Hi Rob,

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I found this in FAQ in Ansys Fluent ( I googled Patch) : Often, Chemical Engineers need to compute the RTD of their Continuous stirred tank reactors (CSTRs). FLUENTâ€™s DPM model is challenging to use in these systems due to difficulty in getting statistically meaningful number of particles at the outlet. A second method is to introduce passive tracer material either with species or user-defined scalars. A method with user-defined scalars (UDS) is outlined here. Approach: 1. Solve for single-phase steady state flow field with inlets and outlets. 2. Switch to unsteady solver. 3. Introduce a UDS with mass flux as convection term and default unsteady term. The UDS represents a passive tracer that is used to determine RTD. 4. Change UDS diffusivity to zero or reasonable values. For water-water system, this is quite low ~ 1e-10 m2/s. 5. For pulse input, patch a known amount of UDS near the inlet; for step input, make UDS value = 1 at inlet. 6. Turn on surface monitor of area-av. UDS value at the outlet. Plot, print/ write to file. 7. Turn off all equations except the UDS equation. Run for the required flow time. The UDS conc. at the outlet as a function of time can be used to extract the residence time distribution. If you use step input of tracer (UDS value = 1 at inlet), the outlet UDS profile when normalized (Coutlet/Cinlet) is called F curve which is a cumulative residence time distribution. If you introduce a pulse, the normalized response is called C Curve which is the RTD function.

This is exactly what I want to do. But I do not know how to add the value at a specific plane.

Thank you

Pratheeba