Downstream Wind Flow Path Diversion and Its Effects on the Performance of Vertical Axis
Wind Turbine
Manganhar A. L, Rajpar A. H, Samo S. R and Luhar M. Ramzan
ABSTRACT
Environmental friendly energy generation from natural source of wind is the crucial issue of this
decade. Vertical axis wind turbine are more suited to urban areas as they are silent and reduced
risk associated with slower rate of rotation. Applications of wind generators employed in lowwind speed zones require some additional integration to generate more power. Efforts in
development of guide mechanism for concentration of wind stream in upstream wind have been
commonly implemented to improve the impact of wind beam on rotor blades. So far limited
efforts have been made to analyze the effect of downstream wind flow in the form of the
improvement in diffuser geometry. Conventional VAWT extract energy from incoming wind
flow up to some extent, however huge amount of energy is carried out by downstream wind flow
due to very short time contact with the rotor. In the present experimental study effect of path
diversion of downstream wind flow on performance of VAWT was analyzed. For the blockage
of downstream wind flow path at various linear displaced positions, a normal erected wall of flat,
Sami-circular and cylindrical shape for path diverting geometries were tested. Performance of
VAWT in terms of improved rotor speed was achieved as reported in this experimental study.
Key words: Upstream flow, downstream flow, Wind diversion, Vertical wall, Wind beam.
1
design
1. INTRODUCTION
suitable for low wind areas to improve the
Urbanization and Industrialization are two
important
processes
of
incoming wind speed was proposed by
socio-economic
Govardhan and Dhanasekaran [4]. Vane
development and are directly dependent of
guided mechanism for upstream wind flow
available energy. Fossil fuels will remain no
for conventional wind turbine to study its
more dependable to bear the load of
effects on turbine performance has been
continuous population growth and their
development
Environment
processes
has
reported in [5] [6]. They have shown that
requirements.
gotten
the efficiency and starting characteristics of
continuously
the Wells turbine (a ducted turbine) has been
polluted through its quality degradation
improved
processes and will provide limited safe
energy
wind flow. It was proved that conventional
wind
capacity and potential to face future energy
and
the
duct to guide and concentrate incoming
wind and solar are dependable and have the
Efficient
with
wind turbine with vertical bucket shaped
generation
processes. Renewable energy sources like
challenges.
compared
Hu and Cheng [7] integrated a vertical axis
is impossible without environmental friendly
dependable
when
respective turbines without guided vanes.
living to its users. Surely sustainable living
and
of ducted or funneled wind turbines
turbine
with
integrated
accelerating techniques has
economic
flow
renewable resource based power generation
around
estimated
is top priority of researchers. They have
the
that
the
wind
improved the
generator
power
and
was
extraction
efficiency can be increased up to 80 % due
been continuously working for improvement
to ducting effect.
in existing systems as well as searching for
Abe etal[8], conducted experimental and
innovation.
analytical investigations on a new diffuser-
In low windy or turbulent wind zones,
shrouded wind turbine assembly. The
Energy generation from wind
by using
characteristic values of the flow fields were
conventional wind turbines would not be
estimated and compared with those for a
suitable for sustainable socio-economic
bare wind turbine. It was concluded that the
development
to be
diffuser-shrouded wind turbine provides
integrated with wind concentrating and
much higher power output as compared to
guide techniques[1] [2] [3]. Modification in
the bare wind turbine. The power coefficient
but it is
needed
2
of the diffuser-shrouded wind turbine
design overcomes the inferior aspect of low
claimed was about four times that of the
wind speed by guiding and increasing the
bare wind turbines. It is also reported that
speed of high altitude wind through the
with a simple momentum theory, developed
ODGV. The expected wind speeds and,
along the lines of momentum theory for bare
therewith, the wind energy potential depends
wind turbines, the power augmentation is
strongly on the local weather conditions and
proportional to the mass flow rate generated
building arrangement. Modeling the flow
at the nozzle of the diffuser augmented wind
field in such urban canyons is difficult [15]
turbine (DAWT) [9]. Such mass flow
[16].
