The simulation volume is divided into a set of control volumes by three sets of grid planes, one in each axis direction.
There is always a current grid working direction, and a selected region of grid planes in this direction. The current working direction is shown in the message area. The lines indicating the selected region is highlighted.
3.5.1 Simulation volume
The Simulation Volume command lets you change the simulation volume extent in all three di-rections. If you increase the volume, the original grid planes are kept, but one additional plane is added in each direction. If you decrease the volume, planes outside the new volume are deleted, and new planes are created on the volume borders.
3.5.2 Direction
The Direction command changes the working direction. Legal input is x, y or z. The Grid menu commands Region, Add, Position, Move, Delete, Smooth, Stretch and List affects the grid planes in the working direction.
3.5.3 Region
The Region command substitutes the selected grid planes by a new set of grid planes. CASD asks you to enter the new number of control volumes in the region.
3.5.4 Add
The Add command adds a new grid plane in the working direction. You are asked to enter the coordinate value for the new plane.
3.5.5 Position
The Position command lets you edit the position for the selected grid plane.
3.5 Grid menu 57
3.5.6 Move
The Move command moves the selected grid planes a specified distance.
3.5.7 Delete
The Delete command deletes the selected grid planes.
3.5.8 Smooth
The Smooth command substitutes the selected grid planes by a new set of grid planes.
For the Smooth command, the sizes of the control volumes at each end of the region is kept unchanged. The sizes of the control volumes between them varies gradually.
This function is typically used when refining the grid around aleak.
3.5.9 Stretch
The Stretch commands substitutes the selected grid planes by a new set of grid planes. This is particularly useful when stretching the grid towards the outer boundaries.
The Stretch command has two sub-choices:
• Negative Direction (typically used at the boundaries at the negative end of the axis)
• Positive Direction (typically used at the boundaries at the positive end of the axis)
You must enter the size of the control volume at one end of the region, default is the current size.
Then you must enter a factor by which the sizes of the control volumes in the specified direction increases/decreases.
Attention:
Note that stretching of the grid should be avoided in areas of the simulation domain where the main combustion is happening. The flame model in FLACS has been validated for cubical control volumes, thus the user should not stretch the grid in areas where accurate results are required. It is however good practice to stretch the grid towards the boundaries, to concerve simulation time and computer memory.
3.5.10 Information
The Information command displays status information about the defined grid, while the List command lists the grid coordinates in the working direction.
3.5.11 List
The Information command displays status information about the defined grid, while the List command lists the grid coordinates in the working direction.
3.5.12 Display
The Display command turns grid display off, displays the grid in the working direction only, or displays the grid in all three directions.
3.5.13 Select
The selected region of grid planes is limited by two planes, the lower and upper limit. If only one plane is selected, the upper and lower limit is the same grid plane. Grid planes are selected using the following commands:
• Lower boundary
– Select the next grid plane: CTRL+RIGHT – Select the previous grid plane: CTRL+LEFT
• Upper boundary
– Select the next grid plane: CTRL+UP – Select the previous grid plane: CTRL+DOWN
3.5.14 Grid-related operations
3.5.15 Importing the grid from another job
Use the Import command in the File menu to import the grid from another job.
If you enter the grid file name in the command input field, the path must be encapsulated in apostrophes, as described in section . If you select the command from the menu bar, or if no name is specified in the command input field, the Import dialog box is displayed, allowing you to specify the path and file name for a grid file. You will be asked to verify that the current grid is overwritten by the grid from the specified file.
3.5.16 Saving the grid
The Save and Save As commands in the File menu saves the grid, together with the rest of the job data. If the grid is changed, you will need to recalculate the porosities.
3.5.17 Grid-related utilities
FLACS is deleivered with a command line tool for creating an manipulating the grid. This tool can be used to quickly edit or get information about the grid. Please see sectiongmfor further information.
3.5.18 Grid guidelines
The grid resolution should be chosen to obtain a simulation result within an acceptable time frame. In most cases a reasonably good result can be obtained on a coarse grid within less than one hour (in some cases 5 minutes), and high quality results can normally be generated in a few hours (or at least over the night).
3.5 Grid menu 59
Never start a project with a calculation on a grid that will be running for days. If such long sim-ulation times are necessary, always start simulating on a much coarser grid [even if this violates guidelines] to check that the scenario and setup are OK.
The user should keep the position of the grid lines in mind while defining the geometry. The geometry details such as walls and decks should be adjusted to the closest grid line when in-putted. Thereby the user keeps track of the positioning instead of having the geometry moved in an unwanted direction by the porosity calculation program.
In the grid embedding process, it is highly recommended to use Grid→Information in Casd to check different aspects of the grid. Grid sensitivity tests are also recommended.
