cfd analysis of different blades in vertical axis wind turbine

CFD Analysis of Different Blades in Vertical Axis Wind Turbine 1Dr T.Mothilal,

2P.Harish Krishna,

3G.Jagadeesh Babu,

4Ashwin Suresh,

5K.Baskar

6S.Kaliappan

7M.D.Rajkamal

1Professor Department of Mechanical Engineering, Velammal Institute of Technology, Chennai 601204,

[email protected]

2,3,4,5 UGStudents, Department of Mechanical Engineering, Velammal Institute of Technology, Chennai 601204,

India6Associate Professor, Department of Mechanical Engineering, Velammal Institute of Technology , Chennai-601204, India. 7Assistant Professor, Department of Mechanical Engineering, Velammal Institute of Technology, Chennai-601204, India.

Abstract–The average wind velocity in urban areas

is not sufficient to operate Horizontal Axis Wind Turbine

(HAWT), hence Vertical Axis Wind Turbines (VAWT)

are sought after. Vertical Axis Wind Turbine is of

Savonius (Drag) type and Darrieus (Lift) type. The

present study is focused on the comparison of the

coefficient of performance (COP) of Savonius and

Darrieus types of Vertical Axis Wind Turbine. The above

mentioned VAWTs are numerically analyzed using

ANSYS Fluent- Computational Fluid Dynamics (CFD)

software. The design of the blades of both the turbinesis

chosen such that it is optimized for the best output for the

given input. For the same input parameters, the output

parameters of the wind turbines are obtained separately

and are compared. This comparison provides a basis for

choosing the type of VAWT to be implemented

according to the function.

Keywords – VAWT, Darriues, Savonius, CFD,

Ansys Fluent.

Nomenclature:

HAWT Horizontal Axis Wind

Turbine,

VAWT Vertical Axis Wind

Turbine,

COP Coefficient of

Performance,

CFD Computational Fluid

Dynamics,

NACA National Advisory

Committee of

Aeronautics,

d Diameter of blades

e Gap (eccentricity)

D Rotor diameter

h Rotor height

𝑢 Flow velocity,

𝜐 Kinematic viscosity

w Specific thermodynamic

work (per unit mass)

I. INTRODUCTION:

A Wind turbine is a mechanical device which

converts the wind’s Kinetic energy into the

Electrical energy. Wind turbines are manufactured

widely in vertical and horizontal axis types. Wind

turbines can rotate about either a horizontal or a

vertical direction. Wind energy is the most readily

available and the cleanest source of renewable

energy. Thus wind turbines are popular and widely

used in renewable energy power production.

VAWTs are a type of wind turbine where the main

rotor shaft is set transverse to the wind [1]. VAWT

have the advantages of being omni-directional i.e.

they have the ability to accept winds from all

directions without yawing and the ability to provide

rotary drive to a fixed load [2]. In comparison to

HAWT, VAWT does not require winds at high

speed to rotate and are less noisy. Hence VAWT is

emerging popularly for domestic use in homes

(roof tops) and offices. VAWT are generally

further classified into various types such as

Savonius type, Darrieus type etc. The basic version

of Savonius rotor has an s-shaped cross-section

formed by two semi-circular blades with a small

overlap between them [3]. The Darrieus rotor

consists of a number of curved aerofoil blades

mounted on a vertical rotating shaft or framework.

High fidelity simulation has become the virtual

wind tunnel of today. With the increasing

complexity of problems being analyzed along with

the high costs of experimental setups, high fidelity

numerical analysis tools, such as Computational

Fluid Dynamics (CFD) provide invaluable insight

into the wind turbine flow dynamics. They provide

valuable assistance in both design and analysis

allowing better decisions to be made. CFD has

been used widely to analyze the performance of

VAWTs. [4]

Early assessments of the CFD technology for

Darrieus rotor aerodynamics, aiming at thoroughly

investigating the complex fluid mechanics of these

machines, made use mainly of a two-dimensional

approach. A 2D simulation of H Darrieus can

provide quite accurate estimations of both the

overall performance and the flow field description

around the rotor with reasonable computational

cost, on condition that proper settings are applied.

In case of the medium-size rotor and low tip-speed

ratios, the use of a transitional model forturbulence

closure is suggested by Alessandro Bianchini. [5]

Savonius type of wind turbine is also called as

S-rotor wind turbine. It was invented and patented

by Finnish engineer, Sigurd J. Savonius in 1931.

