Structured / hexahedral meshes
One of the foundations of CFD simulations is spatial discretisation (meshing / grid-generation) of the simulated geometry into many small so called finite volume cells. The properties of the grid (equi-angle-skew, aspect-ratio, etc.) are key factors for the quality of the simulation. Structured (directly addressable) grids are rarely used nowadays. High quality simulations, especially using complex models, usually require hexahedral grids.
To achieve fast solutions for concrete problems we take great care during grid generation. More cells always enhance the accuracy of the solution, but also cause longer calculation times. Consequently we derive the optimum meshing solution for each model individually (sometimes with different resolutions in different spatial dimensions) as compromise between precise results and manageable calculation time.
As an example we show here the hexahedral mesh of a complex mountain terrain, that we generated for a LES (Large Eddy Simulation) of a wind turbine. LES is a method that is very sensitive to the (changes) of cell size of the mesh and has therefore high quality requirements. That means that the cell size must not change too fast over the entire complex terrain with an area of 10-20 km² and a vertical extent of 1000-2000 meters in order to be able to do a realistic simulation. Especially in the vicinity of the wind turbine itself there must be sufficient mesh resolution to give a meaningful judgement about the wind field influence of a site assessment. Simulations like these can easily have more than 100 Mio cells and do not only require highest mesh quality but are also computationally intense.