| The unsteady Reynolds-averaged Navier-Stokes equations, supplemented by a Reynolds stress model, were solved with relatively high spatial and temporal resolutions to determine the time-averaged mean velocity and turbulent stresses in isothermal, single phase flows in a rectangular duct containing a single rod and in a 37-rod rod bundle. In addition, the time-averaged mean temperature, temperature fluctuation variance and turbulent heat fluxes were computed in the single-rod duct flow in which the rod was heated passively. The computed values were, in general, in fair agreement with available experimental results. The simulations also resolved the formation of coherent structures in the form of counter-rotating vortices with axes alternating on either side of rod-rod and rod-wall gaps. The contributions of these coherent structures on the turbulent and thermal fields and on cross-gap mixing were documented and found to be substantial. The action of coherent structures moderates the mean temperature rise in the gap region but creates substantial local variation of the instantaneous temperature fluctuations at relatively low frequencies. Reduction of the gap size below a critical value resulted in a decrease of the frequency of coherent fluctuations and the cross-channel mixing efficiency, contrary to predictions based on the extrapolation of available empirical correlations. In the case of the 37-rod bundle, the coherent velocity fluctuations were found to be strongly correlated in the entire computational domain. |
