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2D PIV Study of Grid Turbulence 10.17863/CAM.47724 Cardesa Dueñas, José Ignacio 0000000238635285 Dawson, James R. 0000000230696948 Nickels, Timothy Bruce The aim of the current project was to analyse grid turbulence with a technique that allows for a direct measurement of velocity derivative statistics. Two-dimensional particle image velocimetry ($2D~PIV$) of highly resolved grid turbulence at $Re_{M}=U_{\infty}M/\nu = 16000$ where $M$ is the grid rod spacing was analysed. Data spanning the downstream range $65 < x/M < 145$ was captured separately on planes parallel to a water channel's floor and walls with a $PIV$ resolution within three times the Kolmogorov microscale $\eta_{_{K}}$. Isotropic turbulence ($IT$), axisymmetric turbulence ($AT$), homogeneous turbulence ($HT$), all these theoretical frameworks have been checked against velocity derivative data. It was found that the data complies most satisfactorily with $HT$ only, and to a lesser extent with $IT$ and $AT$. The $2\times 2$ velocity-gradient tensor $VGT$ that can be formed from $2D~PIV$ data was used in a novel manner by plotting the joint probability density function ($PDF$) of its trace $p$ and determinant $q$. Under the assumptions of \textit{locally} isotropic turbulence, the observed asymmetry in this joint $PDF$ was explained as the result of the predominance of enstrophy amplification by vortex stretching. The observations were validated when compared with $2D$ slices of $3D$ data from a direct numerical simulation of isotropic flow. Finally, a study of homogenisation was carried out by mapping an area of $14M\times 4M$ in the down- and cross-stream directions respectively starting immediately at the grid. This unprecedented attempt was achieved by using $2D~PIV$ over the stitched fields of view of three cameras put side-by-side. The statistics - based on 10600 vector fields per case - in the wake of two carefully chosen grids were studied, showing that homogenisation is a rapid but gradual process. Collapse of statistics across the different grid geometries was achieved passed the region of initial flow transients by taking into account all relevant parameters: differences in grid geometry \textit{and} work done on the flow. This hints at a unique dependency of grid turbulence dynamics in the region of cross-stream homogenised flow. Two-point spatial correlation functions were used to study the footprint of the grid on the flow statistics, exhibiting complex $Re_{M}-$dependent wake interactions between the grid rods. Their impact, however, is quickly washed away by the turbulence. University of Cambridge 2012-05-19 Thesis Doctoral Doctor of Philosophy (PhD) en EPSRC Doctoral Training Account https://www.repository.cam.ac.uk/handle/1810/300649 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/4a1c4ae5-9740-439d-9c04-d311b12f5ebd/download a3e9b9c5fe7788b34f94028ce883e7cd https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/e7e117b5-fa35-47cf-8a9e-b868f138f947/download 87eda9de84448d1f82354d60eee3eb5f https://www.rioxx.net/licenses/all-rights-reserved/ fluid mechanics turbulence particle image velocimetry velocity gradient tensor invariants isotropic turbulence axisymmetric turbulence homogeneous turbulence