Stability and control of very low density axisymmetric jets

Jets with sufficiently low densities relative to the ambient environment can support a global instability. The current work experimentally investigates the character of globally unstable low density jets and subsequently employs microjets around the jet periphery in an effort to inhibit their formation. Global instabilities were suppressed for microjet mass flow rates of approximately two percent that of the primary stream for low density (S=0.14) jets. Very low density (S=0.03) axisyrrametric jets required four percent mass flow to suppress the pure tone oscillations and three percent to extend the potential core. Low density and very low density jets were established by introducing a helium jet into ambient fluids of air and sulfur hexafluoride, respectively.