RANS-VOF Modelling of Floating Tidal Turbine Concepts Edward Ransley* and Scott Brown School of Marine Science and Engineering University of Plymouth 4 th UK & Éire OpenFOAM® User Meeting April 2016, Exeter University, Exeter, UK

Rationale Development of the ORE sector high national priority for the UK Tidal stream – It’s predictable. But… There are a limited number of viable sites Reduced flow at depth Difficult installation, maintenance etc. So … … Floating tidal turbines. Easier installation and recovery Greater resource near the surface Number of concepts proposed But … Exposed to waves Concerns over survivability & power delivery Bluewater’s tidal energy converter (BlueTEC) Scotrenewables Tidal Power Ltd Tidalys floating tidal turbines TidalStream Triton Toshiba

Engineering analysis tools Complicated system to model Complex hydrodynamics Waves + currents Floating structure behaviour Potential flow/Boundary element models CFD Mooring loads and dynamics Turbine models Actuator-disc/-line Blade Element Momentum theory Bladed resolved – computationally expensive Typically decoupled/linearised not suitable for design assessment Need a fully nonlinear, coupled model Including all parts of the system

Incremental development strategy Following the OOP paradigm Developed an open-source, efficient and sustainable, numerical tool for assessing floating tidal stream devices in real conditions Approach including: Parametric design of the computational domain New expression-based boundary conditions to recreate site-specific hydrodynamics Development of a range of static turbine models Sensitivity analysis of critical parameters Validated against published experimental results, analytical solutions & real data Development of and efficient method for including complex mooring restraints Comparison with simplified industry-standard models for floating platform dynamics Coupling of turbine models, barge motion and mooring restraints Single numerical tool for simulating full floating tidal stream systems

Additional bodyForces library Compiled with any solver Allows for additional body forces in the domain creates an object of class bodyForce with an associated volVectorField called bodyForceField. bodyForce contains new functions applyBodyForces() - updates bodyForceField based on flow field variables and bodyForce type specified bodyForceField() – provides access to bodyForceField Additional source term in the Ueqn Allows for two-way coupling between the fluid momentum (U) and the bodyForceField. Depends on the bodyForce type specified in a bodyForceDict Ueqn.H waveBodyFoam.C

New bodyForce type - turbines Various classes of turbines Disc – actuator-disc theory Bladed – actuator-line theory Models incl. generator behaviour Any orientation Any number of turbines Irregular mesh (support structure) Easily adapted  other applications Propellers Wind turbines … any coupled force model

New sixDoFRigidBodyMotion library Using the sixDoFRigidBodyMotionRestraint as a template Adapted to have access to run-time functions Creates a new type of sixDoFRigidBodyMotion object, with new functions: addbodyForces() – similar to addRestraints() applyBodyForces() – similar to that in the bodyForce object for static bodyForces updateAcceleration() – similar to original but with an additional pointer to the mesh (for coupling) New sixDoFRigidBodyMotionSolver Initialisation - creates volVectorField bodyForceField and stores it in the objectRegistry Every time step – updateAcceleration() calls applyBodyForces() - finds the forces depending on the user specified bodyForce type and body’s motion - updates the bodyForceField accordingly, providing an additional source term for the Ueqn. applies the additional forces and torques to the equation of motion of the rigid body All static turbine classes added as bodyForce types in the new solver Cells selection adapted to use the orientation matrix to rotate the co-ordinate system and track position

A Case – MTG Platform Raft Concept Modular device with removable turbine units SCHOTTEL SIT250 turbine model with over-speed control 4-point hybrid-catenary mooring system Wave and current conditions based on PTEC site In small regular waves (without moorings or the turbine) motion results comparable with industry-standard codes With larger waves and currents and when considering the entire system (barge, turbine & moorings) Far more complex behaviour Significant impacts on power delivery, mooring loads, etc. Important for design purposes and feasibility assessment MODULAR TIDE GENERATORS LIMITED Ransley et al. 2016, ‘Coupled RANS-VOF Modelling of Floating Tidal Stream Concepts’, in Proceedings of the 4 th Marine Energy Technology Symposium, April 25-27, 2016, Washington,DC

MODULAR TIDE GENERATORS LIMITED An animation Ransley et al. 2016, ‘RANS-VOF Modelling of Floating Tidal Stream Systems’, in Proceedings of the 5 th Oxford Tidal energy Workshop, April 21-22, 2016, Oxford, UK

Contact us DR. EDWARD RANSLEY Post-Doctoral Research Fellow (MTG & CCP-WSI) School of Marine Science and Engineering Plymouth University Drake Circus Plymouth PL4 8AA T: +44 (0) E: SCOTT BROWN PhD Student & Researcher (MTG) School of Marine Science and Engineering Plymouth University Drake Circus Plymouth PL4 8AA E: