In HAMMER, what does this user notification mean? "Transition from CONCENTRATED to EXTENDED" or "Transition from EXTENDED TO CONCENTRATED"
This is related to the Extended CAV option for air valves. When using the "Extended CAV" calculation option, the program will automatically switch between the regular (concentrated/elastic) and Extended (rigid) based on the percentage of the adjacent pipe volume that the air pocket occupies. This is mainly because the Extended mode is more suited for sufficiently large volume of air entering and the flow regime evolves from hydraulic transients to mass oscillations. You will see notifications indicating when this occurs. You can see the specific timestep under Reports > Transient Analysis Reports > Transient analysis output log.
User Notification "Concentrated CAV model must be used..."
Modeling Reference - Air Valves [TN]
Does the Extended CAV option apply to vapor pockets in the system or only air?