O.F.Wind Wind Site Assessment Simulation in complex terrain based on OpenFOAM Darmstadt, 27.06.2012 Michael Ehlen IB Fischer CFD+engineering GmbH Lipowskystr. 12 81373 München Tel. 089/74118743 Fax 089/74118749 michael.ehlen@cfd-ing.de
IB Fischer CFD+engineering GmbH CFD+engineering is an engineering consultant company for CFD projects with more than 15 years experience in CFD. projects with commercial software (Fluent, CFX, CD-adapco) projects with OpenFOAM customized OpenFoam workflow development of additional functionalities in OpenFoam CFD+engineering was founded in 2004 About 10 engineers in Germany, Munich Selection of our projects: climatisation of several car projects IC Modelling in OpenFoam free surface calculation in water tanks aerodynamics calculation of trains O.F.Wind 2012 by IB Fischer CFD+engineering GmbH page 2
Agenda What is O.F.Wind? Brief summary on the developments for O.F.Wind: Boundary conditions and Filter Forest model Thermal stability Wake model Transient treatment Post processing 2012 by IB Fischer CFD+engineering GmbH page 3
O.F.Wind In 2009 CFD+engineering found out that traditional site assessment CFD software has lots of restrictions for the user in terms of solution robustness, terrain complexity, model size and control over computational run. For CFD softwares like OpenFOAM these restrictions are unknown! O.F.Wind combines the strengths of a powerful CFD code with a user-friendly GUI. Key features of O.F.Wind you should know: Reliable and robust Unlimited Terrain complexity Based on the open source CFD tool OpenFOAM Unlimited parallel computing, unlimited model size Dedicated easy to use GUI Links to other software: WindPro (.map and time series), wrg, Surfer 2012 by IB Fischer CFD+engineering GmbH page 4
O.F.Wind Being built on the cutting edge CFD tool OpenFOAM O.F.Wind offers a lot more control over what happens in the CFD process than traditional site assessment CFD allowing the expert to perform high quality CFD with full transparency. O.F.Wind features the experts like: complete automated workflow based on a GUI: no command line necessary additional openfoam command are still possible usable for beginners and experts Automatic flexible unstructured meshing with local refinement based on blockmesh and snappyhexmesh Whole CFD part of O.F.Wind is available open source and based on OpenFoam Flexible post-processing in Paraview 2012 by IB Fischer CFD+engineering GmbH page 5
Boundary Conditions Atmospheric boundary layer (ABL) inlet boundary condition Shear stress boundary for top boundary Many filters to read and write wind industry specific formats like orographic map and grid description roughness map files wind resource file (wrg, rsf) time series... 2012 by IB Fischer CFD+engineering GmbH page 6
Forest Model The force & source terms are added to the momentum and turbulence eqn's: Momentum: Turbulence: Coefficient sets proposed by different researchers: Forest model Beta P Beta D C Eps 4 C Eps 5 Svensson & Haeggkvist (1990) 1.0 0.0 1.95 0.0 Green (1992) 1.0 4.0 1.5 1.5 Liu (1998) 1.0 4.0 1.5 0.6 Sanz (2003) 1.0 5.1 0.9 0.9 Lopes Da Costa (2007) 0.17 3.37 0.9 0.9 2012 by IB Fischer CFD+engineering GmbH page 7
Forest Model The model is developed and tested in a master thesis. Different scenarios were calculated. In this case the Liu model shows the best result. This is not a general recommendation due to the reason that for other cases other models show better results. 2012 by IB Fischer CFD+engineering GmbH page 8
Forest Model Multiple forest regions in O.F.Wind: Different forest regions are marked with different roughness values Mathematical model: Force terms cancelled outside forest regions. Solve the standard incompressible NS equations outside the forest. Solve the modified equations inside the forest. 2012 by IB Fischer CFD+engineering GmbH page 9
Wake Model Influence of the rotating blade on the wind flow Actuator Disk Theory: Thin disc offers no friction or resistance to the fluid flow through it apart from the generation of momentum flux and pressure changes. Disc is uniformly loaded over its area (thrust and velocity). Pressure is static free-stream far up and far down stream. Inviscid, incompressible and isentropic (adiabatic & reversible) flow. Velocity deficit determined by the thrust coefficient, C t 2012 by IB Fischer CFD+engineering GmbH page 10
