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Nakagawa, Kosuke, Takahiro Tsukahara i Takahiro Ishida. "DNS Study on Turbulent Transition Induced by an Interaction between Freestream Turbulence and Cylindrical Roughness in Swept Flat-Plate Boundary Layer". Aerospace 10, nr 2 (30.01.2023): 128. http://dx.doi.org/10.3390/aerospace10020128.
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Laminar-to-turbulent transition in a swept flat-plate boundary layer is caused by the breakdown of the crossflow vortex via high-frequency secondary instability and is promoted by the wall-surface roughness and the freestream turbulence (FST). Although the FST is characterized by its intensity and wavelength, it is not clear how the wavelength affects turbulent transitions and interacts with the roughness-induced transition. The wavelength of the FST depends on the wind tunnel or in-flight conditions, and its arbitrary control is practically difficult in experiments. By means of direct numerical simulation, we performed a parametric study on the interaction between the roughness-induced disturbance and FST in the Falkner–Skan–Cooke boundary layer. One of our aims is to determine the critical roughness height and its dependence on the turbulent intensity and peak wavelength of FST. We found a suppression and promotion in the transition process as a result of the interaction. In particular, the immediate transition behind the roughness was delayed by the long-wavelength FST, where the presence of FST suppressed the high-frequency disturbance emanating from the roughness edge. Even below the criticality, the short-wavelength FST promoted a secondary instability in the form of the hairpin vortex and triggered an early transition before the crossflow-vortex breakdown with the finger vortex. Thresholds for the FST wavelengths that promote or suppress the early transition were also discussed to provide a practically important indicator in the prediction and control of turbulent transitions due to FST and/or roughness on the swept wing.
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He, S., i M. Seddighi. "Turbulence in transient channel flow". Journal of Fluid Mechanics 715 (9.01.2013): 60–102. http://dx.doi.org/10.1017/jfm.2012.498.
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AbstractDirect numerical simulations (DNS) are performed of a transient channel flow following a rapid increase of flow rate from an initially turbulent flow. It is shown that a low-Reynolds-number turbulent flow can undergo a process of transition that resembles the laminar–turbulent transition. In response to the rapid increase of flow rate, the flow does not progressively evolve from the initial turbulent structure to a new one, but undergoes a process involving three distinct phases (pre-transition, transition and fully turbulent) that are equivalent to the three regions of the boundary layer bypass transition, namely, the buffeted laminar flow, the intermittent flow and the fully turbulent flow regions. This transient channel flow represents an alternative bypass transition scenario to the free-stream-turbulence (FST) induced transition, whereby the initial flow serving as the disturbance is a low-Reynolds-number turbulent wall shear flow with pre-existing streaky structures. The flow nevertheless undergoes a ‘receptivity’ process during which the initial structures are modulated by a time-developing boundary layer, forming streaks of apparently specific favourable spacing (of about double the new boundary layer thickness) which are elongated streamwise during the pre-transitional period. The structures are stable and the flow is laminar-like initially; but later in the transitional phase, localized turbulent spots are generated which grow spatially, merge with each other and eventually occupy the entire wall surfaces when the flow becomes fully turbulent. It appears that the presence of the initial turbulent structures does not promote early transition when compared with boundary layer transition of similar FST intensity. New turbulent structures first appear at high wavenumbers extending into a lower-wavenumber spectrum later as turbulent spots grow and join together. In line with the transient energy growth theory, the maximum turbulent kinetic energy in the pre-transitional phase grows linearly but only in terms of ${u}^{\ensuremath{\prime} } $, whilst ${v}^{\ensuremath{\prime} } $ and ${w}^{\ensuremath{\prime} } $ remain essentially unchanged. The energy production and dissipation rates are very low at this stage despite the high level of ${u}^{\ensuremath{\prime} } $. The pressure–strain term remains unchanged at that time, but increases rapidly later during transition along with the generation of turbulent spots, hence providing an unambiguous measure for the onset of transition.
