In FEA, I have a complex system onto which I have applied some boundary conditions and then conducted a modal analysis for it. In return, I got a few mode shapes (as much as I requested) for each of the natural frequency from the FEA solver. By definition, a mode shape which is associated with a certain natural frequency is the deformed pattern of a system at which it will vibrate at that natural frequency. A system will have as number of natural frequencies (and thus the mode shapes) as the number of DOFs in the system (basically the total number of DOFs of all the nodes in my Finite Element model).
Now, I want to understand that how can I achieve this mode shape in reality? I mean a natural frequency by definition is the frequency at which the system will vibrate when subjected to disturbance, and without any action of continous force. So how should I disturb my system in order to make the structure vibrate at a specific natural frequency with a certain mode shape?
And in what way should the external excitation frequency be applied onto the structure in order to obtain a certain mode shape?
Plus, is it possible to get more than one mode shape for a specific natural frequency? Or is it possible to see more than one natural frequency for the same mode shape?