[MBDyn-users] FYI: Aerodynamics + BEM specific to Wind Turbines

Rix, Patrick Patrick.Rix at repower.de
Thu Aug 30 11:29:49 CEST 2012

Dear Pierangelo,

to contribute to our talking below about aerodynamic / aero-elastic calculation methods & Blade Element and Momentum (BEM) theory I wanted to point to some useful literature in which most of the aspects are explained being of special interest for the wind energy e.g. explanation of the general annular BEM method and of features for unsteady BEM like  'dynamic stall'  and  'dynamic wake' (which are popular terms in the field of aero-elastic simulation of wind tubines) being implemented in e.g  BLADED, FLEX5 and FAST(->AeroDyn) and which are widely in use in the wind industry (..no secret to mention that we use these tools at REpower, too).
As I am not sure if these terms are understood in the same way in helicopter theory with identical meaning I wanted to point you to some references in order to give you a better understanding of what exactly I mean when talking about dynamic stall, wake and so on.

As you programmed the AeroDyn interface you surely know all the docs on the NREL page <http://wind.nrel.gov/designcodes/simulators/> so the other ones below might be more interesting for you

'unofficial' FLEX5 - documentation

[1] Martin Otto Laver Hansen:  "AERODYNAMICS OF WIND TURBINES"
2nd.Edtion 2008, EarthScan London,
ISBN: 978-1-84407-438-9

In Chapter 9 of [1] about 'Unsteady BEM' , Hansen  describes from page 93 on the dynamic stall and dynamic wake as it is/was implemented in FLEX5 by its programmer Stig Oye. (Hansen & Oye work both at the 'DTU Institut for Vindenergi' at 'Denmarks Tekniske Universitet')

'unofficial' BLADED - documentation

[2] Tony Burton, David Sharpe, Nick Jenkins, Ervin Bossanyi,
2001 Ed.1, ISBN-13: 978-0471489979,
2011 Ed.2, ISBN-13: 978-0470699751,
John Wiley & Sons LTD

(Bossanyi is one of the programmers of BLADED.)

BLADED 3.5 User & Theory Manual



Bladed main page: <http://www.gl-garradhassan.com/en/GHBladed.php>

With kind regards

Patrick Rix
Loads & System Simulation

REpower Systems SE
Albert-Betz-Str. 1
D-24783 Osterrönfeld

Tel.: +49 - (0) - 4331 - 1313 - 9408
Fax: +49 - (0) - 4331 - 1313 - 9954

e-mail: p.rix at repower.de
Internet: www.repower.de

-----Ursprüngliche Nachricht-----
Von: Pierangelo Masarati [mailto:masarati at aero.polimi.it]
Gesendet: Dienstag, 28. August 2012 16:38
An: Rix, Patrick
Cc: Ascar, Philippe-Emmanuel
Betreff: Re: AW: AW: AW: contribution to "CART"-WindTurbine example model & AeroDyn-12.58 interface ==> our CART models

On 08/28/2012 11:45 AM, Rix, Patrick wrote:
> Hi Pierangelo,
> S809 - airfoil data: -------------------- I do not remember exactly,
> but the S809 airfoil data are taken either from the example input
> data subdirectory 'CertTest\AeroData\UAE_VI' contained in NREL's
> FAST-source code package at
> <http://wind.nrel.gov/designcodes/simulators/fast/> or from NREL's
> airfoil library page at <http://wind.nrel.gov/airfoils/> where you
> can find * the shapes
> <http://wind.nrel.gov/airfoils/Shapes/S809_Shape.html> * and the
> coefficients <http://wind.nrel.gov/airfoils/Coefficients/> I think we
> modified the data by resampling re-interpolation or so. In any case
> the data are public (downloaded from the internet) and not
> confidential - so feel free to add them.


> Good to hear that you could run both models ! We discovered the same
> diverging behaviour as you showed in your plot when Philippe ran the
> two cases. Philippe said, he also had numerical problems when running
> the naïve solver but we did not test umfpack. Good news that at least
> umfpack can handle this problem.
> My explanation of the divergence is that the implementations of the
> rotor aerodynamic are ticking slightly different between MBDyn's
> built-in version of the BEM and the one of AeroDyn. Apart from the
> different treatment of a number of wind turbine specific
> gimmicks/features such as tip and hub losses (which were disabled in
> the model) we also found that the calculation of axial induction
> factors (tangential ind.fac. was turned off in the example) seems to
> behave really different between MBDyn and AeroDyn: While AeroDyn
> seems to treat the different BEM-rings of the rotor disc
> independently (i.e. the axial ind.facs can vary significantly along
> the blade span) MBDyn seems to 'balance' the axial ind.facs over the
> rotor disc trying to find a sort of unique 'average' value used for
> all rings - is this the intentional behaviour of MBDyn ? I assume
> this is related to very fast spinning rotors such as for helicopters.
> We played around with MBDyn's different BEM flavors available in the
> induced velocity element but all models very quite similar to each
> other and we found no model showing the same sort of independency
> between the ax.ind.facs as is the case in AeroDyn.

Yes, all induced velocity models in MBDyn treat induced flow as constant
plus sine and cosine terms, which are only meaningful in case of yawed
flow.  This is common practice in helicopter analysis.

> Which of MBDyn's BEM-model would you recommend to get an independent
> treatment of the BEM-rings ?

None, currently.

> BTW: ..could the output of the axial induction factor be added to the
> output of MBDyn's aero-elements (*.aer file) ? I think it's more
> informative than the Mach number and having the ax.ind.fac. value in
> the output file would make comparisons / validation much easier as
> one doesn't have to calculate it from Mach number, sound celerity and
> angle of attack.

Right now, you get that info in column 8 of the ".rot" file, which
contains the constant inflow velocity.  No linear contribution is
available in the output so far.

> I really would like to use MBDyn's aerodynamic (as it contributes to
> the Jacobioan matrix as fas as I understood) with similar wind
> turbine specific aero-features as AeroDyn offers (such as tip and hub
> losses, an optional tangential ind.fac., a dynamic stall model and a
> dynamic wake model [if not already covered by MBDyn's Theodorsen
> function] ).

No, Theodorsen only considers unsteady circulation.

> I think these features would help a lot for wind turbine
> simulations when using MBDyn's built-in aerodynamic which is quite
> comfortable for describing heavily tapered and twisted rotor blades
> as we have on our wind turbines.
> Do you think it would be a good idea to add such a sort of 'wind
> turbine' specific model to the list of available models in MBDyn's
> induced velocity element ?

Well, a wind-turbine specific, annular inflow model could be useful.  I
think it could easily fit into the current general scheme of induced
velocity models.  Note that an induced velocity model could even be
implemented as a run-time loadable module.  I have developed a simple
example that produces the equivalent of the uniform inflow in a slightly
different manner.

More on my reply to your subsequent message.

Cheers, p.

Pierangelo Masarati
Associate Professor
Dipartimento di Ingegneria Aerospaziale
Politecnico di Milano

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