Transient DDES computations of the NREL Phase-VI rotor in ... · Transient DDES computations of the NREL Phase-VI rotor in axial ﬂow conditions Niels N. Sørensen1 and Scott Schreck2

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

Users may download and print one copy of any publication from the public portal for the purpose of private study or research.

You may not further distribute the material or use it for any profit-making activity or commercial gain

You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Downloaded from orbit.dtu.dk on: Apr 19, 2020

Transient DDES computations of the NREL Phase-VI rotor in axial flow conditions

Sørensen, Niels N.; Schreck, Scott

Publication date:2012

Link back to DTU Orbit

Citation (APA):Sørensen, N. N. (Author), & Schreck, S. (Author). (2012). Transient DDES computations of the NREL Phase-VIrotor in axial flow conditions. Sound/Visual production (digital), Retrieved fromhttp://www.forwind.de/makingtorque

https://orbit.dtu.dk/en/publications/625b2d9c-63e5-4356-8978-0c636d0f8eff

http://www.forwind.de/makingtorque

Transient DDES computations of the NREL Phase-VI rotorin axial flow conditions

Niels N. Sørensen 1 and Scott Schreck 2

1 Wind Energy Department · DTU, Risø Campus, DK2NREL’s National Wind Technology Center, Golden, CO 80401, US.

IntroductionBackgroundThe NREL Phase-VI measurements:

The NREL/NASA Ames Phase-VI measurements include time resolvedsignals

For CFD validation main focus have been on mean loads

The data are well suited for investigating unsteady aerodynamicsWe want to test the Transitional DDES approach against measurements andRANS predictions:

With respect to mean quantities (LSSTQ, pressure)

Unsteady loading (Cthrust)

Improved physical understanding (Shedding frequency)

2 of 18 N. N. Sørensen, DTU and S. Schreck, NREL Transient DDES computations

Flow ModelingMethodology

Measurements:

S-series from the Unsteady Aerodynamics Experiment Phase VI

Upwind configuration zero cone and zero yaw angle

Rotor Diameter [m] RPM Blade Pitch [deg] Wind Speed [m/s]10.058 71.9 3 (10,12,13,15,20)

Flow Solver:

DDES turbulence modelling

Correlation based laminar/turbulent transition modelling


Flow ModelingIn-house flow solver, EllipSys3D.

Incompressible Navier-Stokes equations

Rotation enforced through a moving grid option (ready to do dynamicstall)

Turbulence is modelled by DDES model

Transition modeling, γ − Reθ correlation based model

Second order accurate in times

Convective terms is modelled by QUICK + CDS4

Time-step [1000-2000] per revolution, with 12-6 sub-iterations

The computations are accelerated by using a three level grid sequence

6-8 hours per revolution using 136 CPU’s


Flow ModelingTransitional k − ω model DDES

Eddy viscosity

µt =ρa1k

max(a1ω;F2Ω)

Transport equation for turbulent kinetic

ρ∂k∂t

+ ρUj∂k∂xj

= τij∂Ui

∂xjγeff − ρβ

∗kωFDDESΓ +∂

∂xj

[(µ+

µt

σk

)∂k∂xj

]

Transport equation for the specific dissipation rate

ρ∂ω

∂t+ρUj

∂ω

∂xj=

γ

νtτij∂Ui

∂xj−βρω

2+2ρ(1−F1)1

σω2ω

∂k∂xj

∂ω

∂xj+

∂

∂xj

[(µ+

µt

σω

)∂ω

∂xj

]


Flow ModelingDES and DDES modelingThe idea behind the DES modeling is to exchange the turbulent length scalewith the grid size when the turbulent length scale become larger than the gridsize, and the grid is capable of resolving some of the scales.

The turbulent length scale in the k − ω model is given by

Lk−ωt =

k32

ǫ=

√k

β∗ωusing ǫ = β

∗kω

The turbulent length scale in a LES would be

LLESt = ∆CDes with ∆ = max [∆x ,∆y ,∆z]

To enforce the LLESt when the grid allows, we will simply scale the

dissipation term in the k equation by the ratio between Lk−ωt and LLES

t .

FDDES = max

(Lk−ω

t

LDESt

(1 − FShield); 1

)


Flow ModelingThe γ − Reθ Correlation based laminar/turbulent transitionmodel

The model uses two transport equations, one for the intermittency γ and onefor the transition onset momentum thickness Reynolds number Reθt

∂(ργ)

∂t+

∂(ρUjγ)

∂xj= Pγ − Eγ +

∂

∂xj

[(µ+

µt

σf

)∂γ

∂xj

].

