Dealing with flexural vibrations of rotors we have to face different problems. One is the "direct problem", in which the forces are known and in which the vibrational behaviour has to be calculated. This is called more generally "simulation". In this case the exciting causes are known and the model of the system has to be known. Then with the exciting causes and with the model of the system you can calculate the vibration.
Another way to deal with dynamical behaviour rotors is the "inverse problem", which is also the "diagnostics" of rotor system. In this case, the measured vibrations are known and from these vibrations we want to identify the exciting causes. This represents also the identification of the malfunction, or defect, which has caused the increasing in the vibration level. This is the real scope of diagnostics: starting from measured vibrations define the exciting causes.