Characterization of Effective Diffusion within Viscoelastic Fluids with Elastic Instabilities

We considered effective diffusion, characterized by magnitudes of effective diffusion coefficients, in order to quantify mass transport due to the onset and development of elastic instabilities. Effective diffusion coefficient magnitudes were determined using different analytic approaches, as they were applied to tracked visualizations of fluorescein dye front variations, as circumferential advection was imposed upon a flow environment produced using a rotating Couette flow arrangement. Effective diffusion coefficient results were provided for a range of flow shear rates, which were produced using different Couette flow rotation speeds and two different flow environment fluid depths. To visualize the flow behavior within the rotating Couette flow environment, minute amounts of fluorescein dye were injected into the center of the flow container using a syringe pump. This dye was then redistributed within the flow by radial diffusion only when no disk rotation was used, and by radial diffusion and by circumferential advection when disk rotation was present. Associated effective diffusion coefficient values, for the latter arrangement, were compared to coefficients values with no disk rotation, which were due to molecular diffusion alone, in order to quantify enhancements due to elastic instabilities. Experiments were conducted using viscoelastic fluids, which were based on a 65% sucrose solution, with different polymer concentrations ranging from 0 ppm to 300 ppm. Associated Reynolds numbers based on the fluid depth and radially averaged maximum flow velocity ranged from 0.00 to 0.5. The resulting effective diffusion coefficient values for different flow shear rates and polymer concentrations quantified the onset of elastic instabilities, as well as significant and dramatic changes to local mass transport magnitudes, which are associated with the further development of elastic instabilities.

Effects of edge biasing on blob dynamics and associated transport in the edge of the J-TEXT tokamak

Effects of edge radial electric field Er and Er × B flow shear on edge turbulence and turbulent transport, in particular, on large-scale blobs and blobby transport have been investigated in the positive and negative biasing discharges in the J-TEXT tokamak. The results show that under certain conditions, the positive electrode biasing induces better plasma confinement than the negative biasing. Further studies reveal that in addition to flow shear effects on blob dynamics, the local radial electric field at the edge region plays a significant role in repulsion of the blobs and associated transport, leading to improvement of particle confinement when the outward motion of the blobs is blocked. The results are in accordance with theoretical predictions.

Generation of E × B flow shear by finite orbit width effects from heat sources in tokamaks

We present a possible mechanism for the generation of strong E × B flow shear relevant to internal transport barrier formation in tokamak plasmas. From gyrokinetic calculations, we show that strong E × B flow shear can be generated by finite orbit width (FOW) effects associated with a non-uniform heat source and is sufficient to lead to transport barrier formation in the core region with a moderate power level. Two FOW effects inducing neoclassical polarization are shown to be responsible for this: 1) the radial drift of particle orbit center due to the variation of the heat source within orbit width and 2) the non-uniformly evolved orbit width by the non-uniform heating.

Comparison of dynamical features between the fast H-L and the H-I-L transition for EAST RF-heated plasmas

In this paper, a comparison of dynamical features between the fast H-L and the H-I-L transition, which can be identified by the intermediate phase, or ‘I-phase’, has been made for radio-frequency (RF) heated deuterium plasmas in EAST. The fast H-L transition is characterized by a rapid release of stored energy during the transition transient, while the H-I-L transition exhibits a ‘soft’ H-mode termination. One important distinction between the transitions has been observed by dedicated probe measurements slightly inside the separatrix, with respect to the radial gradient of the floating potential, which corresponds to the E×B flow and/or the electron temperature gradient. The potential gradient inside the separatrix oscillates and persists during the stationary I-phase, and shows a larger amplitude than that before the fast H-L transition. The reduction of the gradient leads to the final transition to the L-mode for both the fast H-L and the H-I-L transition. These findings indicate that the mean E×B flow shear and/or edge electron temperature gradient play a critical role underlying the H-L transition physics. In addition, the back transition in EAST is found to be sensitive to magnetic configuration, where the vertical configuration, i.e., inner strike-point located at vertical target, favours access to the H-I-L transition, while the horizontal shape facilitates achievement of the fast H-L transition. The divertor recycling level normalized to electron density is higher before the fast H-L transition, as compared to that before the I-phase, which strongly suggest that the density of the recycled neutrals is an important ingredient in determining the back transition behaviour.