∂(ρReθt )

∂t+

∂(ρUj Reθt )

∂xj= Pθt +

∂

∂xj

[σθt (µ+ µt)

∂Reθt

∂xj

].

Leaving out some details the model delivers an effective intermittency γeff inevery point of the domain, including natural transition, by-pass transition andseparation induced transition.

Γ = min (max(γeff , 0.1),1.0)


Domain and gridComputational Grid

The domain is 20 rotor diameters in diameter

Chord-wise 256, Span-wise 256


Domain and gridComputational Grid

O-O-Topology of 136 blocks of 643∼ 36 Million points

The wall normal y+ is less than two on the blade surface


Results, The influence of transitionLow Speed Shaft Torque

0

500

1000

1500

2000

5 10 15 20 25 30

LSS

TQ

[N

m]

Wind Speed [m/s]

MeasuredComputed, Low TuComputed, High Tu

10 20 30 40 50 60 70 80 90

100 110 120

1.5 2 2.5 3 3.5 4 4.5 5

Dri

ving

For

ce [

N/m

]

Radius [m]

W=12 [m/s]

High Turb.Low Turb

For the power curve the effect of by-pass transition mainly plays a rolearound onset of stall

Locally on the blade the impact of transition is mainly seen at the tipwhere we have the lowest AOA’s


Results, Comparison between RANS and DDESPressure distributions, w=15 m/s

-1.5-1

-0.5 0

0.5 1

1.5 2

2.5 3

3.5 4

0 0.2 0.4 0.6 0.8 1

-Cp

x/chord

r/R=0.30

MeasDDESRANS

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1

-Cp

x/chord

r/R=0.63

MeasDDESRANS

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1

-Cp

x/chord

r/R=0.47

MeasDDESRANS

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1

-Cp

x/chord

r/R=0.80

MeasDDESRANS


Results, Comparison between RANS and DDESTime traces of the loads

1.9

2

2.1

2.2

2.3

2.4

2.5

2.6

2.7

1 1.2 1.4 1.6 1.8 2

Cth

rust

Time [s]

r/R=0.30

MEASDDESRANS

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1 1.2 1.4 1.6 1.8 2

Cth

rust

Time [s]

r/R=0.47

MEASDDESRANS


Results, Comparison between RANS and DDESWake at 15 m/s

Geometry RANS DDES


Results, Comparison between RANS and DDESPower Spectral Density based on Cthrust

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]

NREL Phase VI, w=15[m/s], r/R=0.30

Meas.DDESRANS

1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]


Meas.DDESRANS

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]


Meas.DDESRANS

1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]


Meas.DDESRANS


Results, PSD of CthrustPSD of the Thrust Coefficient, 12 m/s

1e-10

1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]

Nrel Phase-VI, w=12 [m/s]

MeasuredComp. Blade-1Comp. Blade-2

1e-10

1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]



1e-10

1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]



1e-09

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]




Results, PSD of CthrustPSD of the Thrust Coefficient, 20 m/s

1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]



1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]



1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]



1e-08

1e-07

1e-06

1e-05

0.0001

0.001

0.01

0.1 1 10 100 1000

Psd(

Cth

rust

)

Frequency [Hz]




Results, physics of the sheddingShedding at 15 m/s


Results, physics of the sheddingStrouhal Numbers and Frequencies, w=15 m/s

The Strouhal number can be defined by:

St =f c sinα

Ueff

and α is the geometrical AOA for simplicity.

Radius Ueff α [deg] f [1/s] St0.30 18.8 38.5 7.8 0.180.47 23.3 35.5 7.4 0.120.63 28.2 31.0 7.6 0.080.80 33.8 26.7 7.8 0.05


ConclusionConclusions

We have shown that

DDES do not improve the power prediction compared to RANSpredictions

DDES can predict the energy contents of the unsteady flow much betterthan RANS

DDES fails to capture the high frequency fluctuations

DDES can capture the low frequency shedding along the blade in caseswith flow separation

The shedding frequency for the NREL Phase-VI blade seem to becontrolled by the root flow

The by-pass transition process can heavily influence the stallingbehaviour


Transient DDES computations of the NREL Phase-VI rotor in ... · Transient DDES computations of the NREL Phase-VI rotor in axial ﬂow conditions Niels N. Sørensen1 and Scott Schreck2

Documents