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[Display omitted] • Experimental investigation of pressure–velocity fluctuations transport. • Pressure (wall sensors) and velocity (optical) measurements in a 5 × 5 rod bundle. • Pressure fluctuations transported by eddies moving with the flowrate velocity. 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. Experiments are performed using two sets of grids, i.e. configurations with and without mixing vanes, over a wide range of Reynolds number, i.e. from about 10,000 to 120,000. Particle Image Velocity measurements near the central instrumented rod reveal different structures of velocity fields and their fluctuations for the two configurations. Mean fields are in U inverted shape without mixing vanes and in λ shape with mixing vanes. Fields of velocity fluctuations are in butterfly shape without mixing vanes and in Y (low Re) and K (high Re) shapes with mixing vanes. Near the grid, u ′ rms / U flow increases by about 30% for Re Dh varying from 10,000 to 120,000. The presence of eddies is highlighted by visualisations of the fields of the velocity fluctuations. The spacing of these eddies and their sizes are found to be in agreement with the periodic length scales (obtained from frequency peaks) and the integral length-scales measured. Frequencies of the observed frequency peaks are found to be the same for both velocity and pressure fluctuations. Consequently, a new Strouhal versus Reynolds map is built over all existing data for Reynolds numbers between 10,000 and 120,000 and for the two grids configurations. The integral length-scales are found to be about the same for velocity and pressure fluctuations near the grid. It is shown that events of pressure fluctuations are persistent and transported with a speed close to the one of the mean flow. Moreover, this transport is correlated with the displacement of velocity events. The coherence between measured turbulence statistics, the observed eddies and cross-correlations of pressure and velocity fluctuations support the conclusion that the large-scale eddies (about 0.2 D h) are at the origin of main pressure fluctuations and their transport. To the authors' knowledge, this is the first experimental evidence of the origin and transport of pressure fluctuations downstream analytical grids with different geometries relevant to PWR fuel assemblies. [ABSTRACT FROM AUTHOR]

The vibration of rods is of prime importance to estimate their wearing due to the frictions with grids' elements. To predict this wearing, the pressure fluctuations exerted on the rods need to be identified and quantified. Pressure fluctuations are explored using an instrumented rod centred within a 5 × 5 rods bundle. The profiles of mean velocity and velocity fluctuations are explored around the instrumented rod using LDV. Two configurations of spacer-grids are used: with and without mixing vanes respectively noted WMV and NMV. The position of the measurement point is rotated over 360° and its distance from the grids is varied from 0.5D h to 20D h. The results are detailed for the Reynolds number 66000. The results show the non-homogeneity of the azimuthal distribution of the pressure fluctuations in the near wake of the spacer-grids for both configurations. The return to a homogeneous azimuthal distribution of the pressure fluctuations is faster without mixing vanes than with mixing vanes. The intensities of the pressure fluctuations are (in average) higher for WMV than for NMV by 13% and 136% at distances 0.5D h and 20D h from the grids respectively. The correlation functions and energy spectra highlight significant differences in the spatial and temporal structure of the pressure fluctuations for the configurations with and without mixing vanes. For both configurations, pressure fluctuations spectra reveal a frequency peak in the near wake of the grids. These peaks are around f D h / V flow ≈ 0.8 (69 Hz). Maxima are found at f ∗ = 0.78 (66.8 Hz) and f ∗ = 0.87 (74.6 Hz) respectively for WMV and NMV configurations. This frequency peaks persist farther from the grid for NMW than for WMV. The integral scale of pressure fluctuations highlight to different behaviour. The pressure integral length scale stays small compared to D h (ranging from 0.13D h to 0.2D h) for the NMV configuration whilst it increases with the distance from the grid for the WMV case. This is coherent with the persistence of frequency peaks for the NMV configurations. The pressure integral length scale is found to be 2–9 times larger for WMV than for NMV with the increase of the distance from the grid between 0.5D h and 20D h. A significant anti-correlated domain appears for NMW and not for WMV. In addition, LDV results show oscillations of the velocity profiles and a damping with the distance from the grids. Between 2D h and 15D h , the dimensionless ratio of "pressure fluctuations to velocity fluctuations" 〈 P rms ' 〉 θ / 1 2 ρ 〈 v rms ' 2 〉 P s q is found to increase by about 70% and 100% respectively for the configurations NMV and WMV. This increase further reveals the complexity of the turbulent flow in the wake of spacer-grids. [ABSTRACT FROM AUTHOR]