Non-circular Pipes
If the cross section of a pipe is not circular, then all previous relations can not be used directly. A new parameter, hydraulic diameter, is defined to correlate the flow in a non-circular pipes to that of a circular pipe flow. This correlation can be applied to many different pipe configurations as long as their cross sectional shapes are not too exaggerated. For example, width to height ratio is less than 4 for a rectangular pipe. The hydraulic diameter is defined as: