numerical analysis of open-centre ducted tidal turbines · march 30, 2012 oxford tidal energy...

UNIVERSITY OF OXFORD DEPARTMENT OF ENGINEERING SCIENCE CIVIL AND OFFSHORE ENGINEERING TIDAL ENERGY RESEARCH GROUP

Numerical Analysis of Open-Centre Ducted Tidal Turbines

Clarissa Belloni, Richard Willden & Guy Houlsby

March 30, 2012 Oxford Tidal Energy Workshop

C.S.K.Belloni, R.H.J.Willden & G.T.Houlsby Numerical Analysis of Open-Centre Ducted Tidal Turbines

Goal of project

Goal: Understanding the fluid mechanics

of open-centre ducted turbines

Methodology: §  CFD simulation; §  Analysis of fluid mechanics; §  Relate fluid mechanics to device performance.



I. Overview: Open-centre ducted tidal turbine devices

Advantage claims: •  speed up effect with power increase •  improved flow alignment

OpenHydro

Clean Current

Main disadvantage: •  increased material •  reduced turbine area



I. Overview of investigation

Bare turbine Open-centre turbine

•  method validation •  baseline case •  low and high blockage •  yawed inflow

•  low blockage •  various aperture diameters •  yawed inflow



Blockage ratio, B = 3.5%,

II. Method validation: Simulation Setup

Numerical simulations: •  FLUENT (numerical solver, solving RANS equations); •  Turbine modelled as porous disc; •  Constant uniform inflow; •  no hydrostatic pressure variation; •  Symmetry condition at domain boundaries; •  Steady & unsteady flow simulations.

BRIEF ARTICLE

THE AUTHOR

(1) CP =power extracted from fluid

power available for extraction= 4a(1− a)2

(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)



1



II. Method validation: Bare turbine

3D simulation of bare disc: Good agreement with Linear Momentum Actuator Disc Theory (LMADT) corrected for blockage effect*.

*Houlsby et al. (2008): “Application of Linear Momentum Actuator Disc Theory to Open Channel Flow”

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afr

Achannel

(5) Ut = U∞(1− a)



1

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afr

Achannel

(5) Ut = U∞(1− a)



1

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afr

Achannel

(5) Ut = U∞(1− a)



1

DRAFT0 0.3 0.6 0.9 1.2 1.5 1.80

0.2

0.4

0.6

0.8

1

CT

CP

LMADT, B=0.035

LMADT, B=0.2

CFD, B=0.035

CFD, B=0.2

Figure 4: Performance of the bare turbine, CP vs. CT , for blockage ratios of B = 0.035 and B = 0.2 for both

CFD and LMADT for confined flow.

point remix with the by-pass flow. The process introduces further, unavoidable en-

ergy removal from the flow. Additional energy removal is especially important when

comparing bare and ducted devices. The presence of a duct within the flow exerts an

additional thrust on the fluid. This added thrust can lead to significantly more energy

being extracted from the flow than is converted into useful energy. As tidal flows must

be considered as finite resources the total energy removed from the flow represents

an important feature of each device. Thus we introduce a device efficiency definition

which relates the useful power generated to the total power removed from the flow,

defined as the basin efficiency,

ηbasin =useful power

total power removed from the flow . (11)

To retrieve the total power removed from the flow we analyse the energy flux

through domain cross sections at various stages upstream and downstream of the tur-

bine; see Fig. 5.

At each streamwise plane the energy flux is calculated by integrating the product

of the total pressure, p0, and the streamwise velocity, ux, over the cross-sectional area,

11

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)



1



II. Method validation: Performance analysis

Power coefficient

Basin efficiency

BRIEF ARTICLE

THE AUTHOR


power available for extraction

(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)



1

BRIEF ARTICLE

THE AUTHOR

(1) CP =useful power

kinetic flux available in upstream flow

(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)

(6) ηbasin =useful power

total power removed from the flow=

P

G(xinflow)−G(x∞)

1

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)



P


(7) ηbasin =P


1

BRIEF ARTICLE

THE AUTHOR



(2) CP =P

1

2ρAfrontalU

3∞

(3) CT =T

1

2ρAfrontalU

2∞

(4) B =Afrontal

Achannel

(5) Ut = U∞(1− a)



