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CFD Analysis of Flow Physics and Aerodynamic Performance of aCombined Three-Bucket Savonius and Three-Bladed Darrieus TurbineR. Guptaa; Agnimitra Biswasa

a National Institute of Technology Silchar, Silchar, Assam, India

Online publication date: 09 March 2011

To cite this Article Gupta, R. and Biswas, Agnimitra(2011) 'CFD Analysis of Flow Physics and Aerodynamic Performanceof a Combined Three-Bucket Savonius and Three-Bladed Darrieus Turbine', International Journal of Green Energy, 8: 2,209 — 233To link to this Article: DOI: 10.1080/15435075.2010.548541URL: http://dx.doi.org/10.1080/15435075.2010.548541

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International Journal of Green Energy, 8: 209–233, 2011Copyright © Taylor & Francis Group, LLCISSN: 1543-5075 print / 1543-5083 onlineDOI: 10.1080/15435075.2010.548541

CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMICPERFORMANCE OF A COMBINED THREE-BUCKETSAVONIUS AND THREE-BLADED DARRIEUS TURBINE

R. Gupta and Agnimitra BiswasNational Institute of Technology Silchar, Silchar, Assam, India

In this paper, the performance of a combined three-bucket Savonius and three-bladedDarrieus turbine was analyzed computationally by using Fluent 6.2 CFD software. Two-dimensional steady-state CFD simulations were performed for without overlap, and withfive overlaps namely 16.2%, 20%, 25%, 30%, and 35%. The flow physics of the combinedturbine was analyzed with the help of pressure, velocity and vorticity contours. Further, theaerodynamic coefficients were evaluated with respect to angle of attack for various tip speedratios. It was concluded that the power augmentation of the combined turbine occurredfor low overlap in Savonius turbine due to high aerodynamic lift-to-drag coefficient of theSavonius turbine, caused by the increase of dynamic pressure from bucket-vortex interac-tions on the concave face of the returning bucket. And it also occurred for high aerodynamiclift-to-drag coefficient (14.5) of the Darrieus turbine from the increase of velocity differenceacross the Darrieus blade with small overlap (16.2%) in Savonius turbine. For without orwith low overlap (16.2%), coanda flow occurred across the overlap between the inner edgeof bucket and the central shaft that migrated from the downstream side of the advancingbucket towards the upstream side and then reached the concave face of the returning bucketincreasing dynamic pressure on it, and hence augmented power production for the combinedturbine. However, high value of overlap (20% onwards) caused vortex separations from theinner edges of bucket destabilizing the coanda flow for which the aerodynamic coefficientswere lowered.

Keywords: Combined Savonius-Darrieus turbine; CFD analysis; Contours; Aerodynamiccoefficients

INTRODUCTION

Wind energy system is an important power generation technology whose continualdevelopment and proliferation can suffice the energy needs of people and can salvagethe severe climate changes caused by the greenhouse gas emissions. With the improvedtechnology along with superior economics of wind energy systems, experts predict windpower would capture 5% of the world energy market by the year 2020 (GWEC 2006).Over the past ten years, global wind power capacity has continued to grow at an averagecumulative rate of over 30%. According to the figures released by Global Wind EnergyCouncil (GWEC), wind energy developing countries (more than 70) have taken the net

Address correspondence to R. Gupta, National Institute of Technology Silchar, Mechanical Engineering,REC Road, Fakirtila, Silchar-10, Cachar, Silchar 788 010, India. E-mail: r_guptanitsil@yahoo.com

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210 GUPTA AND BISWAS

wind energy installed capacity to a record high of 120,798 MW (GWEC 2008). The 120.8GW of global wind capacity will produce 260 TWh and save 158 million tons of CO2 everyyear. The top ten wind power market in terms of MW installed with market share and thegrowth of installed wind energy capacity from 1996 to 2008 are shown in Figure 1(a) and(b), respectively. The Vertical Axis Wind Turbines (VAWTs) are simple in construction,self-starting, inexpensive, suitable for low wind speed applications, and can accept windfrom any direction without orientation. The VAWTs are of different types, such as Savoniusturbine, Darrieus turbine, combined Savonius–Darrieus turbine, etc. VAWT would be desir-able for wind power production apart from horizontal axis wind turbines, especially in lowwind speed regimes. Wind speed regimes are determined from the study of wind character-istics of a region. Akdag and Güler (2010) studied the wind characteristics such as monthlyaverage mean speeds, power densities, turbulence intensities, gust, wind directions, etc. ofAmasra, a Black Sea region in Turkey, taking 9 years of wind data. They estimated meancapacity factors between 26.8% and 43.9% for 30 commercial wind turbines. Ucar andBalo (2008) conducted a similar study on a site in the western part of Anatolia of Turkey

Figure 1 (a) Top 10 wind power market with market share and (b) growth of installed wind energy capacity from1996 to 2008.

