Effect of turbulence on ultrasound propagation for vibration measurements of PWR assemblies in the HERMES P loop of the POSEIDON platform

This study focuses on the propagation of ultrasonic waves through a turbulent water flow representative of the hydraulic environment near a mixing grid in a Pressurized Water Reactor (PWR). Ultrasonic measurement, which relies on accurate knowledge of the acoustic velocity, is an effective non-intrusive technique for detecting displacements under harsh conditions. However, in turbulent flows, local velocity fluctuations induced by vortices can disrupt wave propagation, leading to measurement dispersion. An experimental setup was implemented to quantify these effects by comparing ultrasonic time-of-flight fluctuations upstream and downstream of a vortex-generating obstacle. The 1σ standard deviation from multiple acquisitions was used to characterize the variability of the measurements. The results show that dispersion increases with vortex density, but remains below 15 µm even in the most turbulent regions. In contrast, the Reynolds number alone does not adequately reflect this variability, which depends more strongly on the local vortex density. These results confirm the feasibility of ultrasonic techniques in turbulent flows, provided that the influence of local flow structures is taken into account—particularly for vibration monitoring of mixing grids in PWR cores.