Physical modeling of vortical cross-step flow in the American paddlefish, Polyodon spathula

Abstract

Vortical cross-step filtration in suspension-feeding fish has been reported recently as a novel mechanism, distinct from other biological and industrial filtration processes. Although crossflow passing over backward-facing steps generates vortices that can suspend, concentrate, and transport particles, the morphological factors affecting this vortical flow have not been identified previously. In our 3D-printed models of the oral cavity for ram suspension-feeding fish, the angle of the backward-facing step with respect to the model’s dorsal midline affected vortex parameters significantly, including rotational, tangential, and axial speed. These vortices were comparable to those quantified downstream of the backward-facing steps that were formed by the branchial arches of preserved American paddlefish in a recirculating flow tank. Our data indicate that vortices in cross-step filtration have the characteristics of forced vortices, as the flow of water inside the oral cavity provides the external torque required to sustain forced vortices. Additionally, we quantified a new variable for ram suspension feeding termed the fluid exit ratio. This is defined as the ratio of the total open pore area for water leaving the oral cavity via spaces between branchial arches that are not blocked by gill rakers, divided by the total area for water entering through the gape during ram suspension feeding. Our experiments demonstrated that the fluid exit ratio in preserved paddlefish was a significant predictor of the flow speeds that were quantified anterior of the rostrum, at the gape, directly dorsal of the first ceratobranchial, and in the forced vortex generated by the first ceratobranchial. Physical modeling of vortical cross-step filtration offers future opportunities to explore the complex interactions between structural features of the oral cavity, vortex parameters, motile particle behavior, and particle morphology that determine the suspension, concentration, and transport of particles within the oral cavity of ram suspension-feeding fish.

Development of helical, fish-inspired cross-step filter for collecting harmful algae

“…The 100µm mesh only had 44% open area, less than the 55% open area for the 140µm mesh, which contributed to the disparity. The higher fluid exit ratio in the Sanderson filter was correlated with the formation of larger sustained vortices, as predicted by Brooks et al (2018). Marshall filter was modified to improve its fluid exit ratio by extending the filter frame and adding additional slots (nine instead of five).…”

“…Fouling of the filter with particles or solutes is a major limitation of many industrial filtration processes (Imbrogno and Belfort 2016;Jaffrin 2012). Previous research by one of the authors introduced a new filtering mechanism called vortical cross-step filtration based on physical models of ram filter-feeding fish (Brooks et al 2018;Sanderson et al 2016). The basic cross-step filter design published by Sanderson et al (2016) has been proposed for the adsorption of biological macromolecules (Zhou et al 2018).…”

“…The basis for this work is the previous introduction of the vortical cross-step filtration mechanism (Brooks et al 2018;Sanderson et al 2016). This filtering mechanism uses a series of solid ribs that form the wall of a porous channel or pipe, with consecutive ribs spaced a relatively short distance apart (d-type roughness, characterized by an aspect ratio of slot width to rib height <~ 3-4, Liu and Chung 2012;Stel et al 2012).…”

Channeling vorticity: Modeling the filter-feeding mechanism in silver carp using μCT and 3D PIV

“…We suspect that at slower speeds (0.25 BL s −1 ), the change in depth from the first region of the model to the second region does not create a strong enough pressure change to induce strong vortical flow. Brooks et al (2018) recently showed that pressure drops created across the filtering apparatus in suspension-feeding fishes are crucial to the development of vortical structures. As a result of the decrease in pressure, vortices that develop across region 2 are slow moving and do not accelerate across the model.…”

“…Forced vortices occur at low to moderate Reynolds numbers and require a continuous external source of kinetic energy to continue moving (Brooks et al, 2018), such as would be provided by buccal pumping (or continuous flow as seen within these flow tank experiments). We hypothesize (Fig.…”