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 211

to evaluate yearly energy output and capacity factors of six commercial wind turbines, andcapacity factor of the turbines was found to be between 25 and 34.

Vertical Axis Wind Turbines are classified as Savonius turbine, Darrieus turbine,combined Savonius–Darrieus turbine. Savonius turbine has high starting torque and itspower coefficient normally lies in between 15% and 35%. Ghosh et al. (2009) studied theperformances of a single-stage and a three-stage modified Savonius wind turbines for waterpumping with the help of mathematical modelling. Single-stage configuration showed amaximum power coefficient of 19% at a Reynolds number of 120,000. The estimated dis-charges for a single-stage turbine of 3 m diameter at design wind speeds of 6 and 8.5m/sec were 5,000 and 6,000 m3/month for roto-dynamic pumps. McWilliam and Johnson(2008) studied the airflows around various Savonius turbine designs by using ParticleImage Velocimetry (PIV) in a closed loop wind tunnel. Results showed the dominance oflift forces on the turbines. Further analysis showed considerable interactions between flowover the forward blade and the wake of the trailing blade. Ozgener, Ozgener, and Dincer(2009) investigated some exergoeconomic parameters (energy and exergy loss ratios) fora 1.5 kW wind turbine system, linking capital costs and thermodynamic losses based onsome operating conditions. The combined Savonius–Darrieus turbine has its advantageover individual Savonius and Darrieus turbines since it combines high starting torque ofSavonius turbine and high power coefficient of the Darrieus turbine. Gavalda, Massons,and Diaz (1990) studied the performance of a self-adapting Savonius–Darrieus turbinehaving two-bucket Savonius and two-bladed airfoil chord Darrieus turbines. They reporteda maximum power coefficient of 0.35 obtained at an overlap of 16.67%, and also showedthat the high starting torque of the combined turbine was caused by the reduction of thezones of negative starting torque. Wakui et al. (2005) performed comparative analysis ofthe performances of two different hybrid configurations of Savonius–Darrieus turbines;one having two numbers of two-bucket Savonius turbines as the starter for a single two-bladed Darrieus turbine with the Savonius turbines placed within the Darrieus turbine, andthe other, same configuration but with Savonius turbines mounted below the Darrieus tur-bine. They observed that the second category of their turbine had performed better than theformer, which produced a maximum power coefficient of 0.18. Gupta, Das, and Sharma(2006) studied experimentally the performance of a combined two-bucket Savonius andthree-bladed Darrieus turbine in an open circuit subsonic wind tunnel for overlap variationsbetween 16.2% and 25%. The maximum power coefficient was obtained as 0.25 at 20%overlap. Again, Gupta, Biswas, and Sharma (2008) made a comparative study betweena three-bucket Savonius turbine and a combined three-bucket Savonius and three-bladedDarrieus turbine through wind tunnel testing for without and with overlap variations of16.2%, 20%, 25%, 30%, and 35%. They showed that the maximum power coefficient ofthe combined turbine was obtained at without overlap and that of Savonius turbine wasobtained at 20% overlap. And the maximum power coefficient of the combined turbinewas higher than the Savonius turbine, which was reported to be 0.51. Elmabrok (2009)evaluated the performance of various designs of combined Darrieus–Savonius turbines.The Darrieus turbine was a three wooden bladed system, which was combined individuallywith different two-bladed Savonius turbine models, such as semi-circular type and Bachtype. It was concluded that the combined Darrieus and Bach type Savonius turbine wouldself-start at low wind speeds, since the power and torque coefficients were higher than theother design. Debnath, Biswas, and Gupta (2009) evaluated the performance of a combinedSavonius–Darrieus turbine of the design of Gupta et al. (2008) with overlap variations by

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212 GUPTA AND BISWAS

using Fluent 6.2 CFD software. The results showed good matching of computational andexperimental power coefficients for all overlap variations.

Thus, the recent works on the combined Savonius–Darrieus turbine have shown thatits power coefficient is much greater than the Savonius turbine. However, its aerodynamicperformances with respect to different design parameters like blade/bucket incidence,overlap variations, tip speed ratios. etc. are still not evaluated. Further, the effects of theindividual components i.e. bucket and blade on the performance of the combined turbineis required to be known. In the present paper, the performance of combined Savonius–Darrieus turbine of the design of Gupta et al. (2008) was analyzed computationally byusing Fluent 6.2 CFD software.

