Abrupt Flow Research Articles

On more insightful dimensionless numbers for computational viscoelastic rheology

Abrupt contraction flows involving viscoelastic fluids represent a longstanding computational challenge within the field of non-Newtonian fluid mechanics. Despite the apparent simplicity of the geometry, these flows have given rise to intricate discussions in the study of viscoelastic phenomena. This study aims to re-examine the numerical solutions for flows through abrupt contractions, offering a fresh interpretation through the lens of reformulated dimensionless numbers. These numbers are designed to consider the characteristic shear rate of the problem, providing a more comprehensive understanding of the underlying dynamics.When investigating models with intermediate levels of complexity, such as the Giesekus and Phan-Thien-Tanner constitutive equations, the usual comparison with the corresponding Oldroyd-B model becomes inadequate because it tends to rely on the nominal relaxation time (λ) and the nominal total viscosity (η) instead of their effective counterparts when defining the Reynolds number (Re), the Weissenberg number (Wi) and the ratio of solvent to total viscosities (β) (β plays a role only in rheological models involving a solvent contribution). If these dimensionless numbers are tailored to account for the characteristic shear rate specific to the problem under investigation, the choice of the corresponding Oldroyd-B flow, at the adequate values of Re, Wi, and β allows for significantly better quantification of the correct effects of nonlinear viscoelasticity of the original model.We show the conventional approach tends to overemphasize the role of the nonlinear parameter in nonlinear constitutive equations, like the Giesekus and PTT models, when examining standard abrupt contraction flow outputs such as the Couette correction and vortex size. This overestimation occurs because the conventional method does not allow the Reynolds and Weissenberg numbers (and possibly β) to carry the portion of the nonlinear effect that can potentially be captured by the linear Oldroyd-B model through the use of characteristic shear rate-based values. We believe the present approach provides a better perspective of the role played by the nonlinear parameter and its extension to more general flows is also discussed.

Riparian cottonwood mortality following compound impacts from river water withdrawal and hydroclimatic variation

AbstractOriginating in Glacier Park, Montana, the transboundary St. Mary River has provided a focus for international water sharing for a century. It was dammed and diverted in the USA, and more extensively in Alberta, where it supports Canada's centre for agricultural irrigation. Following water withdrawals, the riparian cottonwoods (Populus angustifolia) collapsed, and we assessed the woodland decline downstream from the St. Mary Dam, through eight sets of aerial photographs over seven decades. These revealed 88% woodland loss from 1951 to 1999 (R2 = 0.984), including a steeper decline with the 1980s drought. Following the implementation of an environmental flow regime that increased minimum flows after 1993, the remnant woodlands were stabilized, with a slight recovery by 2022. Analyses of the historical hydrology revealed compound contributions to the woodland mortality. (1) Annual river flows declined with increasing water diversion, and (2) late summer flows were particularly depleted. (3) Dam operations resulted in abrupt stage recessions and irregular spikes. (4) Accompanying climate warming, the spring snowmelt advanced and late summer flows declined. (5) The greatest climatic influence involved multiple‐year clusters with high versus low flows, corresponding with the Pacific decadal oscillation (PDO) from 1920 to 1990 (R2 = 0.365), but less coupled thereafter. This long‐term study demonstrates the coordination between hydroclimatic variation, water management and the fate of riparian woodlands, which reveal ecosystem health. An environmental flow regime provided benefit, and for other regulated rivers in dry ecoregions, we recommend functional flow regimes that provide sufficient minimum flows and avoid abrupt flow recession or irregular spikes.

Validation of computational fluid dynamics simulations for determining pressure loss coefficients of ventilation components

Ventilation systems include a variety of components for which necessary pressure loss data is often unavailable. Computational fluid dynamics simulations could substitute for expensive measurements, but validation simulations with suitable data are crucial to assess model uncertainties. Existing CFD validation studies either did not focus specifically on pressure losses, only covered few components, or did not include recent developments in turbulence modelling. In the present work, 33 bends, 4 gates and 2 tees were simulated using a consistent approach. Computational fluid dynamics simulations were validated with published data: rectangular high-edge and wide-edge bends from the experimental dataset of Sprenger, gates and diverging tees from the SMACMA guide. The considered flows cover important basic flow phenomena: deflection, splitting and flow separation. The 39 components were simulated with three turbulence models at 14 Reynolds numbers. The simulations predicted pressure loss coefficients accurately for various components. Cases with strong flow separation regions were most challenging. The model prediction uncertainty was assessed by carrying out simulations with three selected turbulence models. As in the experimental data from Sprenger, the simulations showed a distinct dependence of pressure loss coefficients on the Reynolds number for bends. In contrast, for abrupt deflections and flow separation at sharp edges, the Reynolds number dependency was minor. Technical pressure loss data of ductwork components is needed for the dimensioning, optimisation, and energy assessment of ventilation systems. The present validation study assesses the present state of the art of CFD simulations to determine pressure loss coefficients and the resulting prediction uncertainties.

