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Fluid Mechanics-I Spring 2010

Fluid Mechanics-I Spring 2010. Lecture #02. Viscosity Dependence. Coefficient of Viscosity For Liquids, No effect of pressure on dynamic or Kinematic viscosity. For gases, viscosity varies with pressure as. Viscosity as a function of Temperature. Why it is?.

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Fluid Mechanics-I Spring 2010

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  1. Fluid Mechanics-ISpring 2010 Lecture #02

  2. Viscosity Dependence • Coefficient of Viscosity • For Liquids, No effect of pressure on dynamic or Kinematic viscosity. • For gases, viscosity varies with pressure as

  3. Viscosity as a function of Temperature

  4. Why it is? • A drop of blood forms a hump on a horizontal glass; • A drop of mercury forms a near-perfect sphere and can be rolled just like a steel ball over a smooth surface; • Water droplets from rain or dew hang from branches or leaves of trees; • A soap bubble released into the air forms a spherical shape; • Water beads up into small drops on flower petals

  5. Interface Less dense Molecules Interface High Density of Molecules

  6. Interfacial Forces The liquid molecules tend to minimize their surface area because of surface tension.

  7. Surface Tension • Surface tension is generated due to intermolecular forces at the interface between two immiscible fluids with density difference. • The interface behaves like a stretched membrane.

  8. Examples of Surface Tension Water drops formation on smooth surfaces Insects walking on Water surface

  9. Surface Energy • Amount of energy in a stretched membrane. • Energy in a stretched membrane leads to pressure jump in curved surfaces.

  10. Surface Tension Coefficient ( ) • Surface energy per unit area is called surface tension coefficient.

  11. Surface Tension in a Hatched Cylinder

  12. Surface Tension in a Spherical Droplet

  13. Surface Tension for a General Curved Surface

  14. Contact Angle (Liquid-Solid Interface) If θ<90 Wetting Liquid If θ>90 Non-wetting Liquid

  15. Example of Contact Angle • Water wets soap but does not wet wax. The rise or fall of liquid in a narrow tube is due to the combined effect of contact angle and surface tension. This is known as Capillary Effect.

  16. Capillary Tube Patm Patm Patm

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