A passive heatsink simulation principles

Hi, guys. I have some problem simulating a passive heatsink in Flow Simulation. At first I used Solidworks Simulation module for thermal simulation of the heatsink then I compared the simulated results with real ones. They seemed to be close to each other. So then I used Flow Simulation to do the same thing, and the results proved to be far enough from reality. I think I did something wrong setting up my flow simulation project but still can not understand what exactly. Maybe someone could help me.

SolidworksFlow Simulation