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Why do I need Nesting Grids?
Practically you don't need them. You can restrict your simulation to the main (core) area without adding any grids as nesting area.
In times of large computation power, we recommend to not use Nesting Grids anymore but to include more grid cells at the borders within the “normal” domain.
However, there a still situations where the Nesting Grids can help you to improve the model results (or make the model stable)
The reason why Nesting Cells are an option in ENVI-met is that every numerical model, especially 3D models such as ENVI-met are not working reliably at their model borders and at the grids very close to them. So the best you can do is to move these borders as far as possible away from your area of interest in the core area.
The reason for these problems is resulting from the fact, that the model cannot calculate real values for grid points along the borders (and especially not for grid points that are the border). As these points are missing at least one neighbour grid, only simple assumptions can be used here to obtain values that can be used in the model. This simple assumption can be that the values are constant at these grid points (so called closed boundary conditions or forced bc) or that values from inner grid points are copied to the grids on the border (open boundary conditions). A third method also offered in ENVI-met is to copy the values from the outflow boundary back to the inflow boundary (cyclic boundary conditions). For more information please refer to Lateral Boundary Conditions.
Whatever boundary conditions you use, the main problem still is that grid points on or close to a boundary cannot react on influences in the way grid points in the inner part of the model can. If the boundaries are supplied with constant values, they cannot react on things going on in the inner part at all.
We will illustrate this problem by the example of the flow field: Figure A shows the flow field around two buildings. The flow is entering the model from the right hand side. As you can see, the influence of the two buildings on the flow field (vortex), especially on the lee side (left, yellow markers) extends up to the model border. As it was said before, the model cannot solve the normal equations for those grid cells that are directly at a border. For the example A this means that the lee vortex will not be correctly calculated.
Figure A - Flow around two Buildings with 3 Nesting Grids
Figure B shows a better solution for the problem: Here, the model border is further away from the central model area and the flow field has re-established its more simple structure as it hits the model border. Although there is still an influence recognisable, the simple model assumptions used at the boundaries will be able to ensure a stable numerical simulation.
Figure B: (Better) Flow around two Buildings with 5 Nesting Grids
The Role of Nesting Grids
The only difference between example A and B is that in (A) we have used only 3 nesting grid cells around the core domain whereas in (B) 5 of them are used. As the grid size of the nesting grids is increasing with each grid, the area is extending very fast with each extra Nesting Grid. The grid points inside the nesting area have an increasing horizontal resolution with
$$ \Delta xy (n)= \Delta xy(Main) *n $$
where $\Delta xy(Main)$ is the resolution of the main model and $n$ is the index of the nesting grid starting with 1 at the model borders. The more Nesting Grids you use, the lower is the chance that you will get numerical problems because one or more of your model boders are interfering with internal model dynamics.
ENVI-met will warn you, if the flow field is not stable after 10 sec of integration time and suggest to increase the number of Nesting Grids. If you are not sure what causes your problems, you should always try to increase the number of Nesting Grids to be sure that this is not the point of trouble.
Remember: The Gesting Grids are not included in the Output Files by default, so you will not be able to see troubles there. To see the complete model area, you must include the nesting grids in the Output Files in the Project Wizard
Soil Information needed for the Nesting Grids
As the Nesting Grids are created only in the internal memory of ENVI-met, you cannot place buildings on them or edit the soil profile used in the nesting area.
To assign soil profiles to the nesting area, ENVI-met creates a chessboard pattern of two soil profiles (A and B). These two soils might be of the same type as well as of different types. If, for example, the area surrounding your model consists of a mixed land use containing both sealed surfaces (roads) and free surfaces (e.g. loam) you should use a sealed surface for soil profile A and a free surface for soil profile B.
Handling of Radiation in the Nesting Area
As no buildings are present in the nesting area, the sun radiation can reach the surface without reduction. This might result in an unrealistic overestimation of the surface temperature in the nesting area, especially when only sealed surfaces are used as soil profiles. To overcome this problem, ENVI-met can use radiative values averaged over the main model area instead of the normal radiative fluxes in the nesting area. This function is enabled by default, nut can be disabled (not recommended) in the Project Wizard.
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