Data from: A novel mechanism of mixing by pulsing corals

The dynamic pulsation of the xeniid corals is one of the most fascinating
phenomena observed in coral reefs. We quantify for the first time the flow
near the tentacles of these soft corals whose active pulsations are
thought to enhance their symbionts' photosynthetic rates by up to an
order of magnitude. These polyps are about 1 cm in diameter and pulse at
frequencies between about 0.5 and 1 Hz. As a result, the frequency based
Reynolds number calculated using the tentacle length and pulse frequency
is on the order of 10 and rapidly decays as one moves away from the polyp.
This introduces the question of how these corals minimize the
reversibility of the flow and bring in new volumes of fluid during each
pulse. We estimate that the Peclet number of the bulk flow generated by
the coral as being on the order of 100-1000 while the flow between the
bristles of the tentacles as being on the order of 10. This illustrates
the importance of advective transport in removing oxygen waste. Flow
measurements using particle image velocimetry reveal that the individual
polyps generate a jet of water with positive vertical velocities that do
not go below 1 mm/s and with average volumetric flow rates of about 700
cubic mm per second. Our results show that there is nearly continual flow
in the radial direction towards the polyp with only about 3.3 percent back
flow. 3D numerical simulations uncover a region of slow mixing between the
tentacles during expansion. We estimate that the average flow that moves
through the bristles of the tentacles are about 0.3 mm/s. The combination
of nearly continual flow towards the polyp, slow mixing between the
bristles, and the subsequent ejection of this fluid volume into an upward
jet ensures the polyp continually samples new water with sufficient time
for exchange to occur.