During a cfd simulation the flow volume is divided into small cells (meshing). In each of this cells the navier stockes equations are getting solved and thus the velocity in x,y and z direction and the pressue calculated.
In a cross-section (tube,duct etc.) a minimum of 10 cells is needed to describe a flow.
Complex geometries are tending to need a lot of cells. 2 GB RAM are needed per one milion cells. The picture below shows the flow inside a reactor head of a mixer / heat-exchanger.
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For complex and large geometries it make sense to simulate the behavior of the flow inside the geometrie, before the production phase begins.