Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle

Zied Driss; Olfa Mlayeh; Slah Driss; Makram Maaloul; Mohamed Salah Abid

doi:10.12691/ajme-3-3A-3

American Journal of Mechanical Engineering

Vol. 3, No. 3A, 2015, pp 15-21. doi: 10.12691/ajme-3-3A-3 | Research Article

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Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle

Zied Driss^1,, Olfa Mlayeh¹, Slah Driss¹, Makram Maaloul¹, Mohamed Salah Abid¹

¹Laboratory of Electro-Mechanic Systems (LASEM), National School of Engineers of Sfax (ENIS), University of Sfax (US), B.P. 1173, Road Soukra km 3.5, 3038 Sfax, TUNISIA

	Abstract
1.	Introduction
2.	Geometrical Arrangements
3.	Numerical Model
4.	Results and Discussions
5.	Conclusions
	References

Abstract

In this paper, computer simulation has been conducted to study the aerodynamic structure around an unconventional Savonius wind rotor with a 75° bucket arc angle. The numerical model is based on the resolution of the Navier-Stokes equations in conjunction with the standard k-ε turbulence model. These equations were solved by a finite volume discretization method. The software Solidworks Flow simulation has been used to characterise the flow characteristics in different transverse and longitudinal planes. The good comparison between numerical and experimental results confirms the validity of the numerical method.

Keywords: unconventional Savonius wind rotor, bucket arc angle, wind tunnel, aerodynamic structure, CFD

Received May 20, 2015; Revised June 22, 2015; Accepted July 13, 2015

Cite this article:

Zied Driss, Olfa Mlayeh, Slah Driss, Makram Maaloul, Mohamed Salah Abid. Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle. American Journal of Mechanical Engineering. Vol. 3, No. 3A, 2015, pp 15-21. https://pubs.sciepub.com/ajme/3/3A/3

Driss, Zied, et al. "Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle." American Journal of Mechanical Engineering 3.3A (2015): 15-21.

Driss, Z. , Mlayeh, O. , Driss, S. , Maaloul, M. , & Abid, M. S. (2015). Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle. American Journal of Mechanical Engineering, 3(3A), 15-21.

Driss, Zied, Olfa Mlayeh, Slah Driss, Makram Maaloul, and Mohamed Salah Abid. "Numerical Study of an Unconventional Savonius Wind Rotor with a 75° Bucket Arc Angle." American Journal of Mechanical Engineering 3, no. 3A (2015): 15-21.

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1. Introduction

The Savonius wind rotor has aroused a large credit, not only in research and academic communities but also in industrial appliances. In comparison to that of other kinds, the efficiency of Savonius rotor is lower. The reason of low efficiency mainly rests on the fact that one bucket moves against wind when another one moves in the direction of wind ^{[2, 3]}. In this context, numerical and experimental investigations have been conducted to improve the Savonius wind rotor performance. For example, Khan et al. ^[4] tested Savonius rotor both in tunnel and natural wind conditions with the provision of variation of overlap. Research conducted by Grinspan et al. ^[5] in this direction led to the development of a new blade shape with a twist for the Savonius rotor. They obtained a maximum power coefficient of 0.5 for its model. Saha and Rajkumar ^[6] performed work on twist bladed metallic Savonius rotor and compared the performance with conventional semi-circular blades having no twist. They obtained an efficiency of 0.14. Their rotor also produced good starting torque and larger rotational speeds. Saha et al. ^[7] conducted wind tunnel tests to assess the aerodynamic performance of single, two and three-stage Savonius rotor systems. Both semicircular and twisted blades have been used. Aldos ^[8] studied power augmentation of Savonius rotor by allowing the rotor blades to swing back when on the upwind side. He reported a power augmentation of the order of 11.25% with the increase in Cp. He further concluded that different basic rotors configuration might produce different power augmentation. Sabzevari ^[9] examined the effects of several ducting, concentrators and diffusers on the performance improvements of a split Savonius rotor. A circularly ducted Savonius rotor equipped with a number of identical wind concentrators and diffusers along the periphery of circular housing produced efficiency of the order of 40%. In order to eliminate the low aerodynamic performance of Savonius wind rotors, Mohamed et al. ^[10] studied several shapes of obstacles and deflectors placed in front of two and three blades Savonius turbine. A rounded deflector structure was placed in front of two counter-rotating turbines. An experimental investigation was carried out by Golecha et al. ^[11] to identify the position of the deflector plate to yield higher coefficient of power for single stage modified Savonius rotor. Akwa et al. ^[13] discussed the influence of the buckets overlap ratio of a Savonius wind rotor on the averaged moment and power coefficients, over complete cycles of operation. Kamoji et al. ^[14] compared the helical Savonius rotor with the conventional Savonius rotor. The results indicate that the helical Savonius rotors have positive coefficient of static torque. D’Alessandro et al. ^[15] developed a mathematical model of the interaction between the flow field and the rotor blades. The aim of their research was to gain an insight into the complex flow field developed around a Savonius wind rotor and to evaluate its performance. Irabu and Roy ^[16] improved and adjusted the output power of Savonius rotor under various wind power and suggests the method of prevention the rotor from strong wind disaster.

