# Passive Scalars

Source: https://help.sim-flow.com/documentation/panels/passive-scalars
Last updated: 2026-10-14

## Introduction

![img 5959fb0b413c9](https://help.sim-flow.com/documentation/panels/images/passive-scalars/img_5959fb0b413c9.png)

There are situations when one wants to simulate transport of some species in the flow. In general, you would have to employ a species transport model. This model is rather complex and only applies to compressible flows, which means you would not be able to use it to, for example, simulate contamination in the water. In such cases, it might be appropriate to use the **Passive Scalars** model.

The passive scalar model can be used when changes in scalar concentrations do not influence (hence "passive") the physical properties of the flow like density or viscosity. This condition is usually met when the scalar is used to model relatively small concentrations of some species (e.g. \(CO_2\) in air). The equation solved for each scalar \(\Psi\) is a general convection-diffusion equation of the form:

\(\frac{\partial \Psi}{ \partial t} + \nabla \cdot (\vec U \Psi) = \nabla \cdot (D \nabla \Psi) + S \Psi \)

where:

- \(\vec U\) is velocity obtained from the momentum equation,
- \(D\) is the diffusivity coefficient and
- \(S \Psi\) is an optional source term.

## Adding New Scalars

![img 5959fb52b8448](https://help.sim-flow.com/documentation/panels/images/passive-scalars/img_5959fb52b8448.png)

You can add new passive scalars by simply clicking the **Add New** button below the **Scalars** list. A newly created scalar will be displayed in the list and will be available in all contexts where other equations and fields are available (e.g. boundary and initial conditions).

## Editing Scalars

![scalar rename](https://help.sim-flow.com/documentation/panels/images/passive-scalars/scalar-rename.png)

Scalars are created with a default name which can be, as usual, changed by double-clicking on the scalar name. A single click on the name will expand options available for a given scalar.

**Options:**

- **Custom Diffusivity** – by default the diffusivity coefficient \(D\) is retrieved from the turbulence model but you can override it and manually specify a constant **Diffusivity**
- **\(Sc[-]\)** -
- **\(Sc_t[-]\)** -

## Removing Scalars

![img 5959fbba39b06](https://help.sim-flow.com/documentation/panels/images/passive-scalars/img_5959fbba39b06.png)

Scalars, like other entities, can be removed by simply clicking on the **Delete** button on the right-hand side of the list.