Fluid Mechanics of Inhalant Siphon Flows

Inhalant siphon and suction flows are ubiquitous in marine ecosystems. From biological flows in filter-feeding benthic bivalves and predation by planktivorous fishes, to engineered flows in water samplers and production of hydrodynamic stimuli for laboratory assays, inhalant siphon flows span much of the laminar range (Reynolds number ~ 0.01 – 2,000) and fundamentally influence many transport and exchange processes. Direct numerical simulations (DNS) of inhalant siphon flows with varying Reynolds numbers and geometries have informed design and construction of an index of refraction-matched flow facility (mineral oil, borosilicate glass tubing) in which we are employing particle image velocimetry (PIV) to quantify transient and steady-state flow fields outside and inside the siphon tube. Varying siphon diameter, flow rate, and extraction height allows us to evaluate effects of Reynolds number and siphon geometry on local hydrodynamics. This complementary experimental and numerical modeling investigation of siphon flow hydrodynamics was motivated recently by a colleague whose biologically inspired numerical modeling of inhalant siphons using a boundary condition of constant volumetric outflow (as opposed to the classically assumed uniform inlet velocity profile) revealed nontrivial departures from idealized flows: inviscid potential flows (i.e. point sink) and pipe flows (the classical pipe entry problem), particularly in the low Reynolds number regime. Reduced entrance lengths, larger radial inflows, and modifications to fluid capture zones seen numerically at low Reynolds number are being tested experimentally and may have important implications for both biological and engineered siphons.