Trench effect

The trench effect is a combination of circumstances that can rush a fire up an inclined surface. It depends on two well-understood but separate ideas: the Coandă effect from fluid dynamics and the flashover concept from fire dynamics.

The Coandă effect is the tendency of a fast stream of air to deflect to nearby surfaces. The stream's static pressure tends to decrease, which creates a pressure difference between the wall and areas far from the wall. This bends the stream towards the surface and tends to keep it attached to that surface.