Boundary‐trapped, inhalant siphon and drain flows: Pipe entry revisited numerically

Lay Abstract Many seafloor exchange processes are controlled or strongly influenced by pumping of water into projecting tubes or burrow openings flush with the seabed. Quantitative descriptions of flow inside tubes and burrow openings rely primarily on a model that assumes uniform flow at the pipe entrance and lacks any description of the upstream geometry or flow field. Quantifications of flows in front of the entrance generally assume omnidirectional, convergent flow into the pipe opening. Numerical models of a tube drawing water from a large volume and of a drain flush with the bottom drawing from the center of a large tank explicitly connect flow inside the pipe to the flow outside. They indicate—for slow, viscous, and small‐diameter (of the pipe) flows—that considerable flow adjustment occurs upstream of the pipe entrance and that uniform velocity at the entrance is a poor assumption. For these cases, flow entering the pipe comes from a relatively small, cylindrical region parallel with the pipe rather than being drawn from all possible directions. Uniform entrance flow never becomes a good approximation because much of the flow entering faster, less viscous, and larger‐diameter pipes enters with appreciable radial velocity that affects flow well into the pipe. These results alter estimates of pumping costs, which can represent appreciable fractions of animal energy budgets. They also determine the regions from which food and chemosensory information can be obtained. The results demonstrate the necessity for most applications to replace textbook models based on uniform entry flow.