PHYSICAL MODEL

The combined Savonius–Darrieus turbine had a three-bucket Savonius turbinemounted on top of three-bladed Darrieus turbine, as shown in Figure 2. The buckets ofthe Savonius turbine were 8 cm in diameter, 3 mm in thickness and 10 cm in height. Toremove any mechanical interference between the Darrieus blades and Savonius buckets,the arcs of the buckets were shortened by almost 10◦ on the inner edges. The buckets hadthe provision for five overlap variations, namely 16.2%, 20%, 25%, 30%, and 35%. Theoverlap is the distance of the inner edge of the bucket from the axis of rotation assuming thearc is carried to the full semi-circle. The overlap ratio is the ratio of overlap to the bucketdiameter expressed in percentage. The Darrieus turbine was made up of similar aluminumstrips of width 12 mm, thickness 3 mm, diameter 8 cm, and height 10 cm, with bladesspaced 120◦ apart. Thus for the combined Savonius–Darrieus turbine, the total height was20 cm. The central shaft was made of mild-steel of 15 mm diameter. Washers and nutshaving knurled surfaces were used to change the overlap. The turbine was fixed to theshaft-using nut and bolt arrangement. The central shaft was connected to a base through aball bearing. The central shaft, base and the supports were made from mild steel, and theblades were made from lightweight aluminium. The models were tested in an open circuit

Figure 2 Combined three-bucket-Savonius-three-bladed-Darrieus turbine.

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 213

subsonic wind tunnel available in the department. The brief description of the wind tunneland the experimental procedure were given in the paper of Gupta et al. (2006).

CFD FORMULATION OF THE COMBINED SAVONIUS–DARRIEUS TURBINE

Any computational formulation of a physical process is based on mathematicalmodelling first, which starts with modelling of the physical system with its boundary condi-tions, and then followed by the implementation of the governing mathematical equations.The physical domain of the combined three-bucket Savonius and three-bladed Darrieusturbine along with the boundary conditions is shown in Figure 3. Velocity inlet and out-flow conditions were taken on the left and right boundaries, respectively. The top andbottom boundaries of the computational domain signifying the sidewalls of the wind tunnelhad symmetry conditions on them. The buckets, blades, and the central shaft were set tostandard wall conditions.

The governing equation of the conservative form of Navier–Stokes equation in finitedifference form for incompressible flow of constant viscosity is as shown in Equations (1)and (2).

∂

∂t(ρ �Vr) + ∇.(ρ �Vr �Vr) + ρ(2 �ω �Vr + ⇀

ω �ωr) = −∇Ps + ρ�g + ∇.(τ ). (1)

According to the eddy viscosity concept of the Stokes’ hypothesis for Newtonian fluids,the Reynolds stress tensor of the Equation (1), τ , can be expressed as

τ = μ[(∇ �Vr + ∇ �VTr ) − 2

3∇.�VrI]. (2)

The simplest and most widely used two-equation turbulence model is the standard k–ε

turbulence model that solves two separate transport equations to allow the turbulent kinetic

Figure 3 Computational domain of the combined Savonius–Darrieus turbine.

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214 GUPTA AND BISWAS

energy and its dissipation rate to be independently determined. The standard k–ε modelis particularly suitable for flows though sharp corners, straight and curved edges like theturbine blades as the model uses wall functions based on the law of the wall. The standardk–ε equations of the turbulent model can be represented as

∂

∂t(ρk) + ∂

∂xi(ρkui) = ∂

∂xj

[(μ + μt

σk

)∂k

∂xj

]+ Gk + Gb − ρε − YM + Sk (3)

∂

∂t(ρε) + ∂

∂xi(ρεui) = ∂

∂xj

[(μ + μt

σε

)∂ε

∂xj

]+ C1ε

ε

k(Gk + C3εGb) − C2ερ

ε2

k+ Sε.

(4)The values of the five constants of the standard k-ε turbulence model are taken as

Cμ = 0.09 C1ε = 1.44 C2ε = 1.44 σk = 1.0 σε = 1.3.

