High Wi Research Articles

Numerical investigation on the deformation and breakup of an elastoviscoplastic droplet in simple shear flow

In this paper, direct numerical simulations (DNSs) are performed to investigate the deformation and breakup of an elastoviscoplastic (EVP) droplet in a Newtonian matrix under simple shear flow. The two-phase interface is captured by the volume-of-fluid (VOF) method with adaptive mesh refinement technique. The Saramito model (Bingham model coupled exponential Phan-Thien–Tanner viscoelastic model) is used to characterize the rheological behavior of the droplet. The droplet deformation and conformational state are studied with different Capillary numbers Ca, Weissenberg numbers Wi, and Bingham numbers Bi, which represent the surface tension, elasticity, and yield stress of the droplet, respectively. Our results show that droplet deformation occurs at low Ca, while breakup occurs at high Ca. The droplet non-monotonically deforms with increasing Wi and Bi, while is elongated for higher Ca. In addition, three breakup modes (mid-point pinching, transitional breakup, and homogeneous breakup) are reported for EVP droplets, in which transitional breakup disappears due to the influence of high elasticity. The conformational state of the droplet intuitively demonstrates the change of breakup from horizontal shear to vertical breakup. In spite of the fact that the surface tension always inhibits the deformation of droplets, the present work indicates that Bi has little effect on the deformation with high Wi and high Ca, while the influence is obvious at low Wi and Ca. The observed elastic and plastic effects on droplet deformation and breakup are believed to have significant impacts, as yield stress fluids are widely encountered in industrial applications.

Brownian dynamics simulations of bottlebrush polymers in dilute solution under simple shear and uniaxial extensional flows.

We study the dynamics of bottlebrush polymer molecules in dilute solutions subjected to shear and uniaxial extensional flows using Brownian dynamics simulations with hydrodynamic interaction (HI). Bottlebrush polymers are modeled using a coarse-grained representation, consisting of a set of beads interacting pairwise via a purely repulsive potential and connected by finitely extensible nonlinear springs. We present the results for molecular stretching, stress, and solution viscosity during the startup of flow as well as under steady state as a function of side chain length while keeping the backbone length fixed. In extensional flow, the backbone fractional extension and the first normal stress difference decrease with an increase in side chain length at a fixed Weissenberg number (Wi). Using simulation results both in the presence of and in the absence of HI, we show that this is primarily a consequence of steric interaction resulting from the dense grafting of side chains. In shear flow, we observe a shear-thinning behavior in all cases, although it becomes less pronounced with increasing side chain length. Furthermore, nonmonotonicity in the backbone fractional extension is observed under shear, particularly at high Wi. We contextualize our simulation results for bottlebrush polymers with respect to existing studies in the literature for linear polymers and show that the unique dynamical features characterizing bottlebrush polymers arise on account of their additional molecular thickness due to the presence of densely grafted side chains.

Numerical investigation on the viscoelastic polymer flow in material extrusion additive manufacturing

In this paper, high-fidelity direct numerical simulations (DNSs) are carried out to explore the behavior of viscoelastic polymer flow in direct ink writing (DIW) additive manufacturing. The impact of ink elasticity on the stress for the strand deposition process has been examined. The exponential Phan-Thien Tanner (PTT) model is used to characterize the shear-thinning viscoelastic rheological behavior of polymeric fluids, and the gas-polymer interface is captured by the volume of fluid (VOF) method. Focusing on the extrusion and deposition of polymer, the effects of ink elasticity represented by the Weissenberg number (Wi), normalized printing speed and gap height on the formation of the printed strand have been studied. The results indicate that the polymer flow out of the nozzle is quite stable with time, and the polymeric stress balance is disturbed in the middle of the printed strand. For high Wi, the strand has significant deformation and an obvious large head (the earliest printed bulge in the deposited strand), whereas a larger normalized printing speed leads to a large stress region. In particular, a small normalized gap height has a squeezing effect on the deposited strand, and a visible depression in the middle of the strand can be obtained for large heights. Additionally, the degree of polymer deformation exhibits a major monotonic dependence on the normalized printing speed and gap height. The nonlinear relationships between the maximum stress components and process parameters show that the maximum stress decreases with Wi, but different printing speeds and gap heights have almost no effect on the stress components in the build platform-normal direction. The above analysis contributes to the inference of the deformation effects in different flow states, which can provide guidance to DIW additive manufacturing.

