INTERACTIONS OF TWO AND THREE TURBULENT SIMPLE WAKES

Turbulent complex wakes generated by two and three cylinders in a side-by-side arrangement were investigated experimentally. 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 examine the interactions between turbulent simple wakes and their effects on the momentum and heat transport phenomena. The superposition hypothesis, as proposed by Bradshaw and his co-workers, is examined for its validity and extent. Using the experimental data of a single-cylinder wake as base, the hypothesis is used to assess the turbulence field, up to the third order velocity products, of complex wakes. It is found that the complex interactions 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; its slope has 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.