Blowing pressure stabilization method for the artificial excitation of reed instruments (L).

To investigate the acoustics of reed instruments without the need for a human player, blowing machines are needed, which can generate air pressures up to 8 kPa and flow rates up to 40 liters per minute. Due to reed flexibility and the changing pressure gradient across the reed, the relationship between flow and pressure is highly non-linear. Since the output pressure of ventilators is highly dependent on flow, non-linear pressure regulation is a difficult task that requires a closed-loop approach. Since reed vibration starts suddenly when blowing pressure is gradually increased, an abrupt change in airflow through the instrument is present, resulting in a change in pressure in the artificial mouth. To avoid that, a method is presented to achieve a fast response to abrupt flow changes, which is tested in an existing blowing machine. The enhanced blowing machine exhibits a settling time below 200 ms, which allows for the generation of blowing pressures with linear responses.

Investigation of the hydrothermal phenomena in a wavy microchannel with secondary flow passages through mid-wall inflection points

Advancement in manufacturing processeshas fuelled research in microchannel heat sinks to meet the demands of modern microelectronics. The heat sink design in the present study considers longitudinally wavy microchannels incorporating secondary flow passages. The secondary flow has been investigated for abrupt flow bifurcation (at 60° angle) and smooth flow bifurcation (at 30° angle). Further, these geometries were tested with different inlet–outlet configurations. The main objective of the paper is to study how a secondary flow would affect the flow profile and thermal performance of a wavy microchannel heat sink. The flow profile, pressure drop, temperature profiles, and average Nusselt number for a wide range of flow rates have been obtained and compared with those for the conventional design, and associated trends have been explored. Furthermore, the overall performance of the heat sink designs has been quantified by plotting the thermal enhancement factor (TEF) against a wide range of Reynolds numbers. The heat sink with flow bifurcation of 30° attains the highest value of Nuselt number below the flow rate of 1.8 × 10-6 m3/s, whereas above this flow rate, the Nusselt number value of the conventional wavy heat sink dominates the proposed heat sink designs. The study corresponds to the demands of modern microchips such as the 13th Gen Intel® Core™ processor and serves as a guideline for manufacturing heat sinks with wavy microchannels incorporating secondary flow passages.

Flood patterns in a catchment with mixed bedrock geology and a hilly landscape: identification of flashy runoff contributions during storm events

