Turbulent flow past a circular cylinder- coherent structures and dissipation in the wake

The flow past a circular cylinder has been extensively investigated by researchers due to its widespread application to environmental and engineering flows. In most practical applications, the upstream flow is turbulent, yet this has received less attention in the literature. The influence of turbulence on the wake of a circular cylinder is investigated in this thesis through a combination of experiments and numerical simulations. Focus is placed on the generation of inflow turbulence, and the relationship between coherent structures and dissipation in the wake.

Free-stream turbulence is generated in simulations by projecting grid patterns onto the inlet patch. A new design is developed here which combines the best characteristics of both regular and fractal grid geometries. It has been shown that wakes created by each bar interact in an unpredictable manner, which can lead to vorticity clustering and poor turbulence homogeneity.

A circular cylinder is then placed in the wake of regular and fractal grids. A laminar inflow is also simulated for comparison. A surrogate of dissipation has been developed, which is tested on the circular cylinder wake, and only requires two components of velocity on a two-dimensional slice. In addition to this, a model is proposed which maps the distribution of dissipation and coherent structures. It has been suggested in the literature that dissipation is concentrated in the primary vortex rollers. However, here it is found to reside in between streamwise ribs.

Experiments on the circular cylinder wake are conducted using particle image velocity (PIV) to supplement numerical modelling. A turbulent inflow is generated from a biplane grid upstream of the cylinder. Structures in the wake are decomposed into coherent and stochastic components by a phase averaging procedure. Dissipation is evaluated in the two-dimensional PIV data by using the surrogate method developed in the numerical modelling work. It is concluded that stochastic turbulent fluctuations are more dissipative than coherent motions by an order of magnitude, but the two processes are locked for a short region in the circular cylinder wake. This has also been observed in the numerical simulations, and may indicate a non-classical form of turbulence decay.