Numerical vs experimental pressure drops for Boger fluids in sharp-corner contraction flow

This paper addresses the problem of matching experimental findings with numerical prediction for the extreme experimental levels of pressure-drops observed in the 4:1 sharp-corner contraction flows, as reported by Nigen and Walters [“Viscoelastic contraction flows: Comparison of axisymmetric and planar configurations,” J. Non- Newtonian Fluid Mech. 102, 343–359 (2002)]. In this connection, we report on significant success in achieving quantitative agreement between predictions and experiments. This has been made possible by using a new swanINNFM model, employing an additional dissipative function. Notably, one can observe that extremely large pressure-drops may be attained with a suitable selection of the extensional viscous time scale. In addition, and on vortex structure, the early and immediate vortex enhancement for Boger fluids in axisymmetric contractions has also been reproduced, which is shown to be absent in planar counterparts

Predicting large experimental excess pressure drops for Boger fluids in contraction–expansion flow

More recent finite element/volume studies on pressure-drops in contraction flows have introduced a variety of constitutive models to compare and contrast the competing influences of extensional viscosity, normal stress and shear-thinning. In this study, the ability of an extensional White–Metzner construction with FENE-CR model is explored to reflect enhanced excess pressure drops (epd) in axisymmetric 4:1:4 contraction-expansion flows. Solvent-fraction is taken as =0.9, to mimic viscoelastic constant shear-viscosity Boger fluids. The experimental pressure-drop data of Rothstein & McKinley [1] has been quantitatively captured (in the initial pronounced rise with elasticity, and limiting plateau-patterns), via two modes of numerical prediction: (i) flow-rate Q-increase, and (ii) relaxation-time 1-increase. Here, the former Q-increase mode, in line with experimental procedures, has proved the more effective, generating significantly larger enhanced-epd. This is accompanied with dramatically enhanced trends with De-incrementation in vortex-activity, and significantly larger extrema in N1, shear-stress and related extensional and shear velocity-gradient components. In contrast, the 1-increase counterpart trends remain somewhat invariant to elasticity rise. Moreover, under Q-increase and with elasticity rise, a pattern of flow transition has been identified through three flow-phases in epd-data; (i) steady solutions for low-to-moderate elasticity levels, (ii) oscillatory solutions in the moderate elasticity regime (coinciding with Rothstein & McKinley [1] data), and (iii) finally solution divergence. New to this hybrid algorithmic formulation are - techniques in time discretisation, discrete treatment of pressure terms, compatible stress/velocity-gradient representation; handling ABS-correction in the constitutive equation, which provides consistent material-property prediction; and introducing purely-extensional velocity-gradient component specification at the shear-free centre flow-line through the velocity gradient (VGR) correction

The falling sphere problem and capturing enhanced drag with Boger fluids

In this computational study, the ability of an extensional White–Metzner construction with the FENE-CR model is considered to reflect experimental enhanced drag data of Jones et al. [1]. The numerical drag predictions for three different aspect ratios of sphere:tube radii {0.5, 0.4, 0.2} are obtained with a hybrid finite element/volume (fe/fv) algorithm. Excellent agreement is extracted for all three aspect ratios against the experimental measurements, and at any specified rate, the tighter-fitting the aspect ratio the lower the resulting drag. Moreover, as the Weissenberg number is increased, the transition between steady-state and oscillatory flow is recognised from the instantaneous pressure data, prior to numerical divergence. A main realisation in this study is that it is important to select the same procedure of Wi-continuation across experimental and computational protocols, to extract comparable levels of drag. Clearly the -increase mode (common computational form), is more involved than the Q-increase mode (usual experimental form), and as such, less robust as a reliable method for accurate drag prediction and enhanced drag capture. In general, flow-rate increase (Q-increase) conditions generate larger drag enhancement, when compared to fluid-relaxation time increase ( -increase), at comparable levels of dissipative-factor ( ). The investigation also follows parametric variation in solvent fraction ( ) in one particular geometric aspect-ratio instance. This reveals that at any specific fixed elasticity level, there is an increase in drag observed with rise in . In addition, high solute/low-solvent fractions at low dissipative-factor, were only found to generate drag reduction, consistent with the literature. New and key facets to this fe/fv implementation are summarised, in appealing to: an improved velocity gradient boundary conditions imposed at the centreline (VGR-correction); continuity correction; absolute value of the stress-trace function (ABS-f-correction); increasing flow-rate solution continuation; alongside advanced techniques in fv-time discretisation, discrete treatment of pressure terms, and compatible stress/velocity-gradient representation

Computational Predictions for Boger Fluids and Circular Contraction Flow under Various Aspect Ratios

