The fluid dynamic design of a new acoustic flow meter is shown in this work. The acoustic flow meter is a new addition to the ATLAS silicon tracker cooling system. ATLAS is one of the 7 experiments at the LHC particle collider at CERN. In this instrument, which can also operate with binary mixtures, ultrasonic sound waves are produced with special transducers, and sound transit times are continuously measured in opposite directions in flowing gas for continuous real-time measurement of both the gas binary mixture ratio and the flow rate. Three geometries of the flow meter were studied through CFD (Computational Fluid Dynamics) techniques: the pinched axial geometry, the open axial geometry and the angled geometry. The optimal configurations, in terms of both measurement reliability and pressure drop in the device were found, and the results are shown in this work. The angled geometry was chosen and, after a short prototyping phase, is now successfully operating.