
PNAS paper abstract:
Erosion by flowing fluids carves striking landforms on Earth and also provides important clues to the past and present environments of other worlds.
In these processes, solid boundaries both influence and are shaped by the surrounding fluid, but the emergence of morphology as a result of this interaction is not well understood.
Here, we study the coevolution of shape and flow in the context of erodible bodies molded from clay and immersed in fast flowing water.
Though commonly viewed as a smoothing process, we find that erosion sculpts pointed and cornerlike features that persist as the solid shrinks.
We explain these observations using flow visualization and a fluid mechanical model in which the local shear stress dictates the rate of material removal.
Experiments and simulations show that this interaction ultimately leads to selfsimilarly receding boundaries and a unique front surface characterized by nearly uniform shear stress.
This tendency toward conformity of stress offers a principle for understanding erosion in more complex geometries and flows, such as those present in nature.


abstract:
Wing or fin flexibility can dramatically affect the performance of flying and swimming animals. Both laboratory experiments and numerical simulations have been used to study these effects, but analytical results are notably lacking. Here, we develop smallamplitude theory to model a flapping wing that pitches passively due to a combination of wing compliance, inertia, and fluid forces. Remarkably, we obtain a class of exact solutions describing the wing's emergent pitching motions, along with expressions for how thrust and efficiency are modified by compliance. The solutions recover a range of realistic behaviors and shed new light on how flexibility can aid performance, the importance of resonance, and the separate roles played by wing and fluid inertia. The simple, robust estimates afforded by our theory will likely be valuable even in situations where details of the flapping motion and wing geometry differ.


JNNFM paper abstract:
The flow of viscoelastic fluids is an area in which analytical results are difficult to attain,
yet can provide invaluable information.
We develop a weakcoupling expansion that allows for semianalytical computations of
viscoelastic fluid flows coupled to immersed structures.
We apply the expansion to the transient benchmark problem of a rigid
sphere settling through a viscoelastic fluid using the OldryodB model
and recover previous results and observed behavior.
The theory presented here is in contrast to the retardedmotion,
or low Weissenberg number, expansions that have received much attention,
and one advantage is that the weakcoupling expansion offers information
for Weissenberg numbers larger than one.
The expansion's limit of validity is closely related to the diluteness criterion
for a Boger fluid.
We extend the classical settling problem to include an oscillating bodyforce,
and show how the introduction of a forcing timescale modifies the bodydynamics.


Synopsis:
The vertical motion of bodies through a densitystratified fluid is a problem
with important applications in the ocean and atmosphere. Here we create a "strippeddown"
flow configuration in order to isolate one feature of vertical motion through
stratification  namely the creation of vertical density layers by viscous entrainment.
This flow configuration is created in a tabletop experiment by towing a fiber vertically through a
stratified corn syrup solution and lubrication theory is used to model the dynamic flow configuration.
We find longwave instabilities along the fluid interface for layers that
are larger than a critical lengthscale.


Synopsis:
Here we characterize the brachistochrone path, or path of shortest time, in potential flow past a body,
and we apply our theory to obtain new results related to Darwin's drift volume.
Drift volume is related to a body's added mass and has important practical applications.
Our theory extends previous infinite and semiinfinite results to finite distances and times,
which is an important extension in the context of laboratory experiments.
We also discuss an application to a rigid sphere sedimenting through a sharply stratified fluid in
order to account for an experimentally observed levitation phenomenon.
Further theoretical work related to the potential energy carried by the drift volume is underway.
