REVERSAL IN SPREADING OF A TABBED CIRCULAR JET UNDER CONTROLLED EXCITATION

Detailed flow field measurements have been carried out for a turbulent circular jet perturbed by tabs and artificial excitation. Two ''delta tabs'' were placed at the nozzle exit at diametrically opposite locat ions. The excitation condition involved subharmonic resonance that man ifested in a periodic vortex pairing in the near flow field. While the excitation and the tabs independently increased jet spreading, a comb ination of the two diminished the effect. The jet spreading was most p ronounced with the tabs but was reduced when excitation was applied to the tabbed jet. The tabs generated streamwise vortex pairs that cause d a lateral spreading of the jet in a direction perpendicular to the p lane containing the tabs. The excitation, on the other hand, organized the azimuthal vorticity into coherent ring structures whose evolution and pairing also increased entrainment by the jet. In the tabbed case , the excitation produced coherent azimuthal structures that were dist orted and asymmetric in shape. The self-induction of these structures produced an effect that opposed the tendency for the lateral spreading of the streamwise vortex pairs. The passage of the distorted vortices , and their pairing, also had a cancellation effect on the time-averag ed streamwise vorticity field. These led to the reduction in jet sprea ding. [S1070-6631(97)03012-2].