Conference Paper

NREL/CP-500-47759

May 2010

Rotational Augmentation 

Disparities in the MEXICO and 

UAE Phase VI Experiments

Preprint

S. Schreck

National Renewable Energy Laboratory

T. Sant

University of Malta

D. Micallef

University of Malta and Technical University of Delft

To be presented at the 2010 Torque from Wind Conference

Heraklion, Greece

June 28-30, 2010
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Rotational Augmentation Disparities in the

MEXICO and UAE Phase VI Experiments

S. Schreck

Nat’l Renewable Energy Laboratory

Nat’l Wind Technology Center

1617 Cole Blvd.

Golden, CO 80027

United States

scott.schreck@nrel.gov

T. Sant

University of Malta

Faculty of Engineering

Room 315 MSD

2080 Malta

tonio.sant@um.edu.mt

D. Micallef

Technical University of Delft

University of Malta

Faculty of Engineering

Room 217 MSD

2080 Malta

d.micallef@tudelft.nl

Abstract

Wind turbine structures and components suffer 

excessive loads and premature failures when 

key aerodynamic phenomena are not well 

characterized, fail to be understood, or are 

inaccurately predicted. Turbine blade rotational 

augmentation remains incompletely 

characterized and understood, thus limiting 

robust prediction for design. Pertinent rotational 

augmentation research including experimental, 

theoretical, and computational work has been 

pursued for some time, but large scale wind 

tunnel testing is a relatively recent development 

for investigating wind turbine blade 

aerodynamics. Because of their large scale and 

complementary nature, the MEXICO and UAE 

Phase VI wind tunnel experiments offer 

unprecedented synergies to better characterize 

and understand rotational augmentation of blade 

aerodynamics. Cn means, Cn standard 

deviations, two-dimensional cp distributions, and 

three-dimensional planform surface pressure 

topologies from these two experiments were 

analyzed and compared. Rotating blade data

were evaluated against analogous stationary 

blade data. Rotational augmentation effects 

were found to be pervasive and were present 

over the blade radius and throughout blade 

operating envelopes at all radial locations 

investigated. Rotational effects manifested 

themselves in both mean and time varying 

statistics, in both two-dimensional sectional data 

as well as three-dimensional planform data. 

Comparative analyses of MEXICO and UAE 

data validated and generalized current 

knowledge regarding rotationally augmented 

blade flow fields. In addition to confirming prior 

research, results also provided new insights not 

attainable by considering either data set in 

isolation of the other.

Keywords: Force amplification, rotational 

augmentation, wind tunnel, wind turbine,

unsteady loading 

1 Introduction

Wind turbine service life is shortened and 

operability curtailed when unanticipated 

aerodynamic loads impose excessive stresses 

on wind turbine structural and mechanical 

components. Failure to accurately predict 

turbine aerodynamic loads is due largely to the 

complex nature of wind turbine blade 

aerodynamics and incomplete comprehension of 

the underlying fluid dynamics. At present, key 

wind turbine aerodynamic phenomena remain 

incompletely characterized and understood.

Historically, experimentation and testing have 

occupied a central role in discovering, 

characterizing, and understanding fluid dynamic 

phenomena that govern wind turbine power 

production and structural loading. Early field 

experiments were carried out concurrently at the 

Netherlands Energy Research Foundation, Delft 

University of Technology, National Renewable 

Energy Laboratory, Risoe Wind Turbine Test 

Station, and Imperial College. Notably, these 

efforts successfully carried out research grade 

measurements of turbine aerodynamics and 

structural dynamics in the challenging field 

environment. These efforts were documented 

through IEA Wind Annex XIV [1] and Annex 

XVIII [2]. 

Unfortunately, the accurate, detailed

measurements acquired in these field 

experiments accentuated a long standing 

dilemma. Large scale turbine geometries could 

be densely instrumented and successfully 

tested, but the uncontrollable and sparsely 

characterized atmospheric inflows introduced 

overriding uncertainties. Alternatively, wind 

tunnel testing offered controlled and uniform 

inflows, but test section dimensions constrained 

turbine size, leading to severe mismatches in 

Reynolds number and other similarity 

parameters. These uncontrolled inflows and 

similarity parameter disparities were broadly 

recognized as significant impediments to deeper 

comprehension and more accurate prediction of 

turbine aerodynamics. 

1
This dilemma was first addressed by a series of 

joint projects between the Aeronautical 

Research Institute of Sweden (FFA) and the 

China Aerodynamics Research and 

Development Center (CARDC). This series of 

projects culminated in 1990, when testing was 

completed in the CARDC 12 m x 16 m wind 

tunnel on a two-bladed rotor with a diameter of 

5.35 m. In addition to nacelle and blade root 

moments, surface pressures were measured at 

232 taps distributed over eight radial 

stations.[3,4] Success in these experiments

stimulated interest internationally, and laid the 

foundations for more ambitious plans involving 

still larger turbines and wind tunnels, and more 

elaborate test protocols.

Resource constraints and facility schedules

postponed further wind tunnel testing of larger 

scale wind turbines until the following decade. 

In 2000, NREL completed testing of the 10.1 m 

diameter Unsteady Aerodynamics Experiment 

(UAE) Phase VI turbine in the NASA Ames 24.4 

m x 36.6 m wind tunnel.[5] In 2006, EU Model 

Rotor Experiment in Controlled Conditions

(MEXICO) Project with a 4.5 m diameter rotor 

was tested in the DNW 9.5 m x 9.5 m LLF.[6] 

Notably, these efforts were sufficiently similar to 

enable corroborative comparisons, while specific 

differences showed effects not observed 

previously.