Enhancing rhizosphere soil water retention in wheat through colonization with endophytic fungus Serendipita indica

Effect of S. indica colonization of wheat (Triticum aestivum) on water retention and physical quality of the rhizosphere soil was investigated in a greenhouse study. Colonized and uncolonized seedlings of four wheat cultivars (i.e., Roshan, Ghods, Pishtaz, and Kavir) were grown in rhizoboxes. Plants were harvested and intact samples were taken from the rhizosphere to measure soil organic carbon (SOC), hot-water soluble carbohydrates (HWSC), and soil water retention curve (SWRC). Water contents at field capacity (θFC) and permanent wilting point (θPWP), and plant available water (PAW) were obtained from the measured SWRC data. The integral energy (EI(PAW)) and integral water storage (WI(PAW)), two alternative indices of soil water availability and physical quality, were computed by integration of SWRCs over the PAW range with respect to water content and matric potential, respectively. Low EI(PAW) and high WI(PAW) indicate improved soil physical quality and better soil water availability to plant. The results showed that the SOC storage and HWSC of the wheat rhizosphere were approximately 146 and 83% greater than those of unplanted soils, respectively. Moreover, compared with the uncolonized counterparts, S. indica colonization significantly increased the SOC and HWSC by 76 and 134%, respectively. The presence of plant roots and S. indica colonization improved soil water retention by increasing SOC and HWSC and subsequently enhancing portion of structural porosity (i.e., pore space between the soil aggregates in the size range of 0.5–500 μm, mainly includingmacropores and mesopores). Despite having the same PAW, colonized soil samples had lower EI(PAW) (≈ 133 J kg−1) compared to uncolonized counterparts (≈ 153 J kg−1). This finding implies that plant roots take up water from the colonized wheat-planted soils easier than from the uncolonized counterparts. Compared to uncolonized samples, an increase of 50% in S1 (Dexter's index of soil physical quality) of the colonized Kavir cultivar [with the lowest root extension, SOC (i.e., 0.97 g kg−1), and HWSC (i.e., 0.059 g kg−1)] showed that S. indica can improve soil physical quality due to greater structural pores. Among the four wheat cultivars, the rhizosphere of Roshan (≈ 1.60 g kg−1) and Ghods (1.61 g kg−1) had the highest SOC values. Higher HWSC (i.e., 0.138 g kg−1) and more extensive root distribution for Roshan, especially upon co-cultivation with S. indica, led to greater contribution of structural pores in water retention in the rhizosphere soil when compared with three other cultivars. To conclude, S. indica colonization not only in the presence of plant roots and by increasing SOC and HWSC improves the physical quality of the rhizosphere soil but also the fungus by itself enhances soil physical quality due to enhancing portion of structural porosity involving in water transition and storage.

Master curves for FENE-P fluids in steady shear flow

The FENE-P (Finitely-Extensible Nonlinear Elastic) dumbbell constitutive equation is widely used in simulations and stability analyses of free and wall-bounded viscoelastic shear flows due to its relative simplicity and accuracy in predicting macroscopic properties of dilute polymer solutions. The model contains three independent material parameters, which expressed in dimensionless form correspond to a Weissenberg number ( Wi ), i.e. , the ratio of the dumbbell relaxation time scale to a characteristic flow time scale, a finite extensibility parameter ( L ), corresponding to the ratio of the fully extended dumbbell length to the root mean square end-to-end separation of the polymer chain under equilibrium conditions, and a solvent viscosity ratio, commonly denoted β . An exact solution for the rheological predictions of the FENE-P model in steady simple shear flow is available (Sureshkumar et al., 1997), but the resulting nonlinear and nested set of equations do not readily reveal the key shear-thinning physics that dominates at high Wi as a result of the finite extensibility of the polymer chain. In this note we review a simple way of evaluating the steady material functions characterizing the nonlinear evolution of the polymeric contributions to the shear stress and first normal stress difference as the shear rate increases, provide asymptotic expansions as a function of Wi , and show that it is in fact possible to construct universal master curves for these two material functions as well as the corresponding stress ratio. Steady shear flow experiments on three highly elastic dilute polymer solutions of different finite extensibilities also follow the identified master curves. The governing dimensionless parameter for these master curves is Wi / L and it is only in strong shear flows exceeding Wi / L ≳ 1 that the effects of finite extensibility of the polymer chains dominate the evolution of polymeric stresses in the flow field. We suggest that reporting the magnitude of Wi / L when performing stability analyses or simulating shear-dominated flows with the FENE-P model will help clarify finite extensibility effects. • Asymptotic solutions for FENE-P model in steady shear flow are calculated. • Master curves exist for material functions of FENE-P fluids in steady shear flow. • Governing dimensionless parameter is (Weissenberg number)/(finite extensibility). • Reporting this dimensionless parameter clarifies the magnitude of nonlinear effects.

