Aerodynamic Performance Analysis of Vertical Axis Wind Turbine

(VAWT) Darrieus Type H-Rotor using Computational Fluid Dynamics

(CFD) Approach

Wisnu Wardhana

, and Elyas Nur Fridayana

Department of Ocean Engineering, Faculty of Marine Technology, Institut Teknologi Sepuluh Nopember (ITS), Surabaya

Keywords:

Aerodynamics, Airfoil, Darrieus, CFD Simulation, Wind Turbine

Abstract:

The recent renewable energy grows signiﬁcantly concerning on people awareness of the negative effect to-

ward environment. Based on the advantages that should be offer by Vertical Axis Wind Turbine (VAWT)

becomes an interested object to be observed. Especially VAWT Darrieus, H-Rotor type. Not only known has

simple structure but also has higher efﬁciency than Savonius. Airfoil Proﬁle becomes substantial because its

aerodynamic effects which run on the Darigus turbine. Numerous previous studies observed that the use of

asymmetric airfoil gives advantage toward Darrieus turbine efﬁciency. In this case, this research focuses on

the inﬂuence of cambered airfoil toward the maximum position to the turbine performance. The Investigation

is observed using Computational Fluid Dynamic (CFD) which is two dimensions Darrieus turbine type H-

Rotor turbine model is simulated in transients current condition. In result, the simulation as power coefﬁcient

presented the best turbine performance is conducted by airfoil with the maximum camber position close to the

trailing edge. While the variation on the chord length gives a solidity ratio of 0.6 as the best value between the

above and below.

1 INTRODUCTION

Globalisation now days demanded by hygienic avail-

ability and low-cost resource. Therefore, regarding

human awareness about the negative impact of con-

servative energy resource caused renewable technol-

ogy conversion become vital. In this case, Wind En-

ergy Conversion System (WECS) is the most built

and developed. The furthermost WECS technology is

Horizontal Axis Wind Turbine (HAWT). It has higher

efﬁciency than the other kind of Vertical Axis Wind

Turbine (VAWT). Even though, it offer another ad-

vantage such as the simplest structure. Moreover,

VAWT able to run at any course of the wind and to

provide another beneﬁt such a minimum noise, stable

in wind turbulence condition, integrate with another

building and give easiness in its generator, gearbox

and bearing application (Ahmadi-Baloutaki, 2015).

There are two differences major in VAWT imple-

mentation, those are: Saviours which is the interac-

tion of the wind turbine utilizing drag force and Dar-

rieus which is using lift force of aerodynamic effect,

the second VAWT is the common one to use as wind

turbine electricity (Manwell et al., 2010). Which is in

this case blade design is important to gain the effect

of aerodynamic due to airfoil design. Airfoil is par-

ticular form of geometry design to gain lift force and

to minimize increasing drag force.

In ﬂuid currency, airfoil lift force is occurred be-

cause the differences of upper and lower surface pres-

sure. It caused by angle of attack and camber (?).

Airfoil symmetry often used in darrieus turbine to

minimize the negative torque during single rotor ro-

tation. The other hand, airfoil with camber increasing

the lift force at the zero attack angle and functioning

at higher maximum torque, but at the second half of

rotation it always occurred negative torque. Accord-

ing to the study by Sengupta et al. (2016), Bausas and

Danao (2015), asymmetrical airfoil is giving the im-

provement to the whole of rotor performance. Qamar

and Janajreh (2017) researched the rotor performance

with NACA 4512 and NACA 7512 asymmetrical air-

foil. The result is airfoil with moderate camber has

better performance.

The purpose of this research is to identify the in-

ﬂuence of maximum camber at 4 digit NACA con-

ventional airfoil. The chosen type of airfoil is NACA

4312, 4512 dan 4712 which is the second digit of vari-

Wardhana, W. and Fridayana, E.

Aerodynamic Performance Analysis of Vertical Axis Wind Turbine (VAWT) Darrieus Type H-Rotor using Computational Fluid Dynamics (CFD) Approach.

DOI: 10.5220/0008542700050011

In Proceedings of the 3rd International Conference on Marine Technology (SENTA 2018), pages 5-11

ISBN: 978-989-758-436-7

ation delivers the maximum camber’s value toward

chord line airfoil. The best performing airfoil will

be further investigated to see the effects of the vari-

ation of solidity ratio. Computational Fluid Dynamic

(CFD) two dimensions method is used to obtain the

optimal coefﬁcient power value.

