The effects of excitation on the coherent and random motion in a plane wall jet

Zhou, Ming-De; Heine, C.; Wygnanski, I.

doi:10.1017/s0022112096001711

Cited by 45 publications

(51 citation statements)

References 12 publications

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“…This procedure was repeated iteratively until convergence was achieved. For a detailed description of this method, the reader is referred to Zhou, Heine & Wygnanski (1996) and references therein.…”

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confidence: 99%

On the impact of swirl on the growth of coherent structures

Oberleithner

Paschereit

Wygnanski³

2014

J. Fluid Mech.

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Spatial linear stability analysis is applied to the mean flow of a turbulent swirling jet at swirl intensities below the onset of vortex breakdown. The aim of this work is to predict the dominant coherent flow structure, their driving instabilities and how they are affected by swirl. At the nozzle exit, the swirling jet promotes shear instabilities and, less unstable, centrifugal instabilities. The latter stabilize shortly downstream of the nozzle, contributing very little to the formation of coherent structures. The shear mode remains unstable throughout generating coherent structures that scale with the axial shear-layer thickness. The most amplified mode in the nearfield is a co-winding double-helical mode rotating slowly in counter-direction to the swirl. This gives rise to the formation of slowly rotating and stationary large-scale coherent structures, which explains the asymmetries in the mean flows often encountered in swirling jet experiments. The co-winding single-helical mode at high rotation rate dominates the farfield of the swirling jet in replacement of the co-and counter-winding bending modes dominating the non-swirling jet. Moreover, swirl is found to significantly affect the streamwise phase velocity of the helical modes rendering this flow as highly dispersive and insensitive to intermodal interactions, which explains the absence of vortex pairing observed in previous investigations. The stability analysis is validated through hot-wire measurements of the flow excited at a single helical mode and of the flow perturbed by a time-and space-discrete pulse. The experimental results confirm the predicted mode selection and corresponding streamwise growth rates and phase velocities.

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confidence: 99%

On the impact of swirl on the growth of coherent structures

Oberleithner

Paschereit

Wygnanski³

2014

J. Fluid Mech.

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“…However, the phase jitter of the dominant structures in the present flows, was quite small. Thus the coherent energy and Reynolds stresses obtained from simple, phase-locked data and from the more complex, temporal pattern matching (Zhou et al 1996), were not materially different. Consequently only the conventional phase-locked and ensemble averaged results are presented.…”

Section: Introductionmentioning

confidence: 85%

“…We are fully aware that the coherent portion of the motion may be smeared-out by phase jitter and therefore poorly represented by a phase locked and ensemble averaged quantity (Zhou et al 1996). However, the phase jitter of the dominant structures in the present flows, was quite small.…”

Section: Introductionmentioning

confidence: 86%

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

Zhou¹,

Wygnanski²

1999

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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...

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“…However, it was noticed that in the outer layer, there existed very strong phase jitter which might smear out the coherent waves although the latter could be identified from the instantaneous waves. Thus, the temporal pattern matching technique (Zhou et al 1996) was applied to the same data under 75Hz forcing. The coherent amplitude distributions obtained in this manner are plotted in Fig.25b.…”

Section: J^uw^ujb-uiomentioning

confidence: 99%