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Lecture 2

Lecture 2. Properties of Fluids Units and Dimensions. The elemental volume must be much larger than the free random path of a molecule. Density Dimensions and Units. Dimensions Units:. Specific Weight, Dimensions and Units. Dimensions Units:. Other properties.

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Lecture 2

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  1. Lecture 2 PropertiesofFluids UnitsandDimensions

  2. The elemental volume must be much larger than the free random path of a molecule.

  3. Density Dimensions and Units • Dimensions • Units:

  4. Specific Weight, Dimensions and Units • Dimensions • Units:

  5. Other properties • Temperature, is a measure of molecule kinetic energy, • Specific Gravity (dimensionless)

  6. Viscosity When a fluid is sheared, it begins to move at a strain rate inversely proportional to a property called its coefficient of viscosity, .

  7. Viscosity is responsible for the shear stress which opposes to the relative movement of elemental fluid volumes. Is null if there is no velocity gradient, so it destroys gradients, i.e. generates momentum diffusion.

  8. Viscosity Units

  9. Kinematic Viscosity

  10. Flow between two plates

  11. Viscosity variability • Varies much with temperature and weekly with pressure. • In liquids: • Decreases with temperature, • In gases: • Increases with temperature.

  12. No Slip Condition

  13. Surface Tension • Exists because intermolecular attraction forces between liquid molecules are higher than between the liquid molecules and air. • Is responsible for drop formation and for pressure increase inside the drop.

  14. Spherical droplet • Surface tension decreases with temperature. This is why solubility increases with temperature.

  15. Capilarity

  16. Summary • We have seen the main fluid properties, their dimensions and units. • Viscosity is relevant only when there is movement and velocity gradients are present. • In the next lecture we will see properties of the flow field.

  17. Velocity, momentum, kinetic energy Momentum is the Force applied to a volume multiplied by the action time. A small force acting for long can generate big momentum.

  18. Dimensions consistency/homogeneity • In one equation all terms must have the same dimensions. • In Fluid mechanics this norm is usually respected. • In many empirical laws (often in natural sciences) it is not, being a source of errors. • Manning formula for friction is a tricky example of a non-homogeneous formula.

  19. Other properties computed from velocity

  20. Momentum and kinetic energy • Momentum is often called impulse

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