THE COMPUTATIONAL MODELLING

The CFD simulations were carried out by using Fluent 6.2 software. The meshingwas done in gambit of the package. Both uniform and nonuniform meshing was donewithin the computational domain of the turbine. On the four rectangular edges of the com-putational domain, uniform grid spaces were taken. Nonuniform grid spaces were takenfor the buckets and the blades. The densities of mesh were kept higher on the concave andconvex faces of the buckets than on the ends, since the flow physics on both sides of thecurved faces control the performance of the turbine. Similarly, the mesh density was kepthigher on the faces of the Darrieus blades than its ends. High mesh density was also keptaround the central shaft to capture the vortices that transport across the overlap. On theblades as well as buckets, near wall boundary layers were built in gambit such that the dis-tance of the first row of grid points in direction normal to the solid boundary was 0.004 cm.There were 16 rows of boundary points on each face of the Savonius bucket, such that thetotal number of cells within the boundary layer was around 4,440. The boundary layer wasextended more on the convex face than on the concave face. Similarly there were 10 rowsof boundary points on each face of the Darrieus blade, such that the total number of cellswithin the boundary layer was around 770 up to a total depth of boundary layer of 0.3 cm.Two extreme wind speeds considered for the simulation were 29.4 m/sec and 34.1 m/sec.The tip speed ratios corresponding to these wind speeds were 0.451 and 0.215, respectively.The maximum relative wind speeds between the bucket/blade and the surrounding air were32.26 m/sec and 36.71 m/sec, respectively. And the chord Reynolds numbers with respectto these wind speeds were 1.86 × 105 and 2.12 × 105, respectively. The computations wereinitially carried out with various levels of refinement of mesh. Unstructured (triangular)meshing was done on the face external to the turbine. The computational mesh around thebuckets and blades are shown in Figure 4. The computations were initially carried out withvarious levels of refinement of mesh as shown in Table 1 until grid independent limit mesh(Masson, Ammara, and Paraschivoiu 1997) was reached. For this, CFD code was run for anumber of times with the same set of input parameters. The resolutions of the mesh at all-important areas, like the curved and straight faces were varied in an attempt to reach gridindependent limit mesh. The computed y+ value of the grid independent mesh was within5–20. The pitching moment coefficient was considered as the parameter for which grid

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 215

Figure 4 Computational mesh: (a) around combined Savonius–Darrieus turbine, (b) around the Savonius Buckets(zoomed-in), (c) at the center across shaft and (b) around a Darrieus Blade (zoomed-in).

Table 1 Details of refinement levels of the grid of the combined Savonius–Darrieus turbine.

Refining level No. of nodes No. of cells (Triangular)

1 23, 382 38, 5882 44, 034 75, 6343 51, 778 89, 8704 53, 491 92, 9965 56, 203 96, 2026 60, 024 104, 9467 67, 103 116, 034

independent limit mesh was determined. Figure 5 shows the variation of Cm with differentrefinement levels. The refinement level 7 was selected for final simulation.

In this study, steady state, incompressible flow was considered. The numerical sim-ulation was carried out by solving the conservation equations for mass and momentumwith the help of unstructured-grid finite volume methodology coupled with moving meshtechnique (Fluent User Guide 2005). The finite difference forms of Navier–Stokes equa-tion along with continuity and turbulence equations were solved by the in-built functions

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216 GUPTA AND BISWAS

Grid Independence

0

0.0009

0.0018

0.0027

0 2 4 6 8Refinement Level

Pit

ch

ing

Mo

men

t

Co

eff

Figure 5 Variation of pitching moment coefficient with various refinement levels of the grid of the combinedSavonius–Darrieus turbine.

of the Fluent 6.2 CFD package. The standard k-ε turbulence model with enhanced wallfunction was chosen. The method of dynamic grid or rotating reference frame was imple-mented. Single rotating reference frame was considered in which the buckets, blades andthe central shaft rotate relative to the incoming fluid stream. The sequential algorithm,Semi-Implicit Method for Pressure-Linked Equation (SIMPLE), was used for solving allthe scalar variables. For the convective terms of the momentum equations and also forthe turbulence equations, the second order upwind interpolating scheme (Versteeg andMalalasekera 1995) was adopted in order to achieve accurate results.

CONTOUR PLOT ANALYSIS OF THE COMBINED SAVONIUS–DARRIEUS

TURBINE

The contour plots of pressure, velocity, and vorticity were obtained from the CFDsimulations for the combined three-bucket Savonius and three-bladed Darrieus turbine forbucket angles (or blade angles) of rotation of 110◦, 230◦, and 350◦. The contours wereobtained for without overlap and with overlap variations of 16.2%, 20%, 25%, 30%, and35%. The simulations were carried out for wind speed of 29.4 m/sec. Figure 6(a) to (f)show static pressure contour plots for without and with the above five overlap variations.Figure 7(a) and (b) show dynamic pressure contour plots for without and with 25% overlapvariation. Figure 8(a) to (f) show velocity contour plots for without and with the over-lap variations. For all the angles of rotation, the pressure contour plots show decreaseof static pressure from the upstream side to the downstream side across the turbine. Forwithout overlap, Figure 6(a) shows a decrease of static pressure from 8.07 × 102 Pascal to6.14 × 101 Pascal, from the upstream side to the downstream side of the advancing bucket(at about 1 O’clock position, on top). For 16.2% overlap, the static pressure decreases from5.85 × 102 Pascal to 1.73 × 102 Pascal from the upstream side to the downstream side ofthe advancing bucket, as shown in the Figure 6(b). Therefore, with overlap of 16.2%, thepressure difference between the upstream and downstream sides is lower compared to with-out overlap. And for this, the lift generation of Savonius turbine decreases with overlap.For 20% overlap, the static pressure difference across the advancing bucket further dropsresulting in still lower lift, as shown in Figure 6(c). Moreover, static pressures are negativeon the concave face of the returning bucket and also on the downstream side of the advanc-ing bucket. The negative static pressure means high dynamic pressure, since total pressureis always constant at any location of the flow domain. For without overlap, Figure 7(a)

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 217

Figure 6a Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦ 230◦ &350◦ for without overlap.