Numerical study of viscoelastic flow around an oscillating circular cylinder

The viscoelasticity-induced fluid–structure interaction studies have a significant influence on practical applications. To clarify the lock-in phenomenon and the wake topology of the vibrating cylinder placed in the viscoelastic flow, the Oldroyd-B fluid flows around an oscillating circular cylinder have been numerically investigated at Re = 10 and Re = 60, respectively. The governing equations are solved by the coupling of the square-root-conformation representation approach and the discontinuous Galerkin method in framework of the high-order dual splitting scheme. In addition, the arbitrary Lagrangian–Eulerian formulation is implemented in the coupling procedure in order to account for the interaction between the fluid and the oscillating body in the flow field. With this, complex boundary movements can be tackled simply and efficiently. In numerical simulation, the force coefficients and the wake structures of vortex and stress are discussed in some detail. At Re = 10, when the frequency of cylinder is small, it is obvious that the vortex shedding takes place in the wake. As the frequency increases, almost no obvious vortex shedding is observed. Also, the wake still oscillates at the same frequency of the cylinder for all cases, even for high Wi numbers. However, different wake modes of vortex and stress are found for various frequencies at Re = 60 and Wi = 0.1. In the lock-in region, the 2S mode of wake type are observed. Beyond the lock-in region, the wake type is no longer 2S, but the formation of vortex shedding and stress distribution in the far wake recovers to its natural mode. These numerical results open up a new field of study for viscoelastic fluids.

On swelling behaviours of a bentonite under different water contents

This study starts from a question: how would initial water content (wi) affect equilibrium swelling pressure (peq) of compacted bentonites in swelling pressure tests (ps tests)? However, discussions based on experiments involving a bentonite, Kunigel V1 (K_V1), are extended to the following issues: (a) the effect of wi on peq; (b) pore water density (ρpw); (c) the co-existence of crystalline and osmotic swelling; and (d) relation between peq and compaction tests (pc tests). It is revealed that wi has an insignificant effect on peq in general, although a significant effect may appear for relatively high wi and dry density (ρd) conditions. Two swelling types, crystalline swelling with basal spacing of montmorillonite (d001 = 1·0–1·9 nm) and osmotic swelling (d001 ≥ ∼4·0 nm), co-exist in a w range from 36 to 65%. With the fact that ρpw of K_V1 may range from 1·1 to 1·2 Mg/m3, it is expected that crystalline swelling may mainly govern peq in ps tests if the final dry density of a specimen (ρdf) is larger than 1·46 Mg/m3, while osmotic swelling may mainly govern if ρdf < 1·05 Mg/m3. Together with the results of pc tests, the effects of wi on peq are explained by the proposed conceptual model.

