Challenges in High Reynolds Number Flows for Large Offshore

Challenges in High Reynolds Number
Flows for Large Offshore Wind Turbines
Özlem Ceyhan and Herman Snel
EFMC10, Copenhagen, september 2014
Presented by Gerard Schepers
www.ecn.nl
Outline
• Consequences of upscaling in rotor aerodynamics
• Unavailability of high Reynolds numbers measurements for wind turbine
airfoils
• Possible effects of high Re number
• Computed high Reynolds number effects on airfoils
• Need for validation
• AVATAR measurements in DNW-HDG high pressure tunnel
• Conclusions
• Final Remarks
How large is a 20MW wind
turbine blade? Very!!!
A ‘small’ 73.5 meter blade
Consequences of Upscaling on
Aerodynamics
UPWIND project:
Upscaling from 5MW reference turbine to 20MW wind turbine with Classical Similarity Rules:
•Tip speed is constant
• Rotational speed is therefore inversely proportional to rotor diameter growth
•Local velocities along the blade stay the same.
• Dimensions of the blades are scaled linearly
• increase in the local Reynolds numbers!
The only change in the aerodynamics is the
Re =
U= local velocity
c = chord length
υ = kinematic viscosity
Uc
ν
10,0
8,0
20 MW (252m diameter)
20,0E+06
Chord [m]
7,0
6,0
5,0
4,0
3,0
2,0
1,0
Reynolds Number
5MW (126m diameter;
UPWIND Reference)
24,0E+06
9,0
16,0E+06
12,0E+06
8,0E+06
20MW
4,0E+06
5MW
U=11.5m/s (rated wind speed)
00,0E+00
0,0
0,0
0,2
0,4
0,6
r/R
0,8
1,0
1,2
0,0
0,2
0,4
0,6
r/R
0,8
1,0
1,2
Availability of Cl, Cd and Cm data of the wind
turbine airfoils for high Reynolds numbers
Availability of the wind tunnel test data;
Up to
Re=3x106
Up to Re=6x106, for NACA airfoils
up to Re=9x109 (*)
The effects of very high Reynolds numbers?
(*) Some tests are available for high Re numbers at low Mach numbers of thin NACA profiles coming from 1940’s
1- Loftin, K.L.,Jr., Bursnall, W.J., “Effects of Variations in Reynolds Number Between 3.0x106 and 25x106 upon the Aerodynamic Characteristics of a number of NACA 6Series Airfoil Sections”, NACA-TN-1773, 1948
High Reynolds numbers on aircrafts
90
⇒ Transport
aircraft airfoils are for transonic
speed (Ma about 0.8). wind turbine airfoils
are for low subsonic speed (Ma about 0.3).
o A380
80
Reynolds Number [millions]
70
⇒ Wind turbine airfoils are thicker.
60
⇒ During
the take off and landing, flaps
and/slots are extracted.
o B747
o A350
A340 o o B777
o B787
C17 o
50
Take off and
Landing
40
⇒ High
Reynolds data are absolutely needed
for large WT designs. MEASUREMENTS!!
30
A320 o
o B737
20
10-20 MW
Wind Turbines
10
0
0
0,2
0,4
0,6
0,8
1
1,2
Mach Number
Source : http://www.etw.de; reproduced.
Source of the image: http://en.wikipedia.org/
High Reynolds number effects on airfoils
Reynolds Number Effects: Background
Two basic (and perhaps contradictory) effects, depending on airfoil
shape
1) Generally thinner boundary layer as a result of higher Re number and
less decambering, but:
2) Earlier laminar to turbulent boundary layer transition, which tends to
thicken the boundary layer. The transition position is very difficult to
predict but depends on the pressure distribution and shear and hence
on the airfoil shape!
Hence, prediction methods must be validated by measurements
Intermezzo: The EU FP7 project Avatar
Motivation:
• We simply don’t know if present aerodynamic models are good enough to
design 10MW+ turbines
– “No mature industry will ever design a MEuro machine with
unvalidated tools” (M. Stettner, GE Global Research)
• 10MW+ rotors violate assumptions in current aerodynamic tools, e.g.:
–
–
–
–
Reynolds number effects,
Compressibility effects
Flow transition and separation,
(More) flexible blades
• Hence 10MW+ designs fall outside the validated range of current state
of the art tools.
