A large eddy simulation (LES) of the turbulent flows through the ducts that changes from the square cross-section to the circular one and back to the square one using the immersed boundary (IB) method is carried out in the present work. The distribution of the streamwise mean velocity shows that the characteristic behavior occurs in the region near the corners where the duct cross-section changes. The secondary-flow velocity in the contraction section is larger than that in the expansion section. It is shown that the turbulence is activated and many vortex structures are generated in the expansion section, but there are also low-turbulence regions near the corners.
Masayoshi OKAMOTO (Wed,) studied this question.