It is for a range hood ventDescribe the duct to us, or post a picture of it.
Ducts are whatever shape they need to be to move the air at the proper velocity and pressure drop. The weird shapes are often because that's what fits or what was easy to install or that's what sizes are in stock.Aren't range hoods actually tubular ducting when they pass thru a wall or ceiling? And the retangular shape is just for show.