The gill cover of fish and pre-metamorphic salamanders has a key role in suction feeding by acting as a one-way valve. It initially closes to avoid an inflow of water through the gill slits, after which it opens to allow outflow of the water that was sucked through the mouth into the expanded buccopharyngeal cavity. However, due to the inability of analytical models (relying on the continuity principle) to calculate a fluid flow through a shape-and-size-changing cavity with two openings, stringent boundary conditions had to be used in previously developed mathematical models after the moment of valve opening. By solving additionally for momentum conservation, computational fluid dynamics (CFD) has the capacity to dynamically simulate these flows, but this technique also faces complications to model a transition from closed to open valves. Here, I present a relatively simple solution strategy to incorporate valve opening, exemplified in an axisymmetrical model of a suction-feeding sunfish in ANSYS Fluent software. By controlling viscosity of a separately defined fluid entity at the opercular cavity region, early inflow can be blocked (high viscosity assigned) and later outflow can be allowed (changing viscosity to that of water). Finally, by analysing the CFD solution obtained for the sunfish model, a few new insights in the biomechanics of suction feeding will be discussed.
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