A Natural Ventilation is air circulation is provided by opening barn doors or windows and allowing the wind to draw through the barn. 
Other /More definition:
A Natural Ventilation is air circulation is provided by opening barn doors or windows and allowing the wind to draw through the barn. 
Other /More definition:
A Natural Ventilation is the design and placement of the building to allow adequate ventilation by controlling wind blowing against the building.    
        
        
                                        