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Phani Kumar, P., A. C. Mandal i J. Dey. "Effect of a mesh on boundary layer transitions induced by free-stream turbulence and an isolated roughness element". Journal of Fluid Mechanics 772 (7.05.2015): 445–77. http://dx.doi.org/10.1017/jfm.2015.203.
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Streamwise streaks, their lift-up and streak instability are integral to the bypass transition process. An experimental study has been carried out to find the effect of a mesh placed normal to the flow and at different wall-normal locations in the late stages of two transitional flows induced by free-stream turbulence (FST) and an isolated roughness element. The mesh causes an approximately 30 % reduction in the free-stream velocity, and mild acceleration, irrespective of its wall-normal location. Interestingly, when located near the wall, the mesh suppresses several transitional events leading to transition delay over a large downstream distance. The transition delay is found to be mainly caused by suppression of the lift-up of the high-shear layer and its distortion, along with modification of the spanwise streaky structure to an orderly one. However, with the mesh well away from the wall, the lifted-up shear layer remains largely unaffected, and the downstream boundary layer velocity profile develops an overshoot which is found to follow a plane mixing layer type profile up to the free stream. Reynolds stresses, and the size and strength of vortices increase in this mixing layer region. This high-intensity disturbance can possibly enhance transition of the accelerated flow far downstream, although a reduction in streamwise turbulence intensity occurs over a short distance downstream of the mesh. However, the shape of the large-scale streamwise structure in the wall-normal plane is found to be more or less the same as that without the mesh.
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Han, Sang-Kap, Min-Kyung Joo, Jeon-Kyung Kim, Woonhee Jeung, Heerim Kang i Dong-Hyun Kim. "Bifidobacteria-Fermented Red Ginseng and Its Constituents Ginsenoside Rd and Protopanaxatriol Alleviate Anxiety/Depression in Mice by the Amelioration of Gut Dysbiosis". Nutrients 12, nr 4 (26.03.2020): 901. http://dx.doi.org/10.3390/nu12040901.
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Gut dysbiosis is closely connected with the outbreak of psychiatric disorders with colitis. Bifidobacteria-fermented red ginseng (fRG) increases the absorption of ginsenoside Rd and protopanxatriol into the blood in volunteers and mice. fRG and Rd alleviates 2,4,6-trinitrobenzenesulfonic acid-induced colitis in mice. Therefore, to understand the gut microbiota-mediated mechanism of fRG against anxiety/depression, we examined the effects of red ginseng (RG), fRG, ginsenoside Rd, and protopanaxatriol on the occurrence of anxiety/depression, colitis, and gut dysbiosis in mice. Mice with anxiety/depression were prepared by being exposed to two stressors, immobilization stress (IS) or Escherichia coli (EC). Treatment with RG and fRG significantly mitigated the stress-induced anxiety/depression-like behaviors in elevated plus maze, light-dark transition, forced swimming (FST), and tail suspension tasks (TST) and reduced corticosterone levels in the blood. Their treatments also suppressed the stress-induced NF-κB activation and NF-κB+/Iba1+ cell population in the hippocampus, while the brain-derived neurotrophic factor (BDNF) expression and BDNF+/NeuN+ cell population were increased. Furthermore, treatment with RG or fRG suppressed the stress-induced colitis: they suppressed myeloperoxidase activity, NF-κB activation, and NF-κB+/CD11c+ cell population in the colon. In particular, fRG suppressed the EC-induced depression-like behaviors in FST and TST and colitis more strongly than RG. fRG treatment also significantly alleviated the EC-induced NF-κB+/Iba1+ cell population and EC-suppressed BDNF+/NeuN+ cell population in the hippocampus more strongly than RG. RG and fRG alleviated EC-induced gut dysbiosis: they increased Bacteroidetes population and decreased Proteobacteria population. Rd and protopanaxatriol also alleviated EC-induced anxiety/depression and colitis. In conclusion, fRG and its constituents Rd and protopanaxatriol mitigated anxiety/depression and colitis by regulating NF-κB-mediated BDNF expression and gut dysbiosis.
