IMPROVEMENTS OF VERTICAL AXIS WIND TURBINES USING SUPERCIRCULATION FLOW CONTROL ABSTRACT : Coflow -Jet (CFJ) represents a new supercirculation flow… [603401]

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AERODYNAMIC AND AEROACOUSTIC
IMPROVEMENTS OF VERTICAL AXIS WIND
TURBINES USING SUPERCIRCULATION FLOW
CONTROL

ABSTRACT : Coflow -Jet (CFJ) represents a new supercirculation flow control method able to improve the
aerodynamic performance of the airfoils due to increased circulation on the suction side. Operating wind
turbines generate tonal and broadband noises affecting the living environment adversely; especially small wind
turbines located in the vicinity of human living places. The current study carries out numerical prediction for
aerodynamic performance and noise radiated from an H -Darrieus Vertical Axis Wind Turbine which uses CFJ.
Incom pressible transient simulation is conducted to obtain the instantaneous turbulent flow field. Noise
prediction was performed by the Ffowcs Williams and Hawkings (FW –H) acoustic analogy formulation.
Simulations were performed for three different tip -speed r atios (TSR). First, the mean torque coefficient is
compared with the basic VAWT which does not use CFJ. Then, the study focuses on the broadband noises of
the turbulent boundary layers and the tonal noises due to blade passing frequency .

KEYWORDS: aeroaco ustic, CFJ,supercirculation,momentum, URANS

NOMENCLATURE

1. INTRODUCTION

There are two principal types of VAWT rotors: lift -type and drag -type. Lift -type rotors use airfoil
shaped blades to generate lift, a component of which is in the direction of rotation, thereby producing torque.
These rotors have been shown to be able to achieve power coefficients similar to HAWTs [Paraschivoiu, 2002].
Unlike HAWT rotors, the angle of attack experienced by the blades on a VAWT is a function of azimuth, with
the peak in angle of attack occu rring near the most upstream point of the rotation. The fluctuation of the angle
of attack is a function of the tip speed ratio (TSR), which is defined as the ratio of the blade tip speed to the
wind speed. At lower TSR the angle of attack can exceed the s tatic stall angle, which has a significant impact
on rotor performance due to the occurrence of dynamic stall on the blades.
A second important feature of VAWT performance is the influence of the wake from the upstream
blades. As the lift on the blades flu ctuates, they shed vorticity, and this is exacerbated by dynamic stall. At the
same time the span wise variation in the pressure distribution results in a strong tip vortex being formed. The
vertical wake is then advected through the rotor volume and subse quently interacts with the blades in the
downstream half of the rotation, reducing the blade performance in this region and increasing the noise sources . AoA – angle of attack
CFJ – coflow -jet
CL,Cd,CM – Lift,Drag,Momentum coeficients
HAWT – horizontal axis wind turbine
LE – leading edge
TE – trailing edge
TSR – tip speed ratio
VAWT – vertical axis wind turbine

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2. DYNAMICS OF VAWT

The aerodynamics of a lift -type VAWT are fundamentally unsteady. As the blade rotat es around the
hub the angle between the blade velocity and the wind velocity varies significantly. This creates a variation in
angle of attack and hence a variation in the aerodynamic forces exerted on the blades. The magnitude of this
variation is governe d by the tip speed ratio (TSR) which is defined as the ratio of the blade speed to the wind
speed. An approximation of the variation in blade angle of attack can be calculated using simple geometry by
assuming that the free stream flow is uniform in both m agnitude and direction over the whole rotor. This
‘geometric’ variation in angle of attack is shown in figure 1 for a range of TSR. It is important to note that the
geometric angle of attack varies from the true angle of attack due to the deflection of the incoming flow by the
rotor .

Fig. 1 Blade angle of attack variation

The second key feature of VAWT operation is the fact that the blades in the upstream half of the
rotation shed a wake that passes through the rotor and interacts with the blades in the downstream half of the
rotation. This potentially introduces further unsteady blade loading that could affect the noise radiated by
VAWT rotor.

