FloTHERM收敛控制幻灯片.pptVIP

* * * * * * * * * * * * The false time step sets a transient-like parameter which is used to ‘damp’ or ‘under-relax’ the solution of any variable except pressure. Set to correspond to time-scale of problem, e.g. (typical length)/(typical velocity). Larger values give light damping, small values give heavy damping. Automatic sets False Time-steps based on fans in forced flow cases and the EFCV in natural convection cases. Press [Calculate Automatic Values] to see the automatic value. The slider bar can be used to quickly change the automatic setting by either dividing or multiplying it by a factor of up to 20. Press [Calculate Automatic Values] to see the effect. User Specified allows you to override the automatic settings manually. Calculating the False Time Step False-time step under-relaxation is the principal means that you will use to control convergence of the iterative algorithm. For a given variable, this under-relaxation provides an inertial effect which restrains the amount by which the value in each cell is permitted to change at each outer iteration. The inertial effect is proportional to the mass in the cell and inversely proportional to the false time-step set for the variable. Consequently, the smaller the false time-step the larger the inertial effect, and so the less the variable is permitted to change. By default the program calculates an automatic false time-step which is used for all variables (the one exception being pressure to which the method is not applied). * * * * Remember! In general it is inadvisable to use extreme values for the false time steps. Very large values, where there is no damping, will often lead to divergence. Very small values which lock the variable should not be used. If the values can not change, the solution will not converge. Hint: Before making changes to the false time step to correct dramatic divergence, check the model and the grid for errors. ? 2010 Mentor Graphics Corp. Company Confidential Your Initi