Coherent Structures in the Gaps of Rod Bundles

Abstract

The work presented here is part of a research program at the University of Ottawa aimed at studying the detailed characteristics of turbulent flow and heat transfer near narrow gaps of rod bundles. An experimental investigation was conducted in a simplified system, comprising isothermal air flow in a rectangular channel with a height to width ratio of 2/3 and containing a cylindrical rod with its axis parallel to the channel axis and a diameter equal to half the channel height. The rod was suspended to form an adjustable narrow gap with the channel base. The flow Reynolds number, based on bulk velocity and hydraulic diameter, was 140,000. Previously reported measurements include the distributions of the wall shear stress, the mean velocity and the turbulent stresses, as well as the dependence of the spacing and the convective speed of the observed large-scale, quasi-periodic flow pulsations across the gap upon the gap size. These results are now complemented by conditionally sampled measurements of the phase-averaged characteristics of the coherent vortical structures that produce these flow pulsations and a tentative physical model of their typical shapes.