Wind tunnel testing of large scale turbines 

represents a crucial, though relatively recent 

development for understanding and predicting 

wind turbine aerodynamics. For rotational 

augmentation, prior research extends back 

some decades, encompassing experimental, 

theoretical, and computational work. A review of 

these efforts will not be undertaken herein, but 

can be found in previous documentation.[7] The 

current work will concentrate on characterizing 

and understanding rotational augmentation of 

blade aerodynamic response, using the 

aerodynamics measurements acquired in the 

MEXICO and UAE Phase VI wind tunnel tests. 

2 Experimental Methods

2.1 MEXICO Turbine

MEXICO testing was carried out in the DNW LLF

(Large Low-Speed Facility) 9.5 m x 9.5 m open

jet, and has been documented by Snel, et al. 

[8,9] The MEXICO turbine had a three-bladed 

upwind rotor that was 4.5 m in diameter with 

zero cone angle. The rotor was pitch controlled 

and turned counterclockwise (viewed from 

downwind) at constant speed. Though the 

experiment encompassed multiple rotor speeds, 

data analyzed in the current work were acquired 

at a rotor speed of 325 RPM. A cylindrical tower 

0.508 m in diameter with a spiral strake held the 

nacelle at a hub height of 5.12 m with 2.13 m 

overhang. This situated the rotor axis at the jet 

centerline and placed the rotor well upwind of 

the tower. The MEXICO turbine is shown in the

left panel of Figure 1, in the DNW LLF 9.5 m x 

9.5 m open jet.

Full pressure tap rows were located on all three 

blades, with the tap rows at 0.25R and 0.35R on 

blade 1, the 0.60R tap row on blade 2, and the 

0.82R and 0.92R tap rows on blade 3. A full 

pressure tap row consisted of 25 to 28 taps, 

distributed over the blade pressure and suction 

surfaces. The upper part of Figure 2 shows the 

MEXICO pressure tap distribution on the blade 

suction surface, with all five full tap rows 

depicted on one blade. Pressure taps were 

more densely distributed near the blade leading 

edge to better resolve the pronounced gradients 

typically present there. To validate azimuthal 

uniformity, partial pressure tap rows were 

distributed over multiple blades at each radial 

location.

The MEXICO experiment blades were both 

twisted and tapered. The blade taper 

distribution is evident in Figure 2, with maximum 

blade chord being 0.240 m at 0.20R and 

tapering to 0.011 m at the tip. Figure 3 

documents blade twist, which decreases from 

16.4° at 0.20R to 0.0° at the tip. Between 0.20R 

and the tip, the blade cross section transitioned 

from the DU91-W2-250 airfoil (0.20 ≤ r/R ≤ 0.46), 

to the Risø A1-21 (0.54 ≤ r/R ≤ 0.66), and finally 

to the NACA 64-418 (0.74 ≤ r/R ≤ 1.00 ). The 

blade pitched about an axis located 0.25c aft of 

the leading edge, and centered between the 

blade upper and lower surfaces at that chord 

location.

Pressure taps were flush with the blade surface 

and 0.4 mm in diameter. Close coupled beneath 

each tap was a Kulite XCQ-95 series 

piezoresistive pressure transducer having

sealed gage reference. Close coupling with the 

pressure taps minimized reduction of the 

transducers 150 kHz bandwidth, yielding flat 

frequency response across a broad spectrum. 

Each of the transducer pressure inputs was 

scanned at 5514 Hz. Test section flow speed 

and air properties were measured using the 

DNW LLF wind tunnel air data system.

Turbine blade plane of rotation was maintained 

orthogonal to the test section centerline, yielding 

a yaw angle of 0°. Blade pitch angle was held 

constant at -2.3°. Test section velocity (U∞) was 

varied between 5.4 m/s and 30.0 m/s, at 

graduated intervals, yielding tip speed ratios (λ) 

of 14.1 to 2.5. At each U∞, a 5 s data set was 

2
acquired. Time records of cp were integrated 

over the sectional chord to obtain time records of 

Cn. Time records of cp and Cn were processed 

to obtain means and standard deviations for cp

and Cn. 

2.2 UAE Phase VI Turbine

Several Phase VI UAE configurations were 

tested in the NFAC 24.4 m x 36.6 m wind tunnel, 

and are described by Hand, et al.[5] Data 

analyzed herein were acquired from a two 

bladed upwind rotor, 10.1 m in diameter, with 

zero cone angle. The rotor turned clockwise 

(viewed from downwind) at a constant 71.6 rpm, 

was stall regulated, and had a maximum rated 

power of 19.8 kW. A cylindrical tower 0.4 m in 

diameter supported the turbine at a hub height of 

12.2 m (test section centerline), with 1.32 m 

rotor overhang. This UAE configuration, 

mounted in the NASA Ames 24.4 m x 36.6 m ft 

wind tunnel, is shown in the right panel of Figure 

1.

The black blade on the left side of the rotor in 

Figure 1 was equipped with five full pressure tap

rows to acquire detailed surface pressure data. 

A full pressure tap distribution consisted of 22 

taps distributed over the pressure and suction 

surfaces of the blade. Pressure taps were more 

densely distributed near the blade leading edge 

to better resolve the pronounced gradients 

typically present there. The lower portion of 

Figure 2 shows that UAE full pressure tap 

distributions that were located at r/R = 0.30,

0.47, 0.63, 0.80, and 0.95.

The blades used throughout the NASA Ames 

wind tunnel test were both twisted and tapered. 

