Turbulence structure of velocity and scalar fields over a bed of model bivalves

The Asian Clam Potamocorbula amurensis is a benthic grazer which feeds by actively pumping ambient fluid into its body, filtering out suspended phytoplankton, and then reinjecting the phytoplankton-depleted fluid into the overlying flow. In shallow estuaries which receive large amounts of anthropogenic nutrients, increased phytoplankton biomass (blooms) can have negative environmental consequences, including depletion of dissolved oxygen. The abundance of the filter-feeding clams in San Francisco Bay is thought to exert a strong control on phytoplankton population dynamics, thus preventing nuisance blooms. Benthic grazing only depletes near-bed phytoplankton, so the effectiveness of the filter-feeding process on the water column depends on the mechanics of the scalar mixing processes which supply phytoplankton to the bed. This study used laboratory models of clams to investigate their influence on the local momentum and scalar fields.; A laboratory investigation was performed using an aqueous turbulent boundary layer developing over a bed of 4000 full-scale models of the clam P. amurensis. The model clams incorporated passive roughness, active siphonate pumping, and the ability to filter scalars from the flow. Measurements of vertical and streamwise velocities as well as scalar (dye) concentration were made using a combined laser-Doppler anemometer (LDA) and laser-induced fluorescence (LIF) probe. The resulting measurements enabled direct calculation of momentum and scalar fluxes. Measurements were made for a variety of crossflow velocities, clam pumping rates, and roughness heights. Results show that the effect of active siphonate pumping can be modeled as an effective roughness which depends on the ratio of siphon velocity to crossflow velocity. A conceptual model for the turbulent Prandtl number (Pr{dollar}rmsb{lcub}t{rcub}){dollar} is also developed. The model explains the large measured near-bed Pr{dollar}rmsb{lcub}t{rcub}{dollar} values by relating Pr{dollar}rmsb{lcub}t{rcub}{dollar} to the virtual origins of calculated momentum and scalar mixing length distributions.; A separate study is performed over beds of real P. amurensis clam shells to investigate the role of local hydrodynamics on the recruitment process. Recruitment is the process by which clam larvae settle into favorable sites amongst a group of adult clams. Large episodic stress events can disrupt the process by resuspending the settled larvae. Results show that the process is governed by instantaneous rather than mean stresses, and that the probability of success decreases as the spacing between clams decreases.