This is a cooling tower for on top of Desert Terrain Buildings (40k, 28mm scale)
If you look up desert style buildings, many of them have an old but effective way to cool the buildings. They have these towers on top that let all the hot air rise and bleed out leaving cool air in the building.

This is shown with a bunker design I made, but it can go on top of any building or just on top of a mound.