This work puts forward a modeling study contrasted against experimental, with focus on abrupt circular contraction flow of two highly-elastic constant shear-viscosity Boger fluids, i.e. a polyacrylamide dissolved in corn-syrup PAA/CS (Fluid-1) and a polyisobutylene dissolved in polybutene PIB/PB (Fluid-2), in various contraction-ratio geometries. Moreover, this work goes hand-in-hand with the counterpart matching of experimental pressure-drops observed in such 4:1 and 8:1 aspect-ratio contraction flows, as described experimentally in the literature. In this study, the experimental findings, for Boger fluids with severe strain-hardening features, reveal significant vortex-evolution characteristics, correlated with enhanced pressure-drop phasing and normal-stress response in the corner region. It is shown how such behavior may be replicated through simulation and the rheological dependencies that are necessary to bring this about. Predictive solutions with an advanced hybrid finite-element/volume (fe/fv) algorithm are able to elucidate the rheological properties (extensional viscosity and normal-stress response) that rule such vortex-enhancement evolution. This is accomplished by employing the novel swanINNFM(q) family of fluids, through the swIM model-variant, with its strong and efficient control on elongational properties

Predictions for circular contraction-expansion flows with viscoelastoplastic & thixotropic fluids

In this predominately predictive modelling finite volume/element study, a comparative analysis is performed for time-dependent and viscoelastoplastic flow in a circular contraction-expansion geometry of aspect-ratio 10:1:10. For this, a hybrid finite volume/element scheme is employed. A new and revised micellar model is investigated, under the denomination of BMP+_τp, which reflects a bounded extensional viscosity response and an N1Shear-upturn at large deformation rates (lost in earlier model-variants), a versatile model capable of supporting plasticity, shear-thinning, strain softening-hardening and shear-banding. Many of these features are common to wormlike micellar and polymer solutions. Then, findings are contrasted against a de Souza Mendes model. Two flow regimes are addressed: plastic flow (low flow-rate Q ≤ 1 units, solvent-fraction β  1; minimised plasticity; β = 1/9); as quantified via flow-structure, yield-fronts and pressure-drops. Under the plastic regime, elasticity-increase causes asymmetry about the contraction-plane, whilst yield-stress and enhanced strain-hardening promote solid-like features, apparent through augmented unyielded-regions and rising pressure-drops. Concerning the viscoelastic regime and vortex-structures, extensional-deformation experienced correlates with hardening expectation in uniaxial-extension, whilst streamline activity in vortex-cells correlates with normal-stress response in shear. Adjustment in strain-hardening/softening response with Q-rise, provides translation from weaker salient-corner vortex centres to stronger elastic corner-vortices; yet, when softening finally prevails, asymmetric upstream/downstream salient-corners vortex patterns are recovered. For strong-hardening and solvent-dominated β∼0.8 fluids (as with Boger fluids), an intermediate lip-vortex-formation phase is noted, alongside coexistence of salient-corner vortices. Such a vortex-coexistence phase is distinctly absent in solute-concentrated fluids

On modelling viscoelastic flow through abrupt circular 8:1 contractions – matching experimental pressure-drops and vortex structures

This study compares and contrasts computational predictions against experimental data for some viscoelastic contraction flows. Nigen and Walters (2002) [1], provides the comparative data-set, the specific flow of interest is an 8:1 abrupt circular contraction, and the constitutive model is that of swanINNFM(q) [swIM]. Taken against increasing flow-rate, such a model is observed to capture significant vortex-enhancement in these axisymmetric flows, reflecting well the counterpart experimental findings. In addition, rich vortex characteristics are reflected, through evolving patterns of salient-corner, lip-vortex and elastic-corner vortices. A systematic parametric analysis is conducted over three independent and governing material parameters in the model, whilst attempting to interpret rheological adjustment against such changes in flow-structure. Specifically, this has involved variation in solvent-fraction (β), finite-extensibility parameter (L), and extensional-based dissipative parameter (λD)

Flow past a sphere: Predicting enhanced drag with shear-thinning fluids, dissipative and constant shear-viscosity models

This article tackles the topic of drag detection for flow past a sphere, focusing on response for viscoelastic shear-thinning fluids, in contrast to constant shear-viscosity forms, both with and without extensional-viscous dissipative contributions. The work extends that previously of Garduño et al. [1], where experimental levels of resultant drag-enhancement were captured for Boger-fluids, using a new hybrid dissipative viscoelastic model. This advance was based on Finitely Extensible Non-linear Elastic and White-Metzner constructs, where the level of extensional-viscous material time-scale had to be considerably raised to provide strong strain-hardening properties. The new dissipative model drag findings are: - for low-solvent systems, all such models reflect only significant drag-reduction, with barely any distinction from base-level dissipative-factor response. Such systems consistently gave considerably more pronounced decline in drag than for their high-solvent counterparts. Alternatively, under high-solvent systems (as in Boger fluids), the general observation for all four such dissipative models, is that after an initial-decrease in drag, a second-increasing trend can be extracted. This lies in stark contrast to base-level, null dissipative-factor drag findings, where only drag-reduction could be observed. Yet consistently, the inclusion of shear-thinning is reflected in the overall lowering of drag levels. Nevertheless, strong terminating drag-enhancement can be generated under larger dissipative-factor setting for dissipative-EPTT (shear-thinning, strain-hardening/softening), only slightly suppressed from that for dissipative-FENE-CR (constant shear-viscosity, strain-hardening/hardening-plateau). Other dissipative-{FENE-P, LPTT} variants, showed encouraging trends towards drag-enhancement, but unfortunately suffered from premature solution stunting, and hence, were restricted in accessible range of deformation-rates. In addition, an increase in geometry aspect-ratio, generally provokes elevation of drag, but only under high-solvent state, and hence only then, leads to evidence for stimulating drag-enhancement

