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The mechanism of secondary flow was discussed based on the result of an LES for the turbulence flow parallel to bundled rods. The LES was carried out in the Cartesian grid system using an improved immersed boundary method developed for accurate simulation of complex wall boundary of industrial interest. The method was verified by comparing the LES with a DNS of turbulence flow in a pipe. The LES for bundled rods reasonably reproduced the secondary flow of the second kind. We proposed a theoretical method to analyze the mechanism of the secondary flow. The result of the analysis indicated that the main source of secondary flow was the non-conservative component of Reynolds stress term as was the case for a square duct. The theory proposed that non-conservative force corresponds to the difference between the pressure gradient along the wall and the Reynolds stress term decreasing rapidly near the wall. The present theory will unify the understandings for the secondary flow of the first and second kinds. The present LES technique and theory is useful for investigating the physics in turbulence flow in complex walls.

LES of Turbulence Flow in Bundled Rods and Analysis for the Driving Force of Secondary Flow