Wake Model To apply the Actuator Disk Theory a fine mesh in the area of the wind turbine is necessary. To make the usage more user friendly an automatic mesh refinement is used. On the other hand, an unavoidable drawback of this automatic mesh refinement process is the high computational cost. 2012 by IB Fischer CFD+engineering GmbH page 11
Wake Model Influence of the turbines on the fluid flow: 2012 by IB Fischer CFD+engineering GmbH page 12
Transient Model Time dependent solver which can handle all necessary kinds of boundary conditions based on PimpleFoam Can be used for unstable wind direction where unsteady flow phenomena expected. High order turbulence models can be selected (e.g. LES : large eddy simulation) transient wake region 2012 by IB Fischer CFD+engineering GmbH page 13
Transient Model Transient solutions compared to the steady state solutions steady-state results transient results 2012 by IB Fischer CFD+engineering GmbH page 14
Transient Model Below you see a transient solution. 2012 by IB Fischer CFD+engineering GmbH page 15
Thermal Stability Stability of the atmosphere: Resistance of the atmosphere to the fluid motion in z direction. Temperature equation is necessary Numerically, compressible solvers are impractical for thermally stratified wind flows over complex terrains in OpenFoam: Very slow convergence Very high computational cost Instabilities at low-mach (< 0.3) Mach number: Dimensionless number representing the speed of an object moving through fluid divided by the local speed of sound. 2012 by IB Fischer CFD+engineering GmbH page 16
Thermal Stability Thus, we have to use incompressible equations for thermal stability model. Incompressibility Mass conservation: Time & space independent density Boussinesq approximation: Temperature dependent density Main types of stability classes: Neutral weather condition: Temperature has no influence Stable weather condition: The air gets cooled by the surface. Air temperature > terrain temperature Unstable weather condition: The air gets heated by the surface. This condition may require transient treatment. Air temperature < terrain temperature 2012 by IB Fischer CFD+engineering GmbH page 17
Thermal Stability velocity Friction velocity Horizontal Height 2500 ABL Velocity Distribution neutal stabel_mol=2000 unstabel_mol=-50 2000 Kármán constant Roughness Height atm. stab. function 1500 Height [m] 1000 500 Blue Stable Red Neutral Green Unstable 0 0 2 4 6 8 10 12 14 16 18 Velocity [m/s] 2012 by IB Fischer CFD+engineering GmbH page 18
Micro-/Mesoscale Coupling microscale model Detailed local flow phenomena like flow separation Detailed vertical wind profile Local phenomena like forest regions, turbine models, etc. can be modelled microscale model Inaccurate boundary conditions for inlet, outlet and top boundary Advantages Limitations mesoscale model Possibility to calculate large areas Long term transient calculation Includes weather phenomena Calculate the whole atmospheric condition mesoscale model Don't resolve the local vertical wind profile over complex terrain No mesoscale effects included (weather phenomena, coriolis, etc.) No detailed solution in small local areas 2012 by IB Fischer CFD+engineering GmbH page 19
Micro-/Mesoscale Coupling Steady State Coupling: Extract mean values from the transient mesoscale model and apply it to the microscale model Non uniform boundary conditions for the microscale model Transient Coupling: Extract typical transient diurnal variations and apply these values to a transient microscale simulation Useful to calculate typical days 2012 by IB Fischer CFD+engineering GmbH page 20
Postprocessing The graphical post-processing is done in Paraview. Additional wind industry specific post processing is included: Calculation of additional values like wind shear, wind veer, inclination, speed up, TI, horizontal velocity magnitude, Prediction of time series Weibull fitting of time series Creation of offset planes (e.g. a plane 30m above ground) Wind resource files (wrg, rsf) Gunshoot images to visualize time series... 2012 by IB Fischer CFD+engineering GmbH page 21
Questions? Thank you for your attention! For feedback or questions please contact us! ofwind@cfd-ing.de 2012 by IB Fischer CFD+engineering GmbH page 22