Lecture 1 - Fundamenatal concepts Flashcards
A fluid
A fluid is any substance that deforms continuously when subjected to a shear stress (tangential force per unit area) no matter how small the stress is.
Stress
Stress is defined as force per unit area [N/m2]
Normal stress
Normal stress σ is the normal component to the force acting on a surface per unit area. Normal stress is usually called pressure and N/m2 is also called Pascal (Pa).
Shear stress
Shear stress τ is the tangential component of a force acting on a surface per unit area.
Body forces
All external forces acting without physical contact on all particles of the system and proportional to their mass. Gravitational and magnetic forces are body forces.
Surface forces
All forces acting on a portion of the continuum system through its boundary surface. Pressure and viscosity are examples of surface forces.
For fluids at rest
The shear stress is zero and pressure is the normal stress.
The specific weight
The specific weight γ of a substance is its weight per unit volume [N/m3]
γ=ρ*g
The specific gravity
The specific gravity, SG, of a substance is the ratio of its weight to that of an equal volume of water at a specified temperature, usually 4°C
SG=p/pwater
Newtonian fluids
The rate of deformation is proportional to the shear stress
Non-newtonian fluids
The rate of deformation is not directly proportional to the shear stress.
No slip condition
Experimental observations have shown that a fluid in motion comes to a complete stop at contact with the surface and thus it has a zero velocity, relative to the surface. This is known as the no-slip condition. The no-slip condition shapes the velocity profile: the layer that sticks to the surface slows the velocity of the next adjunct fluid layer.
Incompressible fluids
An incompressible fluid is defined as the fluid whose volume or density does not change with pressure.
Compressible fluids
The property of volume change is called compressibility and a fluid whose volume changes is called compressible fluid
Viscosity
The viscosity of a fluid is a measure of its “resistance to deformation”. Viscosity is due to the internal frictional forces that develops between different layers of fluids as they are forces to move relative to each other. Viscosity is caused by the cohesive forces between the molecules in liquids and by the molecular collisions in gases, and it varies greatly with temperature.