P


(7) ηbasin =P


1



III. Open-Centre Turbine

Various aperture diameters tested



III. Open-Centre Turbine: Flow Field

Flow features: •  No large scale separation on duct exterior

•  Jet flow through the aperture of the turbine

Increased thrust: •  Reduced jet velocity

•  Reduced disc velocity

•  Increase in Δp

DRAFT3.2. Thrust Analysis

When examining the thrust on a device we need to consider not only the thrust on

the disc. The thrust on the duct (and on any support structures if these were modelled)

plays a significant role when assessing the thrust on the overall device. Fig. 13 shows

the thrust coefficient plotted as a function of the spatial mean of the induction factor.

Three thrust coefficients are plotted: CT = CT, turbine, the thrust coefficient for the

turbine disc (the standard thrust coefficient), CT, duct, coefficient for the thrust on the

duct, and CT, total, which is a sum of the two components CT, turbineand CT, duct.

Note that the duct inlet area is used in defining all three coefficients.

!0.4 !0.2 0 0.2 0.4 0.60

0.2

0.4

0.6

0.8

1

a

CT

CT, turbine

CT, duct

CT, total

Figure 13: Thrust coefficients for the open-centre turbine, Dhole = 6 m. All cases B = 0.035.

For very low induction factors, both thrust on duct and disc are of the same or-

der of magnitude. With increasing induction factor, the disc thrust, CT, turbine, in-

creases roughly linearly. While there is a small overall increase, the thrust on the duct,

CT, duct, remains constant over a broad range of induction factors. For high induction

factors the thrust on the duct is significantly smaller than the disc thrust.

22

increasing thrust

increasing thrust



III. Open-Centre Turbine: Thrust Analysis










!0.4 !0.2 0 0.2 0.4 0.60

0.2

0.4

0.6

0.8

1

a

CT

CT, turbine

CT, duct

CT, total







22

Low induction factor (a):

CT, turbine ≈ CT, duct

Increasing induction factor:

CT, turbine α a

CT, duct ≈ constant

High induction factors:

CT, duct << CT, turbine










!0.4 !0.2 0 0.2 0.4 0.60

0.2

0.4

0.6

0.8

1

a

CT

CT, turbine

CT, duct

CT, total







22



III. Open-Centre Turbine: Aperture

Flow features: •  No large scale separation on duct exterior

•  Jet flow through the open-centre of the turbine

Increased aperture: •  Increased jet velocity

•  Increased disc velocity

•  small increase in Δp



III. Open-Centre Turbine: Performance

Power Power density

increasing aperture

•  Power decreases with increasing aperture;

•  Power density increases with increasing aperture;

•  Basin efficiency only slightly influenced by disc aperture.



III. Open-Centre Turbine: Performance

Power

•  Power decreases with increasing aperture;

•  Power density increases with increasing aperture;

•  Basin efficiency only slightly influenced by disc aperture.

Basin efficiency

increasing aperture



IV. Yawed Inflow

Bare and open-centre turbine analysed at various yaw angles



IV. Yawed Inflow: Flow Field

Bare turbine:

•  Reduced projected frontal area

Open-centre turbine:

•  Increased projected frontal area

•  Flow straightening










!0.4 !0.2 0 0.2 0.4 0.60

0.2

0.4

0.6

0.8

1

aC

T

CT, turbine

CT, duct

CT, total







22

A frontal

A frontal



IV. Yawed Inflow: Performance

Power Basin efficiency



IV. Yawed Inflow: Performance •  Increase in power coefficient for open-centre turbine (full disc and with aperture);

•  Slight decrease in power coefficient for bare turbine;

•  Steep drop in basin efficiency for open-centre turbine

Power Basin efficiency



V. Conclusions

Method validation: §  Bare turbine simulations in good agreement with Linear Momentum Actuator Disc

Theory (LMADT). Open-centre turbine: §  Jet flow through aperture increases mass flow and power density, but overall

performance in terms of power reduced significantly compared to the full disc.

Yawed inflow: •  Increase in performance of the open-centre turbine due to increased effective blockage

and flow straightening, at the expense of decreased basin efficiency.



THANK YOU! QUESTIONS?

numerical analysis of open-centre ducted tidal turbines · march 30, 2012 oxford tidal energy...

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