FAST Integrator, step size & precision

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Aonghus.OConnor
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Re: FAST Integrator, step size & precision

Postby Aonghus.OConnor » Thu Oct 27, 2016 2:50 am

Hi all,

I'd like to follow up on Rupert's question about variable time-stepping to get clarification on the following:

Does the ABAM explicit integrator used in FAST insist that a fixed time-step be used?
In other words, is it possible (through modification to the source code) to change the time step of AeroDyn (or other modules for that matter) during a simulation and still expect a valid solution?

Thanks in advance,
Aonghus

Jason.Jonkman
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Re: FAST Integrator, step size & precision

Postby Jason.Jonkman » Thu Oct 27, 2016 6:18 am

Dear Aonghus,

Please note that Rupert's question (and my answer) applied to FAST v7, which was the version of FAST available at the time. The time-integration in FAST v8 differs a lot.

In FAST v8, each module with continuous states has its own time-integrator. The ElastoDyn module allows for explicit RK4, AB4, and ABM4; the BeamDyn modules uses an implicit generalized-alpha, etc. None of the modules currently use a variable time-step integrator, but this is certainly something that could be added in the future. One difficulty in this may be that the FAST driver (glue) code calls all modules at the same fixed (glue-code) time step. Currently, FAST allows modules to have time steps be integer time-step divisors of the glue-code time step, but does not allow for modules to have time steps (for time-integration) larger than the glue-code time step.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Aonghus.OConnor
Posts: 20
Joined: Tue Oct 11, 2016 2:14 am
Organization: self
Location: Ireland

Re: FAST Integrator, step size & precision

Postby Aonghus.OConnor » Thu Oct 27, 2016 8:11 am

Thanks Jason,

I can see it would be a difficulty with respect to ensuring that all FAST modules are called at the same glue code time step.

Apart from this though, do you think that if I were to increase or decrease the time step of the AeroDyn standalone driver mid-simulation I would still expect a valid solution? Or does the theory behind the AeroDyn integrator forbid changing the time step in this way?

Best wishes,

Aonghus

Jason.Jonkman
Posts: 4224
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: FAST Integrator, step size & precision

Postby Jason.Jonkman » Thu Oct 27, 2016 8:48 am

Dear Aonghus,

The unsteady airfoil aerodynamics model of AeroDyn v15 employs a discrete-time update, which requires a fixed time step. There are no other integrators in AeroDyn presently, so, I would think a time-varying time step would be OK when unsteady airfoil aerodynamics are disabled.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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