I would like to derive a relation for shear stress on a conical bore shaped fluid element. Essentially, I have flow going through a converging nozzle. I know variables in this include length of the nozzle, inlet and outlet diameters and pressures and nozzle angle. I've done a force balance on a cylindrical shaped fluid element. All I want to know is if I repeat the same procedure I used on that, how do I treat the shear forces that are acting on the angled part of the element? Is it simply taking the component acting along the same direction as the pressures?