Abstract. With flash flood events having been repeatedly observed in Central and Western Europe in recent years, there is a growing interest in how catchment physiographic properties and hydrological conditions are eventually controlling rapid and concentrated hydrological responses. Here we focus on a set of two nested catchments in Luxembourg (Europe) that have been exposed in 2016 and 2018 to flash flood events and study their seasonal runoff time transfer distributions. Both catchments are of similar size (∼ 30 km2) and have analogous hydrological distance distributions, but their geological bedrock and landscape features are notably different. The upper catchment (KOE) is dominated by a low land area (38 % of the catchment is located less than 30 m above the river network) consisting of variegated marly bedrock (middle Keuper Km3) and moderately steep Luxembourg sandstone outcrops (lower Liassic Li2). The lower catchment (HM) has its drainage network deeply cut into the Luxembourg sandstone, with half of it being covered by marly plateaus (Lower Liassic Li3, located between 80 and 100 m above the river network) featuring heavy clay soil. Based on data generated from a dedicated hydro-meteorological monitoring network, we calculated for 40 rainfall–runoff events observed between August 2019 and July 2021 the corresponding net rainfall transfer time distributions (TTDs) from the hillslopes to the catchment outlet. We then compared the TTD properties and related them to the catchment's hydrological state and rainfall properties. We observed a marked seasonality in TTDs for both catchments. The KOE catchment reacts fastest during the winter period (December–February), while its response time is most delayed and spread out during periods of catchment recharging (October–November) and drying (March–May). The HM catchment exhibits similar TTDs during the mid-October to mid-April period, but they diverge markedly during the remaining part of the year, with opposite variations. During the mid-April to mid-October period, the average response time increases progressively in the KOE catchment. This behavior is in stark contrast to the HM catchment, where response times are significantly shorter (peak discharge delay time decreases by −70 % ± 28 %) and more concentrated (runoff volume occurring in 1 h increases by +48 % ± 87 %) during the mid-April to mid-October, in comparison to the extended winter period. This opposite seasonality leads us to consider different control factors of the runoff transfer processes in relation with the topographic and geological layout of the catchment areas. In the KOE catchment, we found the TTD to be essentially driven by onset and cessation of hydrological connectivity on the flat marly terrain – the latter operating like a variable contributing area in terms of deep soil storage dynamics (except for one summer event). The HM section exhibits contrasted TTDs throughout the year, suggesting threshold-dependent hydrological processes. More specifically, particularly quick runoff transfers seem to dominate under dry conditions (mid-April to mid-October). Correlation analyses compared to the literature on runoff generation on the one hand and our descriptive knowledge of the catchments on the other hand suggest multiple causes for the triggering of these rapid flows. The fractured marly plateaus, but also the hydrophobic forest litter forming during dry conditions on steep slopes, stand as our main hypotheses in this respect. Moreover, the absence of a riparian zone, preventing any dampening of (observed) abrupt and massive flows during extreme precipitation events, also seems to be a key feature of the rapid runoff transfer. For improving our understanding and forecasting capabilities in Luxembourg (and more broadly in the nearby regions of Germany, Belgium, and France with similar physiographic and climate conditions), we recommend further studies focusing on catchments with fractured bedrock and limited riparian zones. Special attention may equally be given to the hypothesized responses of hydrophobic soil surfaces on steep hillslopes and marly soils to heavy precipitation events occurring after extended dry spells.

3D simulation of the hierarchical multi-mode molecular stress function constitutive model in an abrupt contraction flow

• First simulation of the hierarchical multi-mode molecular stress function model. • The model is compared to experiment for a 3D contraction flow of Lupolen LDPE. • Striking agreement shown for vortex opening angles over a large range of flow rates. • Simulated and experimental birefringence show very good correspondence. • Variation of apparent stress optical coefficient with flow rate shown to be low. A recent development of the Molecular Stress Function constitutive model, the Hierarchical Multi-Mode Molecular Stress Function (HMMSF) model has been shown to fit a large range of rheometrical data with accuracy, for a large range of polymer melts. We develop a 3D simulation of the HMMSF model and compare it to experimental data for the flow of Lupolen 1840H LDPE through an abrupt 3D contraction flow. We believe this to be the first finite element implementation of the HMMSF model. It is shown that the model gives a striking agreement with experimental vortex opening angles, with very good agreement to full-field birefringence measurements, over a wide range of flow rates. A method to give fully-developed inlet boundary conditions is implemented (in place of using parabolic inlet boundary conditions), which gives a significantly improved match to birefringence measurements in the inlet area, and in low stress areas downstream from the inlet. Alternative constitutive model parameters are assessed following the principle that extensional rheometer data actually provides a ‘lower bound’ for peak extensional viscosity. It is shown that the model robustly maintains an accurate fit to vortex opening angle and full-field birefringence data, provided that both adjustable parameters are kept such that both shear and extensional data are well fitted.

Residual stresses and shear-induced overaging in boehmite gels

Colloidal gels respond as soft solids at rest, whereas they flow as liquids under external shear. Starting from a fluidized state under an applied shear rate ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p}$, abrupt flow cessation triggers a liquid-to-solid transition during which the stress relaxes towards a so-called residual stress ${\ensuremath{\sigma}}_{\mathrm{res}}$ that tallies a macroscopic signature of previous shear history. Here, we report on the liquid-to-solid transition in gels of boehmite, an aluminum oxide, that shows a remarkable nonmonotonic stress relaxation towards a residual stress ${\ensuremath{\sigma}}_{\mathrm{res}}({\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p})$ characterized by a dual behavior relative to a critical value ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$ of the shear rate ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p}$. Following shear at ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p}>{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$, the gel obtained upon flow cessation is insensitive to shear history, and the residual stress is negligible. However, for ${\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{p}<{\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\gamma}}}_{c}$, the gel encodes some memory of the shear history, and ${\ensuremath{\sigma}}_{\mathrm{res}}$ increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both ${\ensuremath{\sigma}}_{\mathrm{res}}$ and the gel viscoelastic properties increase logarithmically with the strain accumulated during the shear period preceding flow cessation. Such a shear-induced ``overaging'' phenomenon bears great potential for tuning the rheological properties of colloidal gels.

