Chapter 1: Introduction to Fluid Mechanics

Terms and definitions from lectures 1 - 3

Fluid

A substance that deforms continuously when acted on by a shearing stress of any magnitude. Usually a liquid or gas.

Shearing Stress

Represented as 𝜏. The tangential force that acts per unit area on a fluid. Causes fluids to deform continuously and thus flow.

Macroscopic

The view that fluid mechanics takes in order to treat fluid to be a continuum.

Dimensionally Homogenous

The dimensions of the left side of the equation must be the same as those on the right side, and all additive separate terms must have the same dimensions.

General Homogenous

Equations that are valid in any unit system.

Restricted Homogenous

Equations that are only valid in a particular system of units.

Density

Represented as 𝜌. Defined as the mass per unit volume.

Weakly

In gases, the density ______ depends on temperature and pressure.

Strongly

In liquids, the density ______ depends on temperature and pressure.

Specific Volume

Represented as 𝑣. Defined as the volume per unit mass and is the reciprocal of the density.

Specific Weight

Represented as 𝛄. Defined as its weight per unit volume.

Specific Gravity

Represented as SG. Defined as the ratio of the density of the fluid to the density of water at some specified temperature.

4

What specified temperature (in degrees celsius) of water is usually taken for specific gravity?

Pressure

For a fluid at rest, if defined as the normal force per unit area exerted on a plane surface in a fluid and is created by the bombardment of the surface with the fluid molecules.

Absolute Pressure

Pressure in a perfect vacuum. At sea level, it is 101325 Pa.

Gauge Pressure

Pressure with respect to atmospheric pressure. It is found by subtracting atmospheric pressure from absolute pressure.

Viscosity

A fluid’s resistance to flow or internal friction. It relates shearing stress and rate of shearing stress, as well as resistance to motion between layers of fluid.

No - Slip Condition

All fluids “stick” to solid boundaries.

Dynamic Viscosity

Represented as 𝜇. A fluid's internal resistance to flow and shear under an applied force.

Newtonian Fluid

Fluid in which the strain rate is linear with shearing stress.

Shear Thinning

A non-newtonian fluid behavior where viscosity decreases as applied force increases.

Shear Thickening

A non-newtonian fluid behavior where viscosity increases as applied force increases.

Liquid

For ______, Viscosity decreases when its temperature increases.

Gas

For ______, Viscosity increases when its temperature increases.

Kinematic Viscosity

Represented as 𝜈. A measure of a fluid’s inherent resistance to flow under the sole influence of gravity.

Bulk Modulus

A measure of a material’s or fluid’s resistance to uniform compression. A large unit represents incompressibility.

Isothermal Process

Temperature remains constant.

Isentropic Process

Frictionless and no heat exchange with surroundings.

Speed of Sound

Represented as c. Related to the changes in the pressure and density, as per the equation given below.

Mach Number

Represented as M. The ratio between speed and speed of sound.

Vapour Pressure

Represented as Pv. The pressure exerted by a gas in thermodynamic equilibrium with its condensed liquid phase in a closed system.

Boiling

Development of vapour bubbles in fluid. The point in which absolute pressure is equal to the vapour pressure.

Surface Tension

Represented as 𝜎. Unbalanced cohesive forces acting on the liquid molecules at the fluid surface. Is the intensity of molecular attraction per unit length along any line in the surface.

Capillary Action

Occurs when the attraction between the molecules and the walls is greater than the attraction between the molecules themselves.