2LB_Lab_2_ViscosityAndViscousFlow5

2LB_Lab_2_ViscosityAndViscousFlow5 - Physics 2LB Viscosity...

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Physics 2LB Viscosity and Viscous Flow of Fluids (Pre Lab Assignment Included) In this lab we will learn about: The fluid property “viscosity” and its measurement The change in the viscosity of a fluid with temperature Apparatus: Pressure-Flow Apparatus with 50 cm capillary tube ; Computer with Data Studio software ; One liter container or pitcher (for ice/hot water) ; Digital Thermometer , Vernier caliper I. Introduction Think about pouring maple syrup on a stack of pancakes or chocolate syrup on your ice cream. They flow very slowly out of the bottle. In fact, you might need to use hot chocolate syrup to even make it flow. The property of the fluid that prevents it from flowing easily is called its “viscosity”. In your class, when you learn such things as the Bernoulli equation, you deal with mostly “ideal fluids”, you neglect the role played by viscosity in the flow of fluids. When a real fluid flows through a tube (or blood through a blood vessel) its velocity decreases due to “internal friction” between the moving layers of fluid. This internal friction results from the molecule-molecule attraction of the fluid molecules. The measure of this internal friction resulting from the molecular attraction is called the fluids “viscosity”. Thick lubricating oil is more viscous than water which has a viscosity η = 0.00890 poise (dyne-sec/cm 2 ) at 25 o C (1) Even blood is four times more viscous than water. Maple syrup and chocolate syrup are about 3000 and 18,000 times more viscous than water respectively. Note that these syrups don’t weigh thousands of times more than water. So it is the “viscosity” and not the “density” that makes the difference in how a liquid flows. The viscosity of a fluid is very sensitive to temperature. For water, the viscosity changes about 2.5% per o C (it gets less viscous at higher temperature). Some fluids like chocolate syrup have very large changes in viscosity when heated as we noted earlier. Think about lava flow from a volcano. The liquid lava when it cools becomes a solid rock (a great example for viscosity changing with temperature!). When a real fluid flows through a tube, the first layer right next to the wall of the tube sticks to it and does not flow. The layers next to it are dragged back due this first “stuck” layer due to the 1 molecule-molecule attraction. So the velocity and kinetic energy of this second layer is decreased due to this fluid friction. This continues all the way to the center of the tube. Thus in contradiction with the Bernoulli equation (which is valid only for non-viscous ideal fluids 1 ), for a real fluid to flow through a tube, you need a pressure difference at its two ends, as shown in the figure below. Consider the fluid flow through a long narrow tube or capillary as shown in the diagram below (of cross- sectional area π r 2 )* with pressures P 1 > P 2 .
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This note was uploaded on 01/11/2010 for the course PHYS 2b taught by Professor Clare during the Fall '07 term at UC Riverside.

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2LB_Lab_2_ViscosityAndViscousFlow5 - Physics 2LB Viscosity...

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