New features of swirling jets

Important new features are found for a family of swirling jets with velocity v~z(-n), where z is the distance from the jet origin. First, there is a sharp minimum of the pressure coefficient at a certain value of the swirl number Sw which is nearly n independent; this feature can be utilized in technological devices. Second, as Sw increases, a separation zone develops, where the fluid is not at rest in the inviscid limit (contrary to the claims of recent vortex breakdown theories). These results are obtained under the boundary layer approximation for incompressible jets characterized by n and Sw = v(φm)/v(zm), where v(φm) and v(zm) are the maximal values of the swirl and longitudinal velocities at z = const. Unlike prior results viewed in terms of parameter L (which is the v(φ)/v(z) ratio at the outer edge of the jet), the solution dependence on Sw is found similar for both n < 1 and n > 1. For any n, (a) the pressure coefficient is minimum at Sw ~ 0.65; (b) two solutions exist for Sw <Sw(f) (fold value), none for Sw > Sw(f); (c) as Sw decreases, the jets either consolidate near the axis or separate from it, depending on the solution branch; and (d) the flow in the separation zone tends to become swirl-free and potential. (C) 2000 American Institute of Physics.