The Response of a Mixing Layer Formed Between Parallel Streams in a Concomitant Excitation at Two Frequencies

Abstract

1Simultaneous excitation of a turbulent mixing layer by two frequencies, a fundamental and a subharmonic, was investigated experimentally. Plane perturbations were introduced to the flow at its origin by a small oscillating flap. The results describe two experiments that differ only in the amplitudes of the imposed perturbations and both are compared to the data acquired while the mixing layer was forced at a single frequency.Conventional statistical quantities such as: mean velocity profiles, widths of the flow, turbulent intensities, spectra, phase-locked velocity and vorticity fields, as well as streak-lines were computed. The rate of spread of the flow under concomitant excitation at the two frequencies was much greater than under a single frequency although it remained dominated by two-dimensional eddies. The Reynolds stresses and turbulence production are associated with the deformation and orientation of the large coherent vortices. When the major axis of the coherent vortices starts leaning forward on the high velocity side of the flow, the production of turbulent energy changes sign (i.e. becomes negative) and it results in thinning the flow in the direction of streaming. It also indicates that energy is extracted from the turbulence to the mean motion Resonance phenomena play an important role in the evolution of the flow. A vorticity budget showed that the change in mean vorticity was mainly caused by the nonlinear interaction between coherent voracities. Nevertheless the locally dominant frequency scales the mean growth rate, the inclination and distortion of the mean velocity profiles as well as the phaselocked vorticity contours. 14, SIMItCT TtAMSSimultaneous excitation of a turbulent mixing layer by two frequencies, a fundamental and a subharmonic, was investigated experimentally. Plane perturbations were introduced to the flow at its origin by a small oscillating flap. The results describe two experiments that differ only in the amplitudes of the imposed perturbations and both are compared to the data acquired while the mixing layer was forced at a single frequency.Conventional statistical quantities such as: mean velocity profiles, widths of the flow, turbulent intensities, spectra, phase-locked velocity and voracity fields, as well as streak-lines were computed. The rate of spread of the flow under concomitant excitation at the two frequencies was much greater than under a single frequency although it remained dominated by two-dimensional eddies. The Reynolds stresses and turbulence production are associated with the deformation and orientation of the large coherent vortices. When the major axis of the coherent vortices starts leaning forward on the high velocity side of the flow, the production of turbulent energy changes sign (i.e. becomes negative) and it results in thinning the flow in the direction of streaming. It also indicates that energy is extracted from the turbulence to the mean motion. Resonance phenomena play an important role in the evolution of the flow. A vorticity bu...

Nonlinear combustion instability analysis of a bluff body burner based on the flame describing function

“…The external excitation of the shear layers and control of mixing had been investigated by various investigators, including the excitation of shear layers with two frequencies of which one of them was fundamental and the other was subharmonic of the forcing frequency. 37 The results show that the forcing of two frequencies has a great influence on the formation of coherent vortex formation. The use of second frequency forcing to actively control instability has also been found to be achievable.…”