Decreasing the update frequency to 10 iterations until update (IUU) yields in a converged solution (magenta line). However, reducing the update frequency increases the number of iteration steps until convergence is reached and consequently the overall computational time of the simulation. Therefore, an under-relaxation factor τ is introduced, which is used to determine the new boundary condition ζ by mixing the calculated boundary value ζ i with the currently applied one ζ i-1 in the ratio given by τ :