Author(s): Hao Wu; George Constantinescu
Keywords: Freshwater mussels; Turbulence; Numerical simulations; Drag forces
Abstract: Freshwater mussels play several important roles for stream ecology (e.g., for nutrient transport and nutrient availability, transport of contaminated sediments). However, the flow structure at the mussel scale and, in particular, the role played by large-scale turbulence generated by the emerged part of the mussel are still to be elucidated. Understanding the dynamics of the coherent structures and their effect on mean flow, turbulence and sediment entrainment provides additional information on how partially-buried mussels affect the structure of the bottom boundary layer. Information on how the angle of attack and the strength of the active filtering (e.g., mean volumetric discharge through the excurrent and incurrent siphons) affect mean drag forces on the emerged part of the mussel and bed shear stress distribution around it allows understanding how mussels can better resist dislocation at high flow conditions. Such knowledge should contribute to future efforts to increase mussel habitat in natural streams. In the present investigation, three-dimensional large eddy simulations are performed in channels containing a partially-buried freshwater mussel (Lampsilis siliquoidea) with and without active filter feeding for two angles of attack (α=0⁰ when the primary axis of the mussel is aligned with the incoming flow and α=30⁰). Simulations are performed in straight channels with incoming mean flow velocity of 0.3 m/s and an incurrent/excurrent volumetric discharge between 0 and 9*10-6 cm3/s. The height of the semi-buried mussels above the channel bed is 0.03 m and the flow depth in the channel is 0.15 m. Besides drag forces induced by the flow on the mussel shell, local erosion developing slowly in time is the second main factor that can result in the dislocation of mussels from the bed substrate. Numerical simulations are used to understand the effects of the angle of attack and the volumetric discharge of the filtered flow on: 1) the wake structure, large-scale vortices and generation of regions of flow downwelling behind the mussel; 2) the erosion potential of the flow around the mussels; 3) the mean drag force. Results show that for both angles of attack, the total streamwise drag force acting on the mussel’s shell increases as the filtering discharge becomes higher. So, mussels need to shut off active filtering to better withstand increase drag forces at high flow conditions. As expected, the mean drag force increases with the magnitude of the angle of attack. Results also show that streamwise-oriented vortices form behind the mussel for both angles of attack. The larger, more coherent vortex induces strong downwelling on one of its sides and pushes higher streamwise velocity fluid near the bed. As a result, a large part of the near-wake region is characterized by bed shear stresses that are larger than those in the incoming flow.