Physics of Life 3. Fluids

Introduction to the physics of life fluids module. What fluids are. The forces that move fluids. The importance of shearing flows. What shearing flows are. Transfer of momentum within fluids and viscosity. Shear rate, shear stress, and the definition of viscosity. What a Newtonian fluid is. Different types of non-Newtonian fluids.

Interaction of fluids with surfaces. The boundary layer, and what shapes it. Momentum transfer within a boundary layer. Development of the boundary layer along a flat surface. Boundary layer thickness, and what determines it.

KW: boundary layer; momentum transfer; boundary layer thickness; windspeed; characteristic length;

How energy in boundary layers can be captured to do useful work. How sponges use momentum gradient in boundary layers to power feeding. How capture of energy in boundary layers to do physiological work has implications for physiological efficiency, and evolutionary fitness.

KW: sponges; Halichondria; choanocytes; ostia; osculum; spongocoel; food capture; physiological work; evolutionary fitness; energy budget;

What a pitot tube is and how it works. Kinetic energy in a moving fluid is translated into dynamic pressure. How aquatic caddis fly larvae turn their casts into natural pitot tubes for capturing nutrients in flowing streams.

KW: dynamic pressure; pitot tube; fluid kinetics; caddis fly; caddis house; filter feeding; energy capture;

Non-Newtonian fluids are common in biological systems, and are defined as any fluid in which the viscosity differs with shear rate. There are three basic types of non-newtonian fluid behaviors. These are viscoelastic behavior in which fluids have a yield stress, shear thinning, in which the viscosity declines with increasing shear rate, and shear thickening, in which the viscosity increases with increasing shear rate.

KW: non-newtonian fluid; viscosity; yield stress; viscoelasticity; shear thinning; blood; red blood cell; shear thickening; silk; fibroin; spinneret; spider;

The Bernoulli principle as a version of conservation of energy in fluids. The absence of viscosity from the Bernoulli equation. Pressure as fluid flows over the surface. The Bernoulli principle and the relationship between potential energy and kinetic energy. Pressure distributions over the mounds of the fungus building termites. How the Bernoulli principle helps explain how the termite mound can serve as a wind driven lung.

KW: Bernoulli principle; conservation of energy; kinetic energy; potential energy; macrotermes; termite mound; respiratory gas exchange; wind; pressure; fluid velocity;

The Bernoulli principle, viscous entrainment, and induced flow. What the Venturi effect is. How the Bernoulli principle explains suction pressures at the openings of animal burrows. How induced flow explains the ventilation of prairie dog burrows. Mounds as generators of the Venturi effect in prairie dog burrow systems.

KW: the Venturi effect; viscous entrainment; Bernoulli section; prairie dog burrows; ventilation; diffusion; metabolic rate; respiratory gas exchange; oxygen; carbon dioxide;

The Reynolds number, and the ratio of inertial and viscous forces shaping flow. How the Reynolds number determines type of flow. Laminar flow, transitional flow, in turbulent flow as expressed by the Reynolds number. How in any vortex is generated, and how this shapes a vortex street.

KW: Reynolds number; laminar flow; transitional flow; turbulent flow; inertia; viscosity; vortex street; turbulent wake;KW: 

How the Reynolds number can be applied to understanding patterns of flow around objects. Viscosity dominated environments, and inertia dominated environments. Typical Reynolds number ranges for organisms of different sizes. How a fruit fly experiences its world versus how a bird experiences its world. How the Reynolds number can help analyze the physiology of extinct organisms, like the mammal like reptile, Dimetrodon.

KW: Reynolds number; body size; inertia dominated flow; viscosity dominated flow; fruit fly; bird; blackstrap molasses; body temperature regulation; Dimetrodon; mammal like reptile;

Why there are two types of drag. Drag as an expression of transfer of momentum through viscous boundary layers. Drag as a modification of energy content of moving fluids. The difference between pressure drag and viscous drag. Different scaling of pressure drag and viscous drag. What the Bernoulli principle predicts about the origin of drag.

KW: drag; viscous drag; pressure drag; Bernoulli principle; profile area; momentum transfer; viscous boundary layer; scaling;

Drag and terminal velocity. Terminal velocity as a balance between drag and wait. Scaling of terminal velocity. What the drag coefficient is, and how it relates to drag. Scaling of the drag coefficient. Stokes law and terminal velocity. Drag of arrays of fibers. Why milkweed seeds and dandelion seeds sink slower than they should. Parachute locomotion as an aid to seed dispersal.

