Experimental protocol for validation of Computational Fluid Dynamics palaeoecological simulations

Harriet B. Drage , Stephen Pates, Nicholas J. Minter

Computational Fluid Dynamics (CFD) simulations are being used with increasing frequency to test palaeoecological hypotheses. These analyses output simulated velocity and pressure flow profiles, and drag and lift force values acting on a shape, for a given model of laminar or turbulent flow. These outputs are internally consistent, assuming consistently applied parameters. However, many simulations lack validation at the flow speeds and animal sizes modelled, and so the margins of error remain unquantified. Without certainty in the simulated outputs, we risk the resulting palaeoecological hypotheses lacking robustness. Experimental Fluid Dynamics (EFD) analyses using flume tanks can be performed to validate simulated force values, though these have rarely been done for palaeobiological research or at Reynolds numbers reflecting flow speeds and object sizes suitable for extinct invertebrates. We present work to produce a broadly applicable protocol for performing EFD analyses to generate drag and lift forces under different flow regimes, and velocity profiles. We discuss digital and print model production trade-offs, and present the low-cost, open-source force-measuring circuitry used. We test this EFD set-up by carrying out experiments on spheres, which we then compare to theoretical drag calculations. We also present the results of several experiments on animal models, comparing the results to comparable published CFD simulations. Sphere results track expected drag force and coefficient trends reasonably well, though are too low in magnitude. Experimental drag for an ammonite model almost exactly replicated data reported from CFD simulations, though other animal models differed due to experimental conditions. We discuss planned future refinement steps, following which a series of validation values will be produced for early Palaeozoic invertebrate body plans under different flow regimes in varied positions. These values can be used to validate CFD results of future studies, and the protocol replicated to support others in performing EFD validation of palaeoecological hypothesis-testing experiments.