Figure 6b Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦ 230◦ &350◦ for 16 2% overlap.

shows that the maximum dynamic pressure on the convex side of the advancing bucket (at1 o’clock position) is around 1.47 × 103 Pascal. However for 30% overlap, the pressureincreases to 4.13 × 103 Pascal. This increase of dynamic pressure on the convex side of thebucket would have caused by the hitting of separated vortices from the convex face of thebucket on to the Darrieus blade. It can also be observed from the pressure contours thatcircular vortices have formed close to the concave face of the returning bucket (5 O’clock

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218 GUPTA AND BISWAS

Figure 6c Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦ &350◦ for 20% overlap.

Figure 6d Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦ &350◦ for 25% overlap.

position), as shown in Figure 6(a) and (b). Due to the close interactions of the vortices withthe concave face of the bucket, the dynamic pressure has increased for which power willbe augmented. In dynamic pressure contours, high dynamic pressure of around 6.66 × 103

Pascal can be observed on the concave face of the bucket due to the occurrence of bucket-vortices interactions (Figure 7a), which will result in high aerodynamic coefficient for thebucket without overlap thereby augmenting power. However, with overlap of 25%, these

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 219

Figure 6e Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦ &350◦ for 30% overlap.

Figure 6f Contour plot of static pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦ &350◦ for 35% overlap.

vortices tend to migrate towards the downstream side of the advancing bucket increasingdynamic pressure on the upstream of Darrieus blade, as shown in Figure 7(b).

The velocity magnitude contours show that the velocity decreases from the upstreamside to the downstream side of the advancing bucket. For without overlap, Figure 8(a)shows a decrease of velocity from 39 m/sec on the upstream side to 10 m/sec on the down-stream side of the advancing bucket. For 16.2% overlap, Figure 8(b) shows a decrease ofvelocity from 30 m/sec on the upstream side to 9 m/sec on the downstream side of theadvancing bucket. Similarly, for 25% overlap, Figure 8(d) shows a decrease of velocity

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220 GUPTA AND BISWAS

Figure 7a Contour plot of dynamic pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦& 350◦ for without overlap.

Figure 7b Contour plot of dynamic pressure of combined Savonius–Darrieus turbine for blade angle: 110◦, 230◦& 350◦ for 25% overlap.

magnitude from 28 m/sec on the upstream side to 8 m/sec on the downstream side of theadvancing bucket. Therefore, with the increase of overlap, the velocity difference betweenthe two sides of the bucket is decreased resulting in the fall of lift generation for theturbine.

Figure 9 (a) to (d) show the velocity vectors across the Darrieus blade at 60◦ bladeangle for without overlap and with 16.2%, 20%, and 30% overlap variations. These plotsshow that, for both without and with overlaps, the velocities decrease from upstream side of

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 221

Figure 8a Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for without overlap.

Figure 8b Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for 16.2% overlap.

the blade to its downstream side. Further, with the increase of overlap, the flow is acceler-ated on the upstream side of the blade. Figure 9(a) shows that, for without overlap, velocitydecreases from 29.5 m/sec from the upstream side of the blade to only 24.6 m/sec on thedownstream side of the blade. For 16.2% overlap, Figure 9(b) shows that velocity decreasesfrom 29.2 m/sec from the upstream side of the blade to 20.8 m/sec on the downstream sideof the blade. For 30% overlap, Figure 9(d) shows that velocity decreases from 41.6 m/secto 23.1 m/sec from upstream side to downstream side. Hence, the flow is accelerated on theupstream face of the blade, and the velocity difference across the Darrieus blade increaseswith the increase of overlap; therefore, the aerodynamic lift for the blade will also increase

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222 GUPTA AND BISWAS

Figure 8c Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for 20% overlap.

Figure 8d Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for 25% overlap.

simultaneously. However, too high overlap variation in Savonius turbine is undesirablesince it reduces lift for the next Darrieus blade. Figure 10(a)–(d) show the velocity vectordistributions across the Darrieus blade at 180◦ blade angle position. Figure 10(a) showsthat, for without overlap, velocity on the lower side of the blade is about 35 m/sec and thaton the upper side is about 30 m/sec. Figure 10(c) shows that, for 20% overlap, velocity onthe lower side of the blade is about 42 m/sec and that on the upper side is about 30 m/sec.On the other hand Figure 10(d) shows that, for 25% overlap, velocity on the lower side ofthe blade is about 47 m/sec and that on the upper side is about 42 m/sec. Thus, for higher

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 223

Figure 8e Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for 30% overlap.

