In what follows, we restrict ourselves to flows where body forces are neglected (neither a heavy fluid nor an electrical conductor). If the motion is steady and (S) is a closed surface enclosing a finite volume (ϑ) that does not contain a body, then we write the equation of motion of section 4.4 as:
where is the tension vector representing the contact forces exerted by the fluid contained in (S) on the external medium (hence the minus sign to the right-hand side). This relation can also be written as:
The dynalpy vector is given by:
The relation above shows the following result:
which is read as:
In a steady flow, and in the absence of body forces, the flux of the dynalpy vector through a closed surface not containing any body is zero.
Let us consider an impermeable body (no mass exchange between the body and the external medium) placed in a flow and apply the above result to the fluid contained ...