Merging of Aircraft Trailing Vortices

A merging distance criterion for equal-strength corotational vortices is derived from low-turbulence wind tunnel flow visualization data. The vortex separation distance is normalized by defining a vortex core diameter based on circulation defect and angular momentum defect. Merging may take place for larger separation distances than predicted from earlier two-dimensional inviscid calculations, which indicates that viscosity and possibly three-dimensional effects are important factors in the merging phenomenon. Hot-wire velocity distributions and rolling moment measurements show that attenuation of the vortex hazard is associated with vortex merging. Nomenclature bg = generator wingspan bf = follower wingspan CL = lift coefficient cf = rolling-moment coefficient c = wing chord d = vortex separation distance d0 = initial vortex separation distance dc = vortex core diameter (usually based on peak tangential speed) dM = vortex core diameter (based on angular momentum defect equal to that in a Rankine vortex) dr = vortex core diameter (based on circulation defect equal to that in a Rankine vortex) dv —vortex core diameter (based on outer boundary of.the rotational portion of the vortex)