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Stevenson, J. P. J., K. P. Nolan i E. J. Walsh. "Particle image velocimetry measurements of induced separation at the leading edge of a plate". Journal of Fluid Mechanics 804 (9.09.2016): 278–97. http://dx.doi.org/10.1017/jfm.2016.532.
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The free shear layer that separates from the leading edge of a round-nosed plate has been studied under conditions of low (background) and elevated (grid-generated) free stream turbulence (FST) using high-fidelity particle image velocimetry. Transition occurs after separation in each case, followed by reattachment to the flat surface of the plate downstream. A bubble of reverse flow is thereby formed. First, we find that, under elevated (7 %) FST, the time-mean bubble is almost threefold shorter due to an accelerated transition of the shear layer. Quadrant analysis of the Reynolds stresses reveals the presence of slender, highly coherent fluctuations amid the laminar part of the shear layer that are reminiscent of the boundary-layer streaks seen in bypass transition. Instability and the roll-up of vortices then follow near the crest of the shear layer. These vortices are also present under low FST and in both cases are found to make significant contributions to the production of Reynolds stress over the rear of the bubble. But their role in reattachment, whilst important, is not yet fully clear. Instantaneous flow fields from the low-FST case reveal that the bubble of reverse flow often breaks up into two or more parts, thereby complicating the overall reattachment process. We therefore suggest that the downstream end of the ‘separation isoline’ (the locus of zero absolute streamwise velocity that extends unbroken from the leading edge) be used to define the instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak coincides with the location of roll-up, whereas the downstream mode may suggest a ‘flapping’ motion.
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Vaid, Aditya, Nagabhushana Rao Vadlamani, Ananth Sivaramakrishnan Malathi i Vikrant Gupta. "Dynamics of Bypass Transition Behind Roughness Element Subjected to Pulses of Free-Stream Turbulence". Physics of Fluids, 10.10.2022. http://dx.doi.org/10.1063/5.0120241.
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This study explores the dynamics of bypass transition of a zero pressure gradient boundary layer transitioning under the combined influence of an isolated roughness element with pulses of free-stream turbulence (FST). We consider a hemispherical roughness element placed over a flat plate, while the pulses of FST are introduced at the inlet which is in contrast to continuous FST largely explored in the literature. For a fixed turbulence intensity and length scale, a series of eddy-resolving simulations are carried out to examine the effect of varying the pulsing frequency of FST. The flow behind the roughness element remains stable in the absence of FST for the subcritical Reynolds number $Re_k = 400$ considered in this study. We observe that with the pulses of FST, the transition is triggered due to the interaction of the FST-induced Klebanoff streaks with the roughness-induced streamwise vortices. With an increase in the frequency of FST pulses, the boundary layer has less time to relax to its unperturbed state resulting in an earlier onset of transition. The transition onset predicted is in favorable agreement with the correlations proposed in the literature. We analyze the growth of disturbance kinetic energy, the shape of secondary instabilities over the streaks, and their phase speeds in detail. The FST pulse convecting over the roughness element triggers the inner varicose modes in its near-wake region. The varicose modes decay rapidly further downstream and the well-known sinuous instabilities (or the outer modes) trigger transition via transient growth associated with convective instabilities.
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Sengupta, Aditi, Nivedita Gupta i Bryn Noel Ubald. "Separation-induced transition on a T106A blade under low and elevated free stream turbulence". Physics of Fluids 36, nr 2 (1.02.2024). http://dx.doi.org/10.1063/5.0189358.
