Reasonable convergence of the CFD solution can be assumed to have occurred once the plotted normalised residuals have all stabilised (i.e. they are not oscillating) and the values have dropped to around 0.001 or 10-4. If the residuals have dropped to very low levels and the monitored variables are fairly constant (they don't have to be absolutely constant), then you can be confident that you have a good solution (even though a non-convergence message is displayed). The message is always displayed even if the residuals are very slightly above the termination values. These are conservative values and don't need to be strictly adhered to.
The default CFD settings and grid provided in DesignBuilder should enable a converged solution to be found in most cases, but occasionally, adjustments to the settings or to the problem specification may be needed.
One common cause of CFD simulations not converging is when there are insufficient grid cells between an extract and a supply diffuser. When these components are positioned too close together, the flow can become trapped and to avoid this situation, we recommend that there are at least 3 grid cells between the extract and supply objects. You can edit the grid region between these components and manually add the extra cells. An alternative solution is to use a finer grid.
Also, very small dimensions are likely to lead to flow instabilities (e.g. mass residual showing a "flat" line) and preventing convergence.
An initial large deviation in the monitored variable is expected which normally reduces fairly quickly tending towards stability and terminates at a value of 0.0001.
Generally, if you experience a solution that doesn't converge easily where the residuals appear to remain constant, it can help to re-structure the model definition by removing unnecessary obstructions and aligning any repeating component/assemblies with each other in order to reduce the number of grid lines.
In some cases a solution can be very difficult, if not impossible, to achieve using the standard k-e turbulence model. Particularly difficult problems involve very low velocity buoyancy-driven flows where diffusion is dominated by convection and this can cause velocities to fluctuate around a mean value. In these cases, reducing false time steps may not improve the situation. In order to force a solution, you may consider introducing additional boundary conditions such as occupants, which can change the flow sufficiently to obtain a solution without the problem necessarily becoming unrealistic.
Alternatively, you may consider using the constant effective viscosity turbulence model with a relatively high turbulent viscosity or turbulent viscosity multiplier but bear in mind that this will introduce an artificially high level of diffusion into the flow and also, the constant effective viscosity model is incapable of modelling the transport of turbulence.
If the residuals are found to diverge or fluctuate significantly, in many cases the situation can be improved by reducing the false time steps for the velocity components. The recommended procedure is to continuously half the time steps until a more stable solution is found.
You should only need to reduce the velocity false time steps if you experience a very unstable solution which exhibits diverging residuals or high amplitude oscillations. The time steps have a 'damping' effect on the solution which can increase stability but increases the overall simulation time.
Some tips that can help reduce CFD calculation times are: