Publication
Extreme weather events are increasing in frequency in recent years due to global climate change. These events damage infrastructure and pose an open challenge to engineers to design more weather-resistant infrastructure. As engineers, we turn to nature for inspiration. Plants in nature are robust to adverse weather conditions because they deform and experience reduced drag. The drag reduction of plants and other flexible structures has been studied extensively from a structural point of view. In this thesis, we experimentally study this fluid-structure interaction problem from a fluids perspective by focusing on how the deformation of the flexible structure modifies the surrounding flow field.
We translate disks with radial cuts vertically through water. The disk sectors deform axisymmetrically during the motion and allow the formation of an axisymmetric vortex ring behind the disk. We first investigate the changes in the vortex topology, strength, and stability as the disk deforms. More deformed disks reduce the shear rate feeding the vortex and reduce the vortex impulse. The time derivative of the impulse is the vortex-based drag force, which closely matches the true drag measured on the system. Our findings highlight the significant role of vortex structures to the drag force.
In the second investigation, we address the impact of the bending geometry on the onset and rate of drag reduction. The bending geometry is varied by altering the number of radial cuts of the disks. We show that the experimentally-measured transition velocity is a robust indicator of the onset of drag reduction. Disks with more radial cuts are less stiff and have smaller transition velocities compared to disks with fewer cuts. The rate of drag reduction is also larger for disks with more cuts due to secondary stiffening mechanisms, which we quantify using a novel axisymmetric loading test.
Thirdly, we verify the accuracy of two fundamental assumptions made by existing drag prediction models. Existing models assume that the transient deformation history of a flexible structure does not impact the steady state drag force and that the pressure distribution downstream of the object can be neglected when predicting drag. We show that the transient deformation history does influence the drag force at steady state when an axisymmetric vortex ring forms behind the object. Further, the pressure downstream of a deforming structure cannot be neglected, as it results in increased steady state drag.
Lastly, we explore the influence of acceleration on the drag reduction of flexible structures. We measure the drag force that occurs at the end of acceleration and show that it scales by the square root of the acceleration. We introduce a novel Cauchy number that scales the drag reduction of flexible structures in accelerated flows. We also show that highly accelerated flows alter the topology, strength, and stability of the vortex behind the flexible disks by reducin