APPLICATION OF VEGETATION INDECES FOR DOGITAL SOIL MAPPING BASED ON SENTINEL-2 SPACE IMAGES

At present, the works devoted to the creation of digital soil maps using geographic information systems (GIS) and remote sensing (RS) data are relevant. In the work the analysis of vegetation indices (VI) for soil mapping was carried out, the maps of vegetation indices were created: Normalized Difference Vegetation Index (NDVI), Optimized Soil-Adjusted Vegetation Index (OSAVI), Soil-Adjusted Vegetation Index (SAVI), Tranformed Soil-Adjusted Vegetation Index (TSAVI), Enhanced Vegetation Index2 (EVI2) for the territory of ZAO Mirny farm of Kochenevsky District using Sentinel-2 A satellite image (May 10, 2021). ). As a result it was revealed that the vegetation indices OSAVI and EVI2 allow to establish spatial boundaries between the main types of soils of automorphic, half-hydromorphic and hydromorphic moisture regimes.
 Background. Multispectral space and aerial photos for thematic mapping of soil resources are of greater practical application. Sentinel-2 A space image with good spatial and spectrozonal resolution (10 m, 20 m and 60 m) and territorial coverage (290 km) was used in this article. This made it possible to calculate and analyze various vegetation indices for the purposes of digital soil mapping.
 Purpose. аnalysis of vegetation indices for digital soil mapping based on Sentinel-2 A images.
 Materials and research methods. The research was carried out on the territory of CJSC Mirniy, Kochenevsky District, Novosibirsk Region. The methods of digital processing of space images, mapping and geoinformation analysis with the use of Sentinel-2 A satellite image (May 10, 2021) were used. The method of equal intervals was used for comparative analysis of images. This allowed using GIS ArcGIS to make thematic maps of images with the allocation of gradations: very low, low, average, above average, high value.
 Results. Field soil surveys were carried out on the territory of CJSC Mirny farm in Kochenevsky district of Novosibirsk Region. Using SAGA geoinformation system the space image was atmospherically corrected and spatially referenced, NDVI, OSAVI, TSAVI, EVI2 raster maps were compiled. Geoinformation analysis of the large-scale 1:1000 soil map and raster EVI maps revealed that OSAVI allows to establish spatial boundaries between the main types of soils of automorphic, half-hydromorphic and hydromorphic moisture regimes. Very low values of VI are typical for the soils of hydromorphic humidification regime, formed near small lakes, along the banks of the Sharikh river.
 Very low values of UI have objects of hydrography, marsh peaty, meadow-marsh humus soils, marshy and peated marshes, marsh solonchaks, marsh solonchaks, formed in lowered areas of relief with depth of groundwater occurrence less than 0.5 m.
 Ploughed ordinary chernozems, deposited in the upper and middle part of the gentle slope, have low values of WP. These are soils of automorphous moisture regime with depth of groundwater occurrence more than 6 m.
 Average and above average values are characteristic of gray forest saltwort soils under woody vegetation, as well as meadow-chernozem soils under meadow vegetation in the lower part of the gentle slope with groundwater occurrence depth from 3 to 4 m. High WI values were obtained for meadow soils with dense grass cover. Wetting conditions of soils, location in the relief, and vegetation type significantly influence VI values. The obtained VV values can be used at the stage of training data preparation in the form of reference classes for basic soil types required for automatic image recognition.

The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation

In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.

Elongational viscosity scaling of polymer melts with different chemical constituents

Morelly et al. (Macromolecules 52:915-922, 2019) reported transient and steady-state elongational viscosity data of monodisperse linear polymer melts obtained by filament-stretching rheometry with locally controlled strain and strain rate and found different power law scaling of the elongational viscosities of polystyrene, poly(tert-butylstyrene) and poly(methyl-methacrylate). Very good agreement is achieved between data and predictions of the extended interchain pressure (EIP) model (Narimissa et al. J. Rheol. 64, 95-110 (2020)), based solely on linear viscoelastic characterization and the Rouse time τR of the melts. The analysis reveals that both the normalized elongational viscosity and the normalized elongational stress are dependent on the number of entanglements (Z) and the ratio of entanglement molar mass Mem to critical molar mass Mcm of the melts in the linear viscoelastic regime through {eta}_E^0/left({G}_N{tau}_Rright)propto {left({M}_{mathrm{em}}/{M}_{mathrm{cm}}right)}^{2.4}{Z}^{1.4} and {sigma}_E^0/{G}_Npropto {left({M}_{mathrm{em}}/{M}_{mathrm{cm}}right)}^{2.4}{Z}^{1.4} Wi , while in the limit of fast elongational flow with high Weissenberg number Wi={tau}_Rdot{varepsilon} , both viscosity and stress become independent of Z and Mem/Mcm, and approach a scaling which depends only on Wi, i.e. ηE/(GNτR) ∝ Wi−1/2 and σE/GN ∝ Wi1/2. When expressed by an effective power law, the broad transition from the linear viscoelastic to the high Wi regime leads to chemistry-dependent scaling at intermediate Wi depending on the number of entanglements and the ratio between entanglement molar mass and critical molar mass.

