Fluid – A substance which is capable of flowing. A fluid is also a substance which has no permanent resistance to change in shape i.e. a solid can resist a shear stress, τ by static deflection; a fluid cannot, any shear stress applied to a fluid will result in the motion of that fluid for as long as the shear stress is applied. τ=FA Where F = force which is tangent to a surface (shear force), A = area of moving plate in which shear force is applied to.
Velocity gradient – is the change of velocity with distance. If applied shear force is changed, the shear stress is also changed, which results in a new velocity gradient being established. dvdy Boundary conditions of fluids in contact with a wall, the fluid velocity, v = 0.
Fluid Continuum – the variation in properties is so smooth that differential calculus can be used to analyse the substance. The physical problem is approached by treating the fluid as a continuous media and using the average effect of many molecules to solve the problems.
A fluid may be considered a continuum when there is a large volume of molecules and very little space between them, since fluids take the shape of the volume they are in.
Also, the distance between the molecules (mean free path) is very large compared with the molecular diameter.