Session L25: Minisymposium: Life Processes at Biologically Intermediate Reynolds Numbers

Hydrodynamics of inhalant flow at intermediate Reynolds numbers: implications for feeding, sensing, and respiration

Many organisms inhale ambient fluid through an orifice for purposes of feeding, sensing, or respiration. Surprisingly, the dynamics of these fundamental flows, as well as implications for associated ecological processes, are not well understood. I will describe experimental and numerical investigations of idealized inhalant flows through a round tube in the intermediate Reynolds number (Re) range of 1-100. The flow depends strongly on Re due to enhanced viscous effects at lower Re. The ecological function of the flow can be quantified in terms of two simple metrics: the region of influence (ROI), which describes the spatial extent of the active flow field, and the inhalation volume, which describe the original distribution of fluid that is subsequently inhaled. At lower Re, diffusion entrains an increasing volume of fluid over time, enlarging the ROI indefinitely. In some cases, the flow spatially bifurcates, with some fluid being inhaled through the orifice and some bypassing into recirculation. At higher Re, inward advection dominates outward viscous diffusion and the flow remains in an inviscid state. The ROI and inhalation volume are strongly dependent on Re and the height of the orifice above the substratum, suggesting that organisms could alter these parameters to achieve specific inhalation criteria.   

From single swimmers to Swarms: active matter at intermediate Reynolds numbers

In this talk, I will present experiments and simulations of how we use simple geometric objects, model swimmers made out of spheres, to understand characteristics of motility at intermediate Reynolds numbers. Questions we will address are the following. What happens to different types of swimmers as they cross the boundary from Stokes flows to the intermediate Reynolds regime, and can we use our findings to classify intermediate-Reynolds model swimmers? How is the system affected, e.g. the onset of inertia, transition from rest to motility, fluid flows, etc. when we have more than one swimmers present? The fundamental understanding gleaned from these studies builds onto our final goal which is to explore the collective emergent behavior of (slightly) inertial swimmers   

The hydrodynamics of predator-prey interactions in zebrafish

Hydrodynamics govern the behavior of fishes when they operate as predators or prey. In addition to the role of fluid forces in propulsion, fishes relay on flow stimuli to sense a predatory threat and to localize palatable prey. We have performed a series of experiments on zebrafish (\textit{Danio rerio}) that aim to resolve the major factors that determine whether prey survive an encounter with a predator. Zebrafish serve as a model system in this pursuit because the adults prey on larvae of the same species and the larvae are often successful in evading the attacks of the adults. We use a combination of theoretical and experimental approaches to resolve the behavioral algorithms and kinematics that determined the outcome of these interactions. In this context, the hydrodynamics of intermediate Reynolds numbers largely determines the range of flow stimuli and the limits to locomotor performance at dictate prey survival. These principles have the potential to apply to a broad diversity of fishes and other aquatic animals.   

Flight in hairy and sticky situations

The smallest flying insects such as thrips and fairyflies have body lengths less than 1 mm. Despite their ecological importance, the fluid dynamic mechanisms that enable very tiny insects to generate lift at Reynolds number (Re) on the order of 10 remain unclear. Flapping motion in tiny insects is often characterized by 'clap and fling' wing-wing interaction. Further, these insects possess wings consisting of a thin solid membrane with long bristles on the fringes. Why is there a noted biological preference in almost all tiny insects to employ interacting bristled wings under highly viscous conditions that would require large forces to peel the wings apart? In this talk, I will present numerical and experimental studies examining the role of bristled wings in clap and fling aerodynamics. At Re$=$10, bristled wings are observed to reduce both lift and drag forces as compared to geometrically equivalent solid (non-bristled) wings. Recirculating flow through the bristles leads to disproportionally larger drag reduction by bristled wings, as compared to lift reduction between bristled and solid wings. The impact of alterations to bristled wing design variables, including spacing between bristles and ratio of solid membrane to total wing areas, on aerodynamic force coefficients and scalability with Re will be discussed.   

How do fungi measure wind speed?

For successful dispersal, a fungus must push its spores through the boundary layer of nearly still air that clings to it. Some spores pass through this boundary layer by being forcibly ejected. But many spores and fungi have no mechanism for active ejection, and must be carried away passively by the wind. To facilitate dispersal, the spores are borne on the top of aerial structures. Regulating the height of these aerial structures is an engineering challenge; too long and the structures will collapse under their own weight; too short, and they may not reach high enough to cross the boundary layer. A fungus therefore benefits by knowing the wind speed (and therefore the boundary layer thickness). How does it make this measurement? I will show that the model filamentous fungus Neurospora crassa uses water evaporation rate to accurately measure wind speed. In addition to showing that fungi control and optimize even passive mechanisms for dispersal, our findings highlight the importance of physical conditions in controlling fungal growth and behavior.   

The role of flow in the morphodynamics of embryonic heart

Nature has shown us that some hearts do not require valves to achieve unidirectional flow. In its earliest stages, the vertebrate heart consists of a primitive tube that drives blood through a simple vascular network nourishing tissues and other developing organ systems. We have shown that in the case of the embryonic zebrafish heart, an elastic wave resonance mechanism based on impedance mismatches at the boundaries of the heart tube is the likely mechanism responsible for the valveless pumping behavior. When functioning normally, mature heart valves prevent intracardiac retrograde blood flow; before valves develop there is considerable regurgitation, resulting in oscillatory flow between the atrium and ventricle. We show that reversing flows are particularly strong stimuli to endothelial cells and that heart valves form as a developmental response to oscillatory blood flow through the maturing heart.