The blade taper distribution is apparent in Figure 

2, with maximum blade chord being 0.737 m at 

0.25R, and tapering to 0.356 m at the tip. Figure 

3 documents blade twist, which decreases from 

21.8° at 0.25R to 0.0° at the tip. Between 0.25R 

and the tip, blade cross section was uniform, 

corresponding to the S809 airfoil. The blade 

pitched about an axis located 0.30c aft of the 

leading edge, and centered between the blade 

upper and lower surfaces at that chord location. 

Design procedures, constraints, and measures 

of merit for this blade have been documented in 

detail.[10] 

Surface pressure taps were flush mounted at the 

blade surface, and had inside diameters of 0.69 

mm. From the taps, stainless steel hypodermic 

tubes with inside diameters of 0.69 mm 

transmitted surface pressures to the pressure 

transducers. Hypodermic tubing lengths were 

minimized to mitigate pressure delay and 

dispersion effects. Pressures were measured by 

five Pressure Systems Incorporated ESP-32 

electronically scanned pressure transducers 

located inside the blade near the five full 

pressure tap distributions. Each of the 

transducer pressure inputs was scanned at 

520.8 Hz. In conjunction with the tubing 

frequency response, this provided antialiased 

digitization and minimal gain variation out to 55 

Hz.[11] Test section flow speed and air 

properties were measured using the 80 ft x 120 

ft wind tunnel air data system described by 

Zell.[12] 

All data were collected with the turbine rotating 

at a constant speed of 71.6 RPM. Turbine blade 

plane of rotation was maintained orthogonal to 

the test section centerline, yielding a yaw angle 

of 0°. Blade pitch angle was held constant at 

3.0°. Test section velocity (U∞)was varied 

between 5 m/s and 25 m/s, in nominal 

increments of 1 m/s, corresponding to tip speed 

ratios (λ) of 7.5 to 1.5. At each U∞, a 30 s data 

record containing 36 blade rotation cycles was 

acquired. Time records of cp were integrated 

over the sectional chord to obtain time records of 

Cn. Time records of cp and Cn were processed 

to obtain means and standard deviations for cp

and Cn. 

3 Results and Discussion

The results below compare MEXICO and UAE 

Phase VI sectional aerodynamic forces and 

surface pressure distributions, for radial 

locations near the blade root and farther out 

toward the blade tip. These analyses include 

both mean and time varying statistics. To 

provide some integration of sectional flow field 

characteristics, mean surface pressure 

topologies for select operating conditions also 

are included. 

3.1 Local Inflow References

In both the MEXICO and UAE Phase VI 

experiments, local dynamic pressure was 

computed as the difference between test section 

static pressure (p∞) and local total pressure (p0). 

Local total pressure was determined at each full 

pressure tap distribution as the highest pressure 

sensed in the tap distribution.

An inverse free wake lifting line model was used 

to compute angles of attack (α) corresponding to 

the MEXICO and UAE Phase VI experimental 

data at zero yaw. This inverse free wake model 

has been thoroughly validated for both the UAE 

Phase VI [13] and MEXICO [14] databases. 

Thus, it furnished angle of attack data that were 

both accurate and consistent between the two 

experiments. It should be noted that the lifting 

line formulation limits the fidelity with which 

3
three-dimensional flow field features are 

resolved at high angles of attack.

The pressure readings from the MEXICO 

experiment can yield the normal and tangential 

loads acting on the blades of the turbine for 

every spanwise location and azimuth angle at 

which the data are sampled. In the inverse free 

wake model, the force data can be interpolated 

for a desired number of spanwise and azimuthal 

discretization elements. An initial angle of attack 

at each of these elements is assumed and the 

inverse free wake model uses the normal and 

tangential forces to find the lift and drag 

components.

From the Kutta-Joukowski theorem a bound 

circulation distribution is obtained for every time 

step. From this, the trailed and shed circulation 

can be obtained. This enables the velocity field 

due to the turbine wake to be determined and 

the wake is allowed to develop freely under the 

influence of such a velocity field. A new angle of 

attack can thus be obtained which will enable a 

new circulation distribution to be obtained. This 

algorithm is shown schematically in Figure 4. 

The process is repeated until a convergence in 

angle of attack is achieved. This is usually the 

case in two or three iterations. The process is 

an inverse free wake method because rather 

than obtaining the blade loads from twodimensional airfoil information, it uses 

experimental measurements in order to solve for 

the velocity field and hence angles of attack on 

the rotor blades.

In the current work, Cn was analyzed instead of

Cl for two reasons. First, Cn was considered a 

more physically pertinent parameter of interest 

because it decouples flow field activity from 

inflow direction. Second, adoption of Cn permits 

comparisons of results herein with a broad range 

of analyses previously accomplished for other 

experimental data. 

With differences in rotor speed and blade chord 

length offsetting each other, Reynolds numbers 

for the UAE and MEXICO turbines were 

comparable, with both being on the order of 106

. 

Angles of attack (α) for both the UAE and 

MEXICO experiments were computed using the 

same inverse free wake model, and so were 

consistent.

3.2 Mean Cn 

Figure 5 shows mean Cn data corresponding to 

the farthest inboard radial locations on the 

MEXICO and UAE Phase VI blades. In the 

upper panel of Figure 5, UAE Phase VI mean Cn

data acquired at 0.30R for stationary blade

conditions are plotted as a function of α. Here, 

mean Cn initially increased linearly with α at a 

rate of 0.04 per degree. This slope was 

substantially lower than the 0.11 per degree rate

for two-dimensional airfoils, indicating that threedimensional influences were significant for this 

radial location under stationary blade conditions. 

Upon reaching α = 8.90, the slope of the curve

decreased visibly, but Cn continued to increase 

until α = 23.90 deg. At this point, Cn attained a 

maximum of 0.911 and stall occurred. 

