Substituting Internal Forces for Blocked Forces or Free Interface Displacements in Substructured Simulations

Abstract

An efficient method for computing responses of large and complex structures is Dynamic Substructuring. In Dynamic Substructuring methods one decomposes the large and complex structure in several substructures, which are individually modeled. The obtained knowledge of individual subsystems, allows one to create an efficient and compact model of the global system. When the forces can not be measured in the substructure from where the excitation originates, it can however be characterized on the interface either as a blocked force or a free interface motion. In this contribution the idea of blocked forces and equivalent free interface displacement, originally developed in the frequency domain, is extended to the time domain. The method transforms internal force and displacement excitation of linear substructures, into a set of equivalent forces or displacements on the boundary degrees of freedom. The method has a number of advantages; Firstly, it simplifies the reduction of force loaded structures, as it requires no special treatment for the forces acting on the internal parts of the structure. Also, it allows for groups working on different components to couple the dynamics by only sharing the interface excitations and (reduced) component models. Finally, it should be noted that using time-domain approaches compared tot he previously published frequency-domain approaches for equivalent forces and displacements, allows also considering coupling with non-linear receiving substructures. It will be analytically proven that for all other substructures the exact same response will be found. In addition it will be shown how to recover the correct response for the force loaded substructure in the case the internal forces are known. The methods will be demonstrated using the model of an offshore wind turbine that is loaded with (linearized) wave forces on its support structure.