simultaneous use of wind tunnel testing and cfd the...

Suzlon Energy Ltd.

1 Suzlon wind farm in Utah, USA

Simultaneous use of wind tunnel testing and CFD – The numerical wind tunnelC. Mau, G. Tescione, O. UzolSuzlon Blade Science Center

Suzlon Energy Ltd.Outline

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• Motivation for simultaneous use of WT and CFD

• Initial wind tunnel CFD models

• Wall interference

• Wind tunnel CFD framework

Suzlon Energy Ltd.Motivation for simultaneous use of WT and CFD

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• Wind tunnel test campaign of very thick airfoils:• Validation of numerical tools or at least get an idea how much they are off• Use of wind tunnel polars within design process

• What is a very thick airfoil?


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• Wind tunnel test campaign of very thick airfoils:• Validation of numerical tools or at least get an idea how much they are off• Use of wind tunnel polars within design process

• What is a very thick airfoil?

Risø C2 extrapolated – t/c = 42% Risø C2 extrapolated – t/c = 59%


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1.8 m 2.74 m

1.2

5 m

TU Delft LTT cross section DWG wind tunnel cross section

Wind tunnels and test conditions :

• t/c = 42% @ TU Delft - Low turbulence wind tunnel (LTT)• t/c = 59% @ Deutsche WindGuard - Large wind tunnel in

Bremerhaven (DWG)• Re ≤ 3e6 - Chord = 0.5 m• High Re need -> conflicts with low blockage/high AR


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• Not much information available in open literature• High blockage• Flow starts to separated @ low AoA for t/c = 59%• No experience with very thick airfoil testing• High expenses for wind tunnel tests• High uncertainty in validity of the test results

Risk mitigation

1. Step -> Run wind tunnel + airfoil in CFD to check what the flow will look like

Suzlon Energy Ltd.Initial wind tunnel CFD models - setup

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CFD model of t/c = 59% @ DWG

CFD model of t/c = 42% @ LTT

Model settings:

• RANS • incompressible• SST turbulence model• γ-Reθ transition model

Boundary conditions:

• Velocity inlet – Re = 3e6• Pressure outlet• Symmetry BC• No slip smooth wall

Suzlon Energy Ltd.Initial wind tunnel CFD models – First results

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t/c = 59% - AoA = 15 deg @ DWG – Wall shear stress mag. - WT wall highlighted grey

• Large LE radius leads to corner separation due to wall interference

• Corner separation vortex covers large or total area of the airfoil SS

• High uncertainty in polars extracted from WT tests can be expected

t/c = 42% - AoA = 10 deg @ LTT – Wall shear stress mag. - WT wall highlighted grey

Suzlon Energy Ltd.Wall interference: The horseshoe vortex

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Flow around buidingsMartinuzzi, R., Tropea, C.; JFE 1993

Scour of cylindrical pylons in water Das et al., 2013

• The adverse-pressure gradient due to the stagnation on the model leading-edge induces separation in the wind tunnel wall boundary layer, resulting in a vortex system around the airfoilGas turbine blade cascades

Goldstein and Spores, 1988

Fuselage wing junction Boermans et al. 1998

Suzlon Energy Ltd.Wall interference: Remedies

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• Increase model aspect ratio

• Splitter plates

• Boundary layer blowing

• LE strakes

• Boundary layer fences

• Corner VGs

t/c = 59% - AoA = 15 deg @ DWG, Lambda2 criterion

Suzlon Energy Ltd.Wall interference: Remedies - Increase aspect ratio

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1.2

5 m

TU Delft LTT cross section

• Increase airfoil model aspect ratio:

• WT dimensions fixed • reduce chord would give too low Re

Suzlon Energy Ltd.Wall interference: Remedies - Splitter plates

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airfoil

Splitter plate

Win

d t

un

nel w

all

• Splitter plates : • Less developed boundary layer, reduction vortex system around the LE • Confines horseshoe vortex between wind tunnel wall and splitter plate• Quite extensive design task -> not doable within time frame• Additional wind tunnel correction needed

Suzlon Energy Ltd.Wall interference: Remedies - Boundary layer blowing

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• Boundary layer blowing: • Energize BL by injecting additional flow parallel to the

wind tunnel wall and suppress or reduce separation due to horseshoe vortex

• Available @ DWG wind tunnel• Max. blowing ratio (unozzle/u0) of 1.2 -> gives no

improvement• Airfoil is too thick and intersects with possible location

of blowing nozzles

t/c = 59% - AoA = 15 deg with tangential blowing @ DWG top: Lambda2 criterion + Nozzle streamlines bottom left: SS wall shear stress mag. without blowingbottom right: SS wall shear stress mag. with blowing

Suzlon Energy Ltd.Wall interference: Remedies - Corner VGs

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Corner VGs:

• Energize BL close to wall junction and keep the flow attached• Standard method -> also used for semi thick airfoils• Easy to apply and to test during the measurement campaign

t/c = 59% @ DWG with corner VGs

Suzlon Energy Ltd.Wall interference: Remedies - LE strake

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Devenport et al., 1992

LE strakes:

• relieves the adverse-pressure gradient on the wind tunnel wall boundary layer

• effectively decrease gradually the curvature radius of the model LE• different design approaches• might be optimize for AoA• rapid prototyping (3D printed)

Suzlon Energy Ltd.

