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FLACS from Gexcon is the industry standard for CFD explosion modelling and one of the best validated tools for modeling flammable and toxic releases in a technical safety context.

The Cirrus cluster is ideally suited to run multiple FLACS simulations simultaneously, via its batch system. Short lasting simulations (of typically up to a few hours computing time each) can be processed efficiently and you could get a few hundred done in a day or two. In contrast, the Cirrus cluster is not particularly suited for running single big FLACS simulations with many threads: each node on Cirrus has 2x4 memory channels, and for memory-bound applications like FLACS multi-threaded execution will not scale linearly beyond eight cores. On most systems, FLACS will not scale well to more than four cores (cf. the FLACS User's Manual), and therefore multi-core hardware is normally best used by increasing the number of simulations running in parallel rather than by increasing the number of cores per simulation.

Gexcon has two different service offerings on Cirrus: FLACS-Cloud and FLACS-HPC. FLACS-Cloud is the preferable way to exploit the HPC cluster, directly from the FLACS graphical user interfaces. For users who are familiar with accessing remote Linux HPC systems manually, FLACS-HPC may be an option. Both services are presented below.


FLACS-Cloud is a high performance computing service available right from the FLACS-Risk user interface, as well as from the FLACS RunManager. It allows you to run FLACS simulations on the high performance cloud computing infrastructure of Gexcon's partner EPCC straight from the graphical user interfaces of FLACS -- no need to manually log in, transfer data, or start jobs!

By using the FLACS-Cloud service, you can run a large number of simulations very quickly, without having to invest into in-house computing hardware. The FLACS-Cloud service scales to your your demand and facilitates running projects with rapid development cycles.

The workflow for using FLACS-Cloud is described in the FLACS User's Manual and in the FLACS-Risk documentation; you can also find basic information in the knowledge base of the FLACS User Portal (accessible for FLACS license holders).


Compared to FLACS-Cloud, the FLACS-HPC service is built on more traditional ways of accessing and using a remote Linux cluster. Therefore the user experience is more basic, and FLACS has to be run manually. For an experienced user, however, this way of exploiting the HPC system can be at least as efficient as FLACS-Cloud.

Follow the steps below to use the FLACS-HPC facilities on Cirrus.

Note: The instructions below assume you have a valid account on Cirrus. To get an account please first get in touch with FLACS support at and then see the instructions in the Tier-2 SAFE Documentation.

Note: In the instructions below you should substitute "username" by your actual Cirrus username.

Log into Cirrus

Log into Cirrus following the instructions at ../user-guide/connecting.

Upload your data to Cirrus

Transfer your data to Cirrus by following the instructions at ../user-guide/data.

For example, to copy the scenario definition files from the current directory to the folder project_folder in your home directory on Cirrus run the following command on your local machine:

rsync -avz c*.dat3

Note that this will preserve soft links as such; the link targets are not copied if they are outside the current directory.

FLACS license manager

In order to use FLACS a valid license is required. To check the availability of a license, a license manager is used. To be able to connect to the license manager from the batch system, users wishing to use FLACS should add the following file as ~/.hasplm/hasp_104628.ini (that is, in their home directory)

; copy this file (vendor is gexcon) to ~/.hasplm/hasp_104628.ini
aggressive = 0
broadcastsearch = 0
serveraddr = cirrus-services1
disable_IPv6 = 1

Submit a FLACS job to the queue

To run FLACS on Cirrus you must first change to the directory where your FLACS jobs are located, use the cd (change directory) command for Linux. For example

cd projects/sim

The usual way to submit work to the queue system is to write a submission script, which would be located in the working directory. This is a standard bash shell script, a simple example of which is given here:

#!/bin/bash --login

#SBATCH --job-name=test_flacs_1
#SBATCH --ntasks=1
#SBATCH --cpus-per-task=1
#SBATCH --time=02:00:00
#SBATCH --partition=standard
#SBATCH --qos=standard

module load flacs-cfd/21.2

run_runflacs 012345

The script has a series of special comments (introduced by #SBATCH) which give information to the queue system to allow the system to allocate space for the job and to execute the work. These are discussed in more detail below.

The flacs module is loaded to make the application available. Note that you should specify the specific version you require:

module load flacs-cfd/21.2

(Use module avail flacs to see which versions are available.) The appropriate FLACS commands can then be executed in the usual way.

Submit your FLACS jobs using the sbatch command, e.g.:

$ sbatch --account=i123
Submitted batch job 157875

The --account=i123 option is obligatory and states that account i123 will be used to record the CPU time consumed by the job, and result in billing to the relevant customer. You will need your project account code here to replace i123. You can check your account details in SAFE.