Contraction-ratio variation and prediction of large experimental pressure-drops in sharp-corner circular contraction-expansions–Boger fluids

This study is concerned with the continuum modelling of sharp-corner contraction-expansion axisymmetric flows, under contraction-ratio variation, and more particularly, in the precise capture of the large-levels of experimental excess pressure-drops (epd) for Boger fluids. The particular contraction-ratios (α) considered are those studied experimentally by M. Pérez-Camacho, J.E. López-Aguilar, F. Calderas, O. Manero, M.F. Webster, J. Non-Newton. Fluid Mech. 222 (2015) 260-271; of α={2, 4, 6, 8, 10}. Their experimental PAA/corn-syrup Boger fluids have been characterized and modelled with the so-called swanINNFM model through dissipative continuum-scale modelling. This facilitates the precise capture of experimental-levels of epd-data (largest epd=O(6) under α=10 contraction-ratio and sharp corners). The swanINNFM model has already proven capable of reproducing the large excess pressure-drops reported by J.P. Rothstein, G.H. McKinley, J. Non-Newton. Fluid Mech. 98 (2001) 33-63, in their experiments (epd=O(3) for α=4 contraction-ratio and PS/PS Boger fluids); it is also capable of reproducing the Boger-fluid pressure-drop rise, relative to Newtonian-instance, in axisymmetric α=4 contraction-flow, as opposed to the null rise observed in the planar counterpart reported by S. Nigen, K. Walters, J. Non-Newton. Fluid Mech., 102 (2002) 343-359. In the present study, at each contraction-ratio and under De-rise (flow-rate-increase), one may identify two main phases: i) an epd plateauing-region at low deformation-rates, and ii) a sudden epd-rise above the Newtonian unity reference- line. With elevation in contraction-ratio, the first plateaued-epd phase is elongated and the maximum epd-levels rise significantly. Such epd-elevation is captured theoretically and numerically, with counterpart rise in extensional-viscosity. In addition, this position in epd-response correlates well against trends in vortex-dynamics - correctly capturing lip-vortex appearance, lip-vortex and salient-corner vortex co-existence and coalescence, and ultimate elastic corner-vortex domination. In this respect, their presence and transitions, may themselves be linked to increased elastic effects and normal-stress response

Diversification trajectories and paleobiogeography of Neogene chondrichthyans from Europe

Despite the rich fossil record of Neogene chondrichthyans (chimaeras, sharks, rays, and skates) from Europe, little is known about the macroevolutionary processes that generated their current diversity and geographical distribution. We compiled 4368 Neogene occurrences comprising 102 genera, 41 families, and 12 orders from four European regions (Atlantic, Mediterranean, North Sea, and Paratethys) and evaluated their diversification trajectories and paleobiogeographic patterns. In all regions analyzed, we found that generic richness increased during the early Miocene, then decreased sharply during the middle Miocene in the Paratethys, and moderately during the late Miocene and Pliocene in the Mediterranean and North Seas. Origination rates display the most significant pulses in the early Miocene in all regions. Extinction rate pulses varied across regions, with the Paratethys displaying the most significant pulses during the late Miocene and the Mediterranean and North Seas during the late Miocene and early Pliocene. Overall, up to 27% and 56% of the European Neogene genera are now globally and regionally extinct, respectively. The observed pulses of origination and extinction in the different regions coincide with warming and cooling events that occurred during the Neogene globally and regionally. Our study reveals complex diversity dynamics of Neogene chondrichthyans from Europe and their distinct biogeographic composition despite the multiple marine passages that connected the different marine regions during this time

Modelling ozone disinfection process for creating COVID-19 secure spaces

PurposeA novel modelling approach is proposed to study ozone distribution and destruction in indoor spaces. The level of ozone gas concentration in the air, confined within an indoor space during an ozone-based disinfection process, is analysed. The purpose of this work is to investigate how ozone is distributed in time within an enclosed space.Design/methodology/approachA computational methodology for predicting the space- and time-dependent ozone concentration within the room across the consecutive steps of the disinfection process (generation, dwelling and destruction modes) is proposed. The emission and removal of ozone from the air volume are possible by means of a generator located in the middle of the room. This model also accounts for ozone reactions and decay kinetics, and gravity effect on the air.FindingThis work is validated against experimental measurements at different locations in the room during the disinfection cycle. The numerical results are in good agreement with the experimental data. This comparison proves that the presented methodology is able to provide accurate predictions of the time evolution of ozone concentration at different locations of the enclosed space.Originality/valueThis study introduces a novel computational methodology describing solute transport by turbulent flow for predicting the level of ozone concentration within a closed room during a COVID-19 disinfection process. A parametric study is carried out to evaluate the impact of system settings on the time variation of ozone concentration within the space considered