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The separation-induced transition on the suction surface of a T106A low pressure turbine blade is a complex phenomenon with implications for aerodynamic performance. In this numerical investigation, we explore an adverse pressure gradient-dominated flow subjected to varying levels of free stream excitation, as the underlying separation-induced transition is a critical factor in assessing blade profile loss. By comprehensively analyzing the effects of free stream turbulence (FST) on the transition process, we delve into the various mechanisms which govern the instabilities underlying bypass transition by studying the instantaneous enstrophy field. This involves solving the two-dimensional (2D) compressible Navier–Stokes equation through a series of numerical simulations, comparing a baseline flow to cases where FST with varying turbulent intensity (Tu=4% and 7%) is imposed at the inflow. Consistent with previous studies, the introduction of FST is observed to delay flow separation and trigger early transition. We explore the different stages of bypass transition, from the initial growth of disturbances (described by linear stability theory) to the emergence of unsteady separation bubbles that merge into turbulent spots (due to nonlinear interactions), by examining the vorticity dynamics. Utilizing the compressible enstrophy transport equation for the flow in a T106A blade passage, we highlight the various routes of bypass transition resulting from different levels of FST, emphasizing the relative contributions from baroclinicity, compressibility, and viscous terms.
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Mamidala, Santhosh B., André Weingärtner i Jens H. M. Fransson. "A comparative study of experiments with numerical simulations of free-stream turbulence transition". Journal of Fluid Mechanics 951 (14.11.2022). http://dx.doi.org/10.1017/jfm.2022.883.
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To date, very few careful and direct comparisons between experiments and direct numerical simulations (DNS) have been published on free-stream turbulence (FST) induced boundary layer transition, whilst there exist numerous published works on the comparison of canonical turbulent boundary layers. The primary reason is that the former comparison is vastly more difficult to carry out simply because all known transition scenarios have large energy gradients and are extremely sensitive to surrounding conditions. This paper presents a detailed comparison between new experiments and available DNS data of the complex FST transition scenario in a flat plate boundary layer at turbulence intensity level about $Tu = 3\,\%$ and FST Reynolds number about $Re_{{fst}} = 67$ . The leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE are identified as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters is not enough as previously thought, which is illustrated by setting up two experimental FST cases with about the same FST integral parameters at the LE but with different energy spectra. Finally, an FST boundary layer penetration depth (PD) measure is defined using DNS, which suggests that the PD grows with the downstream distance and stays around 20 % of the boundary layer thickness down to transition onset. With this result, one cannot rule out the significance of the continuous FST forcing along the boundary layer edge in this transition scenario, as indicated in previous studies.
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Đurović, Kristina, Ardeshir Hanifi, Philipp Schlatter, Kenzo Sasaki i Dan S. Henningson. "Direct numerical simulation of transition under free-stream turbulence and the influence of large integral length scales". Physics of Fluids 36, nr 7 (1.07.2024). http://dx.doi.org/10.1063/5.0207016.
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Under the action of free-stream turbulence (FST), elongated streamwise streaky structures are generated inside the boundary layer, and their amplitude and wavelength are crucial for the transition onset. While turbulence intensity is strongly correlated with the transitional Reynolds number, characteristic length scales of the FST are often considered to have a slight impact on the transition location. However, a recent experiment by Fransson and Shahinfar [J. Fluid Mech. 899, A23 (2020)] shows significant effects of FST scales. They found that, for higher free-stream turbulence levels and larger integral length scales, an increase in the length scale postpones transition, contrary to established literature. Here, by performing well-resolved numerical simulations, we aim at understanding why the FST integral length scale affects the transition location differently at low- and high turbulence levels. We found that the integral length scales in Fransson and Shahinfar's experiment are so large that the introduced wide streaks have substantially lower growth in the laminar region, upstream of the transition to turbulence, than the streaks induced by smaller integral length scales. The energy in the boundary layer subsequently propagate to smaller spanwise scales as a result of the nonlinear interaction. When the energy has reached smaller spanwise scales, larger amplitude streaks results in regions where the streak growth are larger. It takes longer for the energy from wider streaks to propagate to the spanwise scales associated with the breakdown to turbulence, than for those with smaller spanwise scales. Thus, there is a faster transition for FST with lower integral length scales in this case.