On the basis of the previous studies, it appears important to propose a new design to improve the performance of the conventional Savonius wind rotor. For this purpose, computer simulations have been conducted to study the aerodynamic structure around an unconventional Savonius wind rotor with a 75° bucket arc angle.

2. Geometrical Arrangements

Figure 1 proposes different designs of the Savonius wind rotor. This rotor is constituted by two half buckets characterized by the height H=300 mm, the diameter c=100 mm, the bucket arc angle ψ=75°, and the thickness e=6 mm. The two buckets are collected on a common axis, with a shaft diameter equal to s=10 mm, and they are fixed within screws to make an angle equal to 180°.

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Figure 1. Modified Savonius wind trotor

3. Numerical Model

In this work, the software "Solidworks Flow Simulation" has been used to study the turbulent flow around unconventional Savonius wind rotors. This code is based on solving Navier-Stokes equations with a finite volume discretization method. The technique consists in dividing the computational domain into elementary volumes around each node in the grid; it ensures continuity of flow between nodes. The spatial discretization is obtained by following a procedure for tetrahedral interpolation scheme. As for the temporal discretization, the implicit formulation is adopted. The transport equation is integrated over the control volume [21-25]. The computational domain is defined by the interior volume of the wind tunnel blocked by two planes. The first one is in the tranquillization chamber entry and the second one is in the exit of the diffuser ^[18]. A boundary condition is required anywhere fluid enters or exits the system and can be set as a pressure and velocity. For the inlet velocity, we take as a value V=3 m.s-1, and for the outlet pressure a value of p=101325 Pa is considered which means that at this opening the fluid exits the model to an area of static atmospheric pressure.

4. Results and Discussions

In this section, we are interested to study the aerodynamic characteristics of the flow such as the velocity fields, the average velocity, the total pressure, the dynamic pressure, the turbulent kinetic energy, the dissipation rate of the turbulent kinetic energy, the turbulent viscosity and the vorticity. For thus, three planes were considered defined by z=0 mm, x=0 mm, and y=0 mm (Figure 2). According to the air flow direction, the first is a transverse plane; however, the two others are longitudinal planes.

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Figure 2. Visualization planes

4.1. Velocity Fields

Figure 3 shows the distribution of the velocity field in the planes defined by x=0 mm, y=0 mm and z=0 mm. While examining these results, it can easily be noted that the velocity is weak in the inlet of the collector. In fact, it is governed by the boundary condition value of the inlet velocity which is equal to V=3 m.s-1. In this region, the velocity field is found to be uniform and increases progressively downstream of the collector. At the test vein, an important increase of the velocity value has been noted due to the reduction of the tunnel section that causes the throttling of the flow. While the upstream of the rotor is characterized by the high velocity, a brutal drop is located in the concave surface of the two buckets. Downstream of the rotor, the velocity value keeps increasing till the out of the test section. Then, a sharp decrease has been noted through the diffuser where the minimum velocity values are recorded in the lateral walls of the diffuser. The maximum velocity values are located in the convex surface of the two buckets according to the distribution shown on the y=0 mm plane. The velocity fields on the longitudinal plane, defined by y=0 mm, show a symmetric distribution. Also, a deceleration of the velocity field around the shaft and in the concave surface of the buckets has been observed. Another zone is located in the top and the left wall of the test vein which corresponds to the zone around the advancing bucket.