3.5.18.1 Gas explosion simulations Attention:
The user should always apply cubical grid cells in the combustion region. Deviations from this will give different flame propagation and pressures, and the validation work done is no longer valid. Deviations of the order 10% in aspect ratio is OK, deviations by a factor of 2 in aspect ratio is not OK. If one chooses not to follow this guideline, the results can be somewhat improved by setting a fixed control volume size for the time stepping routine (see sectionThe SETUP namelist, example TIME_STEPPING=" STRICT:L_FIX=1.0" ).
Channels and confined vessels and rooms (filled with gas from wall to wall) must always be resolved by a minimum of 5-6 grid cells in smallest direction if flame acceleration shall be modelled.
This also applies for pipes where flame acceleration along the pipe is of interest.
A pipe connection from one vessel to the next may have less grid cells across the diameter (but preferably more than 1 CV) if only flame transport by pressure difference and not flame accelera-tion along the pipe shall be modelled. Increase the inner diameter of angles and bends somewhat when modelling pipes with cylinder minus primitives. Remember that one full grid cell is re-quired inside the solid walls around " minus primitive holes" to ensure that the walls will not be leaking.
Unconfined gas clouds as well as partially filled clouds should have a minimum of 13 grid cells across the cloud if both sides are unconfined, and a minimum of 10 grid cells in directions where cloud meets confinement on one side (example vertical direction for dense gas cloud in chemical plant).
It is not recommended to use non-cubical grids for explosion simulations. As they are often used for dispersion simulations, the dispersion simulation results should be dumped, thereafter the rdfileutility program should be used to transfer the results from the dispersion grid to a grid better suited for explosions, see example below:
> run9 rdfile rd111111.n001 rd222222.n001
Here 111111 is the dispersion calculation job number and 222222 is copy of the job, in which the grid has been modified to follow explosion grid embedding guidelines. The grid of job 222222 must be completely inside the grid of 111111.
The grid can be stretched outside the combustion region in directions where pressure recordings are not of interest. In directions where pressure wave propagation is of interest, one should not stretch the grid because this will reduce the sharpness and quality of the pressures.
A proper distance to external boundaries is important. At least 5-10 grid cells from vent opening to external boundary should be used in situations where the external explosion is not important (small vent area or strong turbulence inside vessel).
In low-congested vessels with significant vent opening, external explosions and reversing of flow may give a strong feedback into the vessel in connection with venting. To pick up this properly, the distance to the external boundaries should be significant (maybe 3-4 times the length of the vessel).
EULERboundary will reflect negative pressures, which can destroy results when simulating far field pressure propagation. In this case PLANE_WAVE may be recommended (but then the bound-aries must be far away so no products from combustion reaches the boundbound-aries). For unconfined situations, try to have the same distance to boundaries in all directions (use stretching in direc-tions with less interest in results).
3.5.18.2 Blast wave propagation in the far field
Maximum control volume size should be 10% of gas cloud diameter:
max CV =0.1× (gascloudvolume)1/3 (3.1) The grid cells must be approximately cubical in explosion simulations and the cell size should be maintained in directions of interest for blast propagation simulations. For vessel burst the same guidelines applies. If the pressure is much higher than 10 barg, somewhat larger grid cells than this criterion can be acceptable. Remember to use PLANE_WAVE boundary condition and proper distance to the boundaries.
3.5.18.3 Dispersion simulations near field
Calculations of flammable gas requires grid refinement near the leak. The area of the expanded jet (at ambient pressure) must be resolved by one grid cell (ACV < 2×Ajet) except for low-momentum releases of highly buoyant gases such as hydrogen where the guideline (ACV <
1.25×Ajet)should be followed. In most cases, a grid refinement near the leak helps in keeping moderate calculation times while getting acceptable results.
Grid refinement guidelines for efficient simulation of high velocity jets recommend only refine-ment across jet direction (not along). CFLC should be increased by refinerefine-ment factor (CFLV should not be changed). Smoothing from fine grid cells near jet to normal grid cells further away from jet is recommended.
If the jet is not along the axes, is impinging or has a low momentum with positive/negative buoyancy, then extending the refined region of the grid in one or more directions may be re-quired. Refinement along the jet may then also be rere-quired.
If only far field concentrations are of interest, the refinement near the leak may not be needed.
Quicker calculations and less stability problems will be seen without the refinement.
For dense gas calculations, it may be a good idea to use a finer resolution in the vertical direction near the ground than in the other two directions. Increase CFLC by this refinement factor.
Outside the main area of interest, further stretching to the boundaries is recommended to mimize the influence of the boundaries. If in doubt whether the distance to the boundaries in-fluences your results, increase the distance further to check the sensitivity. The general recom-mended procedure for setting up the grid is:
1. Cover the computational domain with a uniform grid
2. Refine the grid in the near region of a jet (perpendicular to the jet axis) 3. Stretch the grid outside the main region towards the boundaries