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In this paper we found, by means of numerical simulations, a transition inthe oscillatory character of the flow field for a particular combination ofbuoyancy and spacing in an array of six circular cylinders at a Reynolds numberof 100 and Prandtl number of 0.7. The cylinders are iso-thermal and they arealigned with the Earth acceleration (g). According to the array orientation, anaiding or an opposing buoyancy is considered. The effect of natural convectionwith respect to the forced convection is modulated with the Richardson number,Ri, ranging between -1 and 1. Two values of center to center spacing (s=3.6d -4d) are considered. The effects of buoyancy and spacing on the flow pattern inthe near and far field are described. Several transitions in the flow patternsare found and a parametric analysis of the dependence of the force coefficientsand Nusselt number with respect to the Richardson number is reported. ForRi=-1, the change of spacing ratio from 3.6 to 4 induces a transition in thestandard deviation of the force coefficients and heat flux. In fact thetransition occurs due to rearrangement of the near field flow in a more orderedwake pattern. Therefore, attention is focused on the influence of geometricaland buoyancy parameters on the heat and momentum exchange and theirfluctuations. The available heat exchange models for cylinders array provide anot accurate prediction of the Nusselt number in the cases here studied.

Buoyancy effect on the flow pattern and the thermal performance of an array of circular cylinders