Flow-induced structure change of rod-like micellar solutions in an abrupt contraction and expansion flow

Owing to investigation of generation for flow-induced structure (FIS) of micellar solutions in an abrupt contraction and expansion flow, we observed the FIS of a rod-like micellar solution passing through a rectangular channel with a symmetric or asymmetric slit. Furthermore, liquid-temperature was changed as experimental conditions (relatively high-, room-, and low-temperature). For relatively high-temperature, a weak FIS exhibited in a symmetric slit-flow. For roomtemperature, agreement between the resultant FIS and the reported FIS was obtained. For relatively low-temperature, drastic FIS in upstream and slit section. In case of an asymmetric slit flow, the same tendency was obtained. However, profile of retardation was asymmetric in slit section. Moreover, degree of spatial constraint was discussed.

An update on the coronary calcium score: a review for clinicians.

The clinical manifestation of coronary artery atherosclerosis is coronary artery disease (CAD) with symptoms ranging from exertional chest pain due to reduction of coronary flow reserve to acute coronary syndrome due to rupture of usually a nonobstructive plaque with abrupt coronary blood flow reduction. CAD is the leading cause of morbidity and mortality worldwide. Therefore, identifying asymptomatic people at risk of CAD is pivotal to guide decision-making for primary prevention. Coronary artery calcium (CAC) is a hallmark of coronary artery atherosclerosis. It can be detected using cardiac computed tomography and quantified by the Agatston method. CAC examination is a cheap, fast and low radiation dose test, without injecting a contrast agent. It provides prognostic information over other traditional cardiovascular risk markers and established scoring systems, especially for low-risk subgroups such as women and younger adults, and indicates the appropriate moment to implement primary prevention, including acetylsalicylic acid and statins. In this review, we discuss the methods of CAC evaluation, the meaning of a zero CAC score (CACS), its conversion to CACS > 0 and the impact of this fact on cardiovascular risk, the effect of statins and proprotein convertase subtilisin/kexin type 9 inhibitor on CAC progression, interpretation of CACS results, and CACS prognostic value in both asymptomatic and symptomatic patients.

Effect of nozzle inlet swirl and air column shape in nozzle on water jet occurring periodic droplets splashing

Water with free surface inside the water tank was pressurized and ejected as a water jet from nozzle with abrupt contraction flow path attached to the bottom of the tank. When the height of the free surface of water makes low into some height from the bottom of the tank, a peculiar air column is generated in the nozzle, a phenomenon occurs in which the droplets are periodically aligned outward in the radial direction and are rapidly splashed from the jet due to the pressure difference from the atmospheric pressure. In order to investigate how water swirling near the nozzle inlet affects this droplet splashing phenomenon, The bottom of the tank was designed to affect the flow of water, and the flow of water was analyzed using visualization photography. As a result, it was found that the circumferential velocity has a great influence on the droplet splashing phenomenon under low pressure. When the circumferential velocity increases, it affects the shape of the air column in the large tube of nozzle, and the shape makes it easier for air bubbles to be generated in the small tube of nozzle. It was found that these processes facilitated the occurrence of the droplet splashing phenomenon.

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

Colloidal gels are out-of-equilibrium soft solids composed of attractive Brownian particles that form a space-spanning network at low volume fractions. The elastic properties of these systems result from the network microstructure, which is very sensitive to shear history. Here, we take advantage of such sensitivity to tune the viscoelastic properties of a colloidal gel made of carbon black nanoparticles. Starting from a fluidized state at an applied shear rate 0, we use an abrupt flow cessation to trigger a liquid-to-solid transition. We observe that the resulting gel is all the more elastic when the shear rate 0 is low and that the viscoelastic spectra can be mapped on a master curve. Moreover, coupling rheometry to small angle X-ray scattering allows us to show that the gel microstructure is different from gels solely formed by thermal agitation where only two length scales are observed: the dimension of the colloidal and the dimension of the fractal aggregates. Competition between shear and thermal energy leads to gels with three characteristic length scales. Such gels structure in a percolated network of fractal clusters that interpenetrate each other. Experiments on gels prepared with various shear histories reveal that cluster interpenetration increases with decreasing values of the shear rate 0 applied before flow cessation. These observations strongly suggest that cluster interpenetration drives the gel elasticity, which we confirm using a structural model. Our results, which are in stark contrast to previous literature, where gel elasticity was either linked to cluster connectivity or to bending modes, highlight a novel local parameter controlling the macroscopic viscoelastic properties of colloidal gels.

