BIEB132_L2_LifeinFluids.pdf

BIEB132_L2_LifeinFluids.pdf - Life in a Fluid Environment...

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Life in a Fluid Environment BIEB 132, Lecture 2, 9/29/09 I. Physical properties of a fluid II. Density and buoyancy III. Generating lift IV. Drag and swimming V. Flow scaling – Reynolds number VI. Motion at low Reynolds Numbers Reading : Levinton, Chapter 5, pp. 189-191, 194-196. I. Density and Buoyancy A. Density ρ (rho) – mass per unit volume 1. Water 830 times more dense than air 2. Seawater density is greater (1025 kg/m 3 at 20˚C) than pure water because of dissolved salts 3. Density increases with decreasing temperature 4. All biological materials are denser than air, most are denser than seawater Material Density (kg/m 3 ) Material Density (kg/m 3 ) Fluids Organic Air 1.2 Mollusk shell 2700 Freshwater 1000 Coral skeleton 2000 Seawater 1025 Bone 2000 Muscle 1060 Organisms Fats and oils 915–945 Dinoflagellate 1080 Inorganic Copepod 1080 Calcium carbonate (CaCO 3 ) 2800 Squid 1070 Magnesium sulfate (MgSO 4 ) 1121 Fish and sharks 1080 Sodium sulfate (Na 2 SO 4 ) 1040 Sodium chloride (NaCl) 1018 Ammonium chloride (NH 4 Cl) 1007 5. Most marine organisms are denser than seawater a) Pelagic organisms need mechanism to prevent sinking (1) Buoyancy mechanisms – reduce density, flotation (2) Generate lift from swimming – fast swimming organisms 6. Importance of higher density of water in aquatic systems compared to air on land a) Allows for suspended organisms – planktonic (drifting) or nektonic (swimming) b) Relaxation of size constraints, greater morphological diversity c) More complex food webs than on land, more trophic levels Fluids p. 1 of 10
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Figures: Assorted plankton, siphonophore Suculeolaria veronica B. Effective density 1. Effective density – density difference between object and fluid ( ρ o ρ f ) Example – 10 kg salmon, V = 0.01 m 3 , ρ = 1080 kg/m 3 (1) Effective density ( ρ o ρ f ) (a) In seawater = (1080 – 1025) kg/m 3 = 55 kg/m 3 (b) In freshwater = (1080 – 1000) kg/m 3 = 80 kg/m 3 (2) Effective weight ( ρ o ρ f ) V (where Weight = ρ V) (a) In seawater, 55 kg/m 3 * 0.01 m 3 = 0.55 kg (b) In freshwater, 80 kg/m 3 * 0.01 m 3 = 0.80 kg 2. Archimedes Principle: “An object partially or wholly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” a) If effective weight > 0 then negatively buoyant b) If effective weight = 0 then neutrally buoyant c) If effective weight < 0 then positively buoyant C. Overview of buoyancy mechanisms – reduce effective weight 1. Reduce sinking in small organisms 2. Change organism density – reduce effective density 3. Improve flotation D. Reduce sinking in small organisms 1. Small organisms have low settling velocity a) Terminal velocity – highest velocity an object reaches when sinking/falling (1) Related to size (radius 2 ) and effective density of object (2) Example – Prochlorococcus 0.5 μm in size, sinks < 0.01 μm/s, negligible (3) Example – dinoflagellate 40 μm in size, sinks 50 μm/s in seawater b) Increase surface area with projections – increase drag and reduce sinking rate
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This note was uploaded on 10/21/2009 for the course BIEB 132 taught by Professor Steven during the Fall '08 term at UCSD.

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BIEB132_L2_LifeinFluids.pdf - Life in a Fluid Environment...

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