Thereafter, Cn decreased slightly to a local 

minimum of 0.861 at 34.00, and finally rose to Cn

= 0.948 at 43.00. MEXICO stationary blade data 

were not available for the 0.25R radial location.

The lower panel of Figure 5 shows mean Cn data 

acquired under rotating blade conditions on the 

MEXICO blade at 0.25R and the UAE blade at 

0.30R. For both data sets, Cn increased with α

in pseudo-linear fashion through the 

approximate range 00 ≤ α ≤ 100, at a rate of 0.09 

per degree. That this slope was more than twice

that for the parked blade indicates that rotational 

influences began to modify the blade flow field at 

low α. At α = 8.90 for the MEXICO data and α = 

10.70 for the UAE, curve slope decreased to less 

than half of its previous magnitude, but 

steepened again at α ≈ 150, to 0.15 per degree

for the MEXICO curve and 0.30 for the UAE. 

These Cn kinematics implied that exceeding the 

two-dimensional static stall α prompted

significant rotational alterations to the blade flow 

field.

These exceptionally steep Cn–α curve

subintervals culminated in Cn maxima (stall Cn), 

which occurred at α = 20.40 and Cn = 2.081 for 

MEXICO, and at α = 20.80 and Cn = 2.436 for 

the UAE. For both data sets, stall Cn was twice 

that generally observed for two-dimensional 

airfoils, and for the UAE was nearly three times 

that for the stationary blade. Rotating blade stall 

α was about 50 higher than that generally 

measured for two-dimensional airfoils, but for the 

UAE rotating blade stall α approximated stall α

for the stationary blade. After reaching 

maximum (stall) Cn, MEXICO and UAE data 

displayed dramatically different kinematics. 

MEXICO Cn dropped precipitously from 2.081 to 

1.564 over the α interval from 20.40 to 26.70, and 

then remained approximately constant. In sharp 

contrast, UAE Cn declined gradually from 2.436 

to 2.270 as α increased from 20.80 to 37.00. 

Notably, neither inboard Cn–α curve exhibited a 

maximum, either absolute or local, in the α

range commonly associated with twodimensional airfoil stall. Rather, the only 

4
maxima, corresponding to stall, occurred at α

levels usually associated with airfoil post stall.

Figure 6 shows data for mid-radius stations on 

the MEXICO and UAE Phase VI blades. The 

upper panel of Figure 6, contains mean Cn–α

data for the stationary MEXICO blade at 0.60R 

and for the stationary UAE blade at 0.63R. 

Through the range 00 ≤ α ≤ 140, both curves 

followed highly similar trajectories. Both rose at 

the same slope of 0.08 per degree until reaching 

α = 80. Thereafter, the slopes of both curves 

decreased progressively with increasing α, and 

both leveled off at Cn = 1.04 upon reaching α = 

140. At this point, the two curves diverged. The 

MEXICO curve decreased briefly to 0.737, and 

then rose sharply to a global maximum of Cn = 

1.185 at α = 20.20. Subsequently, MEXICO Cn

decreased gradually to 0.848 at α = 30.20, and 

then increased gradually to culminate at 0.971 at 

α = 45.20. After diverging from the MEXICO 

curve, the UAE Phase VI curve decreased 

gradually to Cn = 0.706 at α = 27.10, and finally 

increased in equally gradual fashion to end at

0.946, at α = 47.10. 

The lower panel of Figure 6 contains mean Cn

data for rotating blade conditions on the 

MEXICO blade at 0.60R and on the UAE blade 

at 0.63R. For both data sets, Cn increased 

linearly with α until reaching α = 7.20. In this low 

α range, the MEXICO and UAE Cn–α curve 

slopes were 0.11 and 0.08 per degree, 

respectively. As α increased beyond 7.20, Cn–α

curve slopes decreased visibly for both MEXICO 

and UAE data. Subsequently, MEXICO Cn

reached a maximum of 1.251 at α = 14.40, and 

UAE Cn reached a maximum of 1.111 at α = 

12.90. These stall Cn values were higher by 8 

percent and 6 percent, respectively, than those 

for the stationary blade Cn stall. MEXICO stall

took place at marginally higher α and reached

slightly higher Cn than that for the UAE. 

Nonetheless, stalling kinematics were highly 

similar for the two data sets, with Cn decreasing 

gradually after cresting at maximum (stall)

levels. Notably, mid-radius MEXICO and UAE 

stall parameters (α and Cn) for the rotating blade 

did not differ radically from their stationary blade 

counterparts.

Though mid-radius stall kinematics differed little 

between the stationary and rotating blades, poststall responses differed significantly between the 

stationary and rotating states. Following stall, 

UAE Cn declined to a local minimum of 0.935 at 

α = 15.50. MEXICO Cn decreased to a slightly 

lower local minimum of 0.843, at a somewhat 

higher α of 22.60. After this, UAE Cn increased 

to an absolute maximum (post-stall) of 1.35 at α

= 25.30, and then declined slowly and 

nonmonotonically to culminate at Cn = 1.249 at α

= 39.10. MEXICO Cn increased from the local 

minimum for a brief interval, but halted 

prematurely at α = 26.60, where Cn = 1.006. 

3.3 Time Varying Cn 

Figure 7 contains Cn standard deviation (σCn) 

data for the farthest inboard radial locations on 

the MEXICO and UAE Phase VI blades. In the 

upper panel of Figure 7, UAE Phase VI σCn data 

for 0.30R under stationary blade conditions are

plotted as a function of α. Through the range 00

≤ α ≤ 8.90, σCn remained constant at 0.017, and 

subsequently increased in linear fashion to a 

maximum of 0.059 at α = 18.90. After attaining 

this maximum, σCn decreased in intermittent 

steps over the next 30.00 interval, and finally 

reached 0.020 at α = 48.90. MEXICO stationary 

blade data were not available for the 0.25R 

radial location.

