On the structure of vortex breakdown on a delta wing

Abstract

This paper presents a detailed description of the vortex breakdown phenomenon on a delta wing. While there have been a number of earlier studies which have given us a broad, overall picture of these interesting and important flow fields, none has so far yielded the kind of fine structure considered here, in spite of using very fine grids. The present work has been made possible by the use of a new type of grid, called the embedded conical grid, which exploits the special nature of the flow field. Based on the Euler equations, the flow field past a 70 degrees sharp–edged delta wing in a subsonic free–stream is resolved here using this grid. The breakdown is found to begin as a rapid deceleration of the inner core due to a sudden increase in the adverse axial pressure gradient. This leads to a sudden broadening of the core and the formation of a deficit velocity region which becomes intensified causing axial flow reversal. The near–axis vorticity lines get tilted away from the vortex axis and their spiralling direction is reversed. The effect of artificial dissipation and grid coarseness on the computed vortex breakdown has been examined. The results presented here, though obtained for the delta wing geometry, are expected to add to our understanding of vortex breakdown in other flow situations as well.