Exploration of frequencies peaks observed on local wall pressure measurements by time-resolved velocity field measurements in complex flows

In pressurized water reactors (PWR), fluid–structure interaction provokes vibrations of the fuel rods leading to grid-to-rod fretting. To explore the origins of the local excitations of the rods by the flow, analytical experiments are performed within a 5 × 5 rod bundle maintained by spacer grids. New velocity (laser Doppler velocimetry) and pressure fluctuations measurements, taken for various Reynolds numbers, show the presence of frequency peaks in the spectra over a broad range of Reynolds numbers, i.e. 13,000 to 108,000. The Strouhal numbers obtained vary weakly with the Reynolds number and are in the range of 0.21–0.26. The phenomena being more pronounced downstream the dimples, this manuscript explores the flow at the exit of spacer grids downstream the dimples. To see through aligned rods, refractive index matching is implemented on parts of selected rods. The dilated scale of the experiments permits one to perform particle image velocimetry (PIV; grid of 51 × 89 points with a correlation window size of 0.032 Dh) to identify eddies smaller than the rods in the wake of the dimples. PIV measurements reveal the presence of multi-scale eddies with a periodic street of main eddies. The size of these eddies and the periodic length scale of the street are found to be in agreement with the integral length scale and the length scale built on the frequency peaks of pressure fluctuations. To the authors’ knowledge, this is the first experimental evidence of the existence and the role of coherent eddies streets downstream from dimples of spacer grids with geometries relevant to PWR fuel assemblies.