Fig. 2 a) One blade momentum during 1 cycle b) Velocity field around VAWT

A typical torque graph produced by one blade during a full 360 0 cycle is presented in figure 2 a). It
can be seen that most of the power is produced within from half cycle where the kinetic energy extraction is
maximum. The remaining flow has a smaller average velocity due to t his blockage effect as can be seen in
figure 2 b).

2.1. Aeroacoustic noise sources

Aeroacoustic noise sources of a VAWT include several categories which produce both tonal and
broadband noise. The most important are considered: dynamic stall, blade – wake interaction and incoming
turbulence –blade interaction. The use of supercirculation as CFJ should reduce at least two important noise
sources: dynamic stall and blade -wake interaction

2.2. Coflow -Jet supercirculation principle

Aerodynamic and aeroacoustic improvements of vertical axis wind turbines using supercirculation flow control
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CFJ method consists in adding injection slots near the leading edge and suction slots near the trailing
edge of a basic airfoil, as in figure 3. Air is injected near the LE and extracted near the TE using a recirculation
pump, so we have the case of zero net mass flux since the mass o f injected air is equal with the extracted mass

Fig. 3 CFJ principle

The aerodynamic results of such circulation are: increased boundary limit momentum, much higher
stall AoA, decreased wake, much higher lift and a reduced induced drag. It must be mentioned that for some
configurations it is possible to achieve even a negative induced drag which makes the solution highly attractive
for new airplanes design.
A typical performance graph showing Lift and Drag coefficients of CJF and baseline airfoils a re
presented in figure 4.
From an aeroacoustic point of view, the delayed stall produces less wake behind the profiles so it was
expected that in case of VAWT to find a decrease of the blade -wake interaction noise

Fig 4 Performance curves for CFJ airfoils

2.4 Numerical aerodynamic and aeroacoustic simulation

2.4.1 Geometry and mesh

Numerical 2 D study was conducted on 3 blades, H -type VAWT with the following characteristics:
R= 1.2 m; Basic blade profile NACA0021 with C=0.3 m. For CJF modified p rofile we used: hinjection slot =
0.002 m; hsuccion slot =0.004 m. Injection and suction slots are placed at 6% chord and 80% chord distance
from the leading edge as can be seen in figure 5.

Fig. 5 Airfoil design with CFJ on both sides

Injection and suction slots were used on both sides of the airfoils due to the fact that during a complete
rotation the angle of attack oscillates between negative and positive values so the suction side becomes pressure
side. It should be mentioned that i t is possible to use active control of the jets in order to use them alternatively
only for the suction side. The domain was divided in 3 circular parts, two static and one rotating containing the
turbine blades (as seen in figure 6). Mesh was structured f or the static domains and partially structured for the
rotating domain. We used a number of 845000 cells and 853000 nodes. For the boundary layer we used an y+
= max. 2.2 which was calculated after the simulation. Total domain D1 diameter was 6 times the V AWT
diameter in order to keep the acoustic sources information inside.

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Fig. 6 Numerical mesh

2.4.2 Numerical algorithm

Numerical simulation for aerodynamic and acoustic field was conducted with Ansys Fluent software.
For the aerodynamic analysis, we used URANS and SST turbulence model. Transitory analysis was made using
an implicit scheme using a time step which correspond to 1 degree of rotation. Every time step was iterated by
20 times. In the current paper, the discretization algorithm was conduct ed by SIMPLE with second -order
upwind scheme by under -relaxation factors for ensuring convergence of iterative computations. The transient
formulation was performed by second order implicit. Simulations were performed for Basic and CFJ airfoils
at TSR 1.5; 2; 2.5. Injection and suction mass flow were established for each TSR in order to have no stall at
maximum angle of attack for that configuration. This resulted from separated steady flow RANS simulations
using CFJ airfoils at different attack angles and flow speed. This resulted in the minimum injection mass flow
which was later used as a boundary condition for the VAWT simulation.