augmentation can be achieved through two
A
basic principles i.e.
renewable energy and rain water harvester
(i) Increase in the
novel
shrouded
hybrid
diffuser exit to inlet area ratio (ii) by
with
decreasing the negative back pressure at the
(ODGV) for urban high-rise application was
exit.
introduced by W. T. Chong etal[17]. The
Generally, urban areas have weak wind
ODGV surrounds the vertical axis wind
conditions and turbulence due to the
turbine (VAWT) and enhances the VAWT
presence of high rise buildings. The number
performance by increasing the incoming
of obstacles in the path of the wind also
wind speed and guiding it to an optimum
makes it difficult to model the wind resource
flow angle before it interacts with the rotor
[10] [11] [12]. Thus, the wind energy
blades. The experimental results show that
generation systems for urban regions need to
the rotational speed of the VAWT increases
overcome
by about 2 times.
these
disadvantages.
More
an
wind-solar
Omni-directional-guide-vane
effective systems are required to facilitate
From this reviewed study it was observed
these thickly populated low windy regions.
that the performance of wind generator
Grant et al[13], have also reported that
working in low windy areas have been
ducted wind turbine which is attached to the
improved by concentrating and introducing
roof of a building has the significant
guide mechanism for upstream wind flow.
potential for retrofitting onto the existing
So for efforts have not been taken into
building with minimum visual impact.
consideration
Omni-directional-guide-vane was introduced
mechanism for downstream wind flow. The
[14]. It was proposed that the patented
present study is an attempt to extract
3
to
introduce
a
guide
maximum energy from the downstream
3. TEST SETUP
wind flow by introducing a mechanism to
In this experimental study a continuous,
divert the downstream wind flow that may
controlled source of wind was introduced so
be more beneficial as compared to the
that (i) minimize the error in analysis (ii)
conventional outgoing wind flow.
constant
source
of
wind
during
experimentation. For continuous, constant
2.
speed of the wind, a wind tunnel was used as
SYSTEM DEVELOPMENT
of 1m2
wind source. Average of five tests for each
projected area with blade size of 0.92 x 0.46
setup was considered for further analysis.
meter
fixed on 0.6 m tripod stand was
Keeping in view the orientation and type of
developed in workshop of Mechanical
the downstream wind blockage such as flat
Engineering Department of Quaid-e-Awam
wall, Semi circular vertical wall and
University Nawabshah. Low speed open
cylindrical wall were used as flow diverters
wind
as shown in Fig. 2(a, b, c).
Four bladed Savonius rotor
tunnel
comprising
wind
speed
controller with exit area of 0.09 m2 was used
as an auxiliary wind source as shown in Fig.
1.
Rotor
Fig.2. (a)
Low-speed open
wind tunnel
Semi circular
wall
Fig.2.(
b)
Fig. 1 System over View
4
Figure no.3 is the plan view of the rotor and
wind source. The performance of the rotor
in terms of the speed was tested in the
absence of any downstream blockage.
Initially the wind at constant speed of 1m/s
was introduced by rotating the direction of
wind source. In such a way by keeping wind
speed at 3m/s and 5 m/s simultaneous tests
were conducted so that appropriate angle of
Fig.2.(c)
wind source can be decided for further data
Fig.2. Types of blockages (a) Flat wall (b)
analysis
Semi circular wall (c) cylindrical wall
4. DOWN STREAM WIND BLOCKAGE
In this setup four bladed rotor was kept on
4.1 Flat Wall Wind Diverter
fixed position and wind source with constant
wind beam was adjustable to decide the
A flat wall up to the projected area of the
suitable striking angle of the wind source on
rotor was placed vertically parallel to
the blade starting from origin of the rotor
vertical axis of the rotor at the downstream
towards the tip of the blade as shown in
side. Wind from the source, after hitting the
Fig.3.
rotor,
experienced
blockage
on
the
downstream flow and tried to change the
path of flow according to the geometry of
solid wall, resulting change in rotor speed.
