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Research Papers

A Prescribed-Wake Vortex Lattice Method for Preliminary Design of Co-Axial, Dual-Rotor Wind Turbines

[+] Author and Article Information
Aaron Rosenberg

Department of Aerospace Engineering,
Iowa State University,
Ames, IA 50011
e-mail: aaronr@iastate.edu

Anupam Sharma

Mem. ASME
Assistant Professor
Department of Aerospace Engineering,
Iowa State University,
Ames, IA 50011
e-mail: sharma@iastate.edu

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received September 17, 2015; final manuscript received July 29, 2016; published online September 2, 2016. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 138(6), 061002 (Sep 02, 2016) (9 pages) Paper No: SOL-15-1311; doi: 10.1115/1.4034350 History: Received September 17, 2015; Revised July 29, 2016

This paper extends the prescribed-wake vortex lattice method (VLM) to perform aerodynamic analysis of dual-rotor wind turbines (DRWTs). A DRWT turbine consists of a large, primary rotor placed co-axially behind a smaller, secondary rotor. The additional vortex system introduced by the secondary rotor of a DRWT is modeled while taking into account the singularities that can occur when the trailing vortices from the secondary (upstream) rotor interact with the bound vortices of the main (downstream) rotor. Pseudo-steady assumption is invoked, and averaging over multiple relative rotor positions is performed to account for the primary and secondary rotors operating at different rotational velocities. The VLM solver is first validated against experiments and blade element momentum theory results for a conventional, single-rotor turbine. The solver is then verified for two DRWT designs against results from two computational fluid dynamics (CFD) methods: (1) Reynolds-averaged Navier–Stokes CFD with an actuator disk representation of the turbine rotors and (2) large-eddy simulations with an actuator line model. Radial distributions of sectional torque force and angle of attack show reasonable agreement between the three methods. Results of parametric sweeps performed using VLM agree qualitatively with the Reynolds-averaged Navier–Stokes (RANS) CFD results demonstrating that the proposed VLM can be used to guide preliminary design of DRWTs.

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References

Figures

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Fig. 1

A schematic of the DRWT technology by Rosenberg et al. [1]

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Fig. 2

Functions to obtain axial induction factor, a from CP and CT given by 1D momentum theory, and from CT given by Eq. (1)

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Fig. 3

A correlation derived using wake data from a number of RANS CFD simulations

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Fig. 4

Nondimensional chord and twist distributions for the model Tellus T-1995 (Risø) turbine used for validation [16]

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Fig. 5

Comparisons with experimental data and BEM results for the Tellus T-1995 (Risø) turbine: (a) power variation with wind speed and (b) the same information presented as characteristic (CP−λ) curves

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Fig. 6

Comparisons of radial variation of force coefficients at λ=6.0 against BEM results for the Tellus T-1995 (Risø) turbine

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Fig. 7

Trailing wake vorticity behind the two rotors of a dual-rotor wind turbine. Each rotor is three bladed.

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Fig. 8

Predicted variation in % CP of a DRWT with relative angular position between the primary and the secondary rotors

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Fig. 9

Panel code results for the NACA 0030 airfoil at freestream Mach number of 0.2 and zero angle of attack: (a) contours of percentage difference between local and freestream pressure and (b) magnitude of difference between local and freestream pressure and flow speed along the dashed line in (a)

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Fig. 10

Computational grid and result of the RANS/AD method applied to a DRWT: (a) axisymmetric grid (every fifth point shown for clarity) and (b) pressure contours and streamlines (zoomed in near the DRWT)

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Fig. 11

Isosurfaces of Q-criterion (Q = 0.05) computed using the LES/ALM method. The surfaces show the bound and trailing vortices.

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Fig. 12

Comparisons between CFD and VLM of spanwise variations of angle of attack and torque force coefficient for the following parameters of the secondary rotor: rt,s/rt,m=0.25 and λs=8.0: (a) angle of attack, α, and (b) sectional torque force coefficient, cτF

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Fig. 13

Comparisons between CFD and VLM of spanwise variations of angle of attack and torque force coefficient for the following parameters of the secondary rotor: rt,s/rt,m=0.40 and λs=8.0: (a) angle of attack, α, and (b) sectional torque force coefficient, cτF

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Fig. 14

Parametric sweep results—contours of difference in aerodynamic power coefficient, ΔCP=CPDRWT−CPSRWT as predicted by (a) RANS and (b) VLM are shown. The parameters are tip radius and tip speed ratio of the secondary rotor.

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Fig. 15

VLM predictions of aerodynamic power coefficients of a DRWT and the corresponding SRWT as a function of the main rotor tip speed ratio, λm

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