International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 13545-13551ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

13545

The turbine made of two halves cylinder and then

moving the two semi cylinder surfaces sideways

along the cutting plane like the letter ‘S’[6].It is

suggested that a Savonius rotor should not be

referred to solely as a drag-driven rotor, but as a

VAWT whose torque is generated by the combined

effects of lateral and longitudinal forces [7]. A lot

of performance improvement studies have been

done on Savonius rotor in the past by various

researchers as the efficiency of a conventional

Savonius rotor is poor. The importance of reducing

greenhouse gases leads to research more

sustainable energy resource and to investigate more

efficient technologies.

In this article, a dynamic study was conducted

to simulate the transient behavior of the Savonius

and darrieus rotor. -

II. DESIGN OF BLADES

A. Darrieus Rotor

The Darrieus wind turbine consists of a

number of curved aero foil blades mounted on a

vertical rotating shaft. The curvature of the blades

allows the blade to be stressed only in tension at

high rotating speeds. When the Darrieus rotor is

spinning, the aero foils areadvancing through the

air in a circular path.The energy is taken from the

wind by a component of the lift force working in

the direction of rotation. Aerodynamic modelling is

designed using software tools by considering

NACA0012 aero foil whose chord length is 0.12

m.[8]

Fig. 1. NACA0012 air foil

B. Savonius Rotor

The Savonius turbine is one of the simplest

turbines. Aerodynamically, it is a drag-type device,

consisting of two or three blades. Because of

the curvature, the blades experience less drag when

moving against the wind than when moving with

the wind. The differential drag causes the Savonius

turbine to spin. A number of blades will influence

the rotation of therotor of wind turbine models. The

three blades wind turbineproduces higher rotational

speed and tip speed ratio than that two and four

blades. [6]

Dimensions of the blades of Savonius wind

turbine model are diameter of blades (d)= 200 mm;

gap ( e ) = 0.15 x r = 0.15 x 100 =15 mm; rotor

diameter ( D ) = 200 + 200 – 30= 370 mm; [6].

Fig 2. Savonius 3 cup rotor

III. NUMERICAL ANALYSIS

The process of this study is categorized into 3

parts. For simulation of blades, CFD is used which

includes 3 stages pre-processing, simulation, and

post processing.

For an isothermal, 3D incompressible flow,

the governing equations are the conservation

ofmass and the conservation of momentum given

by equations (1) and (2) respectively. Since the

flow velocities in the domain are much smaller than

sound velocity,it can be assumedthat the density

remains constant throughout the flow field.

∇. 𝑢 = 0 …(1)

𝜕𝑢

𝜕𝑡+ 𝑢. ∇ 𝑢 − 𝜈∇2𝑢 = −∇𝑤 + 𝑔 … (2)

Where,𝑢is the flow velocity,𝜐 = 𝜇

𝜌0 is the

kinematic viscosity, w is the specific

thermodynamic work (per unit mass), 𝑔is

acceleration due to gravity.

Creo 3.0 is used for creating a2-Dmodel of

blades. This is used as pre-processorto run the

simulation in ansys fluent.Fluent is used to analyse

the fluid flow properties such as distribution and

separation of the velocity through and around

turbine blade, and variables to describe the fluid

flow.Results of theanalysis are post-processed in a

quantitative and qualitative manner.

IV. MODELING OF VAWT BLADES

A. Darrieus rotor

After selecting the airfoil, its co-ordinates are

imported to creo software for designing the air foil

structure.

International Journal of Pure and Applied Mathematics Special Issue

13546

Parameter Value with limit

Chord length 120mm

Rotating diameter 500mm

Table 1. Design parameters of darrieus rotor

The modelled Darrieus VAWT rotor NACA 0012

is shown in fig 3.

Figure 3. Darrieus rotor

B. Savonius rotor

The model of savonius wind rotor was

designed with following parameters.

Parameter Value with units

Diameter of blade(d) 200mm

Gap (e) 15 mm

Rotor diameter (D) 370 mm

Table 2. Design parameters of Savonius rotor

The modeled Savonius rotor with above

mentioned dimensions is shown in figure 4.

