[MBDyn-users] Placing the middle node of a 3 node beam -> for using AeroDyn

Pierangelo Masarati masarati at aero.polimi.it
Wed Feb 23 15:09:23 CET 2011

Jens.vanSchelve at Emerson.com wrote:
> Ok, I asked that because I have difficulties using the aerodyn-module.
> This is what I want to have (like mbdyn aero3 beam), with 1-5 beeing nodes 
> connected by beam3 elements and A-D beeing AeroDyn elements.
>        __A__ __B__ __C__ __D__
>  ---  |     |     |     |     |
> (HUB) 1-----2-----3-----4-----5
>  ---  |_____|_____|_____|_____|
> But NRELs AeroDyn expects the position and velocity in the middle of the element.
> So if I connect element A to node 2, B to 3 and so on the position and velocities
> of node 2 are passed to element A. This means the velocity of element A is too high.
> Same with the forces: AeroDyn returns the force in the middle of the element, but
> if I connenct A to node 2 the force is applied exactly at node 2 - so I have too
> big blade root moments.
> The conclusion is what I get, but not want, is a model that one can imagine like this:
>           __A__ __B__ __C__ __D__
>  ---     |     |     |     |     |
> (HUB) 1-----2-----3-----4-----5  |
>  ---     |_____|_____|_____|_____|
> So something that is similar to a too long aerodynamic-blade. I did a lot of comparisons
> With NRELs FAST and in fact if I increase the blade and hub size in FAST by half the
> element size, MBDyn-AeroDyn and FAST match very good. Of course if I increase the
> number of beam- and AeroDyn elements in MBDyn the error decreases, but it is still
> not a nice solution.
> So there are two better solutions:
> 1: Change the mbdyn-aerodyn-module to connect two nodes to one AeroDyn element,
>    calculate the mean position and velocity and apply to each node half of the
>    force AeroDyn returns or
> 2. Place the nodes so that one can attach AeroDyn elements without any error, like:
>        __A__ __B__ __C__ __D__ __E__
>  ---  |     |     |     |     |     |
> (HUB) 1--2-----3-----4-----5-----6--7
>  ---  |_____|_____|_____|_____|_____|
> In this case one can attach AeroDyn element A to node 2, B to 3.. and E to 6.
> Node 1 (root) and 7 (tip) do not have an AeroDyn element attached, but are
> still inside the aerodynamic area, the forces are just summed into node 2 and 6
> which is ok in my opinion.
> For the beams you can have three beam3 elements (1-3,3-5,5-7) but node 2 and
> node 6 are not in the midle of the beam or another option would be to have
> two beam2 elements (1-2,5-7) and two beam3 (2-4,4-6). But as far as I understand
> beam2 elements are also not that accurate.
> In a third option one can also introduce additional two nodes, one between node
> 1 and 2 and one between 6 and 7 and than use beam3 elements there instead of beam2
> but also those nodes would not have any aerodynamic force applied - not sooo nice. 
> Does anyone have some other, better options in mind?

In my opinion, you should decouple the structural and the aerodynamic 
models.  You need to place structural nodes where AeroDyn expects them, 
and connect them by beam elements.  The last half aerodynamic patch from 
the last node to the blade tip will have no structural modeling, but 
that's not an issue, as soon as you model inertia correctly (the 
flexibility of the extremity should not be an issue).

Reworking to interface module to pass AeroDyn the average velocity 
between two nodes and split the resulting forces accordingly sounds a 
bit too intrusive and I do not expect significant improvements from it. 
  I admit it could have been a valid design choice from the beginning, 
although more complex than the one we chose.

Cheers, p.

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