The formation of double working surfaces in periodically variable jets

It is a well known result that a periodic ejection variability in a hypersonic jet results in the production of a train of internal working surfaces (one working surface produced by each period of the ejection variability) travelling down the jet beam. This mechanism has been successfully applied to model the knot structures of Herbig-Haro (HH) jets. In this paper we explore the possibility of producing more than one working surface with each ejection variability period. We derive the mathematical criteria that have to be satisfied by the functional form of an ejection velocity variability that produces double working surfaces, and study a family of functions with appropriate properties.