2 BASIC THEORY

2.1 Wind Energy

Energy inside of wind stream is a kinetic energy. This

energy is converted to mechanic energy in wind tur-

bine system. Kinetic energy at a certain wind mass

(m) which moving at (u) speed can be identiﬁed as

equation below:

E =

(1)

If the turbine cross section area is A, where the

air passes through it at the velocity u, then there is a

displacement of volume V per time unit, therefore:

V = uA (2)

Meanwhile, mass current at ρ air density clariﬁed

in below equation:

m = ρuA (3)

Energy which contained in wind P can be identi-

ﬁed as substitute Equation 3 to Equation 1 as turn out

to be this equation :

m = ρuA (4)

As P is power that contained in the wind, u is wind

speed and air density ρ. The amount of air density on

sea surface is 1.225 kg/m

2.2 Tip Speed Ratio

Tip Speed Ratio (γ) is comparison between tip blade

speed and wind speed that get through it (Hemami,

2005).

γ =

ωR

(5)

Which is ω as the rotation speed of turbine, R is

the length of the turbine radius.

Figure 1: Airfoil NACA 4312, 4512 and 4712

2.3 Power Coefﬁcient

The capability of wind turbine to convert wind energy

to mechanic energy called coefﬁcient power (Cp) that

written as equation below:

Cp =

Pmechanic

Pwind

(6)

Cp =

ωT

ρAu

(7)

P mechanic is mechanic power and P wind is wind

power. Meanwhile T is torque which consist at coef-

ﬁcient equation moment (Cm):

Cm =

ρARu

(8)

3 RESEARCH METODOLOGY

3.1 Examine Variable

The purpose of this research is to analyse the char-

acteristic of VAWT 3 blades that use 4 digit NACA

airfoil. First digit showing maximum camber. Sec-

ond digit showing the position of maximum camber

according the leading edge. Last two digits showing

maximum thickness of the airfoil. Those parameters

will be examined is the maximum position of camber

on the airfoil of NACA 4312, 4512 and 4712 as show

in the ﬁgures below:

The tree airfoils have the same maximum thick-

ness and the same maximum camber value, each rep-

resented by the ﬁrst digit and the last two digits in the

four number digit code of NACA airfoil. This aim is

to obtain the lift coefﬁcient (C

) which does not have a

signiﬁcant difference between the examined of three

airfoils. So, the overall rotor performance can be an-

alyzed from the effect of the camber position. The

SENTA 2018 - The 3rd International Conference on Marine Technology

Figure 2: Curve of Cl vs Alpha for Airfoil NACA 4312,

4512, 4712

Figure 3: Aerodynamic force on Darrieus turbine

polar simulation of the three airfoils has been simu-

lated using QBlade software. This software is inte-

grated with the XFOIL, the program used to simulate

the subsonic ﬂow around airfoil and may be able to

predict the performance of airfoil till the last second

before stall (Marten and Wendler, 2013). This simu-

lation comes with the result of C

with the variation of

angel of attack (α), as shows in the Figure 2.

3.2 Geomatry Model

The aim of this research is to analyse Darrieus wind

turbine Straight Bladed with the 3 blades. The aero-

dynamics force of this turbine may be seen in the Fig-

ure 3

The geometry created must be represented the

physical problems, therefore it is important to com-

pare the result of the model with the experimental

data. The model which is used as the standard of val-

idation may be seen in the Table 1. Solidity ratio is

σ = Nc/R with the maximum value 0.5 based on the

recommendation of Ahmadi-Baloutaki (2015). The

initial Azimuthal position is accounted from angular

Figure 4: Azimuthal coordinate of blade number 1 [9]

Table 1: Dimension of validation model [9-10].

rotor

[mm] 1030

rotor

[mm] 1414

n [-] 3

Proﬁl Airfoil NACA 0021

As [m2] 1.45642

Chord lengh (c) [mm] 85.8

σ [-] 0.5

coordinate of the ﬁrst blade, in the half of chord to be

exact. The motion of positive azimuthal is obtained

since the blades start to move counter clockwise, as

show in the Figure 4 below :

3.3 Numerical Model

C-shape Domain is chosen to modeled the computa-

tional domain. The boundary condition and the size

is deﬁned based on Figure 5 where as the red colour

is inlet, blue is outlet, black is the symmetry, green

is the rotor, the purple is the stationary area or silent

area. The orange is blade area, this motion is based

on the rotor motion and the last is gray which is as the

airfoil.

After creating the geometry, mesh and determina-

Figure 5: Boundary layers set up

Aerodynamic Performance Analysis of Vertical Axis Wind Turbine (VAWT) Darrieus Type H-Rotor using Computational Fluid Dynamics

(CFD) Approach

Figure 6: The moment coefﬁcient curve (Cm) as a function

of azimuth angle (θ)

Figure 7: Mesh discretization of whole domain

tion of the boundary condition, the next step is to cre-

ate the certain parameter. To obtain the result of each

tip speed ratio (TSR), the variation of angular is given

with the constant wind velocity at 9 m/s. Computa-

tion is done in each 20 of rotor rotation. The total

rotation of rotor is simulated as 10800 or 3 full cir-

cle. Moment coefﬁcient (Cm) gains from the overall

value of moment in last rotation of 3600. This case

is done to gain the stable result as shown in the Fig-

ure 6. Simulation has been calculated using ANSYS

Fluent software in transient ﬂow conditions. Sliding

mesh technique is used to modeled a rotating turbine.

