Nesting

Introduction

Nesting is a well-known method to reduce computational time for detailed numerical simulations. The idea is that one overall model is forced from an external source. After that, you will zoom in force the more detailed model with the larger coarser overall model. However, nesting can be a time-intensive procedure. The 'Nesting' toolbox allows nesting an already existing finer model in a larger model to increase resolution in the area of interest, in a matter of minutes. Differently, from domain decomposition the two models can not run in parallel but boundary conditions from the coarser (overall) model need to be generated first and then provided to the finer model.

The general idea is that under Nesting – 1 a list of monitoring stations in the overall (coarser) model, needed for the interpolation of the boundary conditions, is generated. The actual boundaries of the nested (smaller / finer) model are generated by Nesting - 2 by interpolating the history files of the overall (coarser) model. For more information about nesting, one is referred to the Delft3D-FLOW manual. The Nesting toolbox is supported for Delft3D-FLOW and Delft3D WAVE models.

Usage of the toolbox

The procedure to generate nested boundary conditions consists of few steps after both models have been

1. Create the overall (coarser) model
2. Create the smaller (finer) model
3. Open the smaller (finer) model
   1. Specify 'open' boundaries. 
   2. Change forcing to 'time series'
4. Open the larger (coarser) model (see Figure below)
   1. Go to the Nesting toolbox and 'Nesting - 1'
   2. Specify the grid, enc and bnd file of the finer model. 
   3. Generate the 'admin' file by clicking on 'Make Observation Points'.
   4. Define the time step of the observation points
   5. Save all your Delft3D-FLOW files
   6. Run the coarser model
5. Open the smaller (finer) model (see Figure 2)
   1. Go to the Nesting toolbox and 'Nesting - 2'
   2. Specify the history and admin file.
   3. Click on 'Run Nesting'. Boundary conditions can be generated for the “Hydrodynamics” and also “Transport” (e,g, salinity)
      
      

The possibilities of Delft Dashboard with nesting is also showed in Tutorial 2: two level nesting.

Figure: With the Nesting toolbox two model are loaded and coupled. The idea is that the Nesting Toolbox will generate observation points in our large (coarser) California model and the output of these observation points is used in the second (finer) model of the San Francisco Bay