Experimental study of formation and development of coherent vortical structures in pulsed turbulent impinging jet

• The effects of flow pulsation on formation of coherent structures are investigated in turbulent confined impinging jet.
• The effects of key parameters such as pulsation frequency and signal shape on behavior of vortex structures studied.
• Significant differences in flow pattern and size of the vortical structures between pulsed and steady jet are observed.
• The increase of frequency, make the vortices more autonomous and organized.
• The size of vortical structures in step type of excitation is larger than those in sinusoidal one.

Coherent structures play a crucial role in enhancement of convective transport phenomena. An experimental investigation was carried out to study the effect of jet flow oscillation on the characteristics of vortex structures in a turbulent confined circular impinging jet. Effects of key parameters such as frequency of pulsation and type of excitation on the development of shear layer coherent vortical structures in time and space and their dynamics were studied using the smoke-wire technique and high speed photography. Results of this study show that flow pulsation in a circular impinging jet leads to the periodic formation of coherent vortical structures which are larger than those in steady impinging jets. Pulsation frequency has significant effects on the formation and size of structures and consequently their dynamics. At higher frequencies, vortex structures are formed in a regular inline pattern and as a result the mixing between the jet fluid and stagnant fluid is reduced. For the step (rectangular) signal shape larger vortices are created in comparison with those in sinusoidal one. The results of flow visualization show that enhanced mixing and transport rates will be achieved for turbulent impinging jets that excited with “step” signal shape.