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Figure 3. Distribution of the velocity field

4.2. Average Velocity

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Figure 4. Distribution of the Magnitude velocity

Figure 4 shows the distribution of the magnitude velocity in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it has been noted that the velocity is weak in the inlet of the collector. In this region, the average velocity field is found to be uniform and increases progressively downstream of the collector. At the test vein, an important increase of the velocity value has been noted due to the reduction of the tunnel section that causes the throttling of the flow. While the upstream of the rotor is characterized by the high velocity, a brutal drop is located in the concave surface of the two buckets. Downstream of the rotor, the average velocity value keeps increasing till the out of the test section. Then, a sharp decrease has been noted through the diffuser where the minimum average velocity values are recorded in the lateral walls of the diffuser. The wakes characteristics of the maximum average velocity values are located in the convex surface of the two buckets according to the distribution shown in the longitudinal plane defined by y=0 mm. However, the wakes characteristics of the minimum average velocity values are located in the concave surface of the buckets and in the bucket sides.

4.3. Total Pressure

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Figure 5. Distribution of the total pressure

Figure 5 shows the distribution of the total pressure in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it has been noted that the total pressure is on its maximum in the intake and is globally uniform in the collector and the upstream of the rotor in the test vein. A brutal drop of the total pressure has been noted in the concave surface of the rotor, downstream of the advancing bucket and around the axis of the rotor. The distribution of the total pressure in the longitudinal planes defined by x=0 cm and y=0 cm shows that the depression zones are located in the downstream of the concave surface of the returning bucket, the convex surface of the advancing bucket and the extremities of the rotor axis. Downstream of the rotor, the total pressure starts to increase gradually in the way out of the test vein and keeps increasing through the diffuser. The transverse planes show the formation of depression zones near the rotor axis and the buckets. Around the rotor, the total pressure is found to be relatively high and is increasing in the way out of the test vein.

4.4. Dynamic Pressure

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Figure 6. Distribution of the dynamic pressure

Figure 6 shows the distribution of the dynamic pressure in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, the dynamic pressure is found to be weak in the collector inlet and increases gradually through the collector as long as the tunnel section gets smaller. When it gets to the test section, the dynamic pressure keeps increasing in the upstream of the rotor and around it. It reaches its maximum in the convex surface of both the advancing and returning buckets. A depression zone is recorded in the concave surface of the rotor buckets and around the shaft. Downstream of the rotor, the dynamic pressure remains relatively low in the test section and through the diffuser.

4.5. Turbulent Kinetic Energy

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Figure 7. Distribution of the turbulent kinetic energy

Figure 7 shows the distribution of the turbulent kinetic energy in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it is clear that the turbulent kinetic energy is found to be very weak in the wind tunnel except in the area surrounding the rotor and the diffuser exit. The distribution of the turbulent kinetic energy shows the increase of the turbulent kinetic energy near the rotor. The wake characteristic of the maximum value of the turbulent kinetic energy is recorded around the axis and along the concave surface especially in the advancing bucket as shown in the distribution in the longitudinal plane defined by y=0 mm.

4.6. Dissipation Rate of the Turbulent Kinetic Energy

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Figure 8. Distribution of the dissipation rate of the turbulent kinetic energy

Figure 8 shows the distribution of the turbulent dissipation rate in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it has been noted that the turbulent dissipation rate is very weak in the wind tunnel except in the area surrounding the Savonius rotor. The wake characteristic of the maximum values of the turbulent dissipation rate of the turbulent kinetic energy appears around the rotor axis and in the end plates of the buckets.

4.7. Turbulent Viscosity

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Figure 9. Distribution of the turbulent viscosity

Figure 9 shows the distribution of the turbulent viscosity in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it has been noted that the viscosity is weak in the collector. It increases upstream and near the rotor and when leaving the collector. The wake characteristic of the maximum values of the turbulent viscosity in the test section is recorded in the concave surface of the buckets. In the rotor downstream, a rapid decrease has been observed.

4.8. Vorticity

Figure 10 shows the distribution of the vorticity in the planes defined by x=0 mm, y=0 mm and z=0 mm. According to these results, it has been noted that the vorticity is very weak in the wind tunnel except in the region around the rotor. The wakes characteristics of the maximum values of the vorticity appear around the rotor axis and in the bucket sides.

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Figure 10. Distribution of the Vorticity

5. Conclusions

Computer simulations have been conducted to study the aerodynamic structure around an unconventional Savonius wind rotor with a 75° bucket arc angle. In this work, it has been observed that the bucket design has a direct effect on the local characteristics. Particularly, it has been noted that the depression zones are located in the concave surface of the bucket and downstream of the rotor. The acceleration zone, where the maximum velocity values are recorded, is formed in the convex surface of the rotor bucket.

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