2. Machine/system transient couplings

To reach a permanent operating state, machines go through transient regimes that result in variations in flow and pressure throughout the system. During these transients, the observed flow and pressure variations can be very different from what can be predicted by simply imagining the transient period as a succession of permanent states on average.

Transients are directly associated with changes in machine and/or system characteristics:

• for the machine, this concerns variations in rotation speed (during start-up or shutdown, for example) and/or variations in some of its geometric characteristics (variations in the pitch of the rotor blades and/or the diffuser and/or the distributor);

• for the system, this mainly concerns valve opening and closing operations.

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