Hi,nYes without a UDF it runs without errors. I have UDFs for my own source terms and such and they compile and run without any errors.nIn the simple DEFINE_UDS_FLUX udf I supplied above, there is only 1 line of code that is doing anything. That is also similar to code which is supplied just before Example that if you had to implement the default advection term in a UDF without the fluid density in the definition of n (see above), you could simply put the following line in your DEFINE_UDS_FLUX UDF: nreturn F_FLUX(f,t) / rho;nnThe problem seems to be that F_FLUX does something different than using the default mass flow rate to specify the flux.n