Flow-Induced Vibration of Elastically Mounted Cylinder-Plate Assemblies

Abstract

The transverse flow-induced vibration (FIV) of an elastically supported cylinder-plate assembly (viz., a rigid splitter-plate attached to the downstream side of a cylinder) is investigated deeply and systematically in this thesis, using the numerical simulation based on Computational Fluid Dynamics (CFD) and the mathematical wake-oscillator model.

To investigate the influence of splitter-plate length (LSP), a circular cylinder-plate assembly is numerically simulated in laminar flow, involving extensive spans of plate length LSP/D = 0–4 (where D is the cylinder diameter) and reduced velocity Ur = 2–30. The simulations demonstrate that LSP substantially affects nearly every aspect of the assembly’s FIV. For structural vibration, the self-limited FIV is induced for LSP/D ≤ 0.5, while a galloping-dominated FIV is triggered for LSP/D ≥ 0.75. For branching behavior, both odd- and even-multiple synchronizations between the structure oscillation and vortex shedding are supported in the assembly. In particular, two new branches (viz., initial galloping branch and still branch) are identified for LSP/D ≥ 2.5. For nonlinear dynamical characteristics, the beating phenomenon of FIV is closely related to some irregular vortices and wake modes unique to the assembly based on the flow analysis.

To investigate the synergy effect of the aspect ratio of cylinder (AR) and plate length, the transverse FIV of an elliptical cylinder-plate assembly is simulated under same conditions, involving various combinations of AR (0.5, 0.67, 0.75, 1, 1.5 and 2) and LSP/D (0.5, 0.75 and 2.5). The two geometrical factors lay different emphasis in influencing the assembly’s FIV. AR determines whether a FIV can be induced on the assembly, with a critical value occurring in the range 0.67 < ARcri < 0.75 at Re = 100. Once the FIV is triggered (AR > ARcri), the fundamental vibration mode (limited or unlimited) depends on LSP , and a change in AR only has a relatively limited impact on the vibration level.

To mathematically predict FIV, the coupled wake-oscillator models based on lift coefficient (CL) and wake angular displacement (θ) are improved from different aspects. Then, a genetic algorithm (GA) optimized nonlinear grey-box estimation framework is proposed to determine free parameters. Based on this, various model structures are discussed in terms of the VIV of a circular cylinder and the galloping of a cylinder-plate assembly. The results suggest that the optimal model has six free parameters with the first-order polynomial expression for the prediction of VIV. While for the complex galloping occurred at larger Ur, more high-order polynomial terms are necessary to predict the non-sinusoidal oscillations, which leads to the optimal model with eight free parameters.

Overall, this thesis offers crucial new perspectives on the nature and physical mechanisms behind the complex transverse FIV response of a cylinder-plate assembly.