3.2.2.1 General

General

The Inlet velocity represents the wind speed entering the wind tunnel.

The [Profile] button enables you to define the wind speed as a function of the height. You can enter the chart values as described in the Chapter 'Wind Profile' below. The value of the Kinematic viscosity describes the resistance of the air against deformation. It is defined as the ratio of the viscosity to the Density of the air.

The Mesh density to be applied around the model is controlled by percentage reference. This specific refinement is utilised for the model simplification and the flow calculation. The default density (20%) normally results in a relatively low number of finite volume mesh cells and a relatively fast calculation. The minimum percentage is 10%. It entails a rather coarse mesh with the smallest number of volumes. The higher the density of the mesh, the smaller the size of the finite volume cells will be. The results are accordingly more precise, but the calculation will need more time due to the greater number of volumes. Setting the maximum mesh density (100%) leads to very fine meshes with millions of volumes. The calculation of 3D flow on such meshes is on the edge of the capabilities of current PCs, with a calculation time of several hours to several days.

You can enter the mesh density or modify it with the slider. The Mesh cell estimation below gives the corresponding number of finite volume cells and the minimum cell size.

The Mesh refinement type can be defined for curvatures of the surfaces (fine mesh only close to sharp edges of the model) or globally for a distance from the surfaces (fine mesh on entire boundary surfaces). The first option is set as default because it produces meshes with a lower number of finite elements.

The Boundary layers option controls whether the finite volume mesh next to the surfaces of the model is refined in a special way. This refinement gives better results near the boundaries of the model. As the number of finite volumes is increased considerably by the layers of small volumes, however, the option is disabled by default. It is highly recommended to activate the boundary layers and define the number of layers NL, however, when the surface roughness is to be taken into account.

The Snap to model edges option enables you to align the mesh with the borders of the model. Please note that this option is only available when the model simplification is off.

This dialog box section controls the parameters of the simulation. There is presently only one Numerical solver option. Alternative solvers are being prepared for upcoming program versions. For more details about the OpenFOAM® solver, see the Chapter 'CFD Solver'.

Furthermore, you can define the Maximum number of iterations. By default, the limit is 500 iterations. If the calculation converges within less iterations, it is stopped. Note that a minimum number of 300 iterations is carried out by default (see the Image 'Program Options'), regardless of whether the convergence criterion (see below) has already been met. The maximum number is useful to avoid infinite loops.

The Convergence criterion represents the stop limit for the calculation. As soon as the residual pressure has fallen below the defined value, the calculation is terminated. The diagram of iterations and residual pressure is shown during the calculation. It is also available in the simulation results (see the Chapter 'Residual Pressure').

If you activate the Use potential flow to calculate initial condition option, a linearised version of non-viscous Navier Stokes equations is used to generate the start conditions.