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Wang, Taiyang, Yaomin Zhao, John Leggett i Richard Sandberg. "Direct Numerical Simulation of a High-Pressure Turbine Stage: Unsteady Boundary Layer Transition and the Resulting Flow Structures". Journal of Turbomachinery, 27.09.2023, 1–28. http://dx.doi.org/10.1115/1.4063510.
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Abstract In the present study, we investigate the unsteady boundary layer transition based on the direct numerical simulation database of a high-pressure turbine (HPT) stage (Zhao and Sandberg, GT2021-58995), focusing on the transition mechanisms on the rotor blade, affected by the incoming periodic wakes and the background free-stream turbulence (FST). Based on the fully-resolved flow fields, we provide detailed analysis of the flow structures responsible for the transition, and two distinctive transition paths have been identified. The first path is the typical bypass transition via the instability of Klebanoff streaks, which happens when the transition region is not directly affected by the wake. The suction-side boundary layer is disturbed at the leading edge, resulting in the formation of streamwise streaks. These streaky structures endure varicose instability in the region with adverse pressure-gradient (APG), then quickly break down into turbulent spots, which then evolve into fully turbulent flow. The other transition path is a consequence of the direct interaction between the wake structures and the blade boundary layer, when the wake apex starts to affect the transitional region. To be specific, the wake structures directly interact with the separation bubble in the APG region, causing sudden breakdown into turbulence. A calmed region is found to follow the wake-induced turbulent boundary layer. It is observed that the recovery to a calmed region can be impacted by the FST, as the calmed region in case with no FST is much longer compared to cases with stronger FST.
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Mohamed Aniffa, S., V. S. Caesar, V. Dabaria i A. C. Mandal. "Characteristics of geometry-and pressure-induced laminar separation bubbles at an enhanced level of free-stream turbulence". Journal of Fluid Mechanics 957 (20.02.2023). http://dx.doi.org/10.1017/jfm.2023.53.
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Responses of a geometry-induced separation bubble (GISB) and a pressure-induced separation bubble (PISB) at enhanced levels of free-stream turbulence (FST) have experimentally been investigated for a comparative study using the particle image velocimetry (PIV) technique. The outlines of separation bubbles based on the dividing streamlines are self-similar for different levels of FST and Reynolds numbers. The spectral analyses of the time-resolved PIV data show that the vortex shedding frequency of a separated shear layer remains unchanged for the GISB cases even with an enhanced level of FST. In contrast, it is different for the PISB cases. We propose a criterion that determines whether the frequency will remain the same even for the cases with FST. Linear stability analyses reveal that the inviscid-inflectional instability dominates the transition process, and the linear stages of transition are not completely bypassed even at an enhanced level of FST. The most amplified frequencies, while scaling with the displacement thickness and the boundary layer edge velocity, collapse in a single curve for all the cases. Furthermore, measurements in the spanwise plane show that the streamwise velocity streak/Klebanof mode at an enhanced level of FST is not a general flow feature for all types of separation bubbles. However, at an enhanced level of FST for the PISB case, the boundary layer streaks are found to distort the two-dimensional vortex structure associated with the Kelvin–Helmholtz instability, eventually leading to a three-dimensional $\varLambda$ -like structure in the spanwise plane.
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Katiyar, S., i S. Sarkar. "Influence of freestream turbulence on boundary layer transition over a controlled-diffusion compressor blade". Physics of Fluids 36, nr 6 (1.06.2024). http://dx.doi.org/10.1063/5.0213250.
