Hevacomp Dynamic Simulation allows you to consider opening windows when defining air flow networks.
A coefficient of the open area is calculated using the area of the window and the free area of the opening. All of the open area is considered, along with wind direction and angle of opening. For a top hung window, for example, the rectangle at the bottom, and the triangles at the sides are considered.
Fig A demonstrating how the opening area is calculated
Fig B demonstrating the calculated free area coefficient compared to the total window area