Figure 8f Contour plot of velocity magnitude of combined Savonius–Darrieus turbine for blade angle: 110◦,230◦ & 350◦ for 35% overlap.

overlap as much as 25%, the velocity difference between the two sides of the blade at 180◦blade angle position is lowered compared to that for 20% overlap, and hence reduces liftfor the blade.

Coanda Flow Analysis of the Combined Savonius–Darrieus Turbine

Coanda flow is defined as the flow of steep vortices across the overlap of the Savoniusturbine that migrates from the downstream side of the advancing bucket to the concave faceof the returning bucket (Fujisawa 1992). As the vortices are accumulated on the concave

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224 GUPTA AND BISWAS

Figure 9 Velocity vectors across the Darrieus blade at 60◦ blade angle for: (a) without overlap, (b) with 16.2%overlap, (c) with 20% overlap and (d) with 30% overlap.

face of the returning bucket and interact with the bucket there, the power production ofthe turbine is augmented by the increase of dynamic pressure on the concave face of thebucket. Figure 11(a) shows the coanda flow around the central shaft for without overlapcondition for bucket (or blade) angles of rotation of 110◦, 230◦, and 350◦. Though it iscalled without overlap, but small gap between the central shaft and the inner edges of thebuckets is considered to accommodate for the space occupied by the fixing bolts of thebuckets. Figure 11(b) shows coanda flow around the central shaft for 16.2% overlap forthe same angles of rotation i.e., 110◦, 230◦, and 350◦. Figure 11(c) shows the coanda flowaround the central shaft for 20% overlap for the same angles of rotation. Finally, Figure11(d)–(f) show the coanda flow across the central shaft for 25%, 30%, and 35% overlapvariations. It is to be noted that the small gap for the fixing bolts is included within theoverlap values. The upstream side, downstream side, advancing bucket, returning bucket,concave face, and overlap are marked in these plots. The coanda flows can be observed tobe migrating from the downstream side across the overlap at the central shaft. In addition,the boundary layer vortices are also observed adjacent to the concave and convex faces ofthe buckets. Figure 11(a), for without overlap and with angles of rotation of 110◦, 230◦, and350◦, shows that steep coanda vortices enter through the two gaps, glide past the centralshaft, and then reaches the concave face of the returning bucket in the upstream side forwhich the dynamic pressure will increase there. Figure 11(b), for 16.2% overlap and with

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 225

Figure 10 Velocity vectors across the Darrieus blade at 180◦ blade angle for: (a) without overlap, (b) with 16.2%overlap, (c) with 20% overlap and (d) with 25% overlap.

angles of rotation of 110◦, 230◦, and 350◦, shows the similar influx of coanda vorticesof increased volume that migrate from the downstream side and flow through the overlapbetween the shaft and the inner edges of the buckets, and finally reach the concave face ofthe returning bucket.

Figure 11(c), for 20% overlap, also shows coanda vortices enter from the downstreamside through the overlap. However a hint of separated vortices can be observed at the inneredge of advancing bucket, which would destabilize the coanda flow coming across theoverlap near that edge. Figure 11(d) and (e), for 25% and 30% overlaps, respectively, showincrease in the amount of separated vortices from the inner edges of the buckets inhibitingthe coanda flow from reaching the concave face of the returning bucket. For this reason,both aerodynamic coefficients and performance of the combined Savonius–Darrieus tur-bine will decrease. Figure 11(f), for 35% overlap, show almost complete destabilization ofcoanda flow. The flow instead of reaching the concave faces re-circulates across the centralshaft thereby lowering its performance.

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226 GUPTA AND BISWAS

Figure 11a Coanda flow for without overlap for bucket angle: 110◦, 230◦ & 350◦

Figure 11b Coanda flow for 16.2% overlap for bucket angle: 110◦, 230◦ & 350◦

AERODYNAMIC COEFFICIENTS OF THE COMBINED SAVONIUS–DARRIEUS

TURBINE

The aerodynamics coefficients of the combined Savonius–Darrieus turbine, likelift (Cl), drag (Cd) and lift-to-drag (Cl/Cd) coefficients were evaluated for without andwith five overlap variations of 16.2%, 20%, 25%, 30%, and 35%. Figure 12(a)–(c) showthe composite plots of the comparisons of Cl, Cd and Cl/Cd between Darrieus blade and

