Investigation of the influence of turbine-to-turbine interaction on their performance using OpenFOAM

Transcription

1 Investigation of the influence of turbine-to-turbine interaction on their performance using OpenFOAM Dr Gavin Tabor, Mulualem Gebreslassie, Prof Mike Belmont CEMPS, University of Exeter

2 Background: Lift/Drag Turbine Novel design for tidal turbine based on cycloidal turbine involving complex rotating airfoil blades Blades act in drag mode on one side; rotate (0.5Ω) to develop lift on other side Unit operates as cross-flow turbine Energy extracted through volume high efficiency (measured efficiency of 50% High blockage factor; suitable for near-surface (eg. esturine) sites. Development backed by AquaScientific Ltd some experimental and esturine testing done

3 Background Computational Methodology Results Conclusions + future directions

4 Project aims Ultimate aim : to model large (100+) farms of units Proximate aim : low-cost CFD model of multiple units to study interactions. However; detailed turbine blade motion too costly; simple actuator disk models insufficiently detailed. Developed new Immersed Body Force technique to treat blade motion LES turbulence formulation need to examine large scale transient motions VOF free surface important for turbine behaviour Computational code used : OpenFOAM

5 LES and VOF Filtered NSE including body force terms :.u = 0, t u +.(u u ) =.(S B) + F S = pi + 2νD B is SGS Stress term : effect of SGS turbulence on GS flow represented by 1-equation eddy viscosity model. F represents artificial body force term.

6 VOF FSF represented by indicator function α α t +.(αu ) +.(α(1 α)u r ) = 0 Final term is artificial compression term active only on interface. Physical properties calculated as weighted average of individual components; µ = αµ w + (1 α)µ a

7 Turbine modelling Immersed body force method : Blades represented by body forces F = F D + F L Compromise between accuracy and efficiency Capable of representing large scale vortexes

8 What is OpenFOAM? OpenFOAM is an Open Source CCM code/code library : Written in C++ Based on FVM on arbitrary unstructured (polyhedral cell) meshes Originally developed by Henry Weller and others at IC ( ); Nabla Ltd ( ) as FOAM Now released (2004 ) under Gnu GPL by OpenCFD Ltd. ( Extensive user community Extensions and variants released by 3rd parties (-dev, pyfoam) Academic and commercial usage.

9 Strictly, OpenFOAM is not a CFD code it is a C++ library of classes for writing CFD codes. OpenFOAM uses the full range of the C++ language inheritance, polymorphism, templating, operator overloading etc where appropriate : Class mechanism define new types for CFD Interface vs implementation : segregation of effort. Operator Overloading provides standard mathematical syntax Inheritance, polymorphism etc encodes relationships between conceptual entities in code Effective result is a high level language for encoding CFD.

10 Validation Laboratory testing carried out in flow channel; flow rate and turbine rotation under a range of mechanical torque conditions : Flow rate measured with rotormeter Torque output using mechanical system Rotation rate recorded optically. Compared with functionally equivalent CFD simulations.

11 Single Turbine simulation

12 Wake profiles

13 Two turbine simulations Reduced efficiency by 18% at 15D turbine spacing Reduced efficiency by at least 7% at 20D spacing

14 Three turbine simulations Performance of middle turbine improved due to blockage effect at 2D lateral spacing As the lateral spacing increased to 4D the blockage effect was reduced

15 Seven turbine simulations 3D spacing inflicted high energy shadowing on downstream row Increased lateral spacing (6D) reduces the wake interaction and the performance of the downstream row improved There was a blockage effect on the performance of the base turbine in the middle row

16 Seven turbine simulations

17 Summary IBF model constructed and successfully validated good comparison with experimental results Successfully generates large scale vortex behaviour LES and free surface simulation to capture other important effects Low cost method allows turbine/turbine interaction to be simulated. Wake decay also shown (single and multiple turbines) Able to examine other turbine issues (eg. effects of venturi plates)

18 Future directions Intention to take this work in two directions higher detail and larger scale. Higher detail PhD project (Matt Berry) to simulate detailed blade motion MRF, GGI, overset meshing Larger scale EPSRC-funded project to develop farm modelling; ROM for individual turbines in linked array, power optimisation and flood risk modelling

19 Any Questions?