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Effects of shape and surfaces on fluid-dynamic performance of organisms at intermediate Re

  • Author(s): Dolinajec, Trevor Hendry
  • Advisor(s): Koehl, Mimi A.R.
  • et al.
Abstract

An organism's performance in relation to the fluid it lives and operates in is important

across size and time scales, but the effects on performance of body shape and

proximity to a surface become particularly nuanced at intermediate Re. This physical

regime in which both viscosity and inertia play important roles has not been studied

as extensively as that of macroscopic animals in which inertia dominates or that of

microscopic animals in which viscosity dominates. However, many ecologically important

animals such as the copepod occupy these intermediate flow conditions, as do

both airborne and aquatic propagules such as the sporocarps of fungi and the larvae of

benthic animals. Through recorded observation and modeling this dissertation arrives

at biological implication regarding these organisms' habitats and life cycles. This work

also creates a fuller understanding of general principles that govern intermediate Re.

Zooplankton contain a range of morphologies, and life cycles that bring them in contact

with surfaces that act as crucibles. The purpose of this study was to determine

how the morphology and orientation of a variety of ecologically-important microscopic

marine animals (adult copepod, snail veliger larva, barnacle nauplius and cyprid larvae)

affect the forces they experience while swimming in the water column, and while

on surfaces (e.g. prey captured on tentacles of benthic predators, larvae settled onto

benthic substrata). Drag, lift, and side forces as well as moments were measured

about three axes for dynamically-scaled physical models of each animal. These forces

and moments can transport and reorient swimming animals, and can push, lift, peel,

or shear animals o surfaces, and thus affect important ecological processes such as

dispersal, predation, and larval settlement. The Reynolds numbers (Re, the ratio of

inertial to viscous forces) for the zooplankton and the models was in the range of 10^2

to 10^3. Body shape and orientation of small animals were found to have significant effects on the magnitudes of fluid dynamic forces and moments at Reynolds numbers

of order 10^3, but were less important at lower Re's. The magnitude and direction of

the net force on an organism was found to change drastically as an organism nears,

and then lands on a surface. The shear stress on the attachment of a small animal to

a surface that is caused by drag pushing the animal downstream is greater than the

shear stress due to rotation of the organism by flow-induced spinning, thus zooplankton

on surfaces are more likely to be pushed than twisted o the surfaces by water currents.

For phytopathogenic fungi in the order Erysiphales, the cause of the diseases called

powdery mildew, reinfection or dispersal to a new host plant is contingent on sporocarps

escaping a fluttering leaf, but the mechanisms that allow for this liberation are

largely unknown and unquantied. The genus Phyllactinia, unlike other members of

the order, has specialized and upwardly bent radial appendages that allow the body of

the sporocarp to extend down from the bottom of the host leaf. This causes the tips

of the appendages to be the only physical connection between the sporocarp and the

leaf with a gap of up to 300 microns, thus creating an arrangement where fluid flow may

contribute to liberation. To test the importance of ambient fluid flow on sporocarp liberation

forces and moments were measured and fluid flow around dynamically-scaled

physical models was observed at Re of 60 - 360. Flow velocities, boundary layer heights,

and sporocarp morphologies were varied to match unsteady flow conditions and sporocarp

maturation. To test the importance of aeroelastically induced inertial forces the kinematics of

fluttering leaves in a wind tunnel were recorded at a range of wind speeds, and samples of sporocarps were weighed. Physically modeled aerodynamic forces and moments alongside recorded inertial forces were compared to measured adhesive forces. The comparative forces strongly suggest that steady wind flow and realistic turbulent wind flow do not exert force necessary for liberation in magnitude or direction, but that unsteady flow can lead to significant pitching moments. The accelerations of fluttering leaves and the resulting inertial forces on sporocarps varied greatly among leaves, with forces large enough to liberate sporocarps occurring in a small subset of leaves with a characteristic flutter frequency of 25 Hz. Pitch-induced overturning of sporocarps can explain the removal of sporocarps observed on wind-exposed leaves, with more sporocarps liberated at greater wind speeds and towards the tips of leaves.

Terminal velocity is an important parameter in the wind dispersal of propagules (seeds,

pollen grains, spores). Aerial righting and aerodynamic stability is common among

vertebrate and invertebrate animals, and some propagules. Fungal sporocarps of the

powdery mildew Phyllactinia have shapes that aect their terminal velocity and aerodynamic

stability while operating at Re 1.0 - 3.3, thus Phyllactinia represents a model

organism for aerodynamic performance at near-unity Re. The reproductive success of

these mildew species is dependent on stability during aerial transport so that a particular

orientation is achieved upon deposition. High speed videography was used to

measure terminal velocity, angular velocities, and angular accelerations of free-falling

sporocarps during aerial righting. Physical models allowed for quantification of forces

and moments acting on sporocarps falling at terminal velocity, as well as providing fine-scale

flow visualization. The morphology of sporocarps is dependent on their maturity,

and experiments carried out with collected sporocarps showed that terminal velocity

is partially a function of morphological parameters. Terminal velocities of sporocarps

ranged from 8 to 28 cm/s. Flow visualizations showed that both the width and length

of the wake formed around a falling sporocarp were dependent on the spread of the

characteristic radial appendages of the genus. Sporocarps were recorded rotating while

falling prior to reaching stability, and angular velocity and angular accelerations decreased

as sporocarps approached zero angle of attack. Models conrmed that a stable

xed point existed at an angle of attack of zero for all tested morphologies of Phyllactinia

sporocarps. However, naturally occurring morphologies that were the most likely

to have smaller terminal velocities also displayed smaller aerial-righting moments, and

sporocarps most likely to have larger terminal velocities displayed larger aerial-righting

moments. This suggests a potential trade-o between sporocarps that are more stable

(larger aerial-righting moments) and those that can disperse longer horizontal distances

(smaller terminal velocity).

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