The name of the submission script here is The queue system returns a unique job id (here 157875) to identify the job. For example, the standard output here will appear in a file named slurm-157875.out in the current working directory.

Options for FLACS jobs

The #SBATCH lines in the script above set various parameters which control execution of the job. The first is --job-name just provides a label which will be associated with the job.

The parameter --ntasks=1 is the number of tasks or processes involved in the job. For a serial FLACS job you would use --ntasks=1. The

The maximum length of time (i.e. wall clock time) you want the job to run is specified with the --time=hh:mm:ss option. After this time, your job will be terminated by the job scheduler. The default time limit is 12 hours. It is useful to have an estimate of how long your job will take to be able to specify the correct limit (which can take some experience). Note that shorter jobs can sometimes be scheduled more quickly by the system.

Multithreaded FLACS simulations can be run on Cirrus with the following job submission, schematically:

#SBATCH --ntasks=1
#SBATCH --cpus-per-task=4

run_runflacs -dir projects/sim 010101 NumThreads=4

When submitting multithreaded FLACS simulations the --cpus-per-task option should be used in order for the queue system to allocate the correct resources (here 4 threads running on 4 cores). In addition, one must also specify the number of threads used by the simulation with the NumThreads=4 option to the run_runflacs.

One can also specify the OpenMP version of FLACS explicitly via, e.g.,


run_runflacs version _omp <run number> NumThreads=20

See the FLACS manual for further details.

Monitor your jobs

You can monitor the progress of your jobs with the squeue command. This will list all jobs that are running or queued on the system. To list only your jobs use:

squeue -u username

Submitting many FLACS jobs as a job array

Running many related scenarios with the FLACS simulator is ideally suited for using job arrays, i.e. running the simulations as part of a single job.

Note you must determine ahead of time the number of scenarios involved. This determines the number of array elements, which must be specified at the point of job submission. The number of array elements is specified by --array argument to sbatch.

A job script for running a job array with 128 FLACS scenarios that are located in the current directory could look like this:

#!/bin/bash --login

# Recall that the resource specification is per element of the array
# so this would give four instances of one task (with one thread per
# task --cpus-per-task=1).

#SBATCH --array=1-128

#SBATCH --ntasks=1
#SBATCH --cpus-per-task=1
#SBATCH --time=02:00:00
#SBATCH --account=z04

#SBATCH --partition=standard
#SBATCH --qos=commercial

# Abbreviate some SLURM variables for brevity/readability


# Form a list of relevant files, and check the number of array elements
# matches the number of cases with 6-digit identifiers.

CS_FILES=(`ls -1 cs??????.dat3`)

if test "${#CS_FILES[@]}" -ne "${TASK_COUNT}";
  printf "Number of files is:       %s\n" "${#CS_FILES[@]}"
  printf "Number of array tasks is: %s\n" "${TASK_COUNT}"
  printf "Do not match!\n"

# All tasks loop through the entire list to find their specific case.

for (( jid = $((${TASK_MIN})); jid <= $((${TASK_MAX})); jid++ ));
  if test "${TASK_ID}" -eq "${jid}";
      # File list index with offset zero
  file_id=$((${jid} - ${TASK_MIN}))
  # Form the substring file_id (recall syntax is :offset:length)

printf "Task %d has file %s id %s\n" "${TASK_ID}" "${my_file}" "${my_file_id}"

module load flacs-cfd/21.2
`which run_runflacs` ${my_file_id}

Transfer data from Cirrus to your local system

After your simulations are finished, transfer the data back from Cirrus following the instructions at ../user-guide/data.

For example, to copy the result files from the directory project_folder in your home directory on Cirrus to the folder /tmp on your local machine use:

rsync -rvz --include='r[13t]*.*' --exclude='*' /tmp

Billing for FLACS-HPC use on Cirrus

CPU time on Cirrus is measured in CPUh for each job run on a compute node, based on the number of physical cores employed. Only jobs submitted to compute nodes via sbatch are charged. Any processing on a login node is not charged. However, using login nodes for computations other than simple pre- or post-processing is strongly discouraged.

Gexcon normally bills monthly for the use of FLACS-Cloud and FLACS-HPC, based on the Cirrus CPU usage logging.

Getting help

Get in touch with FLACS Support by email to if you encounter any problems. For specific issues related to Cirrus rather than FLACS contact the Cirrus helpdesk.