Effect of angle of attack on flow past a partially-burrowed, isolated freshwater mussel

Abstract

Though freshwater mussels in rivers tend to be aligned with the flow, the angle of attack can increase significantly during periods when the stage and discharge vary rapidly (e.g., floods). As the near-bed flow changes, mussels can experience larger drag forces before they reorient themselves parallel to the flow and/or increase their level of burial into the substrate to avoid dislocation. The present paper used a previously validated eddy-resolving numerical model to study the changes in the structure of the near-wake flow, horseshoe vortex system, drag forces acting on the exposed part of the mussel shell, sediment entrainment mechanisms and dilution of the jet of filtered water originating in the excurrent siphon with the increase in the magnitude of the angle of attack, |θ|, for an isolated mussel placed in an open channel with incoming fully-developed, turbulent flow. Given that the mussel may modify its mean volumetric discharge through the two siphons and level of mussel burial as θ changes, simulations are performed with and without active filtering and with two heights of the exposed part of the mussel, h. The raise in |θ| increases the total drag force acting on the exposed part of the shell, the entrainment into the excurrent-siphon jet containing filtered water, the circulation of the horseshoe vortices, the strength of the anti-symmetric wake shedding mode and the circulation of the main streamwise-oriented (base) vortex forming behind the mussel. Results show that, for all angles of attack, the downflow induced by the primary base vortex generates a large region where the bed shear stresses are strongly amplified. So, particulate deposition is expected to be low and significant local scour may occur in the near wake of a partially burrowed mussel.