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 227

Figure 11c Coanda flow for 20% overlap for bucket angle: 110◦, 230◦ & 350◦

Figure 11d Coanda flow for 25% overlap for bucket angle: 110◦, 230◦ & 350◦

Savonius bucket for various overlap variations. These figures show that the values of Cl

and Cl/Cd are higher for Darrieus blade. The highest Cl of 0.079 is obtained for withoutoverlap for Savonius bucket, whereas the highest Cl of 0.089 is obtained for 20% overlapfor Darrieus blade (Figure 12a). However, the highest Cl/Cd for the bucket is 10.5 obtainedfor 16.2% overlap variation and that for the blade is 14.5, which is also obtained for thesame overlap variation (Figure 12c). Figure 13(a) and (b) show the variations of Cl andCl/Cd of the bucket with respect to angle of attack for four tip speed ratios of 0.215, 0.274,

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228 GUPTA AND BISWAS

Figure 11e Coanda flow for 30% overlap for bucket angle: 110◦, 230◦ & 350◦

Figure 11f Coanda flow for 35% overlap for bucket angle: 110◦, 230◦ & 350◦

0.333 and 0.451. Similarly, Figure 13(c) and (d) show the variations of Cl and Cl/Cd ofthe blade with respect to angle of attack for the above tip speed ratios. Figure 13(a) showsthat Cl increases with the increase of angle of attack up to the maximum at certain angle ofattack and then decreases. The maximum Cl of 0.079 is obtained at an angle of attack of81.41◦ for tip speed ratio of 0.451. Figure 13(b) also shows that Cl/Cd increases with theincrease of angle of attack up to the maximum at certain angle of attack and then decreases.The maximum Cl/Cd of 10.41 is obtained at an angle of attack of 111.41◦ for tip speedratio of 0.333. Figure 13(c) and (d), for the blade, show the same trend in the variationsof Cl and Cl/Cd with angle of attack as that for the bucket. The maximum Cl of 0.089is obtained at an angle of attack of 81.41◦ for tip speed ratio of 0.333 (Figure 13c). The

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 229

0

0.015

0.03

0.045

0.06

0.075

0.09

0.105

Overlap

Cl

Bucket

Blade

0 5 10 15 20 25 30 35 40

Figure 12a Comparison of Cl between bucket and blade for various overlap values.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Overlap

Cd

Bucket

Blade

0 5 10 15 20 25 30 35 40

Figure 12b Comparison of Cd between bucket and blade for various overlap values.

02468

10121416

0 5 10 15 20 25 30 35 40Overlap

Cl/C

d

Bucket

Blade

Figure 12c Comparison of Cl /Cd between bucket and blade for various overlap values.

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230 GUPTA AND BISWAS

–0.015

0

0.015

0.03

0.045

0.06

0.075

0.09

Angle of Attack

Cl_

bu

cket

TSR = 0.451

TSR = 0.333

TSR = 0.274

TSR = 0.215

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Figure 13a Variation of Cl for bucket with respect to angle of attack for various tip speed ratios.

–2

0

2

4

6

8

10

12

Cl/C

d_b

ucket

TSR = 0.451

TSR = 0.333

TSR = 0.274

TSR = 0.215

Angle of Attack

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Figure 13b Variation of Cl/Cd for bucket with respect to angle of attack for various tip speed ratios.

–0.04

–0.02

0

0.02

0.04

0.06

0.08

0.1

Cl_

bla

de

TSR = 0.451

TSR = 0.333

TSR = 0.274

TSR = 0.215

Angle of Attack

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Figure 13c Variation of Cl for blade with respect to angle of attack for various tip speed ratios.

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CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 231

–8.1

–5.4

–2.7

0

2.7

5.4

8.1

10.8

13.5

16.2

Angle of Attack

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Cl/C

d_b

lad

e

TSR = 0.451

TSR = 0.333

TSR = 0.274

TSR = 0.215

Figure 13d Variation of Cl/ Cd for bucket with respect to angle of attack for various tip speed ratios.

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.3 0.32 0.34 0.36 0.38 0.4 0.42Tip Speed Ratio

Cp

Expt

CFD

Figure 14 Variation of Cp with respect to tip speed ratio for 16.2% overlap.

maximum Cl/Cd of 14.5 is obtained at an angle of attack of 101.41◦ for tip speed ratio of0.451 (Figure 13d).

Power coefficient (Cp) of a wind turbine is defined as the ratio of power producedby the turbine to the useful power contained in the wind. From our earlier published work(Debnath et al. 2009), it can be seen that there is a good agreement between the experimen-tal and computational results with respect to power coefficient for the same design of thecombined Savonius–Darrieus turbine. Figure 14 is reproduced from the earlier publishedwork of Debnath et al. (2009) showing the agreement at an overlap of 16.2%.

