In fluid dynamics, stream thrust averaging is a process used to convert three-dimensional flow through a duct into one-dimensional uniform flow.  It makes the assumptions that the flow is mixed adiabatically and without friction.  However, due to the mixing process, there is a net increase in the entropy of the system.  Although there is an increase in entropy, the stream thrust averaged values are more representative of the flow than a simple average as a simple average would violate the second Law of Thermodynamics.