The highly unsteady flow around a rowing blade in motion is examined using a three-dimensional computational fluid dynamics (CFD) model which accounts for the interaction of the blade with the free surface of the water. The model is validated using previous experimental results for quarter-scale blades held stationary near the surface in a water flume. Steady-state drag and lift coefficients from the quarter-scale blade flume simulation are compared to those from a simulation of the more realistic case of a full-scale blade in open water. The model is then modified to accommodate blade motion by simulating the unsteady motion of the rowing shell moving through the water, and the sweep of the oar blade with respect to the shell. Qualitatively, the motion of the free surface around the blade during a stroke shows a realistic agreement with the actual deformation encountered during rowing. Drag and lift coefficients calculated for the blade during a stroke show that the transient hydrodynamic behaviour of the blade in motion differs substantially from the stationary case.
The flow around a rowing oar blade during a stroke is highly complex owing to the proximity of the water surface and the rapidly changing blade flow incidence (here, greater than 180° in under 0.75s). This flow is simulated using a computational fluid dynamics (CFD) model with a rotating subdomain for blade rotation coupled to a model of the shell velocity. Based on the shell velocity and a specified oar angular velocity, the CFD model calculates the highly unsteady three-dimensional flow, providing instantaneous drag, lift, and propulsive forces on the blade. The propulsive force drives the shell velocity model, which also accounts for the shell drag and the motion of the rowers relative to the shell. The dynamic blade—water interaction is depicted in six distinct flow regimes, characterized by the relative motion of the blade and the temporal influence of drag and lift. It is seen that the propulsive force generated by the blade is largely lift induced during the first half of the stroke. Dynamic stall behaviour of the blade characterizes the flow during the second half of the stroke, where drag increasingly influences the propulsive force. At the end of the stroke, the propulsive force is once again largely lift induced.
The behaviour of oar shaft bending during the drive phase is examined using a hydrodynamic-based model of the rowing stroke. By modelling the complex time-varying hydrodynamic load on the blade, the amount of shaft bending during the drive can be calculated. It is shown during the first 45 per cent of the drive that the blade rotation rate is up to 30 per cent slower than the oarlock rotation rate as the oar deflects and energy is stored in the flexible shaft. Through the remainder of the drive the shaft unbends, causing the blade to rotate up to 16 per cent quicker than the oarlock as the stored energy is transferred to the water and to shell propulsion. The effects that this bending has on oar blade and handle forces highlight the importance of accounting for oar shaft flexibility when modelling the rowing stroke.
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