 3.4x 105) using four wind tunnels and two water tank facilities. Quantitative measurements have shown a significant drag reduction on modified bodies of up to 47% in comparison with the unmodified form. Results from particle image velocimetry (PIV) and flow visualisation have shown that even a small amount of three-dimensionality can produce large changes in the wake width and structure along the span. It is shown that a common feature of these perturbed bluff body shapes is that they set up significant spanwise velocity components that prevent the wake, from rolling up into the conventional 2D wake of von Karaman vortex shedding. This in turn leads to a steady near symmetrical wake with none of the adverse effects associated with vortex shedding. Applications of the results of this research to practical engineering structures are also discussed. A 3D cylindrical body is developed with elliptically shaped surface protrusions that shows an absence of vortex shedding in the wake regardless of flow direction, achieving a 25% drag reduction over a conventional 2D circular cylinder. Results are also presented which indicate that these bodies may be useful in the attenuation of vortex-induced vibrations as the amplitude of oscillations is significantly reduced when sufficient spanwise three-dimensionality is present.