15Pressure probes location Transition line Separation line

Wall interference: Remedies – LE strake @ t/c = 59%

t/c = 59% - AoA =15 deg @ DWG without LE strake SS wall shear stress mag.

t/c = 59% - AoA =15 deg @ DWG with LE strake SS wall shear stress mag.

Suzlon Energy Ltd.

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t/c = 59% - AoA =15 deg @ DWG Raw IR-camera image - SS from wind tunnel test with strake

Airfoil

Pressure probes location Transition line Separation line

• Strake straightens out transition line

• Strake reduces wash down on the SS and gives a more straight seperation line around the midspan

• Good agreement of the transition and separation line between experiment and CFD

t/c = 59% - AoA =15 deg @ DWG without LE strake SS wall shear stress mag.

t/c = 59% - AoA =15 deg @ DWG with LE strake SS wall shear stress mag.


Suzlon Energy Ltd.

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w/o strakes and vorner VGs (left):• Corner vortices and separation on a ”W” line• Curved transition line• On the mid-span (PP location) separation is

delayed

With strakes and corner VGs (right):• Corner vortices suppressed• Straight transition line• Separation is more uniform (but it starts on the

mid-span)

Wall interference: Remedies – LE strake & corner VGs @ t/c = 59%

t/c = 59% - AoA =15 deg @ DWG Raw IR-camera image - SS from wind tunnel test without strake and corner VGs

t/c = 59% - AoA =15 deg @ DWG Raw IR-camera image - SS from wind tunnel test with strake and corner VGs

Suzlon Energy Ltd.

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t/c = 42% - AoA =10 deg @ LTT Raw IR-camera image - SS from wind tunnel test with strake


• Different design approaches for the LE strake where also tested in CFD for the t/c = 42 % airfoil

• LE strake gave no improvement in CFD and it was also confirmed during the WT test

• LE strake was no option for t/c = 42 % airfoil

• Another solution had to be found

t/c = 42% - AoA =10 deg @ LTT with LE strake SS wall shear stress mag.


t/c = 42% - AoA =14 deg @ LTT without LE strake SS wall shear stress mag.

Suzlon Energy Ltd.

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Wall interference: Remedies - Boundary layer fences

Boundary layer fences:

• Does not prevent HS vortex but it confines it• Prevent the HS vortex to travel spanwise causing

the corner vortices• different design approaches• works for several AoA until limit• rapid prototyping (laser cut)

Suzlon Energy Ltd.

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• Corner vortex is caught between wind tunnel wall and fence

• transition line is straightened out

• Suppresses wash down on the SS

• Separation is more uniform (but it starts on the mid-span)


t/c = 42% - AoA =14 deg @ LTT without LE strake SS wall shear stress mag.

t/c = 42% - AoA =14 deg @ LTT with LE strake SS wall shear stress mag.

Suzlon Energy Ltd.

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• Corner vortex is caught between wind tunnel wall and fence

• transition line is straightened out

• Suppresses wash down on the SS

• Separation is more uniform (but it starts on the mid-span)


t/c = 42% - AoA =14 deg @ LTT Raw IR-camera image - SS from wind tunnel test without fence

t/c = 42% - AoA =14 deg @ LTT Raw IR-camera image - SS from wind tunnel test with fence

Suzlon Energy Ltd.

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Upfront CFD simulations of the wind tunnel:

• identified potential risks before the test campaign• were used to find solutions, which extend the validity and quality of

the tests• saved time in the wind tunnel for the actual test campaign

Wall interference remedies:

• LE strakes in combination with corner VGs on a 59% airfoil succeded in reducing the horseshoe vortex and improving spanwise homogeneity of transition and separation lines

• BL fences on a 42% airfoil succeded in bounding the horseshoe vortex preventing corner separations and improving spanwise homogeneity

Wall interference: Remedies - conclusions

Suzlon Energy Ltd.Wind tunnel CFD framework

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• Development of a ”push button” wind tunnel CFD framework • Used for 1 to 1 comparison to test results after the wind tunnel test campaign is completed• Tune and validate CFD models and setup

