Characterizing the Impact of Geometric Simplification on Large Woody Debris Using CFD

In general, the presence of large woody debris within river systems creates a desirable effect on fish populations. Experimentally, these positive effects are well documented. Unlike experimental observations; however, numerical simulations of these obstructions are relatively sparse. Those that do exist are in general conducted using idealized simplifications of the actual geometries that they are intended to represent. The purpose of this article is to evaluate the effect of this geometric simplification on the flow field dynamics using numerical simulation. For a given flow field obstruction consisting of a fallen root wad, a series of progressively detailed geometries were created. For each geometry, a shape complexity factor was computed and used for quantifiable comparison purposes. As expected, the localized effect of increased Reynolds number resulted in an increase in turbulent kinetic energy. However, an increase in complexity factor resulted in local and downstream decreases in turbulent kinetic energy. For the cases examined, the results indicated that oversimplifying the complexity of an object can result in significant overpredictions of both velocity magnitude and turbulent kinetic energy.