近畿大学次世代基盤技術研究所報告 Vol. 12（2021）73－80

Measurement of flow fields in the wake of upwind and downwind
wind turbines in wind tunnel experiments
Jay Prakash GOIT*1 and Takatsugu KAMEDA*2
The current study compares aerodynamic characteristics and performance of upwind anddownwind wind turbines in wind tunnel experiments. The scaled model of wind turbine, named
KDWT25, is designed and fabricated. Comparison of wake fields of the two configurations showedthat the wind speed recovery in the wake of downwind turbine is faster than the upwind turbine
for the uniform and laminar inflow conditions considered in this study. The tendency of turbulence
profiles for both configurations is similar, with the maximum turbulence intensity observed
around the rotor edge. Power coefficient at design tip speed ratio is 30% higher for the downwind
turbine, even when operated in the same inflow conditions. It is assumed that the nacelle-induced
blockage which deflects the flow toward more efficient outer region of the rotor, may be responsible
for the better performance of the downwind turbine.
Keywords: Wind Tunnel Experiment, Downwind Turbine, Upwind Turbine, Wind Turbine Wake

1．Introduction
Large utility-scale wind turbines are predominantly
horizontal-axis with three-bladed rotors. Thesehorizontal-axis wind turbines have either upwind or
downwind configuration; former have rotor upwind of
towers, while later have rotor downwind of towers.
Although upwind turbine configurations dominate
current wind energy market around the world,
downwind turbines have received significant attention
in the recent years due to their potential advantages forincreasing the rotor size and the rated power.
Current large-scale wind turbines have rated powerof around 5 MW to 8 MW. This is expected to increase
to 10-20 MW, with rotor diameters of 170 to 240 m in
the near future 1. The primary motivation for installing
larger turbines is that they capture more energy fromwind, thus reducing the levelized cost of energy (LCOE).
Recent developments in offshore wind energy willcontribute to the further increase in the sizes ofturbines, since transportation, installation andoperation of large wind turbines at offshore sites pose
fewer challenges compared to the onshore sites.
However, blades of large wind turbines can be as longas 100 m or more, and thus, in conventional upwind
configuration they are prone to tower strikes. To
overcome this issue, blades of large turbines aredesigned with increased stiffness. Furthermore, rotor

cone and tilt angles are increased in order to increase
clearance between blade tip and tower2. Increasing
blade stiffness can significantly increase the cost of
blade manufacturing, and large cone and tilt angles can
reduce the turbine power output. Downwind
configuration, on the other hand can have more flexible
blades, since the axial load acting on blades will bend
them away from the tower. They can also have lower
cone and tilt angles, as they do not require large tower
clearance. This will allow downwind turbines to operate
close to optimum design condition thus allowing themto maintain their performances3.
Several other advantages of downwind turbines havebeen discussed in literatures. Nacelles of downwind
turbines act as blockage to the incoming flow, such thatwind is deflected radially outward on the rotors2.
Because the outer regions of turbine rotors are
responsible for most of the power production, nacelleinduced blockage is believed to improve the output ofdownwind turbines 4. Another advantage of downwind
turbines related to their performance in complex
terrain was discussed by Yoshida 5. He showed that due
to negative tilt angles, downwind turbines experiencesmaller misalignment with respect to up-hill flow, thus
having advantage over upwind turbine configuration.
Downwind turbines have tendency to align themselveswith the incoming flow. This will reduce the yaw
misalignment and can also be exploited for reducing thecost of the yaw control system 4.

Received: 19 May 2021
*1 Department of Mechanical Engineering, Kindai University, Higashi-Hiroshima, Hiroshima 739-2116, Japan
Email jay.goit@hiro.kindai.ac.jp*2 Department of Mechanical Engineering, Kindai University, Higashi-Hiroshima, Hiroshima 739-2116, Japan
Email kameda@hiro.kindai.ac.jp－ 73 －

