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, or discharge coefficient for subsonic flow.

• Sonic losses result when sonic choking occurs. Sonic losses are dependent on the minimum effective flow area (C_dA) and are unrelated to the K factor.

The two loss mechanisms cannot occur simultaneously in nature or in AFT xStream. During a time step if one mechanism is in use, the input data for the other mechanism is ignored by AFT xStream. The loss mechanism in use can switch during the transient as system operation changes.

It is also important to note that sonic choking at a restriction cannot be calculated if the CdA for Sonic Choking is not entered. In this case, AFT xStream will always use subsonic loss calculations if no CdA for Sonic Choking has been entered, even if the actual system would experience choked flow at that restriction. The only exception to this is if the restriction occurs at a valve which is defined in terms of Cv/Kv and XT, which can account for both subsonic and sonic loss at the valve.