Fig.4. Savonius Rotor

V. COMPUTATIONAL MODELING AND

ANALYSIS

The two dimensional computational domain of

the two bladed Darrieus and three buckets Savonius

rotor along with the boundary conditions is shown

in Fig. 5(a). Fig. 5(b). The 2D computational model

of rotors was generated in Ansys fluent of the CFD

software such that the dimensions of the rotor are

exactly same as those of the dimensions mentioned

above.

Velocity inlet is considered on the face AB,

pressure outflow is considered on the outlet CD,

symmetry condition is considered for the free slip

(faces AC & BD). And the freeslip condition is

considered on the blades and buckets. The inlet

velocity considered is 6 m/s, and the solver

calculates the pressure at the outlet by default due

to the pressure outlet condition specified on the

outlet face. Any wind speeds up to the value of 10

m/s can be called low wind speed. [9]

Fig. 5(a). Blade arrangement of Savonius rotor


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Fig. 5(b). Blade arrangement of Darrieus Rotor

The Darrieus rotor is arranged in the form of 2

blades at 180 deg.,opposite in direction. The

savonius is arranged in the form of 3 cup rotor at an

angle of 120 deg between each blade.

For both darriues and savonius rotor, fine

meshing is done in Ansys with aqaud mesh having

10layer inflation near the bladesand domain.

Table 3. Meshing values

Fig.6 (a). Darrieus blade mesh

Fig.6 (b). Savonius blade mesh

The pressure based solve is used to the steady

state condition. The analysis is run by K – omega

SST turbulence model, the material chosen for the

blades is aluminium the boundary conditions are

varied by varying the inlet velocity of the air. The

rotating domain is given moving wall condition

with an angular velocity respect to the inlet

velocity.

The coupled method is used as the pressure

velocity relation equation. The solution is

initialized by the hybrid method. Thepressure

momentum, turbulent kinetic energy,and specific

dissipation rate are got by second order upwind

equation.

The flow courant number is 200 and explicit

relaxation factor for momentum and pressure is 0.5.

The following are the obtained contours,

Fig. 7. Darrieus Velocity Contour

Fig. 8. Darriues pressure contour

Type of

blade Darrieus

Savonius

Domain Nodes Element

s Nodes

Element

s

Rotating

domain 5770 2801

3812 1802

Surface

body

28029

2 139333

28094

2

139555

All

Domain

s

28606

2 142134

28475

4

141357


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Fig. 9. Savonius rotor velocity contour

Fig. 10. Savonious rotor Pressure contour

VI. RESULTS AND DISCUSSION

From the analysis, various results are obtained

and according to that graphs are plotted.

Graph 1. TSR vs COP

Graph 2. Velocity vs COP

The following are the discussions made from the

above graph and obtained results,

1. The Savonius type VAWT shows better

starting torque at low wind speed.

2. The increasing wind speed and tip speed

ratio are better for the Darrieus type of

VAWT.

3. For the Domestic purpose, Savonius type

VAWT shows the better result as the

average wind speed will produce regular

power supply.

4. For regions with higher wind velocity,

Darrieus type VAWT proves to be more

efficient.

VII. CONCLUSION

From the results obtained, it can be concluded

that both the type of VAWT depends on the

application, wind characteristics, etc. Savonius

works under drag forces. Darrieus operates because

of lift forces. Savonius is able to self-start but

darrieus cannot start. [10]

A Darrieus is a high speed, low torque

machine suitable for generating alternating current

(AC) electricity. Darrieus type requiresa manual

push, therefore, some external power source is

required to start turning as the starting torque is

very low.

The Savonius uses drag and therefore cannot

rotate faster than the approaching wind speed. An

area that has strong and gusting winds or when you

need a unit that self-starts, this is the best type [11].

A. Ghosh et.al found that the combined darrieus

and savonius had increased concentration of

vortices. There is a velocity shortage in the

savonius rotor in comparison to darrieus for which

darrieus be able to extract more energy

compensating each other [12]. Therefore a

combination of both is a good decision for low

power application.

REFERENCE

0

0.1

0.2

0.3

0.4

0.5 1 1.5 2 2.5 3 3.5 4

CO

P

TSR

COP VS TSR

DARRIEUS SAVONIUS

0

0.1

0.2

0.3

0.4

5 6 7 8 9 10 11

CO

P

VELOCITY m/s

COP VS VELOCITY

DARRIEUS SAVONIUS


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cfd analysis of different blades in vertical axis wind turbine

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