Initiation, evolution and global consequences of coherent structures in turbulent shear flows

Fiedler, H. E.; Dziomba, B.; Mensing, P.; Rösgen, Thomas

doi:10.1007/3-540-10289-2_29

Cited by 50 publications

(16 citation statements)

References 26 publications

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“…Although not decoupled from the direct interactions between coherent modes and fine-grained turbulence, the production of the fine-grained turbulence by the mean motion' appear both experimentally (e.g., Fiedler, et al 1981) and theoretically (e.g., Liu and Merkine 1976, Alper and Liu 1978, Mankbadi and Liu 1981 to be devoid of the large-scale amplification and negative production as was found for the coherent modes. Consequently, the turbulence energy, excluding the coherent-mode contributions, appears to be developing, if at all, monotonically even in the nonequilibrium region of coherent mode/turbulence/mean flow interactions.…”

Section: Componentmentioning

confidence: 84%

“….-.~-. Fiedler, et al (1981) also showed that the shear layer spreading rate is altered by the enhanced coherent mode. However, we shall ilustrate the similar observed effect of coherent-mode development through the use of results from Ho and Huang (1982).…”

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 91%

“…Although the shear layer in Ho and Huang (1982) (Fiedler, et al 1981, Weisbrot 1984, Fiedler and Mensing 1985; see also Wygnanski and Petersen 1985). The coresponding coherent-mode energy measured by Ho and Huang (1982) is shown in Figure 4, where E(f) is the kinetic energy due to the streamwise velocity fluctuation associated with each of the frequencies, integrated across the shear layer.…”

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 99%

“…In the spatial problem, the coherent-mode amplitudes To illustrate the coherent mode energy production (and destruction) mechanism through its interaction with the mean flow, we show in Figure 2 the measurements of Fiedler, et al (1981) of this mechanism along the line of most intense mean velocity gradient in a controlled, one-sided turbulent mixing layer. Here w is the vertical velocity and z the vertical coordinate, Ua> is the free stream velocity.…”

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 99%

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Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

Liu

1988

Advances in Applied Mechanics

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Section: Componentmentioning

confidence: 84%

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 91%

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 99%

Section: Ill Some Aspecrs Of Quantitative Observationsmentioning

confidence: 99%

See 2 more Smart Citations

Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

Liu

1988

Advances in Applied Mechanics

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“…When it was realized that the coherent structures play a central role in the evolution of the mixing layer, artificial excitation was soon to follow (Oster et al 1978;Oster & Wygnanski 1982;Ho & Huang 1982;Gaster et al 1985;Fiedler & Mensing 1985). Fiedler et al (1981), Oster et al (1982) and Monkewitz and Huerre (1982) used the parallel, linear stability analysis to predict the most amplified frequencies and the amplification rates of the large eddies in the externally excited turbulent mixing layer while others used modeling and numerical simulation to obtain similar results (Patnaik et al 1976;Acton 1976;Ashurst 1979;Riley & Metcalfe 1980, Corcos & Sherman 1984, Inoue & Leonard 1987Inoue 1989). Gaster et al ( 1985) have demonstrated the significance of the flow divergence on the evolution of the large coherent structures and the ability of the stability approach to predict it in great detail.…”

Section: Introductionmentioning

confidence: 99%

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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On large structures and turbulent mixing in confined mixing layers under forcing

Wang

2005

AIChE Journal

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IntroductionFluid mixing is of surpassing interest in science and engineering applications. [1][2][3][4] In many cases, we need fast mixing, such as in combustion, 5 acoustic noise reduction, 6 and protein folding in biotechnology. 7 Traditionally, agitated tank and static mixer, 8,9 jet, and mixing layer 10,11 are often used to enhance mixing. Other mixing augmentation methods include chaotic mixing at relatively low Reynolds number flows 12,13 and an oscillatory flow mixer consisting of tubes fitted with equally spaced orifice plate baffles. 14 Recently, Wang 15,16 introduced a novel extremely fast mixing process in a confined three-dimensional mixing layer in a pipe (that is, the inlet of the pipe is a mixing layer), where the initial two streams can be mixed, at least on the large scale, within one pipe diameter downstream of the splitterplate. The rapid mixing can also be obtained at relatively small Reynolds number flow in this confined configuration, where the Reynolds number is 400 based on the mean velocity and pipe diameter and the original flow is laminar without forcing. For this relatively small Reynolds number flow, the scalar power spectrum can display Ϫ5/3 Oboukhov-Corrsin spectrum in the near field under strong forcing. This could have a substantial impact in technologies that require fast mixing, both in laminar and turbulent flows. Such an interesting flow could also be a new candidate for homogeneous turbulence Nygaard and Glezer,28 Wiltse and Glezer, 29 and many others. An important common conclusion from these works is that the spreading rate, which is one of the most important fundamental parameters and is often used as a mixing criterion in the mixing layer (see, for example, Cantwell 30 and Dimotakis 31 ), can be enhanced through the active forcing.Unfortunately the effect of conventional initial periodic forcing in mixing layers for mixing enhancement is limited by the saturation phenomenon arising from the nonlinear effect. 24,25 For instance, Fiedler and Mensing 27 observed the saturation when the forcing amplitude was sufficiently high, that is, the intensity of the maximum periodic transverse velocity constituents and saturation Strouhal number became constant and independent of forcing amplitude when the forcing amplitude was Ͼ6.5%. They obtained the same mean spreading rate for both strong and weak forcing, that is, twice the neutral value and independent of forcing amplitude. Weisbrot and Wygnanski 32 also investigated strong forcing and reported no distinguishing difference in the spreading rate from that of Fiedler et al. 33 and Oster and Wygnanski. 25 On the foundation of the above-mentioned observations and that of others, Browand and Ho 34 proposed a quasi-universal spreading rate of the forced and unforced mixing layers.In the confined mixing layer, however, Wang 15,16 surprisingly found that the aforementioned saturation limitation of the spreading rate of the shear layer could be overcome when the flow was forced under a specific narrow-frequency band. Thus, the mixing ca...

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Initiation, evolution and global consequences of coherent structures in turbulent shear flows

Cited by 50 publications

References 26 publications

Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

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

On large structures and turbulent mixing in confined mixing layers under forcing

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