Flow and Entrainment Mechanisms Around a Freshwater Mussel Aligned With the Incoming Flow

Abstract

The paper uses information from eddy-resolving simulations to characterize flow and turbulence around freshwater mussels at the organism scale. The focus is on the simplest case of a partially burrowed, isolated mussel aligned with the incoming flow in an open channel. The wake structure and the capacity of the flow to displace the mussel from the bed substrate and to induce local scour around the mussel are investigated as a function of the ratio between the height of the exposed part of the mussel, h, and the total mussel height, d, and as a function of the filtration velocity ratio (VR) between the incoming channel flow velocity, U 0 , and the mean velocity inside the excurrent siphon, U e. As opposed to the flow past most surface-mounted obstacles where the bed shear stresses are reduced in the wake of the body, the capacity of the flow to induce bed erosion behind isolated mussels aligned with the flow is relatively large because of strong downwelling motions inside the horizontal separated shear layers. These flow features are associated with the formation of counterrotating base vortices of unequal coherence that induce upwash behind the mussel. The total circulation of the base vortices increases with increasing h/d and VR. Though both symmetrical and antisymmetrical shedding are observed in the wake, the antisymmetrical mode dominates, and its strength increases with increasing VR. The nondimensional streamwise force acting on the emerged part of the mussel's shell increases with increasing h/d and VR. Finally, the paper discusses the effects of varying h/d and VR on the dynamics and dilution of the jet of filtered water originating in the excurrent siphon, which is important to understand how mussels affect mixing and water quality (e.g., nutrient availability and phytoplankton concentration) in natural streams.

Influence of Bed Roughness on Flow and Turbulence Structure Around a Partially‐Buried, Isolated Freshwater Mussel

“…The rates of decrease of L R / h with h / H for the rough bed cases with d 50 / H = 0.062 in Figure 5b are fairly independent of the value of VR . A similar trend in the variation of L R / h with h / H was predicted by H. Wu, Constantinescu, and Zeng (2020) for an isolated mussel ( L. siliquidea ) placed on a smooth bed.…”

“…The TKE levels are relatively low inside the near‐wake region in the smooth bed case with no active filtering (Figure 7a). The peak values are close to one order of magnitude lower than those observed by H. Wu, Constantinescu, and Zeng (2020) in their corresponding simulations. The main reason is that for the present mussel shape and mussel orientation with respect to the bed, the anti‐symmetric vortex shedding, which is the main mechanism for the amplification of the TKE behind the mussel, is basically suppressed in the absence of siphonal activity (see discussion in Section 5).…”

“…This is the true regardless of the value of VR (e.g., see results for smooth‐bed simulations in Figure 8). For similar cases, H. Wu, Constantinescu, and Zeng (2020) observed the formation of horseshoe vortices in the mean and instantaneous flow fields. The difference can be attributed to the lower degree of bluntness of the mussel used in the present study.…”

“…Wu, Constantinescu, and Zeng (2020) for similar values of h/H and VR. Increasing the bed roughness induces a strong amplification of the antisymmetric vortex shedding mode in the wake (Figure7d-7i), which explains the larger values of the TKE in the rough bed simulations compared to the corresponding smooth bed simulations.…”

“…On the other surfaces, the gradient of C in the direction normal to the surface was set to zero. More details on the boundary conditions are given in H. Wu, Constantinescu, and Zeng (2020) who used a similar approach to conduct simulations past a mussel placed on a smooth bed. The precursor simulation also provided the inlet distributions of the turbulent kinetic energy, k , and the turbulence vorticity, ω , for which transport equations are solved in the present SST‐based DES model.…”