The Navier-Stokes equations represent the flow of a fluid with friction. They consist of the following conservation equations:

 1. Conservation of mass (second line):

 div u=0

 2. Conservation of momentum (first line)

  −μΔu+(u⋅∇)u+∇p=f

 3. Conservation of energy (probably, in the third line, but I don't know the nomenclature):

 u∣Γ=0.