The Kelvin-Helmholtz Instability From the Perspective of Hybrid Simulations

The flow shear-driven Kelvin-Helmholtz (KH) instability is ubiquitous in planetary magnetospheres. At Earth these surface waves are important along the dawn and dusk flanks of the magnetopause boundary while at Jupiter and Saturn the entire dayside magnetopause boundary can exhibit KH activity due to corotational flows in the magnetosphere. Kelvin-Helmholtz waves can be a major ingredient in the so-called viscous-like interaction with the solar wind. In this paper, we review the KH instability from the perspective of hybrid (kinetic ions, fluid electrons) simulations. Many of the simulations are based on parameters typically found at Saturn’s magnetopause boundary, but the results can be generally applied to any KH-unstable situation. The focus of the discussion is on the ion kinetic scale and implications for mass, momentum, and energy transport at the magnetopause boundary.

Bifurcated Current Sheet Observed on the Boundary of Kelvin-Helmholtz Vortices

On May 5, 2017 MMS observed a bifurcated current sheet at the boundary of Kelvin-Helmholtz vortices (KHVs) developed on the dawnside tailward magnetopause. We use the event to enhance our understanding of the formation and structure of asymmetric current sheets in the presence of density asymmetry, flow shear, and guide field, which have been rarely studied. The entire current layer comprises three separate current sheets, each corresponding to magnetosphere-side sunward separatrix region, central near-X-line region, and magnetosheath-side tailward separatrix region. Two off-center structures are identified as slow-mode discontinuities. All three current sheets have a thickness of ∼0.2 ion inertial length, demonstrating the sub-ion-scale current layer, where electrons mainly carry the current. We find that both the diamagnetic and electron anisotropy currents substantially support the bifurcated currents in the presence of density asymmetry and weak velocity shear. The combined effects of strong guide field, low density asymmetry, and weak flow shear appear to lead to asymmetries in the streamlines and the current-layer structure of the quadrupolar reconnection geometry. We also investigate intense electrostatics waves observed on the magnetosheath side of the KHV boundary. These waves may pre-heat a magnetosheath population that is to participate into the reconnection process, leading to two-step energization of the magnetosheath plasma entering into the magnetosphere via KHV-driven reconnection.

Assessment of flow within developing chicken vasculature and biofabricated vascularized tissues using multimodal imaging techniques

Fluid flow shear stresses are strong regulators for directing the organization of vascular networks. Knowledge of structural and flow dynamics information within complex vasculature is essential for tuning the vascular organization within engineered tissues, by manipulating flows. However, reported investigations of vascular organization and their associated flow dynamics within complex vasculature over time are limited, due to limitations in the available physiological pre-clinical models, and the optical inaccessibility and aseptic nature of these models. Here, we developed laser speckle contrast imaging (LSCI) and side-stream dark field microscopy (SDF) systems to map the vascular organization, spatio-temporal blood flow fluctuations as well as erythrocytes movements within individual blood vessels of developing chick embryo, cultured within an artificial eggshell system. By combining imaging data and computational simulations, we estimated fluid flow shear stresses within multiscale vasculature of varying complexity. Furthermore, we demonstrated the LSCI compatibility with bioengineered perfusable muscle tissue constructs, fabricated via molding techniques. The presented application of LSCI and SDF on perfusable tissues enables us to study the flow perfusion effects in a non-invasive fashion. The gained knowledge can help to use fluid perfusion in order to tune and control multiscale vascular organization within engineered tissues.