2.4.3 Boundary conditions

Table 1 . Boundary conditions
Parameter Value
Incoming far field velocity 10 m/s
Turbulent intensity far -field 5%
Flow density 1.225 kg/m3
Dynamic viscosity 1.789×10-5 1.789 x 10-5 Ns/m2
Turbulent intensity injection –suction slots 5%
Injection speed 90 m/s
Suction speed 45 m/s
Walls Non-slip condition

2.5 Results

2.5.1 Aerodynamic performance

The use of CFJ for the VAWT increases the output power by reducing or completely eliminating the
stall of the airfoils and due to the reactive force which appears due to the jets. However, this performance
comes to a cost, which is the power consumption of the jets which must be evaluated and subtract it from the
overall generated power by the VAWT. Since the study was dedicated to the acoustic field, this evaluation was
not conducted here. Data and results regarding the Momentum coefficient ( CM) and the comparison between
Baseline (no jet) and CFJ solution for TSR 1.5; 2; 2.5 are presented below .

Aerodynamic and aeroacoustic improvements of vertical axis wind turbines using supercirculation flow control
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Fig. 7 Momentum coefficient for different TSR values

Aerodynamic performance is dramatically improved by CFJ solution especially for low TSR.
It is worth to mention that this performance is achieved by using a supplementary power to inject the
air. However, it has been previously demonstrated that it is possible to achieve a net gain using the CFJ as a
super circulation method.

2.5.2 Aeroacoust ic performance

The acoustic field around the wind was calculated using the Ffowcs Williams -Hawkings analogy. We
distributed 8 receivers around the turbine at 3 m distance from the rotating axis as can be seen from figure 11.
OASPL values for each receiver were calculated for TSR 1.5 ;2; 2.5 and are presented within Table
2. The values were calculated for overall frequency domain (0 -600 Hz), infrasound (0 -20Hz) and sound (20 –
600 Hz) comparing Base with CFJ solution.

Table 2. Average OASPL
Configuration Sound
pressure
level
[dB] Frequency domain
0-600 Hz 0-20Hz 20-600 Hz
Base CFJ Base CFJ Base CFJ
TSR 1.5 Lp avg 104.5 102.7 104.3 102.6 88.6 86.3
Diff 1.8 1.8 2.3
TSR 2 Lp avg 104.7 103.4 104.6 103.4 88.3 84.6
Diff 1.3 1.3 3.7
TSR 2.5 Lp avg 104.8 99.2 104.7 99.1 90.1 81.2
Diff 5.6 5.5 8.9

It can be seen that it is possible to achieve an important noise reduction on all frequencies and even
more on audible domain. The max average reduction of 8.9 dB was obtained for TSR 2.5 case. The explanation
might be that for this TSR the jets speed was optimised in order to be the minimum speed at which stall is not
present.

Fig. 8 Frequency domain for base and CFJ solutions at TSR 2.5

For smaller TSR it is possible that the jets were to s trong and can add some broadband jet noise. An
interesting finding is that CFJ is capable to strongly reduce the tonal noise as can be seen from the FFT analysis
at receiver P2 and presented in figure 8. This was an expected result since the wake reduction due to CFJ
reduces also the blade -wake interaction which is the main tonal noise source.

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3. CONCLUSIONS

CFJ VAWT represents an interesting option for wind energy harvesting since it is efficient and quieter.
Efficiency is much higher than the base but requ ires additional input power to recirculate the jet. However, it
has been demonstrated by other authors that it is possible to achieve an important net gain in power output.
Several interesting findings about noise field have been revealed by this study. The most important is that it is
possible to get a strong OASPL reduction both broadband and tonal. This is the effect of wake reduction and
thus the blade -noise interaction. These findings require a future investigation for the possibility of optimization
the CFJ VAWT in terms of efficiency and noise production.

ACKNOWLEDGEMENT

We would like to thank INCDT COMOTI for the resources provided, PhD eng. Romulus Petcu for
advice and for acquiring the devices within “Nucleu” Program TURBOPROP, project number PN19.05.02.03 .

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