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Introduction

Nesting is a well-known method to reduce computational time for detailed numerical simulations. The idea is that a smaller detailed model retrieves boundary information from a larger coarser model around it. Nesting can be time consuming. To speed up the process, the 'Nesting' toolbox is introduced. It allows quick nesting of (an already existing) finer model into a larger model

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The Nesting toolbox allows nesting an already exisiting finer model in a larger model to increase resolution in the area of interest. Differently

Different from domain decomposition, the two models can not run in parallel but boundary conditions from the coarser (overall) . First the boundary conditions for the fine model need to be generated at first and then provided to the finer model. For more information we refer to the Delft3D-FLOW manual.
The procedure to generate nested boundary conditions consists of few steps:
1. Open the overall model.
2. Using the tool Nesting – 1 a from the coarser (overall) model. The procedure is as follows: Under Nesting – 1 a list of monitoring stations in the overall model, needed for the interpolation of the boundary conditions, can be is generated. To do that, “Select grid file”, “Select Enc File” (enclosure file), “Select Bnd File” (boundary condition file) of the finer model. “Make observation points” will create a number of observation points (. obs) at the boundary of the nested model. In addition to this, the program generates the nesting administration (.adm), i.e. the link between the boundary support points in the nested model and the monitoring stations in the overall model. Save all to save the so created observation and administration files.
3. Run the overall model with the list of monitoring stations generated by Nesting -1.
4. The actual boundary conditions (* .bct) for the nested model are generated by Nesting -2 using “Select History File” (“*.his”) of the overall model and “Select Admin file” (nesting administration) and then “Run Nesting”. Boundary Then, the actual boundaries of the nested (smaller / finer) model are generated through Nesting - 2 by interpolating the history files of the overall model. For more information about nesting, one is referred to the Delft3D-FLOW manual. The Nesting toolbox supports Delft3D-FLOW and Delft3D-WAVE models.

Usage of the toolbox

The nesting procedure involves the following steps:

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 *.enc and *.bnd file of the finer model grid. 
   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

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1. 1. .g. salinity)
      
      

The possibilities for nesting in Delft Dashboard are also explained in Tutorial 2: two level nesting.

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Figure: With the Nesting toolbox two models 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 San Francisco Bay , salinity).