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Vortex characteristics of sharp and round step cylinders at high Reynolds numbers

Wang, J. (2020). Vortex characteristics of sharp and round step cylinders at high Reynolds numbers. (Unpublished Doctoral thesis, City, University of London)


‘Flow around a uniform circular cylinder’ being one of typical physical issues has been comprehensively investigated in the past few decades, the involved content of which includes but is not limited to surface pressure distribution, surface loading variation and vortex characteristics behind the cylinder. As for actual engineering practises, such a simple structure encounters geometrical variation along the span which gives rise to a series of non-uniform cylinders having circular cross-sections but varying the diameter along the span. Non-uniform cylinders significantly strengthen the complexity regarding fluid dynamics in the wake, which posts a vital threat to ensure service time of relevant engineering buildings and facilities.

A step cylinder consists of two circular cylinders with different diameters, by connecting with each other coaxially. This special shape has a wide range of applications, one of which is to compose ‘buoys-risers’ system in ocean engineering field. The addition of buoys gives rise to geometrical changing along the span, furthermore, causing complicated three-dimensional flow structures in the wake zone and vortex-induced vibration (VIV) of the riser, thereby is greatly worth investigation. It should be borne in mind that the majority of step cylinder models which have been investigated are sharp step cylinders, i.e., sharp corner near the step junction. The present study tests a modified step cylinder (viz. round step cylinder) by replacing the sharp edge with a round corner. Besides, most part of previous research regarding sharp step cylinders successfully and reliably discussed complex vortex interaction in the wake under lower Reynolds numbers, indicating that there can be some improvement for experimental / numerical study on corresponding vortex interaction under higher Reynolds numbers. As a result, this thesis focuses on comparative study on turbulent flow properties (e.g., vortex characteristics) behind sharp step cylinder and round step cylinder under moderate high Reynolds numbers (1.6e+4, 3.3e+4), which can help gain a deep insight on vortex behaver in turbulent state, as well as the impact of shape of step junction.

More specifically, the present study carries out a series of experiments in terms of the aforementioned two types of step cylinder (viz. sharp step and round step) at Re = 1.6e+4, 3.3e+4 by applying particle image velocimetry (PIV) technique. Velocity fluctuation in streamwise and spanwise along two measuring planes parallel to the model axial in the wake is captured and investigated. Different types of vortex shedding, as well as vortex interaction, are quantitatively characterised by investigating flow fields, corresponding spectra, local frequency variation, spatial energy distribution and primary ‘ingredients’ of flow fields. Considering the fact that flow structures near step junctions display strong three dimensionality which cannot be fully depicted based on the present experimental data, i.e., planar velocity fields, corresponding numerical simulations are performed using the open source CFD (computation fluid dynamics) software namely OpenFOAM, with the emphasis on time-averaged content near sharp step junction and round step junction, streamwise vortices, velocity contours along multi-planes, surface pressure distribution and loading variation. Besides, during the numerical simulation, a hybrid mesh strategy is introduced and utilized, with the combination of structured grid and ‘Cartesian grid’, to achieve reduction of cell numbers.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
Q Science > QA Mathematics > QA75 Electronic computers. Computer science
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering
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