토사 물성의 불확실성을 고려한 토석류 퇴적특성 수치해석 연구

Recent climate change often causes heavy rainfall inducing abrupt debris flow that results in many forms of damage and loss. There have been extensive investigations on the characteristics of debris flow mainly by experimental methods. However, the results of the experimental studies always include uncertainly because the material properties of debris flow such as water content and composition of soil used in the studies varies to some extent. In order to identify the uncertainty, this study employs the 3D Navier-Stokes equation with the volume of fluid (VOF) method in order to consider multi-phase materials, and the nonlinear viscous model was implemented. Both variations of water contents and nonlinear viscosity were considered and the results of deposition features were compared with previous studies. The results show that the deposition width of the debris flow tends to decrease as the volumetric water content of the debris flow increases, and increase as the free parameter τSUB00/SUB used to estimate the yield stress increases. It is expected that the results of this study on the uncertainty of the material properties of the debris flow can present the range of affected region and that it can be used for the study of the impact load on the downstream field.

Modal Characterization, Aerodynamics, and Gust Response of an Electroactive Membrane

Flexibility and electric field sensitivity of dielectric elastomer membranes motivate the consideration of their applications in the wings of unmanned aerial vehicles. Applied electric field excitation expands the dielectric elastomer in the in-plane direction by compressing it into the thickness direction, which causes a change in the tension and ultimately affects the dynamic behavior of the elastomer. This study outlines the modal characteristics and fluid-structural dynamic behavior of the very high bond 4910 membrane, a type of dielectric elastomer, especially in an abrupt flow environment. An experimental shaker arrangement along with the digital image correlation system is used for experimental vibration testing. A finite element model is developed to study the modal characteristics and validate the experimental results. A fluid–structure interaction model of the electroactive membrane and its surrounding airflow is also developed to investigate the aerodynamic responses of the structure. Gust environments are predicted using the von Kármán and Dryden gust models, and the effect of different gust velocities on the aerodynamic lift and drag are obtained from the numerical fluid–structure interaction models. A wind tunnel, equipped with a data acquisition system, is used to study the aerodynamic responses and validate the numerical results experimentally. The modal analysis shows that the resonance frequencies decreased as the applied electric field excitation is increased. The coefficients of lift and drag fluctuate with the stochastic gust distribution, and the effects of von Kármán and Dryden gust profiles on aerodynamic characteristics are comparable. The values of the coefficients of lift and drag increase with the increase of applied voltage. The numerical results for the modal and aerodynamic responses show a good agreement with their corresponding experimental findings.

Making China Modern: From the Great Qing to Xi Jinping

Making China Modern: From the Great Qing to Xi Jinping

Effects of hydropeaking on the behaviour, fine‐scale movements and habitat selection of an Iberian cyprinid fish

AbstractHydropeaking is widely known for changing the quantity and quality of the available habitat downstream of hydroelectric facilities, thus affecting all stages of fish life cycles. Hydropeaking impacts on salmonids are widely studied, but knowledge of its effects on cyprinids, which are dominant in Mediterranean areas, is scarce. In this study, 11 Iberian barbel (Luciobarbus bocagei, Steindachner 1864) were tagged with radio transmitters equipped with ElectroMyoGram (EMG) sensors, aiming: to (a) assess the behaviour, fine‐scale movements and habitat selection of the target species in response to periods of artificial and abrupt flow variations (i.e., hydropeaking); and (b) identify, which environmental variables and, in particular, flow components can influence the species behaviour. Results from the six barbel that provided analysable data indicate that fish were more active during the day and in periods of ecological flow. Moreover, during hydropeaking, especially during the increase in flow magnitude, fish activity decreased with the tagged fish showing a refuge‐seeking behaviour. This information can be used to minimize the changes caused in the aquatic habitat and fish communities occurring downstream of hydroelectric dams.