However, downwind configuration has one
significant disadvantage, i.e., they experiences strong
tower shadow effect 6. Tower shadow is simply the windspeed deficit in the wake of the tower. The blades of
downwind turbines experience reduced wind speed
once per revolution, causing impulsive loading andrapid fluctuation in power7. The impulsive loads are thesource of vibration of turbine structure at blade-passing
frequency (BP), resulting in higher fatigue loading.
Blades interaction with the tower wake is also thesource of considerable low frequency noise emission 8.
Higher fatigue loads and noise emission are theprimary factors why most utility-scale turbine
manufacturers do not employ downwind configuration.
At the offshore sites however, noise emission is not
an issue, since turbines can be installed far from thecoast. Therefore, because of the above discussed
advantages, downwind configuration is receiving
considerable interest for large offshore wind turbine
design. In this regard, Ichter et al. 9 and Loth et al.1 has
proposed a morphing downwind-aligned rotor concept
for extreme-scale wind turbines ( � ����� ). This
unconventional morphing design is bio-inspired bypalm trees, which have segmented trunks that areeasily able to deflect downwind under extreme windconditions.
Although several advantages and disadvantages of
downwind configurations have been reported in theliteratures, comparison of performance—in terms of
power output—of the upwind and downwind turbines
has not been reported to date. Difference between theflow fields around the two turbine configurations has
also not received sufficient attention. In the current
work we compare the aerodynamic characteristics andpower performances of upwind and downwind windturbines in wind tunnel experiments. This technicalreport is further organized as follows. Section 2
describes measurement setup, providing details aboutmodel wind turbines, wind tunnel facility andcharacterization of generator used. Flow fields andpower outputs for upwind and downwind wind turbines
are compared in Section 3. Finally, Section 4 provides
summary of the work.
2．Measurement setup
This section describes the model wind turbine andwind tunnel setup used in the experiments.

2.1. Wind turbine model

The model wind turbine (KDWT25) used in theexperiemnts has both rotor diameter ��� and hub
height of 0.25 m. The rotor of KDWT25 was optimized
using BEMTurbine 11, an in-house open source tool.
Following earlier studies, SD7003 airfoil profile wasused along entire blade length 12. The design employed

lift (�� ) and drag (�� ) coefficient profiles at Reynolds
number of 50,000. Chorld length and twist angle—along
the blade length—were initially optimized for tip speed
ratios λ � �� �������. Tip speed ratio is given by:
(1)
��
λ� .
�

where � is the angular velocity of the rotor, R is therotor radius and � is inflow velocity. Since most utilityscale wind turbines have optimum λ value of 7, model
wind turbine should also be optimized for similar λvalue, in order to ensure that it has similar
aerodynamic characteristics as utility-scale turbines.
However, a small wind turbine with higher tip speed
ratio will have very high rotational speed. For instance,
if the model turbine used in this study is designed forλ � �, its optimum rotational speed will become morethat 2100 rpm (at wind speed of 4 m/s). Due to large
contribution of friction loss from motor and othermoving components, it is not practical to attain such ahigh rotor speed. As a consequence, such turbine willshow significantly poor performance in terms of power
output. Therefore, KDWT25 was designed to perform
optimally at the lower tips speed ratio of λ � �. For
the details about the BEMTurbine and the design of
KDWT25, readers are referred to Goit 13.

Figure 1. Design and major parameter of KDWT25
Figure 1 shows the final design of the model windturbine. For the accurate comparison, same rotor has
been used for both upwind and downwind
configurations. In terms of geometrical scaling, NREL
5-MW reference wind turbine is 500 times and DTU 10MW reference wind turbine is 700 times larger than
KDWT25. The model does not have nacelle tilt or rotor
pre-cone angles.
Performance of the rotor design is analyzed using
blade element momentum (BEM) analysis function in
the BEMTurbine. Figure 2 shows power coefficient (�� )as a function of tip speed ratio. �� is defined as:－ 74 －

�� � �

��� � ��

�

,(2)

where � is aerodynamic power of rotor, � is air
density and � � � �⁄� � � is the rotor area. Maximum
power coefficient of 0.47 is attained at the design tip
speed ratio value of � � 5. However, as will be clear
from Section 3, �� � 0��7 is very high and could not be
obtained in the actual measurements.

turbine is quite high (about 1500 rpm) resulting inhigher generator voltage, �� was set to 520Ω.
Output characteristic of the motor was evaluated bycoupling two motors of the same specifications as shownin Figure 3 (b). In this measurement, power was
supplied at a constant voltage to motor I, and motor II
was connected to a variable resistor. When both motors
started to rotate, motor II would operate as a generator
dumping the generated power to the resistor. Rotational
speed of the system was controlled by changing theresistance value of the variable resistor, and rotational
speed, current and voltage of motor II were themeasurement parameters. Power output of the motors
could be computed using� � �� �� � �� �

(4)

where R is the resistance value set in the variable
resistor.
Figure 2. Power coefficient of KDWT25 as a functionof tip speed ratio.

