Intro Method Results Comp Con References

CFD Simulation of a 3-Bladed Horizontal AxisTidal Stream Turbine using RANS and LES

J. McNaughton, I. Afgan, D.D. Apsley,

S. Rolfo, T. Stallard and P.K. Stansby

Modelling and Simulation Centre, School of MACE, The University of Manchester

Oxford Tidal Energy WorkshopOxford, UK.

29 - 30 March 2012

Intro Method Results Comp Con References

Overview

1 Introduction

2 Methodology

3 RANS Results

4 RANS and LES comparison

5 Conclusions & Further work

Intro Method Results Comp Con References

Introduction

ReDAPT (Reliable Data Acquisition Platform for Tidal):

Project designed to accelerate the tidal energy industry.

Aim to inspire market confidence in tidal stream turbines(TST).

Deployment of a full scale 1MW turbine at EMEC in theOrkneys.

This work:

Using Code Saturne to accurately predict loading on tidalturbines.

Assess the influence of turbulence and waves on TSTs.

Current results compare Reynolds Averaged Navier Stokes(RANS) model and Large Eddy Simulation (LES) againstthose for a laboratory scale turbine.

Intro Method Results Comp Con References

IntroductionCase overview

CFD modelling of experimentalstudy of Bahaj et al. (2005).

0.4 m radius, R, turbine pulledthrough a towing tank.

Parametric study of forcecoefficients againsttip-speed-ratio (TSR).

Photo reproduced from Bahajet al. (2005).

Intro Method Results Comp Con References

MethodologyCFD Solver

EDF’s open-source CFD code, Code Saturne (Archambeauet al., 2004), is used to simulate turbulent flow past a TST.

A sliding mesh method is developed in Code Saturne to allowfor the TST rotation.

Second order in space (central differencing).

First-order in time with ∆t ≈ 1.5 of rotation per time-step.

The k − ω SST RANS turbulence model is used to simulatethe flow.

Results are also compared against an LES that is also beenperformed as part of this project.

Intro Method Results Comp Con References

MethodologySliding-mesh method

Start in cell centre, I , with face-centre, F .

I is projected through F to give a halopoint, H.

Search for, J, the nearest cell-centre tothe halo-point.

The face-centre value is given a Dirichletcondition with the value:

φF =1

2(φI + φH),

Where:

φH = φJ +∂φ

∂x

∣∣∣∣J

(JH).

Interface

I H

J

F

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MethodologyMeshing strategy

Geometry built in two parts, innerturbine and outer domain.

Block-structured approach withhanging nodes to controlcell-count.

≈ 2 million cells with15 < y+ < 200 at the walls.

LES mesh is wall refined with ≈7.7 million cells.

Intro Method Results Comp Con References

RANS ResultsFlow-field

Pressure iso-surfaces colouredby velocity are shown for theinstantaneous flow-field forTSR = 6.

Tip effects are clearly visible inthe near wake whilst the mastcreates main structures furtherdown-stream.

Intro Method Results Comp Con References

RANS ResultsFlow-field

Instantaneous velocity on centre-plane shows tip-effects andinfluence of mast on flow.

Intro Method Results Comp Con References

RANS ResultsForce coefficients

Effect of blades passing the mast is clear from instantaneousforce coefficients.

Power Spectral Density (PSD) analysis of the CP showspeaks at 3 and 6 times the blade rotation frequency (f).

0.739

0.74

0.741

0.742

0.743Force coefficients over one full rotation for TSR=6

7.68 7.82 7.96Time (s)

0.404

0.405

0.406

0.407

0.408C

TC

P

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RANS ResultsForce coefficients

k − ω SST predicts force curve against TSR although underpredicted by approximately 10%.

LES shows gain in precision matching experiments within 3%except for the lower values of TSR.

4 5 6 7 8 9TSR

0.4

0.6

0.8

1

1.2

CT

Bahaj et al 2005k-ω - Present workk-ω - McSherry et al 2011LES

4 5 6 7 8 9TSR

0

0.1

0.2

0.3

0.4

0.5

CP

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RANS and LES comparisonFlow structures

Iso-Q 0.5 (ΩijΩij − SijSij) surfaces coloured by vorticity areshown for RANS and LES.

LES maintains the tip vortices and captures structures in thewake better than RANS.

k − ω SST LES

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RANS and LES comparisonPressure coefficients on the blades

Mean pressure coefficients are shown on the blades at quarterlength locations.

Both RANS and LES predict similar behaviour although LEScaptures larger forces on pressure and suction surfaces.

0 0.2 0.4 0.6 0.8 1x / c

-2

-1

0

1

2

-CP

r/R = 0.25r/R = 0.50r/R = 0.75

k − ω SST

0 0.2 0.4 0.6 0.8 1x / c

-2

-1

0

1

2

-CP

r/R = 0.25r/R = 0.50r/R = 0.75

LES

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Conclusions & Further work

Conclusions:

Sliding-mesh method successfully implemented inCode Saturne to simulate a rotating TST.

Flow features are captured well by RANS and LES.

RANS under predicts the force coefficients whilst LES is farmore accurate.

Difference in calculations is near wall modelling and higherorder time-scheme used by Code Saturne for LES.

Further work:

Wall refined RANS simulation to compare with experiments.

Assess influence of waves on full scale MCT comparing withdata from EMEC.

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Acknowledgements

This research was performed as part of the Reliable DataAcquisition Platform for Tidal (ReDAPT) project commissionedand funded by the Energy Technologies Institute (ETI). Theauthors are highly grateful to EDF for additional funding andaccess to its High Performance Computing (HPC) facilities.

Intro Method Results Comp Con References

References

Archambeau, F., Mechitoua, N., and Sakiz, M. (2004). Code Saturne: a finite volume code for the computationof turbulent incompressible flows-industrial applications. International Journal on Finite Volumes, 1(1):1–62.

Bahaj, A. S., Batten, W. M. J., Molland, A. F., and Chaplin, J. R. (2005). Experimental investigation into theeffect of rotor blade sweep on the performance of the Marine Current Turbines. Proceedings of the Institutionof Mechanical Engineers, Part A: Journal of Power and Energy, 215(5):611–622.

Mcsherry, R., Grimwade, J., Jones, I., Mathias, S., Wells, A., Mateus, A., and House, E. F. (2011). 3D CFDmodelling of tidal turbine performance with validation against laboratory experiments.

Intro Method Results Comp Con References

Sliding-mesh methodImplementation: Code Saturne Subroutines

Black - Unchanged.

Red - Modified.

Blue - New.

findha - Search for closestcell-centre to halo-point.

upcoef - Updates Dirichletvalues on interface.

caltri

inivar usiniv

tridim

usclim findha

navsto

preduv

codits

upcoef

resolp

upcoef

upcoef

Intro Method Results Comp Con References

Sliding-mesh methodImplementation: Pressure-Velocity Loop

Code Saturne’s existingpressure-velocity loop isenabled by settingNTERUP > 1.

Loop used to ensurecontinuity over theinterface.

Modification to include theRANS subroutines.

tridim

phyvar

usclim

navsto

turb**

phyvar

P-V Loop

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RANS and LES comparisonComparison of numerical set-up

RANS (k − ω SST) LES

Time-scheme 1st order 2nd order

Rotation per time-step 1.5 0.05

Space discretization Centred Centred

Wall modelling Wall-functions Wall refined

Mesh size 2.2 million 7.6 million