Control of an axisymmetric jet using vortex generators

Abstract

The results of an experimental investigation on the effect of vortex generators, in the form of small tabs at the nozzle exit on the evolution of a jet, are reported in this paper. Primarily tabs of triangular shape are considered, and the effect is studied up to a jet Mach number of 1.8. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices having a sense of rotation opposite to that expected from the wrapping of the boundary layer. This results in an inward indentation of the mixing layer into the core of the jet. A triangular-shaped tab with its apex leaning downstream, referred to as a delta tab, is found to be the most effective in producing such vortices, with a consequential large influence on the overall jet evolution. Two delta tabs, spaced 180° apart, completely bifurcate the jet. Four delta tabs stretch the mixing layer into four ‘‘fingers,’’ resulting in a significant increase in the jet mixing downstream. For six delta tabs the mixing layer distortion settles back to a three finger configuration through an interaction of the streamwise vortices. The tabs are found to be equally effective in jets with turbulent or laminar initial boundary layers. Two sources of streamwise vorticity are postulated for the flow under consideration. One is the upstream ‘‘pressure hill,’’ generated by the tab, which constitutes the main contributor of vorticity to the dominant pair. Another is due to vortex filaments shed from the sides of the tab and reoriented downstream by the mean shear of the mixing layer. Depending on the orientation of the tab, the latter source can produce a vortex pair having a sense of rotation opposite to that of the dominant pair. In the case of the delta tab, vorticity from the two sources add, explaining the strong effect in that configuration.

Numerical simulation of mixing enhancement in a hot supersonic jet

“…The sense of rotation of the \necklace vortex" pair is such that, between the vortex pair uid is moved from the jet into the co ow while for the \trailing vortex" type, between the vortex pair uid is moved from the co ow i n to the jet. In a later study, Zaman, et al [8] were able to show, via ow visualization, that tabs produced di erent o ws depending on whether the apex of the delta tab pointed upstream or downstream. By way of comparison we note that Grinstein, et al [10] use the term \delta-wing vortex" instead of \trailing vortex" and \mushroom vortex" for \necklace vortex".…”

“…Possible physical mechanisms by which tabs produce streamwise vortices has been discussed by Samimy, Zaman and Reeder [5] ; Zaman [6] ; and Zaman, Reeder and Samimy [8] . One possible mechanism is the stripping and rolling up of the boundary layer by the tab.…”

“…In particular, Samimy, et al [5] and Zaman, et al [8] give o w visualizations of the jets with and without tabs. These visualizations are laser sheet illuminated cross sections of the jet.…”

“…In the experiments of Samimy, e t a l . [ 5 ] and Zaman, et al [8] some of the con gurations were, in their words, \nominally laminar" and others \nominally turbulent". In both cases they found that the experimental results were virtually identical.…”

“…Often this is accomplished through the use of a lobed mixer with ejector (Presz [2] ; Tillman, Patrick and Peterson [3] ). Other methods utilize tab-like devices to generate a pair of counter-rotating axial vortices to enhance stream mixing (Ahuja and Brown [4] ; Samimy, Zaman and Reeder [5] ; Zaman [6] ; A h uja [7] ; Zaman, Reeder and Samimy [8] ). Although it has been known for a number of years that tabs can have a substantial e ect on jets [9] , many of the details and the physical mechanisms involved are unclear.…”