Flat wall was set flexible and effect was
analyzed with increase in distance between
rotor and the wall at various positions as
well as changing its orientation as shown in
Fig.4(a, b).
Fig. 3. Top view of the rotor and wind
source
5
were conducted with the clockwise rotation
of wall at every 10o rotation with horizontal
axis of rotor. The tangents drawn at each
rotation step intersect the effective path of
wind and represent its new path after
diversion as shown in Fig.5.
Fig.4 (a) 2D view of rotor at various
position of flat wall.
Fig. 5: 2D view of the Semi circular vertical
wall with rotor
4.3. Cylindrical wall Blockage
Fig.4(b) 2D view of rotor at changing
The Semi circular vertical wall blockage
orientation of flat wall.
discussed in previous section was enhanced
from two to three quadrants, here in this
4.2. Semi Circular Wall Blockage
case. Only wind beam direction side
The curved wall was used to divert
quadrant was left open to intake wind from
downstream wind flow in order to observe
tunnel. It seems to be a cylindrical enclosure
diversion effects on the rotor performance.
for vertical axis wind turbine with horizontal
In this setup a semi circular vertical wall of
opening for inward wind flow and vertical
radius 0.84m (0.3m greater than the radius
opening for outward flow as shown in Fig.
of rotor) was placed vertically to cover half
6. The geometry with all defined dimensions
of the rotor from downstream side. Tests
6
was tested as an outward wind blocker and
fits with the geometry and dimensions of the
direction modifier to study its effects on the
blades of the selected rotor.
performance of the rotor.
Fig. 6. 2D Views of the cylindrical blockage
rotor
Fig. 7. Effect of wind direction change on
rotor performance.
5. RESULTS AND DISCUSSIONS
5.1. Performance of VAWT at Various
In the second proposed setup, wind tunnel
Wind Directions
was adjusted at proposed wind direction.
The effect of wind velocities on the
The performance of vertical axis wind
performance of the rotor was analyzed. It
turbine rotor in terms of rpm was tested at
was observed that the performance of the
various speeds of wind source by changing
rotor increases approximately in linear way
direction of the wind beam striking from
with velocity. It may be noted that the small
center of the rotor to the tip of the rotor
variation in performance was observed due
active side without introducing blockage in
to natural wind interference in the path of
the downstream flow (free stream wind).
effective wind direction. Effect of the wind
Effect of the rotation of wind source on the
velocity on rotor speed is shown in Fig.8.
rotor speed is shown in Fig.7. It was
observed that 15° clockwise rotation of wind
source for this particular rotor was most
effective. This new path of the wind mostly
7
to 1.7 m of this rotor after word increament
was ineffective as shown in Fig.9. All the
readings were taken on effective wind
direction.
Fig 8: Effect of wind speed on rotor speed
Fig. 9. Effect of change in displacement of
5.2 Performance of VAWT with Flow
flat wall blockage on rotor speed
Diverters.
In an other approach
5.2.1 Flat Wall
the flat wall was
rotated anticlockwise and the effect on the
The effect of flat wall blockage at down
performance of vertical axis wind turbine
stream side was analyzed initially by
was observed at varouis angles as shown in
varying the distance between wall and the
Fig.10. It was observed that performance of
rotor placed normally as shown in Fig.9.
the rotor decreases with increase in the
Maintaninig the necesory clearence between
rotation angle from 0° to 90° in response to
wall and rotor blade first test was taken at a
distance of
increase blockage. The maximum reduction
0.02m from the rotor blade.
in its speed was about 20%.
Other tests were conducted maintanig
constant distance variation of 0.3 m.
According to the observations the blockage
effect was realised upto 1.7m. The speed of
rotor increases linearly with the increament
in distance between rotor and blockage up
8
then it returns to its free stream performance
of 66rpm with further rotations. This 70o
rotation angle diverts flow at about 105o
rotation in the direction of rotor.
Fig. 10. Effect of flat wall rotation on the
rotor speed .