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The influence of inlet freestream turbulence (FST) on the boundary layer transition over the suction surface of a controlled-diffusion compressor blade is demonstrated here by employing a well-resolved large-eddy simulation. Inherent to low Reynolds number conditions, a laminar separation bubble (LSB) forms on the suction surface, attributing to substantial flow diffusion. Inlet FST levels ranging from 1.5% to 7.6% are systematically varied, while maintaining a constant Reynolds number based on axial chord and inlet velocity at 2.1 × 105. Transition of the shear layer is initiated via Kelvin–Helmholtz instability with the amplification of selective frequencies until an inlet FST of 2.3%. Secondary instability emerges in the second half of the LSB, attributed to the amplification of perturbations in the braid region, ultimately leading to breakdown near the reattachment. At a moderate FST level of 4.2%, longitudinal streaks in the first half of the blade elongate downstream, causing the LSB to disappear, while the flow becomes inflectional at the mid-chord. Thus, the boundary layer transition in the second half of the blade is attributed to the high receptivity of the inflectional layer and breakdown of streaks, leading to an exponential growth of disturbances. Finally, at an inlet FST of 7.6%, the boundary layer appears pre-transitional in the first half of the blade, exhibiting significant turbulence levels. In the latter half, excitation occurs primarily through the breakdown of streaks, reflecting an algebraic growth of disturbances. Flow features and oscillations in the Nusselt number in this case suggest the outer mode of streak-induced instability.
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Mamidala, Santhosh B., A. Weingärtner i J. H. M. Fransson. "Leading-edge pressure gradient effect on boundary layer receptivity to free-stream turbulence". Journal of Fluid Mechanics 935 (3.02.2022). http://dx.doi.org/10.1017/jfm.2022.19.
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Free-stream turbulence (FST) induced boundary layer transition is an intricate physical process that starts already at the leading edge (LE) with the LE receptivity process dictating how the broad spectrum of FST scales is received by the boundary layer. The importance of the FST integral length scale, apart from the turbulence intensity, has recently been recognized in transition prediction but a systematic variational study of the LE pressure gradient has still not been undertaken. Here, the LE pressure gradient is systematically varied in order to quantify its effect on the transition location. To this purpose, we present a measurement technique for accurate determination of flat-plate boundary layer transition location. The technique is based on electret condenser microphones which are distributed in the streamwise direction with high spatial resolution. All time signals are acquired simultaneously and post-processed giving the full intermittency distribution of the flow over the plate in a few minutes. The technique is validated against a similar procedure using hot-wire anemometry measurements. Our data clearly shows that the LE pressure gradient plays a decisive role in the receptivity process for small integral length scales, at moderate turbulence intensities, leading to variations in the transitional Reynolds number close to 40 %. To our knowledge, this high sensitivity of LE pressure gradient to transition has so far not been reported and our experiments were therefore partly repeated using another LE to ensure set-up independence and result repeatability.
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Feng, Long, Yimiao Hou, Qingqing Hao, Mingxing Chen, Shuo Wang, Xuemin Hu i Wenxiu Yang. "A multi-function textile with pH-induced switch wettability transition for controllable oil−water separation". Textile Research Journal, 14.11.2021, 004051752110569. http://dx.doi.org/10.1177/00405175211056981.
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The deterioration of water ecology caused by the discharge of oil spill wastewater, industrial sewage, and municipal sewage has attracted wide attention worldwide. Thus, it is significant to design a simple, environmentally friendly approach to separate oil–water mixtures. In this work, three different fabrics with pH-induced wettability transition were prepared by a dip-coating process for oil and water separation. The dip-coating fabrics had the advantages of oil–water separation, photocatalytic degradation, and recycling. Polyethylene terephthalate/polyamide nonwoven fabric was used as the substrate materials of the fabric. The carboxylic acid-modified TiO2 endowed the fabric with hydrophilicity–hydrophobicity and photocatalytic properties. The Fe3O4 nanoparticles obtained by the coprecipitation method provided magnetism for the fabric, facilitating the recycling of the fabric and improving the hydrophobicity of the fabric. The fabrics coated with dipping solutions were superhydrophobic in a neutral environment and hydrophilic in an alkaline environment. Among the three coated fabrics, the fabric coated with stearic acid/TiO2-Fe3O4 (FST) had the most satisfying oil–water separation performance and durability. Under the neutral condition, the contact angle of the FST was 151° and the separation efficiency was 98%. Under the alkaline condition, the underwater oil contact angle of the FST was 150° and the separation efficiency was 95%. After 15 cycles, the oil–water separation rate of the FST was still higher than 90%. Due to the presence of TiO2, the coated fabric had an exceptional performance in the photodegradation of organic pollutants (69.9%). In addition, the fabrics can be quickly recovered due to magnetism.
