An investigation of the effects of discrete wing tip jets on wake vortex roll up

A water tunnel experiment and a computational flow field model have shown that discrete wing tip jets can strongly affect the roll up of the wing tip vortex and apparently decrease its rolled i:p strength at moderate levels of blowing. The key factor in vortex alleviation was the extent of the local flow interactions between the discrete jets and the developing wing tip vortex. Vortex trajectory in both the spanwise and vertical directions was influenced by the jets. An outboard shift of the wing tip vortex indicated that discrete wing tip jets may be able to produce improved wing aerodynamics during cruise flight. The counterrotating pair of vortices generated by a jet in a cross flow were clearly seen in the water tunnel and appeared to be very effective in reducing the intensity of the wing vortex system. Two types of periodic secondary vortices were also observed in the water tunnel for heavy jet blowing. These were "spin-off vortices" which periodically developed in the rolling up tip vortex but rapidly spun outboard and above the wing and "entrained vortices" which was a set of periodic vortices laterally connecting the wing tip vortex to the vortices embedded in the jet. These secondary vortices are oriented such that they will greatly accelerate the spreading of wake vorticity through the vortex stretching term of the Helmholtz equation. This influence was confirmed in the water tunnel tests. Guidelines for jet configuration design and distribution of jet blowing were developed based on comparison of observed vortex strengths.