1.1 WHAT IS FLUIDS?
- Liquids and gases are both fluids
- a fluid is any substance that flows and takes the shape of its container.
- If the fluids are at rest, the study of them is called fluid statics.
- If the fluids are in motion, the study of them is called fluid dynamics.
- —The science and technology of the mechanical properties of liquids is called hydraulics.
- Similarly, the science and technology of the mechanical properties of air and other gases is called pneumatics.
1.2 THERMODYNAMIC PROPERTIES OF A FLUID
1.2.1 DENSITY
- The density of a fluid is its mass per unit volume:
- Liquids are essentially incompressible
- Density is highly variable in gases nearly proportional to the pressure.
- Note: specific volume is defined as:
1.2.2 SPECIFIC WEIGHT
- The specific weight of a fluid is its weight, , per unit volume. Density and specific weight are related by gravity:
1.2.3 SPECIFIC GRAVITY
- Specific gravity is the ratio of a fluid density to a standard reference fluid, typically water at 4˚C (for liquids) and air (for gases):
- For example, the specific gravity of mercury is SGHg = 13,580/1000 = 13.6
1.2.4 VISCOSITY
- Viscosity is a measure of a fluid’s resistance to flow. It determines the fluid strain rate that is generated by a given applied shear stress.
- Temperature has a strong and pressure has a moderate effect on viscosity. The viscosity of gases and most liquids increases slowly with pressure.
- A Newtonian fluid has a linear relationship between shear stress and velocity gradient:
- The no‐slip condition: at the wall velocity is zero relative to the wall This is The no slip condition: at the wall velocity is zero relative to the wall. This is a characteristic of all viscous fluid.
- The shear stress is proportional to the slope of the velocity profile and is greatest at the wall greatest at the wall.
- A liquid, being unable to expand freely, will form an interface with a second liquid or gas.
- The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension.
- Surface tension Υ (pronounced upsilon) has the dimension of force per unit length (N/m) or of energy per unit area (J/m2).
- Υair‐water = 0.073 N/m; Υair‐mercury = 0.48 N/m
- Using a force balance, pressure increase in the interior of a liquid half‐cylinder droplet of length L and radius R is:
- Contact angle θ appears when a liquid interface intersects with a solid surface.
- Water is extremely wetting to a clean glass surface with θ≈0. For a clean mercury‐air‐glass interface, θ≈130°.