Кинетика процесса накопления повреждений и разрушения в зонах концентраторов при испытаниях образцов на разрыв

The paper considers the effect of various stress concentrators on processes of damage accumulation, initiation and propagation of crack in steel (steel 3) samples during uniaxial tensile. In the middle of the samples there was a transverse welded joint or a hole with a diameter of 5 mm. The acoustic-emission testing results showed that weight content of location impulses (Wi) in the clusters of low, middle and high energy level and criteria parameters value Wi at the stages of the samples destruction have similar pattern of change despite the various types of concentrators, stress-strain material diagrams, pattern of damage accumulation, number of registered acoustic emission (AE) events and its registration activity.

Universality of steady shear flow of Rouse melts

The data set of steady and transient shear data reported by Santangelo and Roland Journal of Rheology 45: 583–594, (2001) in the nonlinear range of shear rates of an unentangled polystyrene melt PS13K with a molar mass of 13.7 kDa is analysed by using the single integral constitutive equation approach developed by Narimissa and Wagner Journal of Rheology 64:129–140, (2020) for elongational and shear flow of Rouse melts. We compare model predictions with the steady-state, stress growth, and stress relaxation data after start-up shear flows. In characterising the linear-viscoelastic relaxation behaviour, we consider that in the vicinity of the glass transition temperature, Rouse modes and glassy modes are inseparable, and we model the terminal regime of PS13K by effective Rouse modes. Excellent agreement is achieved between model predictions and shear viscosity data, and good agreement with first normal stress coefficient data. In particular, the shear viscosity data of PS13K as well as of two polystyrene melts with M = 10.5 kDa and M = 9.8 kDa investigated by Stratton Macromolecules 5 (3): 304–310, (1972) agree quantitatively with the universal mastercurve predicted by Narimissa and Wagner for unentangled melts, and approach a scaling of Wi−1/2at sufficiently high Weissenberg numbers Wi. Some deviations between model predictions and data are seen for stress growth and stress relaxation of shear stress and first normal stress difference, which may be attributed to limitations of the experimental data, and may also indicate limitations of the model due to the complex interactions of Rouse modes and glassy modes in the vicinity of the glass transition temperature.Graphical abstract

Asymmetric flow of polymer solutions around microfluidic cylinders: Interaction between shear-thinning and viscoelasticity

Viscoelastic flow around a cylinder is a model problem representing a wide range of relevant industrial processing and biological applications. Reducing the cylinder to microscopic dimensions conveniently enables the problem to be examined in the absence of inertia. Recently, we have developed glass microfluidic geometries containing long and slender, yet rigidly fixed, microfluidic cylinders, which present a low blockage ratio β=2r/W=0.1, where r=20μm is the cylinder radius and W=400μm is the channel width. Using a shear-banding viscoelastic wormlike micellar (WLM) solution, we showed how the flow around such a cylinder could destabilize beyond a critical Weissenberg number (Wi=λU/r, where λ is a characteristic time of the fluid and U is the average flow velocity), resulting in the asymmetric division of the fluid around either side of the cylinder [Haward et al, Soft Matter15:1927]. In the present work we investigate this flow instability in greater detail using a range of polymer solutions formulated from hydrolyzed poly(acrylamide) (HPAA) dissolved at different concentrations in deionized water. The test fluids present a range of shear-thinning responses under steady shear, and also a wide variety of characteristic times. At low HPAA concentrations, the flow around the cylinder remains essentially symmetric for all Wi, but as the concentration increases, so does the maximum degree of the flow asymmetry observed with increasing Wi. Interestingly, at intermediate concentrations, the flow can resymmetrize at very high Wi. We understand these effects by considering simultaneously both the degree of shear-thinning of the fluid and the imposed Wi, and our analysis shows that both strong shear-thinning and high elasticity are required for the formation of strongly asymmetric flows. Our results represent the first report of this highly asymmetric flow state in polymer solutions, showing that it is a general phenomenon and not only specific to WLM. Our analysis provides a clear insight into the origins of this unusual flow state and may also be relevant to understanding other instances of asymmetries arising in shear-thinning viscoelastic flows.

Pore-Scale Flow Characterization of Polymer Solutions in Microfluidic Porous Media.

