The interactions of the flow with tiny geometric details of the duct and register geometry can make a big difference in the direction, velocity, and turbulence level of the flow. This affects the spread of the flow over the windshield and side window, which in turn controls the mist film evaporation and melt rates. This complex physics behavior needs to be simulated accurately in order to correctly predict defrost and demist. Defrost and demist are by definition transient, but traditional flow simulation tools are not inherently transient and often have difficulty providing accurate results with complex geometry. The turbulent flow over the windshield and side glass is almost impossible to visualize in a physical test stand, yet these complex structures must be understood in order to optimally design the system.