Physics of fluids (Woodbury, N.Y. : 1994)最新文献

{"title":"On the mechanism of elasto-inertial turbulence.","authors":"Yves Dubief,&nbsp;Vincent E Terrapon,&nbsp;Julio Soria","doi":"10.1063/1.4820142","DOIUrl":"https://doi.org/10.1063/1.4820142","url":null,"abstract":"<p><p>Elasto-inertial turbulence (EIT) is a new state of turbulence found in inertial flows with polymer additives. The dynamics of turbulence generated and controlled by such additives is investigated from the perspective of the coupling between polymer dynamics and flow structures. Direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 (based on the bulk and the channel height) are used to study the formation and dynamics of elastic instabilities and their effects on the flow. The flow topology of EIT is found to differ significantly from Newtonian wall-turbulence. Structures identified by positive (rotational flow topology) and negative (extensional/compressional flow topology) second invariant <i>Q</i>_<i>a</i> isosurfaces of the velocity gradient are cylindrical and aligned in the spanwise direction. Polymers are significantly stretched in sheet-like regions that extend in the streamwise direction with a small upward tilt. The <i>Q</i>_<i>a</i> cylindrical structures emerge from the sheets of high polymer extension, in a mechanism of energy transfer from the fluctuations of the polymer stress work to the turbulent kinetic energy. At subcritical Reynolds numbers, EIT is observed at modest Weissenberg number (<i>Wi</i>, ratio polymer relaxation time to viscous time scale). For supercritical Reynolds numbers, flows approach EIT at large <i>Wi</i>. EIT provides new insights on the nature of the asymptotic state of polymer drag reduction (maximum drag reduction), and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows observed in some polymeric flows.</p>","PeriodicalId":520752,"journal":{"name":"Physics of fluids (Woodbury, N.Y. : 1994)","volume":" ","pages":"110817"},"PeriodicalIF":4.6,"publicationDate":"2013-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1063/1.4820142","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"40276442","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Electro-worming: The behaviors of Caenorhabditis (C.) elegans in DC and AC electric fields.","authors":"Han-Sheng Chuang,&nbsp;David M Raizen,&nbsp;Nooreen Dabbish,&nbsp;Haim H Bau","doi":"10.1063/1.3640009","DOIUrl":"https://doi.org/10.1063/1.3640009","url":null,"abstract":"C. elegans is an attractive animal model for biomedical research and for studying animal locomotion. Electric fields offer a means to control nematodes’ motion and to apply remote, non-contact forces. We study the behavior of nematodes as a function of electric field intensity and frequency. The deliberate attraction of nematodes towards the negative pole of an electric field has been known for some time and dubbed electrotaxis.1 More recently, we have demonstrated that nematodes can be polarized with electric fields.2 \u0000 \u0000We have carried out our experiments with wild type C. elegans. The DC field experiments were conducted in microconduits fabricated with Polydimethylsiloxane (PDMS), whose ends were equipped with electrodes. The AC field experiments were carried out with a pair of spiked electrodes patterned on a glass slide [Fig. ​[Fig.1a1a]. \u0000 \u0000 \u0000 \u0000Figure 1 \u0000 \u0000(Color) The behaviors of C. elegans subjected to electric fields. (a) Schematic diagram of the dielectrophoretic (DEP) trap (top) and the simulated electric field (bottom). (b) (Top): An adult worm trapped by an electric field (40 kV/m, 1 MHz). (Bottom): ... \u0000 \u0000 \u0000 \u0000When a L2-stage or older nematode was introduced in a conduit and subjected to a low frequency, low intensity electric field, it swam deliberately toward the cathode [Fig. ​[Fig.1b,1b, bottom]. This electrotactic motion is neuron-mediated, and is not the result of any electrical forces. When the field frequency exceeds tens of kilo hertz, however, the nematode’s neural system is no longer able to respond to the alternations in the electric field polarity. \u0000 \u0000When subjected to sufficiently intense and non-uniform electric fields that overcome the nematode’s muscular power, the nematode polarizes, aligns with the field’s direction, and experiences a net force directed towards the point of maximum electric field intensity [Fig. ​[Fig.1b,1b, top]. A nematode is trapped between two spiked electrodes when its body length exceeds the gap width between the electrodes. When the gap width is longer than a worm’s body, however, only the worm’s tail is tethered and the worm’s head is free to move [Figs. ​[Figs.1d1d–1f]. \u0000 \u0000A conceptual phase diagram delineating nematode’s behavior as a function of frequency (ω) and intensity (E) was obtained [Fig. ​[Fig.1c].1c]. The range of frequencies and intensities that apparently do not harm the worms is identified. After release, worms that were trapped in this particular E-ω range function as normally as untrapped worms. To examine the effect of trapping on neural functions, we monitored the trajectories traced by a nematode’s head when illuminated with blue (480 nm) and green (545 nm) light. The nematode appeared to maintain its photophobicity [Fig. ​[Fig.1d].1d]. The tethered worm pumps and stirs the fluid around it [Figs. ​[Figs.1e,1e, ​,1f1f]. \u0000 \u0000Better understanding of the effect of electric fields on nematodes would enable us to sort nematodes by size and muscular power, to control the nematod","PeriodicalId":520752,"journal":{"name":"Physics of fluids (Woodbury, N.Y. : 1994)","volume":" ","pages":"91107-911071"},"PeriodicalIF":4.6,"publicationDate":"2011-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1063/1.3640009","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"40121174","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}