Anne Cros

It is often expected that hydrophobic walls reduce viscous dissipation. However, our experimental results show that they instead increase the damping of shallow-water sloshing while leaving the natural oscillation frequency essentially unchanged. Two geometrically identical acrylic containers, differing only in wall wettability, are impulsively excited and the subsequent free-surface decay is recorded using a smartphone camera. Contrary to intuitive expectations, the hydrophobic container exhibits a decay time nearly three times shorter than the hydrophilic container. A shallow-water model reduces the dynamics to a damped harmonic oscillator, allowing students to extract frequency and decay rate independently through nonlinear regression and spectral analysis. The experiment provides a clear and accessible laboratory activity for upper-level undergraduate courses, illustrating the separation between restoring forces and dissipative mechanisms, the role of boundary conditions, and the exponential decay of mechanical energy in oscillatory systems.

In this experimental work, sloshing tests were performed with containers filled 1 with water at 50% of their volume capacity. Two boundary conditions were considered: un- 2 coated containers and containers with hydrophobic coated walls such as in [1]. In addition, 3 several aspect ratios λ (container width over length) were considered. We characterized two 4 regimes: the first one when the container is periodically forced at a frequency lower than its 5 resonant frequency and the second one after the forcing is suddenly stopped. In each case, 6 the amplitude of the waves was measured. Several surprising results were found. First, in 7 the forced regime, the sloshing amplitude was lower in the hydrophobic containers than in 8 the nonhydrophobic walls, despite the free-slip condition in the former case. Second, the 9 free damping after sudden stoppage was much higher with the hydrophobic walls than 10 with the uncoated containers. This behavior is explained by the collision of waves with 11 oil-coated walls, which decreases the load pressure. Finally, it was found that the damping 12 depends on the dimension of the container through λ and is greater when λ = 1.0. These 13 experimental findings open the way for further innovative research.

When a ‐elbowed pipe conveys a flow, the fluid deviation generates a transversal force on the tube. So, if the pipe is flexible, it may be deformed. In this experimental and theoretical work, we study the static deflection of a flexible pipe whose upper extreme is clamped while the other, lower, elbowed end is free. When water discharges, a follower force acts onto the free extreme which is proportional to the square of the fluid's velocity. We establish an analytical expression for the free extreme's deflection when the system's weight is neglected and solve the differential equation when its weight is taken into account. An experimental device is mounted which allows to easily record various deflection measurements as the fluid's velocity varies. Our experimental points are in agreement with the theory. They roughly align around the same line when dimensionless numbers are considered since theoretical deviation from a weight‐negligible system is lower than in our experiments.