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

Abstract

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...

Numerical study of active mixing over a dynamic flow field in a T-jets mixer—Induction of resonance

“…The control of vortex scales in a vortex street from the modulation of a stream was also observed by Wang (2003) and Wang (2006) using an apparatus that joined two parallel streams of water being one of them oscillated using a loud speaker according to a patented apparatus Fiedler and Wang (2002). In a widely used mixing geometry, such as the T-jets mixer, this is the first numerical work reporting a control of flow / mixing scales by means of the parameters of active mixing.…”

There can be turbulence in microfluidics at low Reynolds number

“…20,21 In the present work, we demonstrate that turbulence can be achieved in an electrokinetically forced pressure-driven flow in microchannels with Re on the order of 1.…”

Counter‐rotating vortex shedding generated by acoustic excitations in confined mixing layers

“…This is consistent with the observation in free mixing layers. 2,7 Therefore, at least in the region of the mixing chamber apart from the boundary layer of the side wall, the instability of the confined mixing layer is not different from that of the conventional free mixing layers. Note that no CRVs are observed in the nozzle section.…”

“…In particularly, it is discovered recently that ultrafast mixing can be realized in confined mixing layers under an optimal forcing frequency. 1,2 Investigations in this field have broad applications. 3 In the chemical industry, a slow mixing process can severely influence the quality of the final products.…”

“…Mixing enhancement, as one of the most important topics in fluid dynamics, has attracted increasing attention in the last 50 years. In particularly, it is discovered recently that ultrafast mixing can be realized in confined mixing layers under an optimal forcing frequency . Investigations in this field have broad applications .…”