New features of swirling jets

Important new features are found for a family of swirling jets with velocit y v similar to z(-n), where z is the distance from the jet origin. First, t here is a sharp minimum of the pressure coefficient at a certain value of t he swirl number Sw which is nearly n independent; this feature can be utili zed in technological devices. Second, as Sw increases, a separation zone de velops, where the fluid is not at rest in the inviscid limit (contrary to t he claims of recent vortex breakdown theories). These results are obtained under the boundary layer approximation for incompressible jets characterize d by n and Sw=v(phi m)/v(zm), where v(phi m) and v(zm) are the maximal valu es of the swirl and longitudinal velocities at z=const. Unlike prior result s viewed in terms of parameter L (which is the v(phi)/v(z) ratio at the out er edge of the jet), the solution dependence on Sw is found similar for bot h n < 1 and n > 1. For any n, (a) the pressure coefficient is minimum at Sw approximate to 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. [S1070-6631(00)01311-8].