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Title: Turbulence generated by multiscale inhomogeneous and anisotropic grids
Author: Zheng, Shaokai
ISNI:       0000 0004 7658 1307
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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In this thesis two classes of inhomogeneous multiscale grids are proposed and investigated experimentally. These studies provide further understandings of the idea to generate bespoke turbulence field using fractal grids as proposed by Vassilicos and colleagues. The first part of the work introduces the rectangular fractal grid (RFG). Due to the inhomogeneous grid geometry, the streamwise location of turbulence intensity peak appears different from the wake interaction length scale calculated from the grid bar dimensions, and a region of decreasing length scales with decreasing Reynolds number is observed. This region is shown to be inhomogeneous and anisotropic. Nevertheless, the non-equilibrium scaling is found in this region, where the ratio between the integral length scale and Taylor microscale remains constant. From the second part of the work, turbulent flows with different mean shear rates are studied using the new inhomogeneous multiscale grids. By designing the local blockage ratio and the bar dimensions, this new type of shear generating grid is capable of producing different mean velocity and turbulence intensity profiles at the same time. The mean velocity profiles are shown to match the predictive mean velocity model, and a scaling relation of turbulence intensity is proposed based on the wake interaction length scale. The streamwise evolution of the Reynolds stress is studied, and a new dimensionless time scale is proposed. Finally, the design and scaling methods of the turbulent shear flow generated by the inhomogeneous multiscale grid are tested in a low-fidelity engineering wind tunnel with different size and background flow quality, and the results are consistent. These results perhaps provide a general methodology to produce various types of turbulent flows through the design of one single passive grid, which is desirable for both fundamental studies of turbulence and engineering applications such as the wind engineering experiments.
Supervisor: Vassilicos, Christos ; Graham, Mike ; Bruce, Paul Sponsor: European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral