Author(s): Yannick Marschall; George Constantinescu; Robert Boes; David F. Vetsch

Linked Author(s): David Vetsch, George Constantinescu, Yannick Marschall, Robert Boes

Keywords: Drag Coefficient; URANS; DES; Coherent Flow Structures; Numerical Modelling

Abstract: We conducted unsteady Reynolds-Averaged Navier Stokes (URANS) and detached eddy simulations (DES) to simulate the flow around a boulder-like obstacle placed at the bottom of an open channel at laboratory scale (obstacle width B=7 cm) and at a larger scale (B=21 cm), while keeping the channel Froude number constant. The obstacle shape was generated from a three- dimensional scan of a stone. The study discusses the effects of the turbulence modelling approach (URANS vs. DES) and scale effects. An increase of the drag coefficient of around 10-15% was observed for the simulations conducted at larger scale. Part of this difference can be attributed to differences in the fully developed velocity profiles in the approach flow which are a function of the Reynolds number. Maintaining the boulder size, the drag coefficient predicted by URANS is about 15% smaller than the DES value. URANS overestimates the wake recirculation region leading to a slightly higher pressure acting on the downstream side of the boulder. The results of this investigation show that URANS methods, which are widely used for practical applications in hydraulic engineering, can predict reasonably well the mean flow field and the quantities of engineering interest. Computationally more expensive methods like DES, that resolve a large part of the energetically important eddies in the flow, provide more accurate predictions of the mean quantities and allow investigating the flow physics based on the simulated dynamics of the large-scale coherent structures.

DOI: https://doi.org/10.3929/ethz-b-000675921

Year: 2024