What is drag?
-The rate of removal of momentum from a moving fluid by an immersed body. -also called linear momentum -either the fluid or the object is moving (can be either way)
How do you calculate drag?
linear momentum= mass x velocity -viscosity comes into drag as it determines the velocity
What does drag do?
-the water is displaced by the object
-gets deflect laterally
-theoretically what water would do if around a cylinder
What should happen to idealized fluid when approaching an object (drag)?
-he calculated what should happen to the pressure of the fluid as it approaches the object -should be high as it contacts the object - as you lose velocity= pressure goes up, then the pressure should go down -there should be no net pressure difference before the cylinder or behind it -BUT this does not happen, the pressure does not drop as quickly as you predict and never goes as down as you predicted adn never gets all the way back up -so now actually a pressure differential, at 0
What is d'Alembert's paradox? (what actually happens to a fluid when encountering an object?)
-it will create a net force
-that is why you feel the force on a beach
-generating a displacement force
-the fluid does not behave as idealized is due to friction, due to viscous properties of fluid, the particels of fluid interact with the surface of the object= stick to it a bit
-frictional drag= material properties of the object and the viscosity of the fluid
-pressure drag=also exerts force on the object
What does the amount of drag you experience depend on?
-how much pressure drag has to do with how much water you are displacing. -what the cross sectional diameter, the area of the object is -the more area the more water gets displaced -if water slowed down due to friction, then greater pressure differential= greater pressure drag
How do limpets deal with drag?
-limpets = hunker down, =in the benthic boundary layer the water is slower so the drag less, the pressure gradient is less as the water is slower in the boundary layer, -so they target areas where water not moving as much -being small, rigid and hunkering down in areas where the water is slower (boundary layer)
What is another strategy to deal with drag?
-flexible body form- macroalgae, cnidarians, sponges -macroalgae= larger size thanks to flexibility, so can grow larger in the water beaten environment) -when high water velocity= they bend= as a result= smaller cross sectional area = thus less water is displaced= less pressure drag
How does a sea anemone Metridium deal with drag?
-you can have plastic morphology as well: another strategy
-successive changes in appearance as current increases
- here again reduce the cross sectional area so less water is displaced
-can differentially retract the feeding appendages so the area is smaller, so minimize the pressure drag = dynamic morphology
How does giant kelp deal with drag?
-can have ruffled blade or strap like blade
-selective forces on the leaf size= the drag and the need to photosynthesize
-they will develop ruffled blade morphology in sheltered areas and strap like in exposed areas
= adaptive response, the same plant and the energy at which it occurs will decide what type of blade it will have
-how they change it is that they change the elasticity of the blades
How can giant kelp change material properties during growth?
-they change the material properties of the blades that influence the elasticity
-in wave swept environments another factor is the acceleration not just how fast the water is moving but how fast it accelerates= so must be elastic as well
-so the blades can be more like a rubber band= can absorb the energy of the acceleration
-here no relationship between blade area and environmental stress factors as they are changing their properties
What is the transition of laminar to turbulent flow depend on?
-size of the Reynolds number
What flow is experienced at Reynolds numbers below 10?
What flow is experienced at Reynolds numbers between 10 and 40?
-like corals= recirculation regions= often with filter feeders so get more nutrients,can optimize how they grow, grow thicker etc.
What flow is experienced at Reynolds numbers between 40 and 200 000?
-von Karman trail
-now dettached eddies, not attached anymore
-happens at size from big coral to islands
What flow is experienced at Reynolds numbers above 200 000?
-full turbulent wake
-now fully turbulent flow
What is the importance of turbulence at small scales?
1. Dispersion of eggs and sperm (turbulence is important as it affect the rate of mixing -greater turbulence=more mixing) 2. Dispersion of olfactory cues (help organisms move-detection of river etc, (anadromous and catadromous species)
What is the direction of force in drag and lift?
-While drag is a force that lies parallel to the direction of water flow, lift is a force that lies perpendicular to flow -drag and lift are pressure derived drag is parallel to the flow -lift is perpendicular to the flow
What does lift depend on?
-lift depends on if there is differential deflection of flow
-either on dorsal or ventral surface
-if the force is greater on the top than bottom= greater lift
-the amount of lift you experience is related to the surface area
-lift can be quite strong, is dominant with the really flat things
-drag is the cross sectional area to the flow
How do flounders deal with lift?
-flounders llive in the benthic layer not even the boundary layer -so they do not experience any lift as the water is not moving -both sides of the fish exposed to non moving water layer -bury themselves as ambush predators and to reduce lift
How do sand dollars deal with lift?
-sand dollars have grooves in their bodies -basically holes -that connect the dorsal and ventral part of the animals
-more round on the dorsal side of the animal
-the water goes faster on the top of the animal than the bottom -this sets up the pressure differential of lift= but the slits they have counteracts it
-also change the direction towards the water movement to take advantage of the deflection for food particles into their mouth
-they are using their shape to use the lift
-now the lift is actually horizontal due to their change in direction
How do different limpets deal with drag and lift?
-total force exerted is very similar when add both lift and drag together
-Lottia: more lift, this is better in wave heavy areas
-Acmae: more drag less lift, good in really high flow areas
What is Bernoulli's principle?
-Simply a rephrasing of the principle of conservation of energy. In practical terms this means there is a trade-off between water pressure and velocity.
-An increase in the cross-sectional area of a pipe causes a decrease in velocity and therefore an increase in pressure (centre). An increase in elevation of the pipe (right) causes a decrease in pressure.
-these are proportional increases in velocity lead to proportional decrease in pressure and vice versa
How do fish utilize Bernoulli's principle?
-inertial suction feeding= can feed passively
-water hits the front of the fish= water slow downs so high pressure (as velocity goes down)
-then speeds up around the operculum and the pressure goes down
-similar with sand dollar, water wants to move from high pressure to low pressure, so it opens its mouth and the water passively flows into the mouth
-extending jaw (longer)= larger pressure difference better flow= advantage