The lower panel of Figure 7 shows σCn data 

acquired under rotating blade conditions on the 

MEXICO blade at 0.25R and the UAE blade at 

0.30R. In the low α range, σCn remained low 

and approximately level for both the MEXICO 

and UAE data. Upon reaching α = 15.0 deg, the 

MEXICO σCn began to rise at a modest rate until 

α = 20.40. Over approximately the same α

interval, UAE σCn increased sharply after α = 

13.30, reaching a local maximum of 0.178 at α = 

17.80. 

Beyond α = 20.40, higher α prompted relatively 

small though visible variations in MEXICO σCn, 

with the lowest σCn in this α range being 0.072 at 

α = 37.70, and the highest being 0.091 at α = 

48.50. Over a comparable range of 19.70 ≤ α ≤ 

30.20, UAE σCn climbed to a maximum of σCn = 

0.214 at α = 30.20, though the σCn rate of 

increase through 19.70 ≤ α ≤ 30.20 was 

significantly lower than that over 13.30 ≤ α ≤ 

17.80. Though the σCn magnitudes observed for 

the UAE σCn were appreciably higher than those 

for MEXICO, the two curves were well correlated 

with respect to slope variations and the 

occurrence of minima and maxima.

Figure 8 contains Cn standard deviation (σCn) 

data for the mid-radius locations on the MEXICO 

and UAE Phase VI blades. In Figure 8, the 

upper panel contains σCn data for the stationary 

MEXICO blade at 0.60R and for the stationary 

UAE blade at 0.63R. Clearly, the two σCn–α

curves show striking similarities with respect to 

maximal magnitudes and slope correlations. 

Through the range 0.00 ≤ α ≤ 12.20, neither 

5
curve deviated significantly from the other, with 

both remaining constant at σCn = 0.01. After α = 

12.20, both curves began to rise at moderate 

and similar rates, with the MEXICO curve

peaking at σCn = 0.060 at α = 18.20, and the 

UAE curve peaking at σCn = 0.057 and α = 22.10. 

During the subsequent decrease in σCn, the two 

curves continued to resemble each other, with

the MEXICO curve reaching a minimum of σCn = 

0.038 at α = 30.20, and the UAE curve doing the 

same at σCn = 0.032 and α = 27.10. Thereafter, 

at higher values of α, the correlation between 

the two curves was less evident, though still 

perceptible. 

The lower panel of Figure 8 shows σCn data 

acquired during blade rotation from the MEXICO 

blade at 0.60R and the UAE blade at 0.62R. 

Like the stationary blade data in the upper panel 

of Figure 8, the data for the MEXICO and UAE 

rotating blades show remarkable similarities. 

Below α = 100, σCn remains below 0.01 for 15 of

16 data points in this range. The sole exception 

is the UAE data point at α = 3.20, which 

assumes a value of 0.015. At approximately α = 

100, both the MEXICO and UAE curves begin to 

rise rapidly. Shortly thereafter, at α = 19.20, the 

MEXICO curve peaks at σCn = 0.125, and at α = 

18.20, the UAE curve peaks at σCn = 0.189. 

From this peak, the MEXICO curve decreases to 

σCn = 0.076 at α = 22.60, and then rises to 

culminate at σCn = 0.094 at α = 26.60. Similarly, 

from the UAE peak, the curve descends to σCn = 

0.146 at α = 22.10, and then rises again, 

reaching σCn = 0.165 at α = 27.00. Though the 

MEXICO data end at α = 26.60 and the UAE 

data continue through α = 39.10, the correlation 

between the two data sets through the range 00

≤ α ≤ 270 is dramatic. This implies

correspondingly prominent similarities in the 

unsteady flow physics of the MEXICO and UAE 

blades near mid-radius.

3.4 Sectional cp Distributions

Blade cp data were analyzed to better 

understand the fluid dynamics responsible for 

the MEXICO and UAE Cn kinematics. 

Specifically, MEXICO and UAE Phase VI mean 

sectional cp distributions, for rotating blade stall 

conditions, were compared at the inboard and 

mid-radius locations. 

The MEXICO rotating blade cp data shown in 

Figure 9 were acquired at 0.25R for α = 20.40. 

The MEXICO suction surface cp distribution had

a suction peak at 0.002c where cp = -7.496. Aft 

of this peak, cp magnitude decreased sharply 

over the leading 0.11c, then more gradually from 

0.11c to 0.30c, and finally became virtually 

constant on the aft 0.70c of the suction surface. 

Over the aft 0.70c, cp varied between -0.935 and 

-1.461. On the MEXICO blade pressure surface, 

stagnation was detected at the 0.16c tap 

location. 

Also shown in Figure 9 are UAE rotating blade cp

data, which were measured at 0.30R for α = 

20.80. UAE suction surface cp exhibited no 

leading edge suction peak, and instead 

assumed a virtually constant level of

approximately -2.5 over the chord range 0.0c ≤ 

x/c ≤ 0.56c . Aft of 0.56c, cp level varied in 

pseudo-linear fashion, reaching cp = -0.394 at 

the trailing edge. On the UAE blade pressure 

surface, stagnation was detected at the 0.06c 

tap location. 

Notably, the Figure 9 MEXICO and UAE rotating 

blade stall cp distributions differed dramatically 

from each other, even though they were 

measured at nearly identical α and yielded stall

Cn levels that were comparably elevated (Figure 

5). Thus, it was evident that different cp

distribution features were responsible for 

observed Cn amplifications. Though the 

MEXICO and UAE pressure surface cp 

distributions were slightly unconventional, none 

of the attributes present there could account for 

the Cn amplifications, thus focusing attention on 

the suction surface.

