- The domain must contain all obstacles that affect the wind in your region of interest (ROI).
- The grid must have sufficient resolution in the ROI.
- The grid normally must be stretched outside the ROI.
You can find a definition of the domain and grid in What is the domain and the grid?.
For details on how to set up grids in general, see How to use the quick grid tool and How to set up a grid manually.
In this article
Defining the Region of Interest and selecting a grid size
The Region of Interest (ROI) is the area where you want to evaluate cloud build-up from your dispersion simulations. The ROI is typically a naturally limited area such as an offshore module with wind walls or the inside of a building. Otherwise, it may simply be the congested part of a larger area, or limited by the greatest distance at which your selected leak size can credibly give a flammable concentration.
In the ROI, you should strive to use a uniform grid. For large outside areas such as offshore platforms or onshore plants, a cell size of 1 – 2 meters is typically used. If your ROI is small, make the grid cells small enough so that you have at least 10 grid cells in each direction in your ROI. If you are unsure about the grid cell size to use, you can perform a grid sensitivity simulation for a few cases. The overall pattern of ventilation should not be significantly changed by e.g. doubling grid resolution. Sometimes it may not be possible to use an absolutely uniform grid (meaning all sides of a grid cell are the same length): when trying to represent walls and decks as on-grid, you may end up with slightly elongated cells. But grid cells should ideally not be too far from cubical in the ROI.
When a wall or deck is not exactly on the edge of a grid cell, Porcalc will move the wall to the closest grid cell. This may have unexpected consequences:
- monitor points and panels can end up on the wrong side of a wall, giving wrong results,
- corners that should be air-tight (such as in a building) can be opened when one or both walls are moved outward,
- area and volume porosities may not agree.
Because of these potential pitfalls, it is best to try to get major walls and decks to align with grid lines in the ROI. Skilled CASD users may address this already in placing the walls/decks in anticipation of common grid sizes. Walls and decks can be “rounded” to the nearest 1 m or 0.5 m so that they are treated in a predictable way by FLACS. If not, you should make your grid hit the major walls and decks exactly.
For example, if you aim for a 0.5 m grid in the ROI and the decks are at Z = 7.3 m and Z = 13.6 m, it is better to evenly split the region in 13 grid cells of 0.485 m height than to enforce a 0.5 m height and miss one of the decks by 20 cm. Make sure the grid cells match the decks exactly, then split using thetool in the menu. FLACS will calculate the required grid cell size. It is OK to have slightly different grid sizes on each side of a wall. Slightly means you must make sure that no grid cell is more than 20% wider than its neighbor. The tool in the menu will show you the largest size difference between neighbours in your entire grid.
Selecting the simulation domain
Obstacles may affect the wind over a long distance:
You must select your domain so that obstacles upwind of your ROI are included. As a rule of thumb, the domain should extend at least 1.5 times your ROI in all directions (except ZLO, where it should end at ground level):
However, if there are big obstacles upwind that would be excluded by using this rule, you may have to extend the domain in that direction to include them. For example, a building with height H may have an influence if its distance from the ROI is less than 6-10 H. If you are unsure, you can run a test simulation to see how far an obstacle affects the general wind field. The domain should start a short distance upwind of the obstacle so that the wind field is able to flow around it properly.
Regarding the vertical extent of the domain, based on guidelines for the use of meteorological models in urban areas, the domain should extend to at least 5 H, where H is the maximum hight among the obstacles in the ROI. This large vertical extension ensures correct modelling of the flow acceleration above the top of the obstacles and avoids artificial flow constraints induced by the conditions at the top boundary. If the highest obstacle reaches above the vertical ROI, the height of the domain may be reduced to less than 5 H, but you must keep at least 0.5 H distance from the obstacle's roof to the top boundary to ensure convergence of the simulation.
Very big obstacles such as mountains should already be accounted for in your meteorological data.
Creating a grid using the Quick Grid wizard
The Quick Grid wizard will help you set up a grid. You can find the wizard in CASD’s Grid menu. Main inputs are:
- grid size,
- simulation domain.
The following picture shows where to use these values. The Core Domain should be a little larger than the ROI, since stretching the grid just outside the ROI may make the ventilation patterns inaccurate. You may stretch earlier downwind than upwind, since downwind obstacles are less important for flow in the ROI. Upwind of the ROI, you should keep the grid size sufficiently fine to resolve major obstacles with at least 3x3 grid cells to represent flow around them. As a rule of thumb, stretch after 0.5 ROI widths downstream and 1 ROI width upstream.
When working with the Quick grid wizard:
- Keep the memory consumption smaller than the available memory of the computer where you will be running the simulations. Keep in mind that if you refine the grid around a leak (see below), you will increase the memory consumption. Keep the memory consumption below 1 GB if you can. If the memory consumption is high, simulations will take long to finish.
- Run a test simulation on a coarser grid. This simulation will finish earlier and will allow you to verify that your simulation is set up correctly and properly reports all the data you need. It is better to check this early rather than finding out that you are missing something when you are close to a deadline!
- Document your ROI, grid size, and simulation domain independently (e.g. in a spreadsheet) since these values can be very useful when writing a report, evaluating results, defining monitoring regions, etc.
Reviewing the wind field
Use Flowvis to view the flow field from your ventilation simulations:
- Create a 2D cut plane plot. Select your simulation and the VVEC variable.
- In Z) and boundaries (X and Y) to show the area of interest. , adjust the cut plane height (
- Set the to Fixed with minimum and maximum values that make the plot easy to read: Stagnant areas in white (no arrow) and the highest flow areas in red.
The figure below shows some example settings and the resulting plot.
Verify the following:
- The wind field interaction with obstacles is resolved properly.
- The wind in the ROI is well-resolved.
- Looking at the flow behind obstacles, make sure that the distance that they affect the wind field makes it likely that obstacles you have left outside the domain do not affect results.
- The domain should be large enough to include obstacles that affect flow in the ROI, at least 1.5x the ROI size in each direction.
- The ROI and surroundings must be covered with a grid of around 1-2 m size (or at least 10 grid cells in each direction within ROI).
- The grid may be stretched from 0.5 ROI widths downstream of the ROI and 1 ROI width upstream.
- Use the Quick Grid and Refine grid wizards.
- When in doubt, test your grid with test simulations.