Jet characteristics are most receptive to changes in initial conditions, which play a dominant role in the development of both low [1}3] and high-speed [4] jets. From the literature it is seen that jets from non-circular geometries [1}3, 5, 6] result in enhanced mixing, a characteristic making them far superior to the conventional circular jet. This is attributed primarily to the axis-switching phenomenon caused by the non-uniform self-induction process predominant in the development of these jets [6]. Further, improvement of jet #ow "eld in non-circular jets was achieved by Schadow et al. [1] in triangular and square nozzle jets wherein the small-scale eddies shed from the corners interact strongly with the large-scale vortices shed from the plain sides. Some similar observations have been made for rectangular-slot jets [3,7].
Flows from modi"ed circular/axisymmetric nozzles have been studied by a number of investigators. Gross distortions in the jet structure have been observed using intrusive means in circular jets, e.g., tabs both in subsonic [8] and supersonic regimes [9] which introduce gross distortions in the jet #ow "eld and bring about increase in entrainment. In supersonic jets, tabs help to weaken the shock structure considerably and, hence, reduce the overall jet noise [10}12]. Non-intrusive means were also explored by Pannu and Johanneson [13] using triangular notches in a conical circular nozzle. The study revealed a much higher spread in the notched plane with the shedding of vortices from the edges of the notches. This vortical structure was found by them to act as a shielding device to the shock structure resulting in noise reduction. Norum [11] and McDaniel et al. [14] used "ngers/slots in the nozzle parallel to the jet axis. The pressure relieving e!ect of the "ngers/slots greatly weakens the shock structure and hence, reduces shock-associated noise.
The present study aims at understanding the changes introduced by the presence of notches in a circular-slot jet for the purpose of mixing enhancement and noise reduction. The source of noise has been reported by Hammitt [15], Glass [16] and McDaniel et al. [14] to be located approximately at the end of the third shock cell, and as such relieving the excess pressure through notches may help weaken the