Turbulent structures in the wake of circular cylinders.

摘要

This thesis presents experimental studies of turbulent structures in the wake of circular cylinders in the cross flow. The objective of the studies is to gain, through experimental investigation, the knowledge to improve our understanding of turbulent structures in the wake of single cylinder and two tandem cylinders. The phase-averaged technique is the major analysis method to extract the coherent and incoherent turbulent structures in the cylinder wake and investigate the three-dimensional vorticity due to the Reynolds number effect. Three topics are studied in this thesis.; Firstly, the flow structures, momentum and heat transport in the wake of two tandem circular cylinders have been experimentally investigated. Measurements were conducted at x/d = 10, 20 and 30 at a Reynolds number of 7000 using a three-wire (one X-wire plus a cold wire) probe, in conjunction with an X-wire. The cross-stream distributions of the Reynolds stresses and heat fluxes vary at the same x/ d between the regimes. So do the coherent contributions to the Reynolds stresses and heat fluxes.; Following the investigation of the flow topology of the two tandem circular cylinders, three-dimensional vorticity in a turbulent cylinder wake is examined. A circular cylinder with a diameter of 12.7mm was used to generate the wake flow. Measurements were conducted at x/d = 10, 20 and 40 at a Reynolds number of 2500. The three components of the vorticity vector in the intermediate region of a turbulent cylinder wake were measured simultaneously using a multi hot-wire probe. This probe has an improved spatial resolution compared with those reported in the literature. The behaviour of the instantaneous velocity and vorticity signals is examined. Both coherent and incoherent vorticity fields are investigated using a phase-averaged technique.; Finally, the Reynolds number effects on three-dimensional vorticity in a turbulent wake are studied. When Reynolds number, Re, varies from 103 to 104, there is a large change in the turbulent near-wake dynamics (e.g. the base pressure coefficient, fluctuating lift coefficient and vortex formation length) of a circular cylinder, which has previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. (Abstract shortened by UMI.)