The Effect of Fluid Flow Shear Stress and Substrate Stiffness on Yes-Associated Protein (YAP) Activity and Osteogenesis in Murine Osteosarcoma Cells

Osteosarcoma (OS) is an aggressive bone cancer originating in the mesenchymal lineage. Prognosis for metastatic disease is poor, with a mortality rate of approximately 40%; OS is an aggressive disease for which new treatments are needed. All bone cells are sensitive to their mechanical/physical surroundings and changes in these surroundings can affect their behavior. However, it is not well understood how OS cells specifically respond to fluid movement, or substrate stiffness—two stimuli of relevance in the tumor microenvironment. We used cells from spontaneous OS tumors in a mouse engineered to have a bone-specific knockout of pRb-1 and p53 in the osteoblast lineage. We silenced Sox2 (which regulates YAP) and tested the effect of fluid flow shear stress (FFSS) and substrate stiffness on YAP expression/activity—which was significantly reduced by loss of Sox2, but that effect was reversed by FFSS but not by substrate stiffness. Osteogenic gene expression was also reduced in the absence of Sox2 but again this was reversed by FFSS and remained largely unaffected by substrate stiffness. Thus we described the effect of two distinct stimuli on the mechanosensory and osteogenic profiles of OS cells. Taken together, these data suggest that modulation of fluid movement through, or stiffness levels within, OS tumors could represent a novel consideration in the development of new treatments to prevent their progression.

High-Fidelity Simulations of a High-Pressure Turbine Stage: Effects of Reynolds Number and Inlet Turbulence

We present the first wall-resolved high-fidelity simulations of high-pressure turbine (HPT) stages at engine-relevant conditions. A series of cases have been performed to investigate the effects of varying Reynolds numbers and inlet turbulence on the aerothermal behavior of the stage. While all of the cases have similar mean pressure distribution, the cases with higher Reynolds number show larger amplitude wall shear stress and enhanced heat fluxes around the vane and rotor blades. Moreover, higher-amplitude turbulence fluctuations at the inlet enhance heat transfer on the pressure-side and induce early transition on the suction-side of the vane, although the rotor blade boundary layers are not significantly affected. In addition to the time-averaged results, phase-lock averaged statistics are also collected to characterize the evolution of the stator wakes in the rotor passages. It is shown that the stretching and deformation of the stator wakes is dominated by the mean flow shear, and their interactions with the rotor blades can significantly intensify the heat transfer on the suction side. For the first time, the recently proposed entropy analysis has been applied to phase-lock averaged flow fields, which enables a quantitative characterization of the different mechanisms responsible for the unsteady losses of the stages. The results indicate that the losses related to the evolution of the stator wakes is mainly caused by the turbulence production, i.e. the direct interaction between the wake fluctuations and the mean flow shear through the rotor passages.

Transcription Factor β-Catenin Plays a Key Role in Fluid Flow Shear Stress-Mediated Glomerular Injury in Solitary Kidney

Increased fluid flow shear stress (FFSS) in solitary kidney alters podocyte function in vivo. FFSS-treated cultured podocytes show upregulated AKT-GSK3β-β-catenin signaling. The present study was undertaken to confirm (i) the activation of β-catenin signaling in podocytes in vivo using unilaterally nephrectomized (UNX) TOPGAL mice with the β-galactosidase reporter gene for β-catenin activation, (ii) β-catenin translocation in FFSS-treated mouse podocytes, and (iii) β-catenin signaling using publicly available data from UNX mice. The UNX of TOPGAL mice resulted in glomerular hypertrophy and increased the mesangial matrix consistent with hemodynamic adaptation. Uninephrectomized TOPGAL mice showed an increased β-galactosidase expression at 4 weeks but not at 12 weeks, as assessed using immunofluorescence microscopy (p < 0.001 at 4 weeks; p = 0.16 at 12 weeks) and X-gal staining (p = 0.008 at 4 weeks; p = 0.65 at 12 weeks). Immunofluorescence microscopy showed a significant increase in phospho-β-catenin (Ser552, p = 0.005) at 4 weeks but not at 12 weeks (p = 0.935) following UNX, and the levels of phospho-β-catenin (Ser675) did not change. In vitro FFSS caused a sustained increase in the nuclear translocation of phospho-β-catenin (Ser552) but not phospho-β-catenin (Ser675) in podocytes. The bioinformatic analysis of the GEO dataset, #GSE53996, also identified β-catenin as a key upstream regulator. We conclude that transcription factor β-catenin mediates FFSS-induced podocyte (glomerular) injury in solitary kidney.