Whether you want to model a high momentum jet releasing pressurised gas into the domain or an extraction fan sucking gas out of the domain, they are both defined as leaks in FLACS. You can specify leaks in FLACS by expanding the Leaks section in the Scenario settings. Here you can define both sources and sinks of fuel, air, or inert gas, as well as momentum sources like fans; all these are modelled as leaks in FLACS.

How to define a leak

You can define a leak by clicking on the Add button in the Leaks section in the scenario menu. In this case, you will need to enter all the input required manually.

You find the Leaks section in the scenario menu in CASD.

Alternatively, you can right-click in the white area of the Leak wizard to define a leak. If you choose to use the Leak wizard, you can either enter the input manually or use the Jet utility to calculate a steady-state or time-dependent leak based on the inventory characteristics and the leak size.

Click Add to create a leak or right-click into the list to run the leak wizard.
More details on defining point and area leaks are available in separate articles.

Leak type

You can define up to 50 leaks in a given scenario; the different types of leaks are briefly described below.

The various types of leaks defined in FLACS.


A Diffuse leak is a no-momentum release of gas into the grid cell chosen.


A Jet leak specifies a low or high momentum release. A Jet leak can be either a point or area leak.


Same as Jet but with modelled entrainment. The entrainment functionality is used to define the evolved leak conditions and positions.


An Air leak will release air (or, in general, the mixture defined by the equivalence ratio ER9) instead of fuel (or, the mixture defined by the equivalence ratio ER0); otherwise it is similar to a Jet leak.


A Suction leak is a negative point source, that is, gas/air will be removed at the leak location.

The leak direction for a fan leak is reversed, for example a +X suction leak generates a suction flow towards the negative X direction).


A Fan leak does not add or remove mass, but assigns a fixed momentum to the fluid in the concerned control volume.


A leak of Inert type releases the specified inert gas rather than fuel (only in the Inert simulation type).

Please refer to the FLACS User’s Manual for additional information.