"Axial Fan" calculate pi differentials Vu, or known β1, β2 angle, you can import and export speed impeller to draw a triangle, since the impeller Importers have the same peripheral speed and shaft surface speed, so to study the issue of convenience, often the inlet and outlet velocity triangle painted together. Because Cascades fluid flow around an airfoil and flow around a single view of different type, Cascades will affect the size and direction of flow velocity, so as to derive formulas and demonstration of simplicity, it is desirable around Cascades relative velocity w1 and w2, geometric mean w∞ as infinity (undisturbed fluid) to flow velocity.