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

Turbulent complex wakes generated by two and three cylinders in a side-by-s ide arrangement were investigated experimentally. In the present context, t he complex wake refers to the flow formed by two or more simple wakes behin d side-by-side cylinders. One cylinder was slightly heated; the temperature difference is about 1 degrees C so that the temperature could be treated a s 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 inte ractions between turbulent simple wakes and their effects on the momentum a nd heat transport phenomena. It is observed that the cross-stream distribut ions of the Reynolds normal stresses can be asymmetrical at a small spacing -to-diameter ratio. The Reynolds shear stress and its lateral transport dis tributions however remain symmetrical. This is explained in terms of the ga p flow deflection behind side-by-side cylinders and the transport character istics 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 s cales in the inertial sub-range. On the other hand, the temperature spectra in the inertial sub-range have been affected; their slopes have been appre ciably increased compared with the single-cylinder data. The gradient trans port assumption is found to be valid for the turbulence field, but not for the temperature field. The heat flux and temperature gradient do not approa ch zero simultaneously near the centerlines of simple wakes, thus giving ri se to a substantial variation in the heat transport. This leads to a signif icant drop in the turbulent Prandtl number. The superposition hypothesis, a s proposed by Bradshaw and his co-workers, is also examined for the present complex wakes. (C) 2000 Elsevier Science Inc. All rights reserved.