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Mi, Shu, Ye Gao, Ze Jiang, Junwei Zhang, Wenhui Cao, Jinjin Li, Yutong Wang i in. "Electric‐Field‐Induced in‐Plane Anisotropy of Superconducting Transition Temperature for FeSe0.5Te0.5 Thin Films". Advanced Functional Materials, 25.03.2024. http://dx.doi.org/10.1002/adfm.202404450.
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AbstractSuperconductor/ferroelectric heterostructures are very important for fundamental research and applications since superconductivity can be controlled by electric fields via ferroelectric polarization/piezostrain. Superconductivity is a macroscopic quantum phenomenon and it is commonly believed that the in‐plane superconducting transition temperature (Tc) should be isotropic for superconductors with crystal structures of stacking layers (quasi 2D). Here, the observation of piezostrain‐induced in‐plane anisotropy of Tc in FeSe0.5Te0.5 (FST) thin films grown on ferroelectric single crystals is reported. The as‐prepared FST shows a tetragonal phase with equal in‐plane lattice parameters and an absence of in‐plane anisotropy of Tc. Upon applying electric fields, piezostrain induces a difference in the in‐plane lattice parameters (a‐b) and in‐plane anisotropy of Tc. The in‐plane anisotropy of Tc correlates with a‐b and becomes more remarkable for larger values of a‐b. Some possible extrinsic effects are ruled out by experiments and analysis. A possible mechanism is proposed involving electronic nematicity and inhomogeneous superconductivity in FST to account for this unusual phenomenon. This work is significant for uncovering the exotic nature of unconventional superconductors, as well as applications.
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Scillitoe, Ashley D., Paul G. Tucker i Paolo Adami. "Numerical Investigation of Three-Dimensional Separation in an Axial Flow Compressor: The Influence of Freestream Turbulence Intensity and Endwall Boundary Layer State". Journal of Turbomachinery 139, nr 2 (26.10.2016). http://dx.doi.org/10.1115/1.4034797.
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Regions of three-dimensional separations are an inherent flow feature of the suction surface-endwall corner in axial compressors. These corner separations can cause a significant total pressure loss and reduce the compressor's efficiency. This paper uses wall-resolved LES to investigate the loss sources in a corner separation, and examines the influence of the inflow turbulence on these sources. Different subgrid scale (SGS) models are tested and the choice of model is found to be important. The σ SGS model, which performed well, is then used to perform LES of a compressor endwall flow. The time-averaged data are in good agreement with measurements. The viscous and turbulent dissipation are used to highlight the sources of loss, with the latter being dominant. The key loss sources are seen to be the 2D laminar separation bubble and trailing edge wake, and the 3D flow region near the endwall. Increasing the freestream turbulence (FST) intensity changes the suction surface boundary layer transition mode from separation induced to bypass. However, it does not significantly alter the transition location and therefore the corner separation size. Additionally, the FST does not noticeably interact with the corner separation itself, meaning that in this case the corner separation is relatively insensitive to the FST. The endwall boundary layer state is found to be significant. A laminar endwall boundary layer separates much earlier leading to a larger passage vortex. This significantly alters the endwall flow and loss. Hence, the need for accurate boundary measurements is clear.
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Cao, Li-Hua, Yuan-Yuan Zhao, Ming Bai, David Geliebter, Jan Geliebter, Raj Tiwari, Hong-Juan He i in. "Mechanistic Studies of Gypenosides in Microglial State Transition and its Implications in Depression-Like Behaviors: Role of TLR4/MyD88/NF-κB Signaling". Frontiers in Pharmacology 13 (15.03.2022). http://dx.doi.org/10.3389/fphar.2022.838261.
