Fluid Mechanics L3 - Problem Solving

C. Upper plate stationary, lower plate moving left

And what is Patm?

Left: free surface (exposed to air); Patm = 0

System (Sys)

A specific quantity of matter or fluid

Surroundings

Everything external to the system

Boundary

The real or imaginary surface that separates the system from its surroundings

Closed system or control mass characteristics

• Energy can cross the boundary

• Volume doesn’t have to be fixed

• Isolated system

Open system or control volume (CV)

• Control surface

• Good for studying flow through a device

• Mass and energy can cross the boundary

Dimensions

Physical quantities that can be measured

What surround a symbol to mean “the dimension” of a unit?

Braces { }

Primary of fundamental dimensions

Mass, length, timeS

Secondary or derived dimensions

Speed, force, stress

Primary SI system units

kg, m, s

Secondary SI system units

1 N = 1kg * 1m/s²
1 kgf = 9.807 N

Primary English or BG system units

lbm (1 slug = 32.174 lbm), ft, s

Secondary English or BG system units

1 lbf = 1 slug * 1 ft/s²

What is weight and what is its formula?

A force; W = mg

Values of g or gravitational acceleration

9.807 m/s²
32.174 ft/s²

Specific weight

Weight per unit volume of a substance/fluid

Specific weight formula

λ = ρg

Dimensionally homogeneous

Every term in an equation must have the same dimensions

What are the dimensions of Reynold’s Number

None; dimensionless

Drag force (Fd) units

kg*m/s²

Density (ρ) units

kg/m²

Accuracy error

Repeatable errors

Precision error

Variety in measurements