In the field of vibrations of complex structures, energy methods like SEA and a series of mid-frequency methods, represent an important resource for computational analysis. All these methods are based in general on a linear formulation of the elastic problem. However, when nonlinearities are present, for example related to clearance or stiffening of joints, these methods, in principle, cannot be applied.
Entropy and therefore the second principal of thermodynamics has been proposed originally from the Sapienza group to complete energy methods in vibroacoustics. The idea is to add entropy beside energy in making models for complex mechanical systems, especially in the field of high frequency vibrations. In fact, energy is naturally involved in the first law of thermodynamics (the energy conservation principle), but it does not involve any flow directions and "energy flow speed" information, governed indeed by inequality principles. However, this information are crucial.
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