Heavy Side Function

 

With the initial values of the concentration of A and B set to zero in the reactor numerical problems occur when adding an inlet flow of 500 mol/ dm­³ of A and B. It becomes a big discontinuity when 500 molars in the feed meet 0 molar in the reactor. In COMSOL Multiphysics this numerical noise will show as negative concentrations for the first couple of time steps. In order to solve this, a built in function can be used called 'heavy side function'. Instead of using a boundary condition of C_A0(t_{0 ­­}= 0) = C_A0, an expression like C_A0(t₀ = 0) = C_A0*flc2hs(t-10,5) can be used, where the first argument tells where to start and the second how wide the step should be. To start before t = 0 the function smoothens the concentration to a desired positive value. Since the concentration difference is big it is feasible to have a large step.