Ram suspension-feeding fish such as herring use gill rakers to separate small food particles from large water volumes while swimming forward with an open mouth. Fish gill raker function was tested using 3D-printed conical models and computational fluid dynamics (CFD) simulations over a range of slot aspect ratios. (See CFD simulation output in Dryad dataset: case and data files for ANSYS Fluent 19.1 CFD simulations.)

Our hypothesis predicting the exit of particles based on mass flow rates, dividing streamlines (i.e., stagnation streamlines) at the slots between gill rakers, and particle size was supported by the results of experiments with physical models in a recirculating flume. (See R code with embedded comments in Dryad dataset to test Hypotheses 1 and 2 for particle exit/trapping.)

Particle movement in suspension-feeding fish gill raker models was consistent with the physical principles of lateral displacement arrays (“bump arrays”) for microfluidic and mesofluidic separation of particles by size. Although particles were smaller than the slots between rakers, particles skipped over the vortical region that was generated downstream from each raker. Particles “bumped” on anterior raker surfaces during posterior transport. (See particle movement data in Dryad dataset: three files for particle contact, exit, and retention in the physical models.)

Experiments in a recirculating flume demonstrated that the shortest distance between the dividing streamline and the raker surface preceding the slot predicts the maximum radius of a particle that will exit from the model by passing through the slot. This theoretical maximum radius is analogous to the critical separation radius identified with reference to the stagnation streamlines in microfluidic and mesofluidic devices that use deterministic lateral displacement and sieve-based lateral displacement. These conclusions provide new perspectives and metrics for analyzing crossflow and cross-step filtration in fish with applications to filtration engineering. 

The following information is also provided as a "README" file.

"CFD Simulation Output" folder, containing ANSYS Fluent 19.1 simulation output consisting of case and data files (.cas.gz and .dat.gz files).

These files can be opened and data explored using the free Ansys Student software, currently available for download at:

https://www.ansys.com/academic/students/ansys-student 

This installer package includes the software CFD-post for viewing the results.

4 CFD result files are included:

1.35 mm - deep model:

1p35_deep_cas.gz and 1p35_deep_dat.gz

1.35 mm - shallow model:

1p35_shallow_cas.gz and 1p35_shallow_dat.gz

1.80 mm - deep model:

1p8_deep_cas.gz and 1p8_deep_dat.gz

1.80 mm - shallow model:

1p8_shallow_cas.gz and 1p8_shallow_dat.gz

"R Code to Test Hypotheses" folder, containing the R code with embedded comments to test Hypotheses 1 and 2 for particle exit/trapping.

"Particle Movement" folder, containing spreadsheets for particle contact, exit, and retention in the physical models:

File titled “Exit trajectories and contacts” 

Column A: physical model type

Column B: experimental trial date

Column C: individual particle number

Column D: category of exit trajectory assigned

Column E: number of contacts for each individual particle

Column F: average number of slots between contacts  (Note that “NA” in Column F means “Not Applicable” because Column E states that those particles had only 0 or 1 contact.)

File titled “Horizontal movement”

Column A: physical model type

Column B: individual particle number

Column C: experimental trial number

Column D: slot number of first contact

Column E: slot number of exit

Column F: number of horizontal slots moved by particle

File titled “Slots of exiting and retained particles”

The sum of the retained particles and the sum of the exiting particles in each slot are tabulated for each of four trials using the four physical models.

The total exiting and retained particles per trial in each slot is also presented as a percentage for each trial.

Ansys Fluent 2019 R1 .cas and .dat files can be opened with Ansys Fluent 2019 R1 or later versions, or the results included in these files can be viewed with CFD-post 2019 R1 (Ansys) or later versions. CFD-post software included in 'Ansys Student' software can be used for this. This software is currently provided for free to run on Windows, and can be downloaded from https://www.ansys.com/academic/students/ansys-student