Organized structures and their interactions in the turbulent wake of circular cylinders

Abstract

This thesis presents an experimental study of organized structures in the turbulent near wakes of single, two and three side-by-side circular cylinders, covering four topics. Firstly, the relationships between logitudinal (ribs) and spanwise vortical structures in the turbulent near wake of a circular cylinder were investigated. Three components of vorticity in the wake of a circular cylinder were measured simultaneously with a combination of four X-wire probes. The most probable rib orientation was estimated based on the probability density function distribution of the inclination of the vorticity vector. With the WAG (window average gradient) detection method, conditional averaging and structural averaging have been made. A schematic representation of the spatial relationship between the spanwise structures and rib structures was proposed, and contributions from these two types of vortical structures to the vorticity variance were also been determined based on spanwise structure detections. Secondly, interactions between spanwise vortical structures, momentum and heat transport in the turbulent wake of two side-by-side circular cylinders were studied. Spacing T between the cylinder axes was varied from 1.5d to 3d (d is the diameter of the cylinders). Both cylinders were slightly heated. While a three-wire probe (an X-wire plus a cold-wire) was used to measure the velocity and temperature fluctuations, another X-wire was used to provide a phase reference. Following the detection of vortical structures based on the lateral velocity fluctuation, the contribution from the structures is quantified and discussed in the context of the topology of the velocity and temperature fields. The results point to a significant effect of T/d on the topology and transport characteristics. The streamwise evolution of the vortex street is also discussed in detail. Thirdly, interaction between turbulent simple wakes is determined. Using the same three-wire probe mentioned above, velocity and temperature fluctuations were measured in the turbulent wakes generated by single, two and three side-by-side circular cylinders (one of them was heated slightly). It is observed that the interactions between simple wakes do not seem to have any effect on the fine-scale turbulence, at least up to the scales in the inertial sub-range. On the other hand, the temperature spectra in the inertial sub-range have been affected; their slopes have been appreciably increased compared with the single-cylinder data. The gradient transport assumption is found to be valid for the turbulence field, but not for the temperature field. The heat flux and temperature gradient do not approach zero simultaneously near the centerlines of simple wakes, thus giving rise to a substantial variation in the heat transport. This leads to a significant drop in the turbulent Prandtl number. The superposition hypothesis, as proposed by Bradshaw and his co-workers, is also examined for the present complex wakes. Finally, the effects of unequal cylinder spacing on the vortex streets behind the three side-by-side cylinders were studied. Various techniques were used in this experimental study, including hot-wire technique, laser Doppler anemometry (LDA) and laser-illuminated flow visualization. When the cylinder centre-to-centre spacing is identical, i.e. T1/d = T2/d = 1.5, the flow is symmetric about the centreline, with a wide wake behind the central cylinder and two narrow wakes on each side of the wide wake. The dominant frequency in the narrow wakes is about 5.4 times that in the wide wake. The observation is consistent with previous reports, thus lending credence to the present measurement. As T2/d is slightly increased to 1.6, a remarkable change occurs in the flow. A comparison is made between the cases of the equally and unequally spaced cylinders in terms of the pressure around the cylinders, drag, lift, dominant frequencies, vortex formation and downstream evolution. The flow topology (patterns) is also discussed in detail. Eleven publications, including 5 refereed journal papers and 6 refereed conference proceedings, have been produced out of this work.