Space-time modulation of turbulence in co-flow jets

This paper focuses on the effects of a space-time dependent periodic stirring of a moderately turbulent planar co-flow jet configuration. The baseline flow is agitated in time and in space by small-scale turbulent perturbations in combination with large-scale modulation imposed at the inflow plane of a rectangular domain of size L x L x 2L in the x, y and z directions respectively. The prescribed large-scale modulation is characterized by a single modulation frequency omega and modulation wave-number, K. A parametric study at different modulation frequencies and wave-numbers is performed. We evaluate the system response to the external agitation in terms of key dynamic properties of the flow, e.g., the total kinetic energy E-T, the global averaged dissipation rate and additional flow mixing properties. For low modulation frequencies, e.g., omega = 0.5 omega(0), where omega(0) is the large scale-turn over frequency, omega(0) = U-1/D, with U-1 and D being relevant velocity and length scales, and at given wave-number K, we observe that E-T follows the imposed oscillation with a periodic amplitude response that is sustained at locations further from the inflow plane, whereas for higher frequencies, the response amplitude rapidly decays. Results of the global dissipation rate show the development of a definite maximum value of the response amplitude at frequencies on the order of omega(0) for any modulation wave-number K. To investigate in more detail the effects of modulated turbulence on the jet mixing properties, a passive scalar was injected at the inflow plane. The spreading of the scalar surface in the agitated jet was monitored for a wide range of modulation frequencies. In general, results show enhanced mixing efficiency when the main jet is modulated at frequencies near omega(0) and low K values.