Three-dimensional wake development and structural loading on dual step cylinders in cross-flow
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Many cylindrical structures in engineering applications undergo flow-induced periodic loading due to the vortex shedding phenomenon. This can lead to early fatigue failure and be a source of unwanted noise. To mitigate or reduce the mean and fluctuating loading, the flow development can be altered by adding geometrical modifications to the structure. This thesis is focused on the dual step cylinder geometry, which consists of a finite-span circular cylinder (the large cylinder) attached co-axially to the mid-span of a uniform circular cylinder of smaller diameter (the small cylinder). The dual step cylinder is a geometrical modification to a uniform cylinder that is ubiquitous in engineering applications, but has not been investigated extensively for its potential use in flow control. Accordingly, there is a need for developing an understanding of the flow development and structural loading on dual step cylinders in cross-flow. This thesis provides a comprehensive characterization of the effects of dual step cylinder geometry and Reynolds number on wake development and structural loading in the turbulent vortex shedding regime. Experiments were carried out in the Water Flume Facility at the University of Waterloo and an open jet wind tunnel at TU Delft. Experimental measurements included flow and surface visualizations, wake velocity measurements with Tomographic Particle Image Velocimetry (PIV), planar PIV, and Laser Doppler Velocimetry (LDV), and lift and drag force measurements. Through an analysis of the qualitative and quantitative results, six distinct flow regimes have been identified based on observed changes in the flow development downstream of the large cylinder. The results are summarized in a two-dimensional map which has both practical and fundamental importance. Specifically, the map can be used to select dual step cylinder geometries which produce desirable flow and loading characteristics, and the map provides detailed insight into the turbulent wake development of dual step cylinders and other similar complex cylindrical geometries including cantilevered cylinders, single step cylinders, coin-like cylinders, and cylinders with two free ends. For complex bluff body geometries, such as that of a dual step cylinder, the flow development is highly three-dimensional. In order to investigate such flows experimentally, quantitative measurements of the three-dimensional flow field are required. The presently available techniques of three-dimensional velocity field measurement, e.g., Tomographic PIV, Particle Tracking Velocimetry, and Holography, are expensive to implement and are not yet commonplace in experimental facilities world-wide. The present study employs multi-plane PIV measurements in order to investigate the three-dimensional wake development of a dual step cylinder. Two techniques of multi-plane PIV data analysis are proposed in order to reconstruct the dominant vortical structures and vortex interactions in the wake: (i) a Proper Orthogonal Decomposition (POD) based phase-averaging approach, and (ii) a pattern recognition based conditional averaging approach. Through a comparison with Tomographic PIV results, both techniques are shown to successfully reconstruct the wake vortex shedding and vortex interactions downstream of a dual step cylinder. The techniques developed can be applied to complex bluff body flows and can accommodate the presence of more than one dominant frequency.