Effect of aspect ratio on the unlimited flow-induced vibration of an elliptical cylinder-plate assembly

The transverse flow-induced vibration (FIV) of an elastically supported elliptical cylinder-plate assembly is investigated numerically for a laminar flow at a Reynolds number of 100. The aspect ratio (AR) of the elliptical cylinder is varied over a range of values (namely, $\text{AR}=0.5$, 0.67, 0.75, 1, 1.5, and 2). In addition, two normalized splitter-plate lengths, $L_{\text{SP}}/D=0.75$ and 2.5, are investigated (where $L_{\text{SP}}$ is the splitter-plate length, and $D$ is the equivalent diameter of the elliptical cylinder). A low mass ratio of 10 and zero structural damping are used in the numerical simulations to induce larger oscillations in the assembly. The numerical results show that all cases investigated exhibit a FIV over an unlimited range of reduced velocity. An increase in the AR promotes the vibrations of the assembly through a reduction in the reduced velocity associated with the onset of FIV and a concomitant increase in the vibration amplitude. In addition, a larger AR facilitates the transition from a pure galloping (for $\text{AR}\le 1$) to an integrated VIV-galloping response (for $\text{AR}>1$) for an assembly with $L_{\text{SP}}/D=0.75$. Moreover, a larger AR significantly decreases the onset velocity of galloping for an assembly with $L_{\text{SP}}/D=2.5$. The AR determines the nature and width of the synchronization branch in the amplitude response. In general, a larger AR leads to the inception of higher-order synchronization branches in the amplitude response and to the suppression of some branches (e.g., still and initial galloping branches) for assemblies with long splitter plates. Finally, with respect to the flow dynamics associated with an unlimited FIV, increasing AR promotes the shedding of more complex vortices in the wake of the assembly (e.g., the emergence of a tail-shaped vortex and a slender vortex)—despite this, the wake mode remains unaltered.