Polymer solutions are frequently used in enhanced oil recovery and groundwater remediation to improve the recovery of trapped nonaqueous fluids. However, applications are limited by an incomplete understanding of the flow in porous media. The tortuous pore structure imposes both shear and extension, which elongates polymers; moreover, the flow is often at large Weissenberg numbers, Wi, at which polymer elasticity in turn strongly alters the flow. This dynamic elongation can even produce flow instabilities with strong spatial and temporal fluctuations despite the low Reynolds number, Re. Unfortunately, macroscopic approaches are limited in their ability to characterize the pore-scale flow. Thus, understanding how polymer conformations, flow dynamics, and pore geometry together determine these nontrivial flow patterns and impact macroscopic transport remains an outstanding challenge. This review describes how microfluidic tools can shed light on the physics underlying the flow of polymer solutions in porous media at high Wi and low Re. Specifically, microfluidic studies elucidate how steady and unsteady flow behavior depends on pore geometry and solution properties, and how polymer-induced effects impact nonaqueous fluid recovery. This work thus provides new insights for polymer dynamics, non-Newtonian fluid mechanics, and applications such as enhanced oil recovery and groundwater remediation.

Low Reynolds number friction reduction with polymers and textures

We present an experimental investigation with surface textures and a Non-Newtonian polymer solution (polyisobutylene in oil) to determine synergistic effects on laminar hydrodynamic friction reduction. Gap controlled experiments were performed on a custom parallel disk tribo-rheometer to systematically vary the Reynolds number (0.1–16), Weissenberg number (0.1–30), and Deborah number (0.005–0.5) in bi-directional sliding motion with five different texture geometries: cylindrical holes with varying asymmetric angle β. Cavitation effects are not present, thus normal force is produced solely by the textures and the lubricant rheology. Contrary to Newtonian fluids without cavitation, the normal force is positive, independent of the direction of motion when shear normal forces dominate purely viscous effects. We observe that the normal thrust can be larger than a simple superposition of forces due to viscous hydrodynamics and shear normal stresses. This synergistic coupling between the asymmetric textures and the polymer solution (at high Wi) produces an optimal angle of texture asymmetry β for dramatically decreasing the effective coefficient of friction compared to the Newtonian oil without polymer.

Transient dynamics of turbulence growth and bursting: Effects of drag-reducing polymers

The transient process of turbulence development and vortex breakdown from a marginal flow state dominated by streaky velocity patterns is not only essential for understanding the bypass transition into turbulence, but – in the context of viscoelastic fluids – also offers unique insight into the dynamics at high-extent and maximum drag reduction (HDR and MDR). Shooting trajectories connecting the edge state and following its unstable manifold to the turbulent basin are generated. In Newtonian flow, the growth of turbulence starts with the intensification of velocity streaks and a sharp rise in the Reynolds shear stress. It is followed by a quick breakdown into high-intensity small-scale fluctuations before entering the basin of statistical turbulence. Adding drag-reducing polymers does not affect the initial growth of turbulence but stabilizes the primary streak-vortex structure. As a result, the vortex breakdown stage is circumvented. Polymer deformation is insignificant until the vortex breakdown, after which polymer stress rapidly shoots up. At high Weissenberg number Wi, loss of turbulent kinetic energy through polymer elastic conversion is comparable to viscous dissipation. Beyond bypass transition, the transient process studied here closely resembles the bursting phase of the self-sustaining cycle of turbulence. Our results indicate that at high Wi (i.e., HDR) polymer effects can significantly reduce bursting by rerouting the trajectory of turbulent dynamics.

Potential Clinical Benefits and Limitation of Foetal Virtuopsy Using a High-Field MRI of 7 Tesla to Assess First Trimester Foetuses Versus Stereomicroscopic Autopsy

Background: Performing accurate autopsies on small foetuses is a difficult process to achieve, especially when a certain degree of autolysis is involved. High-field MRI of 7 T has proven to be a useful tool to describe small foetuses anatomy, therefore the purpose of our study was to establish its diagnostic usefulness assessing first trimester foetuses compared to stereomicroscopic autopsy or microscopic evaluation. Methods Nine consecutive cases of first trimester foetuses were divided in two groups according to gestational age and crown-rump length (Embryo Group cases of 9-10 gestational weeks and Foetal Group cases of 13 gestational weeks) and scanned using a 3D Fast Imaging with Steady Procession 3D (FISP 3D) respectively 2D Turbo High Resolution T2 WI protocol. Images were interpreted by a radiologist, and two embryologists. All cases were submitted to invasive autopsy, blinded to the imagistic results. Findings Virtuopsy brought important additional information in all cases from the Embryo Group and had similar diagnostic capacity as the conventional method in evaluating the cases from the Foetal Group. The Cohen's kappa coefficient of agreement was k=0.92 (95% CI 82-97), highlighting a very good concordance between foetal virtuopsy and stereomicroscopic autopsy, also supported by the McNemar test p =1.00. The imagistic method presented a better capacity in visualising embryonic structure especially embryonic brain with lesser method disadvantages (p=0,005.) and similar capacity for visualising foetal structures in the second group. Interpretation Virtual autopsy using high field MRI at 7T is emerging as a promising alternative approach to stereomicroscopic autopsy in the assessment of malformed first trimester human foetuses. Funding: Not applicable. Declaration of Interest: The authors report no declarations of interest. Ethical Approval: The study protocol was approved by the “Iuliu Hatieganu” University of Medicine and Pharmacy Ethics Committee and written informed parental consent was obtained before the imagistic and conventional autopsy procedures.

