Structure to reduce turning losses in angled conduit

A duct for carrying a fluid in a downstream direction, said duct having an internal surface defining an internal flow passage including a corner portion for turning the flow, said corner portion having an upstream end and a downstream end, said internal surface of said corner portion defining an inner corner surface of said flow passage, said inner corner surface including means to generate large scale vortices and produce a flow variation which disrupts the eddy flow in the separation region to reduce turning losses, said means comprising a plurality of downstream extending, adjoining alternating troughs and ridges, wherein said troughs and ridges extend from said upstream end of said corner portion to said downstream end of said corner portion, forming convolutions in said inner corner surface, said troughs and ridges increasing in depth and height, respectively, from said upstream end to a maximum depth and height and decreasing from said maximum depth and height to zero depth and height at said downstream end.