Some observations in a large two dimensional shear layer

Fiedler, H. E.; Thies, Helga

doi:10.1007/3-540-08765-6_9

Cited by 11 publications

(4 citation statements)

References 3 publications

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“…It seems at first that the narrow band of f 02 is similar to the frequency of an organ pipe (quasi‐excitation) observed by Fiedler and Thies 41. However, no unusual phenomena mentioned earlier were observed in that work.…”

Section: Discussionsupporting

confidence: 73%

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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Section: Discussionsupporting

confidence: 73%

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

Wang

2005

AIChE Journal

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“…Investigations by Fiedler & Thies [41], Birch & Eggers [42], and Hussain & Kleis [43] confirm that notion.…”

Section: Effects Of Boundary_conditions On Flow Developmentsupporting

confidence: 69%

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

Fiedler

Dziomba

Mensing

et al.

The Role of Coherent Structures in Modelling Turbulence and Mixing

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“…The forcing might be applied by a loudspeaker (Mensing 1981 ; Bechert 1982; Wygnanski, Oster & Fiedler 1979) or by a vibrating flap (Oster 1980) if done deliberately. But a vibrating splitter plate (Pui & Gartshore 1979), blower frequencies (Drubka & Nagib 1981) or wind-tunnel resonance frequencies (Fiedler & Thies 1978) are able to affect the flow in a similar, but more moderate, way. The shear layer reacts on this periodic disturbance only if the excited waves are close or identical to the natural wavelength in the flow.…”

Section: Discussionmentioning

confidence: 99%

“…Furthermore, Zaman & Hussain (1981) were able to suppress the shear-layer turbulence level by applying high-frequency forcing. Fiedler & Thies (1978) found that wind-tunnel frequencies may influence the shear-layer spreading behaviour, thus adding one more possible explanation for the reported discrepancies. One of the basic problems in experiments is the exact definition and identification of the self-similar region.…”

Section: B Dziomba and H E Fiedlermentioning

confidence: 95%

Effect of initial conditions on two-dimensional free shear layers

1985

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The influence of periodic perturbations on the development of two-dimensional free shear layers generated by a splitter plate was investigated in cases where the ratios of the two velocities u1 and u2 either side of the splitter plate were such that 0 < u1/u2 < 1. Investigations were carried out in both a suction and a blower wind tunnel. Results show that even very weak periodic perturbations caused by the wind tunnel may cause significant nonlinear spreading in the downstream development of the shear layer, a behaviour which is also observed when the shear layer is deliberately excited. Other things being equal, the effect of the disturbance is greater when flow separation at the splitter plate is turbulent than when it is laminar.No self-induced feedback frequencies were measured in the test section. All tonal components that were detected in the flow could be traced to external sources.The influence of trailing-edge thickness on the shear-layer development is found to become significant when it exceeds 50% of the sum of boundary-layer displacement thickness at the point of separation. As the trailing edge becomes thicker, the range over which the shear layer is self-similar is shifted farther downstream. This behaviour may be crucial for predicting the evolution of shear layers in high-speed flows having thin boundary layers at separation.The momentum thickness criterion for estimating the development length of the flow as suggested by Bradshaw is shown to be insufficient for two-stream layers, where additional parameters, e.g. the trailing-edge geometry, have to be taken into account. Discrepancies between previously published observations of shear layers, as well as the considerable scatter in reported measurements, may therefore, to a large extent, be attributable to contamination of the experimental facility.

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Some observations in a large two dimensional shear layer

Cited by 11 publications

References 3 publications

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

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

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

Effect of initial conditions on two-dimensional free shear layers

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