Direct numerical simulation of forced turbulent round jet: Effect of flow confinement and varicose excitation

Direct numerical simulation of a turbulent round jet subjected to varicose excitation has been carried out. The effect of domain size and waveform used for providing varicose excitation have been studied with the help of time-averaged mean, fluctuating quantities, and instantaneous isosurfaces of the Q-criterion. Initial evolution of the jet suggests that the secondary instability is delayed in time with an increase in the domain size irrespective of the waveform. It has also been observed that the secondary instability manifests stronger for the square wave based excitation as compared to sinusoidal excitation for the smaller and medium domains. In addition, simulations demonstrate that the aforementioned secondary instability is sustained in the long term for small and medium domains. In the case of a confined domain, simulations indicate that square wave based excitation leads to greater enhancement in mixing and entrainment characteristics of the jet when compared to sinusoidal excitation. We demonstrate that sine pulsing at the inlet excites energy up to the second harmonic of the preferred mode while square pulsing excites energy (at least) up to the fifth harmonic which results in more energetic small-scales structures in the far field which in turn augment the mixing characteristics of jet. Qualitative assessment of vortical structures indicates that differently excited jets gradually become similar in the far field of large domains owing to the availability of sufficient amounts of fluid for entrainment. This behavior has also been quantitatively established by means of axial and lateral profiles of both time-averaged as well as fluctuating quantities characterizing the pulsed jet.