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2D PIV Study of Grid Turbulence


Type

Thesis

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Authors

Cardesa Dueñas, José Ignacio  ORCID logo  https://orcid.org/0000-0002-3863-5285

Abstract

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 ReM=UM/ν=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 ηK. 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 2×2 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×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 ReMdependent wake interactions between the grid rods. Their impact, however, is quickly washed away by the turbulence.

Description

Date

2012-02-29

Advisors

Dawson, James R.
Nickels, Timothy Bruce

Keywords

fluid mechanics, turbulence, particle image velocimetry, velocity gradient tensor, invariants, isotropic turbulence, axisymmetric turbulence, homogeneous turbulence

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
EPSRC Doctoral Training Account