Shear-flow rheology and viscoelastic instabilities of ethanol gel fuels

This study presents a rheological investigation to understand the instability mechanism in newly formulated ethanol gel (EG) fuels when subjected to simple shear flow in a cone-plate rheometer (CPR). Shear-thinning EG fuels are formulated using an organic gellant, Hydroxypropyl methylcellulose (HPMC). EG fuels are sub-classified into concentrated, semi-dilute or dilute gels based on the consistency and elasticity number (El) that describes an interplay between the elastic effects and the viscous effects. The extent of shear-thinning effect in the zero-shear regime is found out by comparing shear-thinning parameter β, with an elastic parameter λ∗. The relation proves the existence of minimal elastic component even in the dilute gels. Ratio of normal stress to shear stress variation with Re, shows the existence of critical Re only for the semi-dilute and dilute gels (EG1-EG4) that marks the onset of elastic instability, whereas this transition is absent in concentrated EG5. During the shear process, two distinct phenomena are identified. First, in the low Wi regime, the presence of Normal stress N1 induces the secondary flow based elastic instability, which is counterbalanced by the presence of another Normal stress N2 to stabilize the flow. Secondly, in the high Wi regime, the polymer shows the maximum extension beyond which polymer breaks down with increase in the applied shear rate.

A Lattice Boltzmann Method and Asynchronous Model Coupling for Viscoelastic Fluids

The numerical algorithms of viscoelastic flows can appear a tremendous challenge as the Weissenberg number (Wi) enlarged sufficiently. In this study, we present a generalized technique of time-stably advancing based on the coupled lattice Boltzmann method, in order to improve the numerical stability of simulations at a high Wi number. The mathematical models of viscoelastic fluids include both the equation of the solvent and the Oldroyd-B constitutive equation of the polymer. In the two-dimensional (2D) channel flow, the coupled method shows good agreements between the corresponding exact results and the numerical results obtained by our method. In addition, as the Wi number increased, for the viscoelastic flows through contractions, we show that the prediction of our presented method can reproduce the same numerical results that were reported by previous studies. The main advantage of current method is that it can be applied to simulate the complex phenomena of the viscoelastic fluids.

Shear-Induced Migration of Rigid Particles near an Interface between a Newtonian and a Viscoelastic Fluid.

Simulations of rigid particles suspended in two-phase shear flow are presented, where one of the suspending fluids is viscoelastic, whereas the other is Newtonian. The Cahn–Hilliard diffuse-interface model is employed for the fluid–fluid interface, which can naturally describe the interactions between the particle and the interface (e.g., particle adsorption). It is shown that particles can migrate across streamlines of the base flow, which is due to the surface tension of the fluid–fluid interface and a difference in normal stresses between the two fluids. The particle is initially located in the viscoelastic fluid, and its migration is investigated in terms of the Weissenberg number Wi (shear rate times relaxation time) and capillary number Ca (viscous stress over capillary stress). Four regimes of particle migration are observed, which can roughly be described by migration away from the interface for Wi = 0, halted migration toward the interface for low Wi and low Ca, particle adsorption at the interface for high Wi and low Ca, and penetration into the Newtonian fluid for high Wi and high Ca. It is found that the angular velocity of the particle plays a large role in determining the final location of the particle, especially for high Wi. From morphology plots, it is deduced that the different dynamics can be described well by considering a balance in the first-normal stress difference and Laplace pressure. However, it is shown that other parameters, such as the equilibrium contact angle and diffusion of the fluid, are also important in determining the final location of the particle.