CONCLUSION

In this paper, the flow physics and aerodynamic performance of a combined three-bucket Savonius and three-bladed Darrieus turbine was analyzed computationally basedon the contour plot analysis by using Fluent 6.2 CFD software. From the analysis, thefollowing conclusions are summarized:

i. Pressure and velocity magnitudes decrease from upstream to downstream side of theadvancing bucket of Savonius turbine resulting in overall lift for the turbine. However

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232 GUPTA AND BISWAS

with overlap, both pressure and velocity magnitude difference across the bucket isdecreased resulting in decrease of aerodynamic lift for the Savonius turbine. For theDarrieus turbine, velocity also decreases across the blade causing aerodynamic lift forthe turbine. With increase in overlap, however, velocity difference across the bladeincreases due to the flow being accelerated on the upstream side of the Darrieus bladedue to the increase of dynamic pressure on the downstream of Savonius bucket.

ii. Strong circular vortices are formed close to the concave face of the returning bucket forwithout overlap and with low overlap (16.2%) resulting from the migration of Coandavortices from the downstream side of the advancing bucket on to the concave face ofthe returning bucket increasing dynamic pressure on it. Due to the close interactions ofthe vortices with the returning bucket, the power will be augmented for the combinedturbine. However, the advantage is lost with high value of overlap (20% onwards) dueto the separation of vortices from the inner edges of bucket, which migrate towards thedownstream side thereby destabilizing the Coanda flow, and hence lowering the aero-dynamic coefficients after 16.2% overlap point. Power augmentation is also causedby high aerodynamic lift-to-drag coefficient (14.5) of the Darrieus turbine from theincrease of velocity difference across the Darrieus blade with small overlap (16.2%)in Savonius turbine.

REFERENCES

Akdag, S. A., and Ö. Güler. 2010. Wind characteristics analyses and determination of appropriatewind turbine for Amasra—Black Sea region, Turkey. International Journal of Green Energy7(4): 422–33.

Debnath, B. K., A. Biswas, and R. Gupta. 2009. Computational fluid dynamics analysis of a com-bined three-bucket Savonius and three-bladed Darrieus rotor at various overlap conditions.International Journal of Renewable and Sustainable Energy 1:033110–13.

Elmabrok, A. M. 2009. Estimation of the performance of the darrieus Savonius combined machine.Proceedings of Ecologic Vehicles and Renewable Energies, Monaco, March 26–29.

FLUENT Inc. Fluent 6.2 documentation: user’s guide; 2005.Fujisawa, F. 1992. On the torque mechanism of Savonius rotor. International Journal of wind

Engineering and Industrial Aerodynamics 40: 277–92.Gavalda, J., J. Massons, and F. Diaz. 1990. Experimental study on a self adapting Darrieus- Savonius

wind machines. International Journal of Solar and Wind Technologies 7(4):457–61.Ghosh, P., M. A. Kamoji, S. B. Kedare, and S. V. Prabhu. 2009. Model testing of single- and three-

stage modified Savonius rotors and viability study of modified Savonius pump rotor systems.International Journal of Green Energy 6(1):22–41.

Global wind energy market as of 2006. Press release of Global Wind Energy Council (GWEC).Global wind energy market as of 2008. Press release of Global Wind Energy Council (GWEC).Gupta, R., A. Biswas, and K. K. Sharma. 2008. Comparative study of three-bucket Savonius turbine

with combined three-bucket-Savonius-three-bladed-Darrieus turbine. International Journal ofRenewable Energy 33:1974–81.

Gupta, R., R. Das, and K. K. Sharma. 2006. Experimental study of a Savonius-Darrieus windmachine. Proceedings of the International Conference on Renewable Energy for DevelopingCountries, University of Columbia, Washington DC.

Masson, C., I. Ammara, and I. Paraschivoiu. 1997. An aerodynamic method for the analysis ofisolated horizontal-axis wind turbines. International Journal of Rotating Machinery 3: 21–32.

McWilliam, M., and D. A. Johnson. 2008. Velocity measurement of flow around model vertical axiswind turbines. International Journal of Green Energy 5(1):55–68.

Downloaded By: [Biswas, A.] At: 09:51 14 March 2011

CFD ANALYSIS OF FLOW PHYSICS AND AERODYNAMIC PERFORMANCE 233

Ozgener, O., L. Ozgener, and I. Dincer. 2009. Analysis of some exergoeconomic parameters of smallwind turbine system. International Journal of Green Energy 6(1):42–56.

Ucar, A., and F. Balo. 2008. A seasonal analysis of wind turbine characteristics and wind powerpotential in Manisa, Turkey. International Journal of Green Energy 5(6):466–79.

Versteeg, H. K., and W. Malalasekera. 1995. An introduction to computational fluid Dynamics, thefinite volume method. New York: Longman Scientific & Technical, p. 132.

Wakui, T., T. Yoshiaki, H. Takumi, and N. Toshio. 2005. Hybrid configuration of Darrieus & Savoniusturbines for stand-alone wind turbine-generator system. International Journal of ElectricalEngineering in Japan 150(4):13–22.

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