Hydrodynamic analysis of bioinspired vortical cross-step filtration by computational modelling

Abstract

Research on the suspension-feeding apparatus of fishes has led recently to the identification of novel filtration mechanisms involving vortices. Structures inside fish mouths form a series of ‘backward-facing steps' by protruding medially into the mouth cavity. In paddlefish and basking shark mouths, porous gill rakers lie inside ‘slots’ between the protruding branchial arches. Vortical flows inside the slots of physical models have been shown to be important for the filtration process, but the complex flow patterns have not been visualised fully. Here we resolve the three-dimensional hydrodynamics by computational fluid dynamics simulation of a simplified mouth cavity including realistic flow dynamics at the porous layer. We developed and validated a modelling protocol in ANSYS Fluent software that combines a porous media model and permeability direction vector mapping. We found that vortex shape and confinement to the medial side of the gill rakers result from flow resistance by the porous gill raker surfaces. Anteriorly directed vortical flow shears the porous layer in the centre of slots. Flow patterns also indicate that slot entrances should remain unblocked, except for the posterior-most slot. This new modelling approach will enable future design exploration of fish-inspired filters.

Particle separation mechanisms in suspension-feeding fishes: key questions and future directions

“…While there is a rich history of research on filtration mechanisms in SF invertebrates (reviews in Jørgensen, 1966;Riisgård and Larsen, 2010;Hamann and Blanke, 2022), relatively few studies on the biomechanics and fluid dynamics of vertebrate SF have been conducted, with most published after Sanderson and Wassersug (1993). Relevant data and models for SF fishes are rare, particularly with respect to the 3D spatially and temporally variable size, shape, and fluid dynamics of the oral cavity (e.g., Divi et al, 2018;Paskin et al, 2022;Van Wassenbergh and Sanderson, 2023;Witkop et al, 2023).…”

“…Obstacles as diverse as rocks in a river, automobiles, and buildings form backward-facing steps that generate downstream vortices (Chen et al, 2018;Montazer et al, 2018). In CFD simulations and flow tank experiments with physical models using the crossflow configuration, the BAs and GRs acted as backward-facing steps that generated a vortical recirculation zone when flow separation (not to be confused with particle separation) occurred at the downstream edge of each step (Figure 9) (Sanderson et al, 2016;Van Wassenbergh and Sanderson, 2023;Witkop et al, 2023;Xu et al, 2023). As flow travels over each step, a trapped vortex forms directly downstream near the step due to the sudden expansion of cross-sectional area in the channel there (Chen et al, 2018).…”

Biomimetic models of fish gill rakers as lateral displacement arrays for particle separation

“…All current filtration models at the level of the gill rakers in suspension-feeding fish rely on steady flow with intra-oral velocities, pressures and streamlines that are not time dependent [15,17,20,21]. This contrasts with the swimming kinematics of ram suspension feeding, during which yaw can generate cyclical intra-oral flow speed and pressure variations that could affect vortex formation and shedding downstream from the rakers [58,59].…”

“…Vortical cross-step filtration in suspensionfeeding fish is a modification of cross-flow filtration that takes advantage of the vortices generated downstream from the filter elements to suspend, concentrate and transport particles with reduced clogging (supplementary figure S1(b)) [12][13][14][15]. This application of vortices inside the gaps between filtration structures is distinct from cross-flow filtration modes used for industrial processing of beverages and foods, wastewater and pharmaceuticals [16].…”

Gill arch and raker morphology of common carp (Cyprinus carpio, Linnaeus, 1758) sampled in aquaculture system

“…However, it is emphasized that the flow dynamics of liquids cause various vortices in the liquids taken into the mouth and provide cross-flow filtration. The distance between the rakers plays a major role in filtering particles from the suspension ( Van & Sanderson, 2023). Also, gill rakers are the anatomical part studied in the discovery of new species (Konhamkakkada et al, 2023) and a model for the industrial design of commercial filtration systems (Van & Sanderson, 2023).…”