High-resolution numerical analysis of turbulent secondary motion in open duct flow

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Fluid flow in a straight duct with rectangular cross-section exhibits turbulence-induced secondary motion of small amplitude, but with large consequences for momentum, heat and mass transport. The corresponding open duct flow (featuring a free surface) is characterized by a distinct secondary flow pattern, leading to such practically important effects as the so-called „dip phenomenon'' (referring to the fact that the maximum of average stream wise velocity is not found at the fluid surface). In the present project we are proposing to investigate the mechanism of secondary flow formation in open channel flow, where rigid/rigid and mixed (rigid/free-surface) corners exist. First, we will generate high-fidelity data by means of pseudo-spectral direct numerical simulation. Second, we will analyze the flow fields with particular emphasis on the dynamics of coherent structures and the consequences for Reynolds-number and aspect-ratio scaling.Third, we will consider the effect of secondary flow in open ducts upon the process of sediment transport by means of Lagrangian particle tracking (in the framework of a point-particle approach). The present project will fill existing knowledge gaps concerning the precise mechanism of secondary flow formation in the presence of mixed corners. Due to the coherent structure approach, it can be expected that our results will pave the way for the design of strategies for flow control. One example of an application where these flow processes are of direct relevance is the appearance of longitudinal ridges forming from mobile sediment particles in man-made canals and rivers.