Documentation

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.

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:

*Convective Heat Transfer**Conjugate Heat Transfer**&**Conjugate Heat Transfer v2.0*- Immersed Boundary (internal flow analysis)

The UI for defining the settings for this boundary condition changes as per the analysis types and compressibility.

**Incompressible**

For incompressible flow simulations, the settings panel for the *Natural convection inlet/outlet* boundary condition is as follows:

The user cannot define the value for the reference pressure explicitly and it is taken from the initial conditions.

**Compressible**

For compressible flow simulations, the user needs to define the ambient **total absolute pressure**.

For conjugate heat transfer simulations, user needs to define the ambient **total absolute pressure** similar to Figure 3 above.

**Incompressible**

For incompressible flow simulations involving the conjugate heat transfer v2.0 and the immersed boundary solver, the user has to define the ambient **total gauge pressure**.

**Compressible**

For compressible flow simulations involving the conjugate heat transfer v2.0 and the immersed boundary solver, the user has to define the ambient **total absolute pressure** same as in Figure 3.

Note that the *Ambient temperature* can be defined parametrically for experiments. This article expands on the topic of parametric studies in SimScale.

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: February 15th, 2023

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