numerical spikes in pitch moment

does anyone observes some numerical spikes in the pitching moments as shown in the plot?

It appears always when the turbine has already stopped.

This is a simulation for a small wind turbine. accordingly we have already a relatively small step size. Here the general solver settings in fst-file:
0.001 DT - Recommended module time step (s)
2 InterpOrder - Interpolation order for input/output time history (-) {1=linear, 2=quadratic}
0 NumCrctn - Number of correction iterations (-) {0=explicit calculation, i.e., no corrections}
99999 DT_UJac - Time between calls to get Jacobians (s)
1E+06 UJacSclFact - Scaling factor used in Jacobians (-)

If I further reduce the step size the spikes get smaller but the simulation time gets too long and obviously I don´t see the need to reduce when the turbine is in rotation.


morewind engineering solutions GmbH

Dear Florian,

I’m not sure; I’ve seen these spikes show in FAST simulations that make use of BeamDyn, but not ElastoDyn, which is what I’m assuming you’re using. Have you tried to disable the unsteady aerodynamics model within AeroDyn to see if the spikes are related to that?

Best regards,

Dear Jason,

Thank you for your prompt reply. I tested the same load case with Beddoes switched to steady in aerodyn and indeed it removes the spikes, see fig (blue with beddoes / red without beddoes).

Anyhow it changes also the behavior of the turbine in the normal operation part of the time serie. Hence I don´t want to switch beddoes off. Do you have any suggestion what else we could do to avoid the spikes?

By the way this is a small wind turbine (20m diameter) with very stiff blades. We therefore are not using Beamdyn here.

Best Regards,

morewind engineering solutions GmbH

Dear Florian,

I gather you are using FAST v8 with AeroDyn v14–is that correct?

It’s been a while since I looked at the BEDDOES option of AeroDyn v14, so, I’m not sure what would drive those spikes. Have you set the few unsteady-aerodynamics-related inputs available in the airfoil data files of AeroDyn v14 appropriately e.g. derived using AirfoilPrep? AeroDyn v15 offers many more inputs in the airfoil data input files that allow the user to tailor the unsteady airfoil hysteresis loops, but it takes a bit of experience to be able to set these inputs properly.

A quick fix in AeroDyn v14 may be to see if disabling the pitching-moment coefficient eliminates the problem (UseCm = “NO_CM”).

I hope that helps.

Best regards,