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Technical Brief

Trapped Cylindrical Flow With Multiple Inlets for Savonius Vertical Axis Wind Turbines

[+] Author and Article Information
Aaron S. Alexander

Mem. ASME
Department of Engineering Technology,
Oklahoma State University,
567 Engineering North,
Stillwater, OK 74078
e-mail: aaron.s.alexander@okstate.edu

Arvind Santhanakrishnan

Department of Mechanical and Aerospace Engineering,
Oklahoma State University,
218 Engineering North,
Stillwater, OK 74078
e-mail: askrish@okstate.edu

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 25, 2017; final manuscript received September 22, 2017; published online December 4, 2017. Assoc. Editor: Matevz Dular.

J. Fluids Eng 140(4), 044501 (Dec 04, 2017) (7 pages) Paper No: FE-17-1117; doi: 10.1115/1.4038166 History: Received February 25, 2017; Revised September 22, 2017

Savonius vertical axis wind turbines (VAWTs) typically suffer from low efficiency due to detrimental drag production during one half of the rotational cycle. The present study examines a stator assembly created with the objective of trapping cylindrical flow for application in a Savonius VAWT. While stator assemblies have been studied in situ around Savonius rotors in the past, they have never been isolated from the rotor to determine the physics of the flow field, raising the likelihood that a moving rotor could cover up deficiencies attributable to the stator design. The flow field created by a stator assembly, sans rotor, is studied computationally using three-dimensional (3D) numerical simulations in the commercial computational fluid dynamics (CFD) package Star-CCM+. Examination of the velocity and pressure contours at the central stator plane shows that the maximum induced velocity exceeded the freestream velocity by 65%. However, flow is not sufficiently trapped in the stator assembly, with excess leakage occurring between the stator blades due to adverse pressure gradients and momentum loss from induced vorticity. A parametric study was conducted on the effect of the number of stator blades with simulations conducted with 6, 12, and 24 blades. Reducing the blade number resulted in a reduction in the cohesiveness of the internal swirling flow structure and increased the leakage of flow through the stator. Two unique energy loss mechanisms have been identified with both caused by adverse pressure gradients induced by the stator.

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Figures

Grahic Jump Location
Fig. 1

Schematic of the stator model

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Fig. 2

(a) Mesh distribution around the stator assembly and (b) result of the grid sensitivity study between the low (15 ×106 cells), mid (18 ×106 cells), and high (22 ×106 cells) cell count cases

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Fig. 5

CFD time evolution results of the tangential velocity inside the stator at the indicated polar positions for V = 33.5 m/s and Rmax = 105 mm (central plane of stator)

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Fig. 4

Bypass rate versus Re

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Fig. 3

Contours of normalized tangential velocity inside the stator at the central plane with velocity vectors overlaid on (d) as representative of the shape of the flow field. Flow is from left to right and is normalized by freestream velocity (V). Re = (a) 278,311, (b) 347,888, (c) 417,466, and (d) 466,171. Reynolds number (Re) is defined based on external diameter of the stator and freestream velocity (V).

Grahic Jump Location
Fig. 9

Pressure coefficient (Eq. (2)) and normalized velocity (V = 33.5 m/s) contours for (a) 6 and (b) 12 blades (mid-height of the stator)

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Fig. 8

Depiction of the variation of the vorticity contour (V = 33.5 m/s) with height for 5 cm below the centerline, the centerline, and 5 cm above the centerline (left to right)

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Fig. 7

Depiction of the variation of normalized velocity contour (V = 33.5 m/s) with height for 5 cm below the centerline, the centerline, and 5 cm above the centerline (left to right)

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Fig. 6

Pressure coefficient (Eq. (2)) contours for (a) 20 m/s and (b) 33.5 m/s (mid-height of the stator). Adverse pressure gradients (APG) 1 and 2 are shown in (a) while APG3 is shown in (b).

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Fig. 10

Slot concept shown on a single blade

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