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Natural Convection Inlet/Outlet

The Natural Convection Inlet/Outlet boundary condition is unique to SimScale and is used to represent a boundary surface that is open to an external environment allowing the fluid to pass through in and out.

For example, this boundary condition type can be used for open windows when modeling a room.

warehouse model using natural convection inlet-outlet bc
Figure 1: Warehouse wireframe model with faces highlighted (blue) where natural convection takes place.

Figure 1 shows a real-life warehouse with natural convection happening at the exhausts (top) and the entry doors all highlighted in blue.

These surfaces will not be defined as pure inlets or outlets. They will behave (partially or totally) in one way or the other, depending on the local fluid conditions. As a result, recirculations and eddies are allowed at these patches, always complying with the continuity equation for the whole domain.

The natural convection inlet/outlet boundary condition is available for the following analysis types:

Defining in the Workbench

Inside the Workbench, you should come across the following settings panel while applying this boundary condition.

  • If the compressbility toggle is active → User needs to define the absolute pressure.
  • Else (inactive) → No need for pressure assignment, since the boundary condition takes the value from the initial conditions.
natural convection inlet-outlet bc as it appears in simscale depending on the compressible switch is on or off
Figure 2: Natural convection inlet/outlet boundary condition settings as seen in the SimScale Workbench, when compressibility is enabled (left) and when disabled (right).


The (Pt) Ambient pressure will be asked for only when the simulation has Compressible switch toggled-on. In this case. your pressure is absolute and not gauge pressure which works for incompressible simulations using Boussinesq Approximation.

Additional options for radiation will be present too for radiation toggled-on.


Mathematically, when this condition is operated the boundary conditions applied on each fluid variable are:

  • Velocity: It is zero gradient at all times, combined with a zero value for the tangential velocity. While it is the standard condition for outflows, the normal velocity for inflows is calculated depending on local conditions.
  • Pressure: A value called Ambient Pressure can be specified at the surface defined as
    • the total pressure when the local velocity vector is pointing inside the domain;
    • the static pressure when the local velocity vector is pointing outside.
      By doing this, we can specify the far-field static pressure of the environment, since Bernoulli’s Principle is satisfied for subsonic and incompressible cases. For example, when defining a window, you can specify the outer atmospheric pressure here.
  • Temperature: The temperature takes an inlet-outlet behavior which means that it becomes zero gradient when the velocity vector aims outside the domain and switches to a fixed value when it aims inside the domain. This fixed value is the one that you will specify as Ambient temperature in Figure 1.
  • Turbulent variables: The turbulent variables like \(k\), \(\epsilon\), \(\omega\) have the same behavior as inlet-outlet. The fixed value is taken from the initial value at every cell.

Last updated: September 9th, 2022