The realizable K-ε turbulence model has been applied

to the solver setup. Convergency is set with overall

equation 10-4 and maximum of iteration is 80.

3.4 Discretization of Numerical Model

The meshing strategy is created base on existent do-

main, which is the area has direct attraction to the air-

foil must be gained the detail mesh. In this model the

mesh is resulted around 140.000 cell. Samples of 2D

mesh discretization of tested models shown in Figure

7 and Figure 8.

Figure 8: Mesh discretization of rotational domain and air-

foil

Figure 9: The coefecient power (Cp) as a function of total

mesh

4 RESULT AND DISCUSSION

4.1 Study of Mesh Sensitivity

The strategy in making mesh is one of important point

in CFD simulation. Generally the more ﬁner of mesh,

the result will be gained will be better. Somehow the

time consuming of the simulation will be longer. In

this study, the simulation based on validation model

with TSR = 3.3 where the Cp result from the experi-

mental data is compared with simulation result. The

amount of the mesh for the ﬁrst simulation is 80.000

cell, then it increases by 30.000 cell. The result of

170.000 cell is still in enhancement, but the result of

140000 simulation sel and 170000 only have small

different. The result of mesh sensitivity study is in

the Figure 9.

4.2 Validation

The present CFD simulation is proven to be able to

show the same behaviour compared to experimental

SENTA 2018 - The 3rd International Conference on Marine Technology

Figure 10: Validation CFD model to be compared with ex-

perimental and the previous CFD data (Raciti Castelli et al.,

2011)

data. Even CFD simulation shows a little higher re-

sult, this is because of 2-Dimension that is used by

the CFD, where the effect of 3D such as vortex inter-

action and tip vortice which inﬂuence tip loses cannot

be catch by 2D model (Subramanian et al., 2017).

In 2D the inﬂuence of singularity (source or vor-

tex) only around cross each segment. However, in the

3-Dimension modelling which is used singularity dis-

tribution (source or vortex) in panel. Therefore, the

inﬂuence is not only happen between the panel but

also it happens in all sides of panel. Trough this ef-

fect it inﬂuence the result of modelling (Chan, 1990).

4.3 Effect of Airfoil Variation

The simulation result as coefﬁcient moment counted

as equation 5, 7 and 8 to gain the result of the power

coefﬁcient as the representation of Darrieus rotor per-

formance. For every rotor within differences of airfoil

proﬁle is simulated in each Tip Speed Ratio (TSR).

Here are the chosen TSR: 2; 3; 3.5; 4 and 5. The di-

mension and the size of the rotor are keep using model

validation data as Table 1.

CFD simulation shows various of Darrieus curve

performance at lower TSR. But, on the upper TSR 4

mostly has the same result. Figure 11 shows that the

airfoil proﬁle of NACA 4712 has the higher maximum

result of coefﬁcient power than another proﬁle.

Interstingly, the NACA 4712 airfoil also has the

widest curve, which means the best operating range.

The proﬁle of NACA 4312 has the lower perfor-

mance. Whereas, the NACA 4512 with the maximum

camber position in the middle of the airfoil coinciden-

tally gives a symmetrical Cp-TSR curve. The Maxi-

mum Cp is at TSR 3.5 with an almost equal gradi-

ent rise and drop curve. Those three examined air-

Figure 11: Cp vs TSR Curve at arotor Darrieus with airfoil

variation

Figure 12: Maximum Cp for each airfoil

foils have the same power coefﬁcient values at TSR 4,

whereas the differences are contrasting at Cp values

seen in TSR 3. Based on the results showed clearly

the position of camber gave the signiﬁcant change to-

ward the whole Darrieus rotor performance.

By shifting the camber’s position toward the trail-

ing edge then the maximum Cp is increased and the

TSR operation ranged becomes wider. Figure 12

showed the maximum value of C p for each airfoil pro-

ﬁle, while, each values of Cp airfoil through TSR 3 is

showed in Figure 13.

Further review were held on TSR 3 in it compared

the instantaneous of coefﬁcient moment through az-

Figure 13: Cp for each airfoil on TSR=3

Aerodynamic Performance Analysis of Vertical Axis Wind Turbine (VAWT) Darrieus Type H-Rotor using Computational Fluid Dynamics

(CFD) Approach

Figure 14: Cm vs Azimuthal angle for NACA 4312, 4512

and 4712 (TSR= 3)

Figure 15: Moment coefﬁcient of NACA 4712 at TSR= 3

on all blade

imuthal angle on blade 1 as showed in Figure 14. The

Cm has almost the same value while the three kinds of

airfoils leading the ﬁrst half of the rotation. But, while

the second half rotation; the negative variation mo-

ment signiﬁcantly occurred. Which blade proﬁle of

NACA 4312 produced the highest negative moment.

