Dynamics of a free-hanging riser

Flow past multiple cylinders, placed in tandem, side-by-side, or staggered arrangements, has been studied extensively owing to its industrial applications in several engineering fields, such as offshore oil drilling rigs, heat exchangers, cooling towers in the chemical processing industry and tall chimneys, which are usually subjected to flows at different speeds. Lot of numerical and experimental researches have indicated that, when the cylinder rotates, wake characteristics are significantly different from those of a non-rotating cylinder. For example, 2-D numerical simulations at Re = 200 demonstrated that an isolated rotating cylinder sheds the primary vortices, like a non-rotating isolated body, for rotating speed < 1.91, whereas the vortex shedding is suppressed at higher α. However, the second flow instability is observed for rotating speed ranging from 4.34 to 4.70 that leads to one-sided vortex shedding. The actively-rotating cylinder is usually regarded as a typical example of the Magnus effect. Although some researchers have reported the associated flow details of the rotating cylinder, there is still a lack of understanding on the multiple-arranged rotating cylinders. Extra lift forces can be generated and applied on the single rotating cylinder due to the Magnus effect. How about the two cylinders? More net lift forces or less? What can we utilize the extra lift forces? Promoting the ship or helping to turn back? This is a problem needed to be solved.

This research will focus on flow wakes and hydrodynamic forces around an actively-rotating circular cylinder as well as two staggered rotating circular cylinders The rotating speed (a = 0, 0.5, 1, 1.5, 2, 2.5, and 3), rotating direction (counter-rotating and co-rotating), spacing ratio (S = 1.5, 2.5, and 4.5), and attack angle (from -90 degree to 90 degree with a step of 5 degree) are planned to be examined. The software ANSYS fluent is required and used to solve this problem at a relatively low Reynolds number of 200. The following results are expected:

1. the variations of hydrodynamic forces with rotating speeds, spacing ratios and attack angles;

2. the flow details around the rotating cylinders;

3. the flow mechanicsm of the two rotating cylinders;

4. the phase difference between the upstream and downstram cylinder;

5. a map of wake patterns.

Through conducting numerous simulation work, the flow past two staggered rotating circular cylinders are potentially understood, and the results may provide a useful guidance on the design of flettner rotor ship and thus energy-saving.

For more information about the project contact Dr Qing Xiao (qing.xiao@strath.ac.uk), Professor at the Department of Naval Architecture Marine and Ocean Engineering at the University of Strathclyde.

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