5.2.2. Semi Circular Vertical Wall
At the initial setup half of the rotor at
downstream side was covered with semi
circular vertical wall to divert the wind after
Figure 11: effect of Semi circular wall
leaving the rotor. From this experimental
blockage on rotor speed
setup it was observed that performance of
the rotor in terms of the speed increases
linearly with change in wind velocity but its
overall performance as compared to free
stream wind was about 4% lesser. Effect of
the semicircular blockage on the speed of
the rotor is shown in Fig.11.
At the second setup, clockwise rotation was
given to the Semi circular vertical wall and
observations were recorded at every 10°
rotation angle as shown in Fig.12. It is
obvious
from
the
figure
that
Fig.12. Effect of Semi circular wall
15%
clockwise rotation on the rotor speed.
improvement in the performance of rotor
speed was recorded at 70° rotation angle and
9
5.2.3. Cylindrical Wall
At the initial stage, for this setup, tests were
conducted at wind direction normal to the
vertical axis of the rotor. The performance
was tested by increasing wind velocity from
0 to 10m/s as shown in Fig.13. It was
recorded that the performance of the system
decreases up to about 45% as compared to
free stream performance. In the second stage
Figure 14: Effect of cylindrical diversion on
this setup was tested by changing the wind
the rotor rpm with respect to wind direction.
direction and was observed that 150 rotation
of source gives 65 rpm as shown in Fig.14
It has been analyzed that the bluff bodies
with the change in their orientation take
aerodynamic
characteristics
and
can
improve the performance of VAWT when
placed at the downstream side of turbine
with their proper orientation.
The performance of rotor with and without
flow diverting geometries in terms of rpm at
source wind speed of 5 m/s angle of source
rotation (θ) of 15° and other given
conditions are shown in Table.1 as well as in
Fig. 13: Effect of the cylindrical blockage on
Fig.15. Here Φ is diversion angle of various
the rotor speed
types of tested blockage.
10
Table.1 Performance of rotor at various
degree angle of attack. Smaller
blockage rotations
angles can be used for flow diversion
with minimum loss, if necessarily
Type of
Blockage
required.
Flat wall
Curved wall
Cylindrical
wall

Semi circular vertical wall diverts
and guides flow in the direction of
Rotation
angle (Φc)
Rotor
speed
(RPM)
(min )
(max)
(min )
(max)
0°
90°
0°
70°
68
54
60
76
rotor rotation, extracts more energy.
(No
rotation)
15%
improvement
in
the
performance of conventional rotor in
64
terms of rpm was observed with 105o
diversion angle at 5m/sec constant
wind velocity.

45% decrease in the performance of
rotor was observed in housing it in a
cylindrical wall with wind direction
normal to its projected area. Setting
it at the best value of proposed
source wind direction of 15o. The
loss of circular type blockage on
motor speed nearly recovered.

This research study is applicable
only for proposed four bladed rotors
with known parameters.
Fig.15 Performance of motor at various
types of Blockages
7. RECOMMONDATIONS
These flow path diverting geometries
6. CONCLUSION

Flow path diversion with flat wall
may be used with VAWTs in urban
shows maximum blockage effect
applications to avoid turbulence
(generate
with
generated by adjacent buildings at
turbulence) at 90o angle of attack and
downstream wind flow in order to
redevelops stream line flow at zero
improve their performance.
back
pressure
11
Environments,” Refocus 2004, Vol.
8. ACKNOWLEDGEEMNT

The
authors
acknowledge
5, No. 3, pp. 32-35.
and
appreciate co-operation and technical
[4]
help provided by Mr.Akhter Hussain
Dhanasekaran T S, Govardhan M.
(2005), “Computational analysis of
Mughul Workshop Superintendent
performance and flow investigation
Mechanical Engineering Department
on wells turbine for wave energy
workshop QUEST, Nawabshah and
conversion”,
his staff in the development of this
Renewable
Energy,
2005, 30(14): 2129−2147.
project.
[5].
Grassmann, H., Bet, F., Cabras,
G., Ceschia, M., Cobai, D. and
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