Whereas, the smallest is NACA 4712. Although the

second half of the azimuthal angle position always

generates a negative torque, however the impact of 3

blades as a whole always generates positive torque as

shown in Figure 15.

4.4 Effect of Solidity Ratio

In the previous section it was found that the NACA

4712 proﬁle has many advantages than another tested

proﬁle. So, those proﬁle is simulated with chord

length variations. Based on this variation, the value

of solidity ratio (σ) also changed. This ratio is deter-

mined by the number of blades (n), chord length (c)

Figure 16: Cp vs TSR for each σ

Figure 17: Maximum Cp with variation σ

and radius length (R) with the following equation:

σ =

(9)

The examined solidity ratio is in the range of val-

ues 0.4 to 0.7. The simulation result curve of Cp-TSR

is showed in Figure 16.

The simulation results show that the maximum Cp

value of 0.511 is owned by VAWT for a solidity ratio

of 0.6. Whereas, the Cp-TSR curve with solidity ra-

tio upper and lower of 0.6 has smaller Cp value. The

difference in solidity ratio also has an effect on tur-

bine range operation. Where, a high solidity has C p

value at a low TSR range. While the solidity decreas-

ing the Cp-TSR curve shifted to the right. This case

also worked on the maximum value of the Cp. Which

is the solidity ratio 0.7 has maximum Cp in TSR = 3.

Within, if the solidity ratio is reduced, the Cp maxi-

mum will also move to higher TSR. More details are

presented in Figure 17 below.

4.5 Conclusion and Suggestion

Numerical analysis with 2-Dimensional Computa-

tional Fluid Dynamic (CFD) approach have been used

in this research. The result has been taken is quite

valid to know the characteristics of Vertical Axis

Wind Turbine, Darrieus type, even though the results

are slightly larger than experimental data.

SENTA 2018 - The 3rd International Conference on Marine Technology

The simulation results on the airfoil asymmetri-

cal variation gave the signiﬁcant differences in Dar-

rieus rotor performance. To bring the maximum cam-

ber position to trailing edge then the Cp maximum is

increased. The examined airfoil proﬁle NACA 4712

gave the best result and the highest maximum Cp re-

sult as best result is 0.495 toward TSR 3.25.

The result of Cp-TSR curve shape also has the best

result; because the wide curve means have good oper-

ation of the speed range. The worst rotor showed by

NACA 4312. The investigation on TSR 3 has shown

that the examined variation performance of three air-

foils is spread caused by the difference of negative

value as long as half second of azimuth angle posi-

tion. While the variation of solidity ratio by changing

the chord length found that the maximum value of the

highest Cp obtained in solidity ratio 0.6 is by value

0.51.

The maximum camber position change has been

shown to provide improved aerodynamic perfor-

mance. The changeover of Cp-TSR curve looked con-

sistent and united. The variable of this work is eligible

for the further research combining with other related

variable.

REFERENCES

Ahmadi-Baloutaki, M. (2015). Analysis and improvement

of aerodynamic performance of straight bladed vertical

axis wind turbines.

Bausas, M. D. and Danao, L. A. M. (2015). The aerody-

namics of a camber-bladed vertical axis wind turbine in

unsteady wind. Energy.

Chan, H. S. (1990). A three-dimensional technique for pre-

dicting ﬁrst-and second-order hydrodynamic forces on a

marine vehicle advancing in waves.

Manwell, J. F., McGowan, J. G., and Rogers, A. L. (2010).

Wind Energy Explained: Theory, Design and Applica-

tion.

Qamar, S. B. and Janajreh, I. (2017). Investigation of Ef-

fect of Cambered Blades on Darrieus VAWTs. In Energy

Procedia.

Raciti Castelli, M., Englaro, A., and Benini, E. (2011). The

Darrieus wind turbine: Proposal for a new performance

prediction model based on CFD. Energy.

Sengupta, A. R., Biswas, A., and Gupta, R. (2016). Stud-

ies of some high solidity symmetrical and unsymmetrical

blade H-Darrieus rotors with respect to starting charac-

teristics, dynamic performances and ﬂow physics in low

wind streams. Renewable Energy.

Subramanian, A., Yogesh, S. A., Sivanandan, H., Giri, A.,

Vasudevan, M., Mugundhan, V., and Velamati, R. K.

(2017). Effect of airfoil and solidity on performance of

small scale vertical axis wind turbine using three dimen-

sional CFD model. Energy.

Aerodynamic Performance Analysis of Vertical Axis Wind Turbine (VAWT) Darrieus Type H-Rotor using Computational Fluid Dynamics

(CFD) Approach