On the development of a turbulent jet subjected to aerodynamic excitation in the helical mode

• Active jet flow control with aerodynamic excitations.
• Laser Doppler anemometry measurements and stereoscopic flow visualisation.
• Two excitation modes and their control mechanism compared.
• Helical excitation modes improve dramatically the flow mixing at the near field.
• Flatness and skewness analysis provides insight information on the jet flow.

Aerodynamic excitation has been used to introduce energy into the free shear layer at the outer edges of an axisymmetric free jet by injecting a pulsating secondary jet through a thin annular nozzle that surrounds and is concentric with the primary nozzle. In the present study, it has been found that the rate of mixing can be increased if the pulsations are applied in a sequential fashion around the circumference of the jet, so as to create helical vortices in the region close to the nozzle exit plane. Flow visualisation confirms the helical nature of the vortices and shows how these vortices are convected downstream with the main flow, strongly influencing the development of the primary jet. Two-component laser Doppler anemometry (LDA) was used to make measurements of the flow distributions in the developing jet for a number of flow conditions. These results have been analysed to extract various measures of the turbulent mixing, including the time averaged velocity distributions, and the higher order moments and spectral distributions of the turbulent velocity fluctuations. The present data confirm that introducing the excitation in a helical mode leads to an increased rate of entrainment and more rapid mixing, when compared with the toroidal excitation used in previous studies. These changes significantly increase the rate of development of the jet in the axial direction.