Control of an axisymmetric turbulent jet by multi-modal excitation

Experimental measurements of naturally occurring instability modes in the axisymmetric shear layer of high Reynolds number turbulent jet are presented. The region up to the end of the potential core was dominated by the axisymmetric mode. The azimuthal modes dominated only downstream of the potential core region. The energy content of the higher order modes (m is greater than 1) was significantly lower than that of the axisymmetric and m = + or - 1 modes. Under optimum conditions, two-frequency excitation (both at m = 0) was more effective than single frequency excitation (at m = 0) for jet spreading enhancement. An extended region of the jet was controlled by forcing combinations of both axisymmetric (m = 0) and helical modes (m = + or - 1). Higher spreading rates were obtained when multi-modal forcing was applied.