A Study on the Delineation of Flushing Sections and Simulation Method to Verify Appropriate Flushing Velocity for Applying Flushing

Recently, water supply networks have experienced deterioration because of various factors, which result in the accumulation of a large amount of scale on the inside surface of pipes. The scale may enter houses when a surge occurs owing to abrupt velocity or flow direction change in pipes, and eventually, water quality accident such as red-water can occur. The scale should be removed by applying pipe-cleaning practices, such as flushing, to prevent this type of accident. Currently, most studies have suggested only the desirable flow velocity to remove the scale through flushing. In our opinion, it will be more critical to develop a method for determining whether it would be possible to achieve the desired flow velocity. Hence, we developed a method to delineate a flushing section and modify a water supply network in considering the minor loss on the hydrant using the head loss-flow rate curve for the hydrant. Next, we simulated a flow condition using the emitter coefficient to achieve the desired flow velocity for flushing each section. The proposed method was applied to a part of the water supply network of a city in Korea. From the results, the pressure head at the primary water source point of the network was sufficient such that each flushing section experienced the desired flow velocity to complete the flushing process. We lowered the pressure head by 20 m at the primary water source point and simulated a flow condition for insufficient pressure head. Under these conditions, some of the flushing sections could not experience the desired flow velocity, and hence, the scale could not be removed as desired. In this case, we believe that the utility authority should decide whether flushing is still a suitable option for cleaning the pipe, or whether other cleaning methods should be considered.

An international model comparison study of controlled fault activation experiments in argillaceous claystone at the Mont Terri Laboratory

We present results from an international model comparison study involving a series of controlled fault activation experiments in Opalinus Clay at the Mont Terri Laboratory, Switzerland. The fault activation experiments were conducted in situ by water injection at variable pressure from boreholes targeting different parts of the Main Fault crossing several tunnels and galleries of the Mont Terri Laboratory. The model simulations focused on (1) an experiment activating a discontinuity (fracture or minor fault) within the damage zone of the main fault, and (2) an experiment activating discontinuities close to the core of the Main Fault. The experimental data consist of coupled hydraulic and mechanical responses monitored at an injection borehole and a monitoring borehole located a few meters away. After overcoming several modeling issues along with necessary model developments, a reasonably good agreement was achieved between the modeling results and the field observations. The fault activation experiments displayed an abrupt flow rate increase associated with a sudden fracture opening and rupture propagation after the injection pressure reached above the estimated normal stress on the fracture. This was followed by an abrupt flow rate decrease, indicating hydraulic closing, once the injection pressure decreases to an estimated 1–2 MPa below the stress normal to the opened fracture. The models were able to capture this abrupt hydromechanical behavior, including an observed dominant opening behavior along with the rupture propagation, while the spatial extent of the shear rupture and the quantity of peak injection flow were the most challenging to predict.

The Chair’s Summing Up Independent Evaluation Office - IMF advice on Capital Flows Executive Board Meeting 20/93 September 18, 2020

Executive Directors welcomed the report of the Independent Evaluation Office (IEO) on IMF Advice on Capital Flows. Directors appreciated the high quality of the report, and its thematic and background country studies. Directors welcomed the finding that the adoption of the Institutional View (IV), along with the development of other frameworks and additional tools, had represented a major advance in the Fund’s policy framework to provide systematic advice to member countries on the management of capital flows and capital account liberalization. Directors also noted the conclusion that, in its application, the Fund had generally followed the IV and other policy frameworks to ensure that the advice was consistent, tailored to country circumstances, and evenhanded across countries. Directors welcomed that capital flow management measures (CFMs) have generally not been used to substitute for warranted policy adjustments. Directors also welcomed the finding that most authorities broadly support the IV’s sequenced framework to capital account liberalization and appreciated the Fund’s specific advice in many cases, especially in the context of technical assistance. More recently, faced with the abrupt capital flow reversals during the COVID-19 crisis, Directors noted that emerging markets and developing economies generally followed a multi-pronged approach broadly consistent with the IV framework and made relatively little use of CFMs.

An experimental and simulation comparison of a 3-D abrupt contraction flow using the molecular stress function constitutive model

ABSTRACT The Molecular Stress Function (MSF) constitutive model with convective constraint release (CCR) mechanism has been shown to accurately fit a large range of viscometric data, and also shown to give strong vortex growth in flows of LDPE through planar and axisymmetric contractions. This work compares simulation and experimental results for 3-D flows of Lupolen 1840H LDPE through a contraction slit. Comparisons are made with vortex opening angles obtained from streak photography, and also with stress-birefringence measurements. The comparisons are made with two versions of the CCR mechanism. The simulated vortex angles for one version of the CCR mechanism are found to approach what is seen experimentally. The best-fit value for the stress optical coefficient was found to vary between CCRs and to decrease with flow rate. This is partially explained by different centreline elongational rates with the two CCRs, which in turn is related to different opening angles.