An explanation of the instability of the free vortex cores occurring over delta winds with raised edges

By rolling up the surfaces of discontinuity originating from the leading edge of delta wings, free vortex cores are formed above the wing. In case of greater angles of incidence, the flow in these vortex cores shows an instability which abruptly produces strong turbulence. In the present paper an explanation is given of this instability being a "frictionless instability" of the vortex core flow by increasing helical interference vortices. The occurring vortex core flows are calculated and investigated for stability by means of a stability criterion concerning flows with helical streamlines given by H. Ludwieg.