Abstract
dc:description.abstractThe 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 ReM=U\inftyM/\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 \etaK. 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 ReM-dependent wake interactions between the grid rods. Their impact, however, is quickly washed away by the turbulence.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2012
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Cardesa Dueñas, José Ignacio
- Advisors dc:contributor.advisor
-
- Dawson, James R.
- Nickels, Timothy Bruce
Subjects
dc:subject × 8Rights
dc:rightsIdentifiers
dc:identifier.*- Author Identifier
-
0000-0002-3863-5285
0000-0002-3069-6948 - OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/300649