In Ansys Fluent® fluid simulation software, why is the Speed of Sound property required for materials when applying UDF for temperature-dependent density?
When density is pressure-dependent, an extra term related to the speed of sound is needed in the pressure correction equation in order to stabilize the pressure solution for compressible flows.
Consequently, when density UDF is detected, Ansys Fluent software automatically adds the Speed-of-Sound property input, as there is no way to distinguish whether the UDF will include a pressure dependency or not.
If the flow is incompressible (density a function of temperature only), you can simply set the Speed of Sound function to None.