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Depression is a prevalent psychiatric disorder. Microglial state transition has been found in many neurological disorders including depression. Gypenosides (Gypenosides I-LXXVIII, Gps) are saponin extracts isolated from the traditional Chinese herb Gynostemma pentaphyllum (Thunb.) Makino that exert anti-inflammatory and neuroprotective activities and regulate depression-like behaviors. However, its effect on microglial state transition in depression remains unknown. We aimed to evaluate the potential relationship between Gps and TLR4/MyD88/NF-κB signaling in microglial state transition in vitro and in vivo. First, BV-2 cells (microglial cell line) were exposed to lipopolysaccharides (LPS) and treated with 10 or 5 μg/ml Gps. Second, the chronic unpredictable mild stress (CUMS)-induced depression mouse model was used to investigate the antidepressant-like behaviors effects of Gps (100 or 50 mg/kg). We determined depression-like behaviors using the open-field test (OFT), forced swim test (FST), and sucrose preference test (SPT). Proteins and inflammatory factors in the TLR4/MyD88/NF-κB signaling pathway and the different microglial reaction states markers were subsequently conducted using enzyme-linked immunosorbent assay, immunocytochemistry, immunofluorescence, qPCR, or Western blotting analyses to evaluate the anti-inflammatory and antidepressant properties of Gps and the underlying molecular mechanisms. We found that Gps regulated the microglial cell line state transition in LPS-exposed BV-2 cells, as evidenced by the significantly decreased expression of inflammatory parameters iNOS, IL-1β, IL-6, and TNF-α and significantly promoted anti-inflammatory microglial phenotypes markers CD206 (Mrc1) and IL-10. More importantly, Gps protected against the loss of monoamine neurotransmitters and depression-like behavior in a mouse model of depression, which was accompanied by a regulation of the microglial state transition. Mechanistically, Gps inhibited TLR4/MyD88/NF-κB signaling, which reduced the release of downstream inflammatory cytokines (IL-1β, IL-6, and TNF-α) and promoted microglial phenotype transition, which all together contributed to the antidepressant effect. Our results suggest that Gps prevents depression-like behaviors by regulating the microglial state transition and inhibiting the TLR4/MyD88/NF-κB signaling pathway. Thus, Gps could be a promising therapeutic strategy to prevent and treat depression-like behaviors and other psychiatric disorders.
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Palumbo, Andrea, Onofrio Semeraro, Jean-Christophe Robinet i Luigi de Luca. "Boundary layer transition induced by low-speed synthetic jets". Physics of Fluids, 5.12.2022. http://dx.doi.org/10.1063/5.0128798.
Pełny tekst źródłaStreszczenie:
The effect of low-speed, circular synthetic jets (SJ) on the turbulent transition of a laminar boundary layer (BL) is studied through Direct Numerical Simulations (DNS). The SJ capability in fixating the streamwise location of transition onset is analyzed in terms of its operation parameters (reduced frequency $F^+$ and momentum coefficient $C_\mu$). The effect of free-stream turbulence (FST) on the near-wall vortical structures generated by the synthetic jet is analyzed as well, to mimic the actual operation of the control system. Velocity spectra, phase portraits and dynamic mode decomposition (DMD) allow to investigate flow unsteadiness and transition to a chaotic state. In most of the investigated cases, SJs successfully promote transition, as the result of varicose-symmetric hairpin-like vortices generated at the jet exit. In particular, it is found that increasing the momentum coefficient always reduces the size of the laminar region; a non-monotonic behaviour of the laminar fetch is noted as the reduced frequency is increased, suggesting the existence of an optimal frequency value. Combination of FST and SJ actuation results in spanwise-asymmetric vortical structures, with little difference in the location of the transition onset as compared to the previous case. The present analysis can be used to gather information on the practical implementation of low-speed SJ actuators as active turbulators.