FP7-ENERGY-2013-1/ n° 608396
18-9-2014
9
Avatar: Main objective
To bring the aerodynamic and fluid-structure models to a
next level and calibrate them for all relevant aspects of
large (10MW+) wind turbines
FP7-ENERGY-2013-1/ n° 608396
18-9-2014
10
High Reynolds number effects on Cl and Cd of thick
airfoils (predicted by RFOIL, eN method for transition )
2
2
DU91-W2-250
1.5
1.5
1
Cl
1
Cl
Re=7mil. Clean
Re=7mil. Clean
0.5
Re=7mil. Rough
Re=7mil. Rough
Re=20mil. Clean
0.5
Re=20mil. Clean
Re=20mil. Rough
Re=20mil. Rough
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0
-10
-5
0
5
10
15
20
-0.5
-0.5
-1
AoA
Cd
2
0
DU97-W-300
2
1.5
0.01
0.02
0.03 Cd 0.04
0.05
0.06
0.07
1.5
Re=7mil. Clean
1
1
Re=7mil. Rough
Cl
Re=20mil. Clean
0.5
Cl
Re=7mil. Clean
Re=7mil. Rough
0.5
0
Re=20mil. Clean
Re=20mil. Rough
-0.5
0
-5
0
5
10
15
20
-1
-0.5
AoA
-1.5
Re=20mil. Rough
0.08
NB: only usable
measurements
found in literature
for high Re are for
a 18% thick
symmetrical airfoil,
Re = 20M.
AVATAR: Comparison results ECN/other partners
Large differences in computed 2D Polars show the need for model improvement at
high Reynolds numbers
Courtesy: N Sorensen
Need for validation by measurement:
the AVATAR project
• A high Re effect assessment was done using RFOIL
• Different partners in the AVATAR project use 2D CFD with existing
transition models
– Differences are large
• Results must be validated to be used with confidence in designs.
• There is a possibility that the high Re effects enable thicker airfoils without
drag penalties, so that weight increase can be limited for the very large
blades and gravitatonal loads can be reduced.
• The AVATAR project includes measurements in the DNW-HDG high
pressure wind tunnel, from Re = 3M upto 18M, DU00-W-212 section. In
order to check Re influence, lower Re measurements (upto 6M) will be
performed in LM wind tunnel.
AVATAR measurements
Measurements are scheduled to start this very week,
with the following special features:
• Concurrent measurement of the wind tunnel
turbulent SPECTRUM with a hot film at entrance
• Estimation of transition location using embedded
Kulite pressure sensors as high frequency
microphones
• Pressure distribution measurements (90 sensors)
• Wake rake for drag determination, sidewise
traversabel
• Flourescent oil flow visualization
• Profile: DU00-W-212, c=15 cm
Conclusions
• Due to the growing sizes of the rotors, higher Reynolds numbers (up to 25
million) are introduced.
• There is a lack of measurement data at high Reynolds numbers of the thick
wind turbine airfoils at incompressible conditions.
• RFOIL is used in order to estimate the effects of Reynolds numbers.
• Within the AVATAR project, measurements will be performed in the DNWHDG wind tunnel in Göttingen, to validate or correct the desing calculations
• Results are forthcoming, measurements starting this week.
Final Remarks
• Reynolds number is reduced with slender blades. It is increased again with higher
tip speed operations.
• Those effects should already be included in the existing or the next generation (510 MW) wind turbines!
• More detailed design work is necessary to be performed in order to design the
best airfoils with right thickness. (stall, dynamic effects, stability etc.)
• There are only a few atmospheric wind tunnels in the world that are able to reach
such high Re numbers in incompressible flow - lower than Mach=0.3. In this
tunnels, the chord length of the model should be extremely large! Therefore the
number of facilities to be used for this purpose is quite limited.
Thank you for your attention
This project has received funding from the European Union’s Seventh Programme for research, technological development and
demonstration under grand agreement No FP7-ENERGY-2013-1/n° 608396 .
Questions...
[email protected]

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