KW: Stokes law; terminal velocity; drag; weight; milkweed seed; dandelion seed; sinking rate; seed dispersal; pressure drag; adaptation; parachute locomotion;

A more detailed look at buoyancy. Buoyancy in air, and buoyancy and water. How a sinking rate depends upon the fluid density differences. How densities of animals compared with densities of either air or water. How plankton reduce sinking rate, and why reductions of sinking rate alone are not a sufficient adaptation. How plankton interact with large-scale patterns of ocean circulation to keep plankton near the surface. How physics of sinking interacts with a basic ecological population model.

KW: buoyancy; density difference; negative buoyancy; positive buoyancy; neutral buoyancy; plankton; diatoms; dinoflagellates; eddy circulation; photic zone; photosynthesis; population models; parachuting; ballooning;

The physics of locomotion through fluids. The Reynolds number and the physical problems of fluid locomotion. How scale affects the function of locomotory engines. Reynolds number and continuous versus intermittent locomotion.

KW: locomotion; swimming; bacterial flagellum; eukaryotic flagellum; cilia; flagella; undulipodium; stator; rotor; proton motive force; sliding microtubule theory; electromechanical vibration theory; microtubule doublet; traveling wave

How viscosity complicates locomotion at small scales. The concept of the added mass. How added mass complicates ciliary locomotion. How the complex bending of cilia helps overcome the problem of added mass. How cilia work in concert to exploit added mass in ciliary locomotion. Coordination of ciliary beating into the metachronal wave.

KW: locomotion; swimming; added mass; ciliary motion; power stroke; return stroke; metachronal wave;

The diverse modes of bacterial locomotion. Spirochaete bacteria and corkscrew locomotion. How spirochaete bacteria are able to use their large size and unique mode of locomotion to overcome immune defenses. What gliding locomotion is. Two mechanisms for gliding locomotion.

KW: locomotion; bacterial locomotion; gliding locomotion; spirochaete bacteria; barrier immunity; periplasmic flagella; tank tread locomotion; motility proteins;

What are vortices? Translation of linear momentum in fluids into angular. My turbulence reduces drag. Frequency of vortex shedding. The meaning of the Strouhal number. The principle of the Aeolian harp. Wind induced shaking and ballistic seed dispersal.

KW: swimming; vortex shedding; Strouhal number; retrograde force; drag; wind shaking; ballistic seed dispersal

Swimming and vortex shedding. How fast should the fish wave its tail fin? The Strouhal number an optimum swimming speed. How vortex shedding helps a fish swim. The similarity between swimming and flying.

KW: swimming; flying; Strouhal number; vortex shedding; swimming frequency; optimum swimming; flapping flight;

How do wings generate lift? Why is the standard model for lift is almost certainly wrong? Vortex shedding and the generation of lift.6 of vortex shedding off of wings. Lift as a combination of linear and angular momentum.

KW: lift; airfoils; Bernoulli model; differential pressure; balance of forces; drag; thrust; weight; vortex shedding; Bernoulli principle

The Flettner rotor. The Magnus force. How the Flettner rotor generates thrust through asymmetries in flow separation and vortex generation. Why the Bernoulli principle is not the explanation for lift.

KW: Flettner rotor; Flettner ship; Magnus force; flow separation; symmetry; asymmetry; vortex shedding; Anton Flettner;

The basic principles of flying with an airfoil. Aspect ratio and designer wings for gliding. The relationship of aspect ratio to the lift to drag ratio. Aspect ratio and the gliding path. The independence of glide path from weight. Wing design in three different birds.

KW: gliding; gliding path; aspect ratio; lift to drag ratio; gliding angle; albatross; petrol; gliding speed;

The aspect ratio and turning radius. How the turning radius of a bird is determined by the design of its wings. High aspect ratio and high second moment of area related to the energetics of banking and turning.

KW: gliding; stability; maneuverability; second moment of area; aspect ratio; turning radius; banking; turning;

Bird wing airfoils can generate both lift and thrust. The variable pitch wing, and the wing beat cycle of hummingbirds. How the pattern of wing shape and orientation enables the hummingbird to generate consistent lift, and to hover. The importance of variable wing pitch during the wing beat cycle. Different patterns of vortex formation at different points of the wing beat cycle

KW: hummingbird; variable pitch wing; fixed wing; thrust; lift; stroke angle; pitch angle; figure 8; wing beat cycle; hovering;

Very large animals can fly. But what about small animals? How small can they be and still fly? This question is complicated by the low Reynolds number regimes at which small animals fly. If lift depends upon the shedding of vortices from wings, flapping flight should be difficult for small creatures flying at low Reynolds number regimes. Small flying animals push this limit by developing an entirely new means of flapping flight known as clap and fling.

KW: Reynolds number; vortex generation; fruit fly; mosquito; thrips; wingbeat frequency; flapping flight;