Dear Yingyi Liu,
The integration to find the total blade and tower masses in ElastoDyn is quite simple. I’ll explain for the blade; the tower is similar. First, the mass distribution from the blade file (BMassDen) is linearly interpolated to the analysis nodes, which are located in the centers of the analysis elements. When using AeroDyn v14 (CompElast = 1), the blade discretization specified in AeroDyn v14 will be used for discretization of the blade structural model of ElastoDyn (in this case, input BldNodes in ElastoDyn is unused). When using AeroDyn v15 (CompElast = 2), ElastoDyn will use equally spaced elements/nodes (defined by BldNodes) and the the aerodynamic blade discretization is independent of the structural discretizations in ElastoDyn. ElastoDyn then uses a simple rectangular integration by summing the mass/length times the element length for all elements. The location of the blade analysis nodes, the element lengths, and the interpolated mass/length are written to the ElastoDyn summary file for convenience.
I hope that helps.
Best regards,