The flow regime is laminar (the layers of the fluid don’t mix together), until the Reynolds number () reaches its critical value. The governing system () is ready to solve laminar type of flow. There are not too many real cases of laminar flows. On the other hand, solution of this flow cases provides us valuable information on fluid flow behavior. Laminar cases^21.7 are for example the flows through tubes or capillaries (Poiseuille flows), or the flow over flat plate, or between two parallel flat plates (Couette flows).