Rowing is a competitive sport where victory is determined by fine margins. In the past, the focus has been on optimizing the shell. Due to the increasing regulations placed on the shell design, the scope for manoeuvring in this area is greatly limited. This increases the necessity to focus on the less explored option of propulsion in rowing. The propulsion is caused by the momentum transfer from the rowers to the water with the help of oar blades. The design of these oar blades has also been extensively studied. However, knowledge on the effect of the air-water interface on the drag force on the oar blades is still lacking. Therefore, visualizing the flow around the oar blades will lead to better understanding of the flow structures, which would help in optimizing the drag force acting on the oar blades.

In the present study, a simplified scenario of a rectangular flat plate moving normally to its plane along a straight line and parallel to the free surface has been studied using hydrogen bubble visualization and Particle Image Velocimetry (PIV). Using a 4-axes industrial robot and a force/torque transducer, the effect of the air-water interface on the drag force on the plate has been studied at a Reynolds number, based on the longest edge of the plate, of 6×10⁴. The analysis of the drag force profiles at different plate depths led to the identification of a high drag case, which occurred at a plate depth h of 20 mm. The hydrogen bubble visualization indicated that in this specific case, the high drag was related to the formation of a compact wake behind the plate.

Hydrogen bubble visualizations were also performed at two other plate depths, h = 0 and h = 100 mm, to determine the effect of the air-water interface on the flow structures and ultimately on the drag force acting on the plate. The visualization of the deep water case (h = 100 mm) captured the formation of a vortex ring, which was found to have unique effects on the drag force. The presence of the air-water interface was found to significantly influence the drag force on the plate. Additionally, an extensive decomposition of the drag force profile showed a time-dependent added mass force and a decaying force acting on the plate.

Particle Image Velocimetry (PIV) measurements were carried out in the horizontal mid-plane of the plate. The tracking of the starting vortex and its disintegration has been identified using the swirling strength analysis. Finally, combining the hydrogen bubble visualization and PIV results, the formation and the development of the starting vortex was identified.