2.2 Generator performance

The generator used in the model wind turbine wasbrushed DC motor (Xikit XGM-RA). Usually, small DC
motors are designed to perform optimally at therotational speed of several thousand rpm. However, thisDC motor can generate fairly high power at lower rpm
without any gear system, thus, making it suitable forthe small model wind turbine employed in this study.
But there was no data available on the performance ofthe motor. Therefore, prior to the wind tunnel
measurements, internal resistance and output
characteristics of the motor was evaluated in thecurrent study.
Figure 3 shows the schematics of the experiments forthe evaluation of the motor performance. Figure 3 (a)shows the schematic of internal resistancemeasurement setup. In this setup, rotating disk was
attached to the motor axis, and tests were conducted bysupplying power at three different voltages: 5 V, 10 V
and 12 V. During the measurement the disc was fixed(not allowed to rotate), so that any power loss would
only be due to the internal resistance of the motor.
Internal resistance (�� ) can then be computed using�� � ���

(3)

where V and I are measured voltage and current. In
order to improve the accuracy of the measurements,
tests were conducted at four different disc azimuth
angles: 0° , 90° , 180° and 270° for each input voltage.
It was found that �� varies between 516Ω to 550Ω.
For the higher voltage though, �� converged to 516Ω
to 520Ω. Since the rotational speed of the model wind(a) Measurement of internal resistance(b) Measurement of motor characteristics
Figure 3. Schematic of motor performance evaluation
tests.
Figure 4 shows the power output of motor II as afunction of resistance. The measurements wereconducted for four different voltages ranging from 15 V
to 24 V of the motor I. The selected voltage range
corresponds to the voltage range of the generator during
the wind tunnel experiments. It can be appreciated thatthe maximum power output was generated aroundresistance value of 900Ω . Following this results, allwind tunnel experiments in this study were conductedby fixing the resistance—connected to the generator—to－ 75 －

900Ω.

Figure 4. Motor power as a function of resistance.

2.3 Wind tunnel

Measurements were conducted in a variable windspeed type wind tunnel at School of Engineering,
Kindai University. It is a closed-circuit type wind tunnel,
and has 1.38 m long test section with square crosssection of 0.59 m long sides. The blockage ratio—definedas the rotor area to the test section cross-sectional area—
is 14%. Although the blockage ratio is slightly higher
than what is recommended for wind tunnel
experiments, in this report we do not discuss about theeffect of blockage on the accuracy of the measurements.
Figure 5 shows the schematic and an actual picture of
the test section with the model wind turbine installed.
All the measurements were conducted at an average
free stream velocity (�� ) of 4 m/s. The study used hot
wire anemometry for the measurement of wind speed in
the turbine wake. To that end, I-type hot wire probe(Kanomax 0248R-T5) was used. Differential
manometer (Okano Seisakusho DMP-202N12) wasused to calibrate the hot wire anemometer. Wind speed
data was collected for the period of 10 second at 10 kHz
sampling rate. Rotational speed required for the
analysis of power output was measured using an
external optical tachometer, while voltages andcurrents of the generator were measured using regularmultimeters.

(a) Schematic of the wind tunnel test section(b)Picture of model wind turbine inside the wind tunnel.
Figure 5. Wind tunnel test section.

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3．Results and discussion
Before the measurement and comparison of wakefields, flow characteristics of the wind tunnel wasinvestigated. Figure 6 shows the inflow profiles ofvelocity and turbulence intensity before wind turbine
was installed in the test section. It is appreciated thatwind speed is uniform over the entire measurement
span. Average velocity is 4.09 m/s which is close to thetarget inflow velocity of 4 m/s. Average turbulence
intensity over the entire horizontal span is 0.76%. Even
the highest turbulence intensity measured in thisexperiment is 0.84%. Therefore, it can be concluded that
freestream turbulence of the wind tunnel is negligibly
small.
Note that utility scale wind turbines operate inatmospheric boundary layer which is always turbulent.
Turbulence
intensities–attheheightrange
corresponding to the rotor region—at offshore sites are
generally around 5%, while that at onshore sites can behigher than 10%. However, for the accurate comparisonof the aerodynamics characteristics of the two turbine
configuration, in this study experiments were
performed in uniform laminar inflow conditions.

(a) Mean velocity profile

Figure 7. Comparison of horizontal mean velocity
profiles at the hub height level in the wake of the
upwind and downwind turbines. Dashed-horizontal
lines indicate position of center and edges of the rotor.

(b) Turbulence intensity profile
Figure 6. Horizontal inflow profiles of velocity andturbulence intensity.

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Figure 8. Comparison of horizontal turbulence
intensity profiles at hub height level in the wake ofthe upwind and downwind turbines. Dashedhorizontal lines indicate position of center and edgesof the rotor.

