At various points in the flow solver operation, the calculation requires information such as the rate of flow at an inlet boundary, or the viscosity in some part of the fluid. This dependence can be specified in a simple manner, but more complex dependencies can be developed by using CEL.

Consider the case of flow through a pipe. You may want to set a velocity profile at the inlet, which will typically be greatest at the center. The profile can be expressed mathematically in terms of a function of x, y, and z, which are Ansys CFX system variables known to the CFX-Solver. The mathematical definition of the profile is called an expression, and the value of this expression is a term that is input to the flow solver. You can define many terms in Ansys CFX by way of expressions: fluid properties, subdomain sources, boundary conditions, and initial values are all Ansys CFX terms that can be described using expressions.

A key feature of CEL is that it is used dynamically by the CFX-Solver. For example, you can set viscosity to be dependent on temperature using CEL, and the current heat transfer CFD solution will be computed for this relationship.