Large eddy simulation of the flow in tight-lattice rod bundles at low Reynolds number

Tightly packed rod-bundles are looked upon as possible configurations for the core of a nuclear reactor. Unfortunately the flow in such rod bundles is accompanied by oscillations and strong turbulent mixing. In particular it exhibits large scale coherent structures in the gap regions between the rods. In the present work we propose an LES, with Dynamic Smagorinsky modeling, in boundary fitted coordinates aimed at reproducing such flow features. The algorithm is based on the fractional-step framework. Time advancement is carried out through a second order scheme, while the convective term is discretized through a second (or fourth) order scheme. The geometries under consideration are an infinite triangular lattice of cylindrical pins and an infinite triangular lattice of an innovative pin design we will refer to as "exotic" pins. The Volume Fraction in both cases is equal to 0.67.The Reynolds number under investigation (6,400) is not comparable to current reactor designs but it is relevant to the investigation of innovative designs and incidental conditions.