Which time averaging technics are used for unsteady solutions and where are the differences to a steady calculation?

If somebody can give a comprehensive answer, that would be great.

Thankful, CFD-NewBe

RANS models are for the time averaging steady solution. For unsteady solution the equation will be changed to Unsteady-RANS (URANS) by participating the time derivative. The techniques of RANS and URANS can be different, related to the simulation approach. It is possible to contribute the time derivative for RANS and reach the steady solution by time marching. But in this case the simulation becomes slower. It is the case many programmers ignore the time derivative and solve it iteratively. If you consider the time derivative, the phenomena, and also the geometry will determine your final solution, either steady or unsteady. If the final solution is steady, you will have a RANS solution. But it is not always possible to have an steady solution. For example the vortex shedding behind a cylinder is always unsteady, and the solution must be done by considering the time term.