Abstract: Fluid flow diffuser has a first divergent duct connected by a sudden enlargement of flow area to a second divergent duct. The enlargement is defined by a fence situated downstream of the downstream end of the first duct. The top of the fence is on a flow perimeter intermediate between the downstream end of the first duct and the upstream end of the second duct, the latter end being defined by the bottom of the fence. An opening defined by the downstream end of the first duct and by the top of the fence gives access to a chamber connected to a pressure less than that at latter downstream end thereby to generate a vortex having a surface bridging the opening and assisting transition of flow from the first duct to the top of the fence. A second vortex formed immediately downstream of the fence assists transition of flow from the top of the fence to the surface of the second duct. The first duct has a rate of divergence low enough to avoid boundary separation.