I am running a micro-structure simulation with my characteristic velocity as 1e-04 m/s and characteristic length as 7.64e-06 m. My delta x for the simulation is 5e-07 m and using BGK collision operator I am calculating the pressure forces at the outlet.My forces are coming out to be in the order of 2e-15 N and the value to which it is comparing has an order of e-14. I tried increasing the tau (relaxation-parameter) from 0.6 to 0.8 to 0.999 and found out that my value is approaching to the reference value. However, in the book The Lattice-Boltzmann Methods by Timm Krüger, it is said that the tau value should not be much greater than 1. So I had the question should I increase my tau to greater than 1 or should I change my collision operator from BGK TO TRT Or MRT operator. Is there any upper limit on the tau value?