Direct numerical simulation of open-channel flow over a rough wall covered with benthic algae

Filamentous benthic algae grow ubiquitously on the sediment-water interface (SWI) in streams, rivers, and lakes. They support fluvial food webs, engage in nutrient cycling, attenuate flow structures, and affect bed stability, thereby modifying the aquatic habitat. In this contribution, we are going to present some preliminary results of the direct numerical simulation (DNS) of turbulent open-channel flow over a layer of spherical sediment particles covered with benthic algae canopy in the transitionally rough regime. The flow motion is governed by Navier-Stokes equations and solved with a standard fractional-step method. The filamentous benthic algae are modeled as elastic rods and solved by an efficient and physically accurate finite difference scheme. Both the rods and particles are fully resolved and coupled with the flow using the direct-forcing immersed boundary technique. The flow structures and turbulence statistics in both the roughness sublayer and logarithmic region will be thoroughly analyzed and compared to previous studies on rough wall open-channel flow in order to investigate how benthic algae affect the flow field. The results presented here may provide physical insights and implications into sediment incipient motion and mass/momentum transfer across the sediment-water interface in natural rivers where benthic algae play dominant role and will serve as a first step for better understanding and modeling of the dynamics in hyporheic/benthic zone and entire river ecosystems.