Numerical simulations of an inverted flexible plate in linear shear flows

In each rowing sport, the oars have their very own characteristics most of the time selected through a long time experience. Here we address experimentally and theoretically the problem of rowing efficiency as function of row lengths and blades sizes. In contrast with previous studies which consider imposed kinematics, we set an imposed force framework which is closer to human constraints. We find that optimal row lengths and blades sizes depend on sports and athletes strength, and provide an optimisation scheme.

https://iopscience.iop.org/article/10.1088/1367-2630/ab4226/pdf

Physics of rowing oars

Force modeling of zero/low-velocity fin stabilizer and hydrofoil profile optimization

When an object is accelerated in a fluid, a primary vortex is formed through the roll-up of a shear layer This primary vortex does not grow indefinitely and will reach a limiting size and strength Additional vorticity beyond the critical limit will end up in a trailing shear layer and accumulate into secondary vortices The secondary vortices are typically considerably smaller than the primary vortex Here, we focus on the formation, shedding, and trajectory of secondary vortices generated by a rotating rectangular plate in a quiescent fluid using time-resolved particle image velocimetry The Reynolds number (Re) is varied from 840 to 11150 At low Re, the shear layer is a continuous uninterrupted layer of vorticity that rolls up into a single coherent primary vortex At Re=1955, the shear layer becomes unstable For Re>4000, secondary vortices are discretely released from the plate tip First, we demonstrate that the roll-up of the shear layer, the trajectory of the primary vortex, and the path of secondary vortices can be predicted by a modified Kaden spiral Second, the timing of the secondary vortex shedding is analysed The time interval between the release of successive secondary vortices is not constant during the rotation but increases the more vortices have been shed The shedding time interval also increases with decreasing Reynolds number The increased time interval under both conditions is due to a reduced circulation feeding rate

/pdf/discrete-shedding-of-secondary-vortices-along-a-modified-3cde3khtmf.pdf

Discrete shedding of secondary vortices along a modified Kaden spiral

We present results on the drag on, and the flow field around, a submerged rectangular normal flat plate, which is uniformly accelerated to a constant target velocity along a straight path. The plate aspect ratio is chosen to be to resemble an oar blade in (competitive) rowing, the sport which inspired this study. The plate depth, i.e. the distance from the top of the plate to the air-water interface, the plate acceleration and the plate target velocity are varied, resulting in a plate width based Reynolds number of . In our analysis we distinguish three phases; (i) the acceleration phase during which the plate drag is enhanced, (ii) the transition phase during which the plate drag decreases to a constant steady value upon which (iii) the steady phase is reached. The plate drag force is measured as function of time which showed that the steady-phase plate drag at a depth of plate height (20 mm depth for a plate height of 100 mm) increased by 45 % compared to the plate top at the surface (0 mm). Also, it is shown that the drag force during acceleration of the plate increases over time and is not captured by a single added mass coefficient for prolonged accelerations. Instead, an entrainment rate is defined that captures this behaviour. The formation of starting vortices and the wake development during the time of acceleration and transition towards a steady wake are studied using hydrogen bubble flow visualisations and particle image velocimetry. The formation time, as proposed by Gharib et al. (J. Fluid Mech., vol. 360, 1998, pp. 121-140), appears to be a universal time scale for the vortex formation during the transition phase.

/pdf/drag-force-on-an-accelerating-submerged-plate-4oip5wg5e8.pdf

N. B. Vijayaragavan

Papers

Drag force on an accelerating submerged plate