Suzlon Energy Ltd.Wind tunnel CFD framework - Input

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Input:

• Geometry: airfoil shape, wind tunnel geometry (LTT, DWG, LM, Poul la Cour) or 2.5d• Test conditions: Re, T, ρ, Tu, TVR, AoA, clean/dirty, chord• Booleans and list for pressure probes at the airfoil• Booleans and geometry parameters for potential addons, e.g. slat, LE strake, fence, VG’s, etc.• Mesh size settings for global and local scaling, e.g. local refinement around transition location• Solver settings: steady/transient, time step size, inner iterations, number of iterations/time steps to average over

Suzlon Energy Ltd.Wind tunnel CFD framework - Output

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1D Output:• Cl, Cd based on pressure probes and surface

integral• Transition location• VG drag penalty and forces• …

2D Output:• cp distribution along pressure probe location• cf distribution at different spanwise positions• Velocity distribution downstream of VG array• …

3D Output:• Airfoil surface streamlines• Velocity and pressure field around the airfoil• Flow field downstream of VG array• …

Suzlon Energy Ltd.Wind tunnel CFD framework - Examples

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• Airfoil tested @ DWG• Running AoA sweep in 3D CFD• Tuning of CFD model (mesh settings, model parameters, etc.)• Good agreement in terms of Lift and transition location

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

0 2 4 6 8 10 12 14 16

AoA

Cl

raw Exp. PP

3D CFD PP

corr. Exp.

2D CFD

0.00

0.02

0.04

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0 2 4 6 8 10 12 14 16

AoA

Cd

raw Exp. PP

3D CFD PP

corr. Exp

2D CFD

0.0

0.1

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0 2 4 6 8 10 12 14 16

x/c

AoA

Transition SS/PS

Exp. SSExp. PS3D CFD SS3D CFD PS2D CFD SS

Suzlon Energy Ltd.Conclusions

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Before wind tunnel test – CFD First:• Identify risks for testing of unconventional geometries• Find solutions, which extend the validity and quality of the tests• Helps to find preferable position of pressure probes• Saves time in the wind tunnel for the actual test campaign

After wind tunnel test:• 1 to 1 comparison to wind tunnel test data• Helps to find explanations, why there are deviations between tests and simulations • Very useful to tune the settings and models of 3D CFD setup

Suzlon Energy Ltd.

Suzlon wind farm in Utah, USA

Thank you!

Simultaneous use of wind tunnel testing and CFD – The numerical wind tunnelC. Mau, G. Tescione, O. UzolSuzlon Blade Science Center

Suzlon Energy Ltd.Using IRT for transition location detection

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• Very accurate detection of transition location from WT test

• Live monitoring allows to detect disturbances and help to increase the validity of the test results

Raw images processing:

• find max. grey scale gradient along each horizontal pixel line

• Transition location is dertermined with help of calibration images correlating pixel and chordwise position

Suzlon Energy Ltd.Using IRT for flow visualization

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AoA = 7.5 deg

AoA = 12.5 deg

AoA = 15 deg0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20 25 30

Cl

AoA

Exp. PP

CFD PP Fine

Risø C2 extrapolated t/c = 42 % @ LTT without BL fence:

• Drop in lift polar around 7.5 deg in CFD - Why?• Corner vortex is already present at 7.5 deg in the CFD, but is not detected

in the experiment• At 12.5 deg corner vortex is present on one side in the experiment• At 15 deg there is a the corner vortex at both wind tunnel end walls• The difference in the slope of the polar between 7.5 and 12.5 deg is

related to a earlier development of the corner vortex in CFD

Suzlon Energy Ltd.Wind tunnel CFD framework - Examples

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0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10 12 14 16AoA

Cl

Exp. PP

CFD PP Trimmed QCR

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

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0.20

0 2 4 6 8 10 12 14 16AoA

Cd

Exp. PP

CFD PP Trimmed QCR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

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0 2 4 6 8 10 12 14 16

x/c

AoA

Transition SS/PS

Exp. SS Exp. PS

CFD SS Trimmed QCR CFD PS Trimmed QCR

• Airfoil tested @ DWG• Running AoA sweep in 3D CFD• Tuning of CFD model (mesh settings, model parameters, etc.)• Good agreement in terms of Lift and transition location

Suzlon Energy Ltd.Initial wind tunnel CFD models – First results

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CFD model of t/c = 59% - AoA = 15 deg @ DWG CFD model of t/c = 42% - AoA = 10 deg @ LTT

• Large LE radius leads to corner separation due to wall interference

• Corner separation vortex covers large or total area of the airfoil SS

• High uncertainty in polars extracted from WT tests can be expected

simultaneous use of wind tunnel testing and cfd the...

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