In my understanding, blade is modeled using assumed mode method in FAST and BEM theory is used to analyze the aerodynamics. BEM is a strip theory whereby a blade is discretized into elements/nodes where the aerodynamic loads are calculated. To build a wind turbine rotor/blade model, we need both structural and aerodynamic part. Therefore the structural part of blade needs to be discretized into same elements/nodes as BEM does. But assumed mode method is a continuous method, how can we discretize a blade using this method?
Just to be clear, the blade structural model in FAST v7 and in the ElastoDyn module of FAST v8 uses the user-specified mode shapes as shape functions in geometrically nonlinear beam model. More information on the FAST theory basis is provided in the following forum topic: http://forums.nrel.gov/t/coupled-blade-modes-in-fast/314/1.
In FAST v7 and in the ElastoDyn module of FAST v8 currently, the blade structural model is discretized using the same discretization defined for blade aerodynamics within AeroDyn (see user inputs BldNodes, RNodes, and DRNodes in the AeroDyn primary input file). The structural discretization is used to compute the generalized stiffness matrix for elastic loads and the generalized mass matrix for inertial loads.
I am trying to compare a FAST7 model to a FAST8 model, after I have generated the FAST8 model out of the FAST7 model, just to be shure they behave the same way.
Using AeroDyn14 this is working very well, but changing to AeroDyn15 blade mass, inertia and center of gravity differ significantly in the *.sum-file.
Reading the ElastoDyn source code I can see clearly, that this is because of the different assumtions about mass distribution.
With AeroDyn14 the RNodes from the *_AD.inp are used to calculate DRNodes and with this values the blade element mass is determined
whereas AeroDyn15 assumes equal distribution of the blade elements as ElastoDyn is not reading the new *_AD_blade.dat.
Is there any possibility to quickly adapt the souce code to have the same mass distribution with AeroDyn15 as with AeroDyn14?
There is no easy way to fix this problem in the source code. Instead, I would correct the blade mass by changing the discretization (BldNodes) in the ElastoDyn primary input file, or the mass distribution (BMassDen) or mass scaling factor (AdjBlMs) in the ElastoDyn blade file.