The MEXICO suction surface cp distribution 

displayed two attributes that differed appreciably 

from stationary airfoils. First, while suction peak 

height was not significantly greater than that 

observed on stationary airfoils, suction peak 

chordwise extent was substantially broader. 

Second, cp values of -0.935 to -1.461 in the 

0.30c ≤ x/c ≤ 1.0c chord region were two to three 

times greater than those produced by stationary 

airfoils. In contrast, the entire UAE suction 

surface was nonstandard in conformation and 

was augmented in cp magnitude, and was 

responsible for amplifying Cn. As shown in

previous research, these contrasting cp

distributions imply conspicuously different flow 

field topologies.[15,16] The Figure 9 MEXICO cp

distribution is consistent with a flow field 

containing a trailing edge separation, while cp

distributions like that for the UAE have been 

linked to leading edge separation and 

downstream shear layer impingement.

Figure 10 shows rotating blade cp data for the 

MEXICO blade at 0.60R, and for the UAE blade 

at 0.63R. The MEXICO data correspond to α = 

14.40 and the UAE data to α = 12.90, both of 

which represent stall conditions consistent with 

the rotating blade data in Figure 6. Both cp

6
distributions are highly conventional, and 

strongly resemble those for stationary airfoils. 

Specifically, both the MEXICO and UAE 

pressure distributions have very narrow suction 

peaks, with minimum cp values of -5.816 and -

3.874. From 0.02c to midchord, cp magnitudes 

for both blades decrease gradually until reaching 

the midchord. Between midchord and trailing 

edge, cp for both blades remains virtually 

constant at -0.3 to -0.4, again similar to

stationary airfoils. The cp distributions in Figure 

10 both are consistent with trailing edge 

separation [15,16], and significant augmentation 

relative to stationary airfoil cp distributions is not 

evident. 

While Figure 10 data represented mid-radius 

stall, Figure 11 contains cp distributions for midradius post-stall conditions on the MEXICO and 

UAE rotating blades. The Figure 11 UAE cp

distribution corresponds to the Figure 6 UAE Cn

maximum of 1.35 at α = 25.30. The Figure 11 

MEXICO cp distribution corresponds to the final 

data point in the Figure 6 MEXICO curve, where 

α = 26.60 and Cn = 1.006. As shown in Figure 

11, suction peaked near the leading edges, 

where cp values were -1.182 for MEXICO and -

1.436 for the UAE. Proceeding aft on the blade 

chord, cp magnitude decreased slowly for both 

the MEXICO and UAE, yielding nearly level cp

distributions. Relative to cp magnitudes 

commonly associated with stalled twodimensional airfoils, MEXICO cp magnitudes 

were marginally greater, while UAE cp

magnitudes were appreciably larger. Notably, 

the Figure 11 UAE mid-radius cp distribution 

was strongly reminiscent of the UAE inboard cp

distribution in Figure 9. Consistent with the rest 

of the current work, pressure surface cp

distributions in Figure 11 strongly resembled 

those of stationary two-dimensional airfoils. 

3.5 Planform Pressure Topologies

To analyze flow field kinematics over the entire 

blade in more cohesive fashion, surface 

pressures (not cp) were contour plotted over the 

blade suction surface. Operating conditions at 

or near stalled operation were selected for 

analysis. Time records of cp data were time 

averaged over the entire data set, and the 

resulting mean cp data were converted to 

dimensional pressures (Pascals) referenced to 

test section static pressure. The results are 

shown in Figures 12 and 13.

Figure 12 shows surface pressure contours on 

the MEXICO blade for U∞ = 19.8 m/s. Contours 

were plotted using all suction surface pressure 

taps at r/R = 0.25, 0.35, 0.60, 0.82, and 0.92. 

Pressure contour interval (∆p) is 500 Pa. 

Lowest pressures were observed at the leading 

edge nearest the blade tip. Moving along the 

blade leading edge from 0.92R to 0.35R resulted 

in progressively higher pressure levels. Once 

maximum pressure was reached at 0.35R, 

further inboard movement along the leading 

edge produced lower pressure levels. Contours 

showed that surface pressures at r/R = 0.60, 

0.82, and 0.92 increased steeply over the 

forward half chord, and then underwent little 

change over the remaining aft chord. This same 

trend was evident at 0.25R, although steep 

pressure increases were confined to the forward 

quarter chord, with little apparent change aft of 

there. The 0.35R location differed fundamentally 

from the other four radial stations, in that surface 

pressures remained level over the entire blade 

chord.

Figure 13 shows UAE blade surface pressure 

contours corresponding to U∞ = 15.1 m/s. 

Contour plots include data from all suction 

surface pressure taps at r/R = 0.30, 0.47, 0.63, 

0.80, and 0.95. Pressure contour interval (∆p) is 

100 Pa. As for the MEXICO blade, lowest 

pressures were observed at the blade leading 

edge nearest the tip. Progressing along the 

blade leading edge from 0.95R to 0.80R yielded 

steep pressure increases. Further movement 

inboard from 0.80R to 0.47R continued to 

produce pressure increases, though at a slower 

rate of growth. This trend reversed from 0.47R 

to 0.30R, in which leading edge surface 

pressure decreased. Along the chordwise 

direction, contours showed that surface 

pressures at 0.95R increased steeply over the 

forward quarter chord, and then exhibited 

negligible change aft of this location. At r/R = 

0.80, 0.63, and 0.30, surface pressures 

increased in approximate linear fashion from 

leading to trailing edge. Analogous to the 

MEXICO blade at 0.35R, the UAE 0.47R 

location differed fundamentally from the other 

four radial stations. Here, surface pressures 

appeared approximately level from leading to 

trailing edge.

