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Make sure air is selected under Fluid Materials. Set Density to ideal-gas and make sure Cp is constant and equal to 1006.43 j/kg-k. Also make sure the Molecular Weight is constant and equal to 28.966 kg/kgmol. Selecting the ideal gas option means that FLUENT will use the ideal-gas equation of state to relate density to the static pressure and temperature.
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Click Change/Create.
Define > Operating Conditions
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