The residuals are the error magnitudes for equations as iterations progress. The equations include the governing equations; I.e. the Navier-Stokes momentum equations for each direction (x, y, and z if 3d, or just x and y if 2d), the continuity equation (conservation of mass), and if heat transfer is applicable, the energy equation. The equations may also include equations of the turbulence model defined under models-viscous. The residual is the difference between the previous result and the current result. As these errors are decreasing the equation results are reaching values that are changing less and less. This is what is known as convergence. That is the solutions are converging. If these errors begin to increase, the solution is then said to be diverging. Let me give a simplified example of convergence: Initial value = 2, 1st iteration value = 1.5 (residual value is equal to 2 - 1.5 = .5), 2nd iteration value = 1.2 (residual value = 1.5 - 1.2 = .3), 3rd iteration value = 1.05 (residual value = 1.2 - 1.05 = .15),... 1000th iteration value = 1.00001, and 1001th iteration value = 1.000005 (residual value = .00001 - .000005 = .000005). Perhaps the solution is converging to a value of 1. An opposite trend could be given as an example of divergence. A solution can converge and then diverge or vica versa. Does this help? Is my explanation clear and accurate?
