There are two separate pressure loss mechanisms when gas flows through a restriction, expansion, or end point.

• Subsonic losses are standard "form" losses and are dependent on the geometry of the restriction, expansion, or end point. This is the common K-factor, discharge coefficient for subsonic flow, or one of the other loss models discussed in the Junctions section.

• Sonic losses result when sonic choking occurs. Sonic losses are dependent on the minimum effective flow area and are unrelated to the K-factor. This is the C_dA for sonic choking.

The two loss mechanisms cannot occur simultaneously in nature or in AFT Arrow. If one mechanism occurs during the solution, the input data for the other mechanism is ignored by AFT Arrow.

It is also important to note that sonic choking at a restriction cannot be calculated if the C_dA for Sonic Choking is not entered. Thus, Arrow will always use subsonic loss calculations if no C_dA for Sonic Choking has been entered, even if the actual system would experience choked flow at that restriction.

Note: The C_dA for sonic choking may be different from the subsonic C_dA loss model option in Arrow. The discharge coefficient can vary at different pressure ratios due to the vena contracta moving closer to or farther from the orifice restriction. For the highest accuracy the C_dA used for subsonic and sonic losses should be tested and entered separately. See the "Modeling Choked Flow Through an Orifice" white paper on AFT's website for more information.