Viscosity |
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The coefficient of viscosity is a measure of the resistance of a fluid to shearing forces (stirring), or more simply to the passage of a sphere through the body of the fluid under conditions of laminar flow. The drag force on a sphere is given by Stokes' law (at very low speed) as f.=.6ph rv. Terminal velocity for a sphere (when 6ph rv = mg) is proportional to r. The unit of viscosity is the Poiseuille (Ns/m2) or sometimes the centa-Poise (0.001 Pl). Water at 1.0x10-3 Ns/m is more viscous than air at 1.8x10-5 Ns/m and less than peanut butter at ~250 Ns/m2. Stokes' theorem was of critical importance when Millikan determined the quantized nature of the charge on tiny oil drops moving through air but in most common situations (even slowly falling leaves) flow is not laminar but turbulent. Under these conditions the drag force is given by ... f = r Av2 or more accurately by ... ... where CD.is the drag coefficient. |
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