Though obscured by MEXICO and UAE 

planform differences, careful comparisons 

revealed that the two pressure topologies were 

congruent in two key respects. First, along the 

spanwise direction, surface pressures attained 

maxima at locations just inboard of mid-radius, 

and decreased both inboard and outboard of 

there. Second, at this maximal pressure span 

location, chordwise surface pressures were 

level, while chordwise pressure distributions at 

other radial locations displayed significant

variation.

7
4 Conclusions

The MEXICO and UAE Phase VI and 

experiments were complementary in many 

crucial respects, and thus offer unprecedented 

synergies to better understand and predict 

rotational augmentation of blade aerodynamics. 

Comparisons of the two surface pressure data 

sets included both Cn mean and Cn standard 

deviation statistics and encompassed stationary 

blade baseline data as well as rotating blade 

data. Angles of attack were computed for both 

data sets using the same validated inverse free 

wake model, which provided an accurate and 

consistent inflow reference. Analyses of inboard 

and mid-radius blade locations presented herein 

support the following conclusions.

• Rotational effects on mean Cn are active

across the entire blade performance 

envelope. At low and moderate α where 

inviscid influences dominate, blade rotation 

steepens Cn–α curves. At elevated α where 

viscous effects play a major role, blade 

rotation delays stall to higher α, and 

produces higher stall Cn. 

• At each radial location, MEXICO and UAE 

Cn–α curve steepening, stall α delay, and Cn

amplification are closely comparable. 

However, as indicated by Cn–α curve 

conformation in the stall and post-stall α

range, MEXICO and UAE stall dynamics 

differ appreciably for inboard locations and 

to a lesser extent at mid-radius, consistent 

with differences in respective airfoil shapes. 

• Consistent with disparate inboard Cn–α

curves, inboard cp distributions also are 

dissimilar. The MEXICO cp distribution is 

consistent with a trailing edge separation, 

while the UAE distribution implies a leading 

edge separation followed by shear layer 

impingement.

• Blade rotation amplifies Cn standard 

deviation levels above those for stationary 

blades. UAE Cn standard deviation levels 

consistently exceed those for MEXICO. 

However, strong correlations exist between 

MEXICO and UAE Cn standard deviation 

level variations with respect to α. 

• The MEXICO and UAE blade planforms 

differ substantially. Nonetheless, threedimensional surface pressure topologies

corresponding to peak stall operation share 

common features, testifying to the 

robustness of the rotationally modified flow 

field.

The current work has comparatively analyzed

MEXICO and UAE data to validate and 

generalize knowledge regarding rotationally 

augmented blade flow fields. Some results 

presented herein provide confirmation of prior 

research, while other results provide insights not 

previously grasped in analyses of either data set 

alone. Future inquiry using the MEXICO and 

UAE data will foster more complete 

understanding of rotationally augmented blade 

flows, and thus facilitate more accurate 

prediction and improved turbine design. 

5 References

[1] Schepers, J. G., Brand, A., Bruining, A, 

Graham, J., Hand, M., Infield, D., Madsen, H., 

Paynter, J., Simms, D., “Final Report of IEA 

Annex XIV: Field Rotor Aerodynamics”, ECN-C-

-97-027, Energy Research Centre of The 

Netherlands, Petten, The Netherlands, June 

1997. 

[2] Schepers, J. G., Brand, A., Bruining, A, 

Hand, M., Infield, D., Madsen, H., Maeda, T., 

Paynter, J., van Rooij, R., Shimizu, Y., Simms, 

D., and Stefanatos, N., “Final Report of IEA 

Annex XVIII: Enhanced Field Rotor 

Aerodynamics Database”, ECN-C--02-016, 

Energy Research Centre of The Netherlands, 

Petten, The Netherlands, February 2002.

[3] Dexin, H., and Thor, S.-E., “The Execution 

of Wind Energy Projects 1986-1992,” FFA TN 

1993-19, Aeronautical Research Institute of 

Sweden, Bromma, June 1993.

[4] Ronsten, G., “Geometry and Installation of 

Wind Tunnels of a STORK 5.0 WPX Wind 

Turbine Blade Equipped with Presssure Taps,” 

FFAP-A 1006, Aeronautical Research Institute of 

Sweden, Bromma, February 1994. 

[5] Hand, M., Simms, D., Fingersh, L., Jager, 

D., Cotrell, J., Schreck, S., and Larwood, S., 

“Unsteady Aerodynamics Experiment Phase VI: 

Wind Tunnel Test Configurations and Available 

Data Campaigns,” NREL/TP-500-29955, 

December 2001, Golden, CO: National 

Renewable Energy Laboratory.

[6] Snel, H., Schepers, J., and Montgomerie, 

B., “The MEXICO project (Model Experiments in 

Controlled Conditions): The database and first 

results of data processing and interpretation,” 

The Science of Making Torque from Wind 

Conference, Journal of Physics: Conference 

Series 75 (2007) 012014 doi:10.1088/1742-

6596/75/1/012014.

[7] Schreck, S., “Low Frequency Shedding 

Prompted by Three-Dimensionality under 

Rotational Augmentation,” AIAA-2010-0640, 

January 2010.

[8] Snel, H., J. Schepers, and B. 

Montgomerie, 2007, “The MEXICO project 

8
(Model Experiments in Controlled Conditions): 

The Database and First Results of Data 

Processing and Interpretation,” The Science of 

Making Torque from Wind Conference, Journal 

of Physics: Conference Series 75. 

[9] Snel, H., Schepers, G., and Siccama, N., 

“MEXICO Project: The Database and Results of 

Data Processing and Interpretation,” AIAA-2009-

1217, January 2009.

