Complex turbulent wakes generated by two and three side-by-side cylinders

Abstract Turbulent complex wakes generated by two and three cylinders in a side-by-side arrangement were investigated experimentally. In the present context, the complex wake refers to the flow formed by two or more simple wakes behind side-by-side cylinders. One cylinder was slightly heated; the temperature difference is about 1°C so that the temperature could be treated as a passive scalar. A combination of an X-wire and a cold wire was used to measure the velocity and temperature fluctuations. The present objective is to document the turbulence field of the complex wakes and examine the interactions between turbulent simple wakes and their effects on the momentum and heat transport phenomena. It is observed that the cross-stream distributions of the Reynolds normal stresses can be asymmetrical at a small spacing-to-diameter ratio. The Reynolds shear stress and its lateral transport distributions however remain symmetrical. This is explained in terms of the gap flow deflection behind side-by-side cylinders and the transport characteristics of vortical structures. 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.