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Zoology 427 BiomechanicsLecture 18. Shape and drag•A comment about poster projects•Recap drag and the Reynolds number•Drag and its coefficient•How size, shape, and the Reynolds number determine drag•Streamlined bodies and swimming energetics•When does drag matter? (P2 - P1) /ρ = (u12- u22 )/2PE + KE + W + Ediss = const new stagnation point where the flow separatesWakeBecause of viscosity, velocity cannot increase as much as in the inviscid case. Pressure stress: P ~ ρ u2Shear stress: τ ~ µ du/drρ u LµRe = Drag force arises from pressure and frictional stresses D ~ ρ u2/2D = Cd ρ S u2/2drag coefficientprojected surface areasizeshapeReynolds number!"=$12'()*! Shape and Drag1.251.140.50.30.045!"=$12'()*!Flow Log Reynolds numberCd-1 0 1 2 3 4 5 6 7 81001010.1!"=$12'()*!streamlined body: same surface arearoughness!"=24&'+61+&'+0.4!von Karmanvortex street