Figure 7 and 8 compare mean velocity and turbulence
intensities in the wake of the upwind and downwind
turbines. Measurements were conducted at hub height
and at the spanwise interval of 20 mm. Furthermore,horizontal profiles were measured four streamwise
positions, 1D, 2D, 3D, 4D downstream from the rotor. It
is assumed that the difference between the wakeprofiles will be more conspicuous in the near-wakeregion considered in this study. In the far wake region,
profiles of both upwind and downwind turbines shouldlook similar.
The mean velocity profiles of both upwind anddownwind configurations show similar tendencies, i.e.,
the largest velocity drop occurs at the center, and it
gradually increases towards the edges. At 1D, velocity
defects are almost same over the entire rotor region, but
at downstream locations (3D and 4D) the profiles tend
to have Gaussian distributions. The main difference
between the wake of upwind and downwind turbines is
the rate of wind speed recovery in the wake. Wakerecovery is faster for the downwind turbines, resulting
in the higher wind speed at 2D and further downstream
positions. For example, at 4D, mean wind speed at thecenter of upwind turbine wake is 1.78 m/s, while thatfor downwind turbine wake is 2.31 m/s. This
characteristic of the downwind turbine can be exploitedin large wind farms. It may be possible to reduce theinter-turbine spacing if downwind turbines are used inwind farms (since their wake recovers faster). This may
reduce the cost of energy from such wind farms as therequired area of land lease will be smaller. However,current study only employed a single turbine. It is notpossible to predict how upwind and downwind turbines
will perform in a wind farm and how will they interact
with the boundary layer. Detail experiments orsimulations of wind farms with upwind and downwind
turbines are necessary to understand the difference inthe dynamics of these two configurations when installedin wind farms.
Tendency of turbulence profiles in figure 8 for both
upwind and downwind configurations is also similar.
Maximum turbulence intensity can be observed at therotor edges. This is possibly due to the tip vortices
resulting from the strong velocity gradient in the rotor
tip region. It is observed that when turbines are placedin uniform laminar flow, added turbulence intensity due
to rotor can be between 5 to 10%. In the present
measurement, turbulence intensities were largest at 1D.
From 2D through 4D, turbulence level in the flow did
not show significant reduction. This may be due to thefact that wind turbine wakes are able to sustain its
characteristics for longer downstream distance in thelaminar or low turbulent ambient flow. This is thereason why the wake interaction is more significant inoffshore wind farms.

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rotor edges. For the downstream distance from 2D
through 4D, turbulence level in the flow did notshow any significant reduction.
(3) The comparison of the performance of two turbines
showed that power coefficient of the downwind
configuration was 30% higher than that of the
upwind configuration.

Figure 9. Comparison of power coefficients of the
upwind and downwind turbines.
Finally, performance of the two turbine configurationis compared in figure 9. Its shows power coefficient (‫ܥ‬௉ )as a function of tip speed ratio. It is appreciated thatboth turbines have the peak ‫ܥ‬௉ values around thedesign tip speed ratio, ɉ ൌ ͷ. The peak ‫ܥ‬௉ of upwind
turbine was 0.172 while that of downwind turbine was
0.24. Downwind turbine has more that 30% higher ‫ܥ‬௉compared to the upwind turbine. Better performance ofdownwind configuration can be attributed to its design.
As also discussed in Section 1, nacelle of the downwind
turbine induces blockage to the incoming flow, such thatthe wind is deflected towards the outer region of therotor. Because the outer region of the rotor is
responsible for most of the power production, nacelleinduced blockage may be responsible for higher
efficiency of the downwind turbine.
Peak ‫ܥ‬௉ in the experiments were significantly small
compared to desing power coefficient ‫ܥ‬௉ ൌ ͲǤͶ͹ values
obatined from BEM analysis (cf. figure 2). The reason
for the descripancy lies in the fact that the rotational
speeds of wind turbine in the experiments were
between 800 rpm to 1800 rpm, resulting in large friction
loss. Because of the higher fraction of friction loss—
especially inside generators—in small model windturbines, their ‫ܥ‬௉ values obained from measurements
are generally smaller than the design ‫ܥ‬௉ values.

4．SummaryIn this study we have compared the aerodynamic
characteristics and power performances of upwind anddownwind wind turbines in wind tunnel experiments.
To that end, the scaled model wind turbine, named
KDWT25 was designed using an in-house open source
tool BEMTurbine. Findings of the study aresummarized as follows:(1) Wind speed recovery in the wake of downwind
turbine was faster than the upwind turbine for theuniform and laminar inflow conditions consideredin this study(2) The tendency of turbulence profiles for both upwind
and downwind turbines was similar, with the
maximum turbulence intensity observed at the

Acknowledgement
The authors acknowledge Hirose International
Scholarship Foundation for supporting this work．The
authors would like to thank Mr. Hiroki Sunakawa and
Mr. Naoyuki Kageoka of Kindai University for theirassistance during the experiments. Finally, the authors
are extremely grateful to Mr. Koji Umenishi of Kindai
University for his help and support in the
manufacturing of various components of the model
wind turbines.
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