[8] Giguere, P., and M. Selig, "Design of a 

Tapered and Twisted Blade for the NREL 

Combined Experiment Rotor," NREL/SR 500-

26173, Apr. 1999, Golden, CO: National 

Renewable Energy Laboratory.

[9] Butterfield, C., W. Musial, and D. Simms, 

(1992). “Combined Experiment Phase I Final 

Report” NREL/TP-257-4655. Golden, CO: 

National Renewable Energy Laboratory.

[10] Zell, P., “Performance and Test Section 

Flow Characteristics of the National Full-Scale 

Aerodynamics Complex 80- by 120-Foot Wind 

Tunnel,” NASA TM 103920, January 1993.

[11] Snel, H., J. Schepers, and B. 

Montgomerie, 2007, “The MEXICO project 

(Model Experiments in Controlled Conditions): 

The Database and First Results of Data 

Processing and Interpretation,” The Science of 

Making Torque from Wind Conference, Journal 

of Physics: Conference Series 75. 

[12] Snel, H., Schepers, G., and Siccama, N., 

“MEXICO Project: The Database and Results of 

Data Processing and Interpretation,” AIAA-2009-

1217, January 2009.

[13] Sant T., van Kuik G. and van Bussel G. J. 

W., “Estimating the angle of attack from blade 

pressure measurements on the NREL Phase VI 

rotor using a free wake vortex model: axial 

conditions,” Wind Energy, v. 9, n. 6, pp. 549 –

577, 2006.

[14] Micallef, D., Kloosterman, M., Ferreira, C., 

Sant, T., and van Bussel G.J.W., “Validating 

BEM, Direct and Inverse Free Wake Models with 

the MEXICO Experiment,” AIAA-2010-0462, 

January 2010.

[15] Schreck, S., and Robinson, M., “Boundary 

Layer State and Flow Field Structure Underlying 

Rotational Augmentation of Blade Aerodynamic 

Response,” J. of Solar Energy Eng., v. 125, pp. 

448-456, November 2003.

[16] Schreck, S., Sørensen, N., and Robinson, 

M., “Aerodynamic Structures and Processes in 

Rotationally Augmented Flow Fields,” Wind 

Energy, v. 10, n. 2, March/April 2007, pp. 159-

178.

9
PIX #: 17306 PIX #: 17305

Figure 1. MEXICO turbine mounted in DNW 9.5 m x 9.5 m wind tunnel 

(left) and UAE Phase VI turbine mounted in NASA Ames 24.4 m x 36.6 m 

wind tunnel (right).

Figure 2. MEXICO and UAE Phase VI blade planform drawings with 

suction surface tap locations. Leading edge is at the top of each planform.

10
Figure 3. Sectional blade twist for aerodynamically 

active parts of MEXICO and UAE Phase VI blades.

Pressure 

Measurements 

from Experiment

Assume initial 

AOA

Calculate Bound 

Circulation

Solve for wake 

velocity field

Update wake 

node positions

Calculate new 

AOA

Figure 4. Schematic of inverse free wake model algorithm used to 

compute angles of attack for MEXICO and UAE Phase VI data.

11
Figure 5. MEXICO and UAE Phase VI mean Cn at 

inboard radius, for parked (upper panel) and 

rotating (lower panel) blades.

12
Figure 6. MEXICO and UAE Phase VI mean Cn at 

mid-span radius, for parked (upper panel) and 

rotating (lower panel) blades.

13
Figure 7. MEXICO and UAE Phase VI σCn at 

inboard radius, for parked (upper panel) and 

rotating (lower panel) blades.

14
Figure 8. MEXICO and UAE Phase VI σCn near 

mid-span radius, for parked (upper panel) and 

rotating (lower panel) blades.

15
Figure 9. MEXICO and UAE Phase VI stall cp

distributions at inboard radius, for rotating blades. 

MEXICO α = 20.40 and UAE α = 20.80. 

Figure 10. MEXICO and UAE Phase VI stall cp

distributions near mid-span radius, for rotating 

blades. MEXICO α = 14.40 and UAE α = 12.90.

16
Figure 11. MEXICO and UAE Phase VI post-stall cp

distributions near mid-span radius, for rotating 

blades. MEXICO α = 26.60 and UAE α = 25.30.

Figure 12. MEXICO suction surface pressure contours. U∞ = 19.8 m/s, 

contour ∆p = 500 Pa. Leading edge is at the top of planform.

Figure 13. UAE Phase VI suction surface pressure contours. U∞ = 15.1 

m/s, contour ∆p = 100 Pa. Leading edge is at the top of planform.

17
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4. TITLE AND SUBTITLE

Rotational Augmentation Disparities in the MEXICO and UAE Phase 

VI Experiments: Preprint

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

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6. AUTHOR(S)

S. Schreck, T. Sant, and D. Micallef

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Wind turbine structures and components suffer excessive loads and premature failures when key aerodynamic 

phenomena are not well characterized, fail to be understood, or are inaccurately predicted. Turbine blade rotational 

augmentation remains incompletely characterized and understood, thus limiting robust prediction for design. 

Pertinent rotational augmentation research including experimental, theoretical, and computational work has been 

pursued for some time, but large scale wind tunnel testing is a relatively recent development for investigating wind 

turbine blade aerodynamics. Because of their large scale and complementary nature, the MEXICO and UAE Phase 

VI wind tunnel experiments offer unprecedented synergies to better characterize and understand rotational 

augmentation of blade aerodynamics.

15. SUBJECT TERMS

Force amplification; rotational augmentation; wind tunnel; wind turbine; unsteady loading 

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