TURBULENCE SUPPRESSION IN A JET BY MEANS OF A FINE RING

Studied are the effects on the velocity field of a fine circular wire ring placed axisymmerically downstream of an air jet of Reynolds numbe r 1.4 X 10(4). When placed close to the jet exit (x/D(j) < 3 where D(j ) is the jet diameter) the ring caused a pronounced reduction in the r ms longitudinal velocity fluctuations, the maximum reduction (of appro ximately 30% based on the peak value) occurring when the ring is very close to the jet (x/D(j) = 0.07) and in the shear layer although rings of smaller diameter located in the core also produced the same qualit ative effect. The ring also caused a shift in the virtual origin of th e mean flow and an increase of the momentum thickness of the shear lay er. There resulted a reduction of the magnitude of the fundamental rol l-up frequency, and it saturated later than for the undisturbed flow. The spatial growth rate of the disturbances in the shear layer were al so drastically reduced. Spectra showed that vortex pairing was inhibit ed by the ring and there was an earlier transition to fully developed turbulence. These effects are attributed to the ring causing a change in the form of the mean velocity profile in the shear layer. The effec ts of vortex shedding by the ring were discounted by showing that the results did not qualitatively change when the ring wire Reynolds numbe r, Re(w), was varied from sub to super critical values (30 < Re(w) < 1 20). These findings are compared with previous results, using acoustic excitation to reduce the turbulence intensity in a jet, with which th ey have much in common.