The momentum sources given by Eqns. (7), (9) and (10) may be scaled so that the operating point of the fan lies on a user-supplied fan performance curve. Consider the figure below, which shows a typical fan performance curve. Also shown in this figure are several points on the "load curve", each of which corresponds to a particular fan momentum source scale factor.
The procedure for adjustment of the momentum source scale factor begins with an initial guess of the scale factor. STAR-CCM+ then solves for the flow field for a number of iterations, and stores the resulting fan flow rate and pressure rise. The scale factor is then changed by 25% in a direction that will bring the operating point closer to the fan curve. STAR-CCM+ then solves for the flow field again for a number of iterations, and again stores the resulting fan flow rate and pressure rise.
With the new scale factor, STAR-CCM+ once again solves for the flow field for a number of iterations, to compute a new operating point, which may or may not be the desired operating point. Using the two most recent operating points, a new intersection point is found, and the momentum source scale factor is re-adjusted. This procedure is followed repeatedly for the rest of the analysis, until the solution is within the pressure tolerance.
A key to this procedure is that for each adjustment of the scale factor, STAR-CCM+ must perform a sufficient number of iterations to achieve a stable operating point. Unfortunately, the flow field responds somewhat slowly to a change in the momentum scale factor, so that typically hundreds of iterations are required between adjustments. You can specify the number of iterations between updates with the IUpdate property. The default value is 250 iterations.