Effect of tabs on the flow and noise field of an axisymmetric jet

Abstract

The effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric jet is investigated experimentally over the jet Mach number range of 0.3-1.81. The tabs eliminate screech noise from supersonic jets and alter the shock structure drastically. They distort the jet cross section and increase the jet spread rate significantly. The distortion produced is essentially the same at subsonic and underexpanded supersonic conditions. Thus, the underlying mechanism must be independent of compressibility effects. A tab with a height as small as 2% of the jet diameter, but larger than the efflux boundary-layer thickness, is found to produce a significant effect. Flow visualization reveals that each tab introduces an "indentation" into the high speed side of the shear layer via the action of streamwise vortices. These vortices are inferred to be of the "trailing vortex" type rather than of the "necklace vortex" type. It is apparent that a substantial pressure differential must exist between the upstream and the downstream sides of the tab to effectively produce these trailing vortices. This explains why the tabs are ineffective in the overexpanded flow, as in that case an adverse pressure gradient exists near the nozzle exit which reduces the pressure differential produced by the tab.

Numerical simulation of mixing enhancement in a hot supersonic jet

“…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 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.…”

“…Either mechanism could produce streamwise vortices. For either mechanism, the direction of rotation of the vortex pair could be controlled by the orientation of the tabs [5] . Based on their experimental observations, Samimy, e t a l .…”

“…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.…”

Joule heating flow control methods for high-speed flows

“…The first was the generation of streamwise vorticity in a similar manner to that of micro ramps 63 or tabs. 64 The second is the manipulation of the unsteadiness apparent in SBLI using the broad frequency range capability of LAFPA's. Dupont et al 65 showed that the upstream leg of a lambda shock wave oscillates at a Strouhal number of 0.03 while the downstream leg oscillates at a Strouhal number of 0.5.…”