Constriction Geometry Research Articles

Efficient Emission of Highly Polarized Thermal Radiation from a Suspended Aligned Carbon Nanotube Film.

A polarized light source covering a wide wavelength range is required in applications across diverse fields, including optical communication, photonics, spectroscopy, and imaging. For practical applications, high degrees of polarization and thermal performance are needed to ensure the stability of the radiation intensity and low energy consumption. Here, we achieved efficient emission of highly polarized and broadband thermal radiation from a suspended aligned carbon nanotube film. The anisotropic nature of the film, combined with the suspension, led to a high degree of linear polarization (∼0.9) and great thermal performance. Furthermore, we performed time-resolved measurements of thermal emission from the film, revealing a fast time response of approximately a few microseconds. We also obtained visible light emission from the device and analyzed the film's mechanical breakdown behavior to improve the emission intensity. Finally, we demonstrated that suspended devices with a constriction geometry can enhance the heating performance. These results show that carbon nanotube film-based devices, as electrically driven thermal emitters of polarized radiation, can play an important role for future development in optoelectronics and spectroscopy.

Super-Suppression of Long-Wavelength Phonons in Constricted Nanoporous Geometries.

In a typical semiconductor material, the majority of the heat is carried by long-wavelength, long-mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean free paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessen's rule. In general, however, long-wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale molecular-dynamics simulations, non-equilibrium Green's function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries that create narrow constrictions in the passage of phonons lead to anticorrelated heat currents in the phonon spectrum. This effect results in super-suppression of long-wavelength phonons due to heat trapping and reductions in the thermal conductivity to values well below those predicted by Matthiessen's rule.

Three‐dimensional simulation of dripping and jetting phenomenon in a flow‐focusing geometry

Abstract3D simulations have been achieved on a flow‐focusing geometry employing the VOF method to study the consequence of viscosity, surface tension, wettability, and geometry on drop generation for the dripping regime. Here the dispersed phase is the PDMS oil (polydimethylsiloxane), and the continuous phase is the water. Simulations were performed at different oil‐to‐water viscosity ratios of 3, 12, 27, and 50. The interfacial tension between PDMS oil and water is 0.0118 N/m. It has been abridged to 0.008 N/m, 0.005 N/m, and 0.002 N/m, and simulations were performed. The walls of the microchannel are considered to be PMMA surfaces. The contact angle of an oil droplet on the PMMA surface in the presence of water is 140°. The effect of wettability was shown at various contact angles (angle created by water droplet on the PMMA surface in the presence of oil) of 0°, 40°, 90°, 135° and 180°. The frequency of droplet generation (1/s), non‐dimensional droplet length (L/Wc), droplet volume (nl), and droplet velocity (m/s) have been calculated for each of the cases. A flow pattern map has been industrialized classifying the dripping and jetting regimes. A comparison between normal geometry and two constricted geometries (having different orifice lengths) based on the frequency of droplet, non‐dimensional drop length, drop volume, and drop velocity has been made for both dripping and jetting regimes. Prediction of simulated non‐dimensional droplet length has also been made using dimensional analysis.

Characterizing jamming of dilute and semi-dilute fiber suspensions in a sudden contraction and a T-junction

The clogging or jamming of particle suspensions is a ubiquitous problem, hindering the efficiency of particle–liquid and particle–particle separations. Motivated by pressure screening in the pulp and paper industry, we characterize jamming of dilute and semi-dilute mono-disperse rigid-rod suspensions passing through channels mimicking dead-end and cross-flow filtration membranes, experimentally, using particle-tracking velocimetry. We observe that jams nucleate by either bridging of isolated particles across the constriction, or by localized mechanical entanglement of the particles, i.e., flocculation. Uniquely, we observe floc-formation during acceleration into the aperture and report this as primary mechanism for jamming events. We characterized the accumulation-release cycles of the jamming event using an exponential probability distribution; this distribution is indicative of a Poisson process. For jams nucleated by single-particle bridging, the distribution is (primarily) related to the number of fibers passing through the aperture; this is similar to dry, granular materials. For floc-based nucleation events, the distribution is (primarily) related to the suspension concentration with the average time between jams decreasing inversely with the square-root of the initial suspension concentration. For the conditions tested, the distribution was insensitive to changes in constriction geometry.

CFD investigation of the effect of flow field channel design based on constriction and enlargement configurations on PEMFC performance

In this study, a three-dimensional CFD, non-isothermal model was developed to identify an optimal layout for key geometrical parameters of PEMFC flow field designs with constriction and enlargement cross-sections located at the middle of the channel. For this purpose, the effect of channel configurations with both constriction and enlargement cross-sections was explored and compared with a conventional constant channel segment, under varying hydraulic diameter and lengths. As results, the constriction configuration channel with a hydraulic diameter reduction of 50% has shown the best performance among the enlargement and straight channels for all considered lengths. The constriction configuration enhances fluid velocity, resulting in improved flow distribution and relatively increased pressure drop across the flow channels. Moreover, a better temperature distribution, higher oxygen consumption, improved membrane humidification, and water removal were observed. This leads to an increased net power output of the PEMFC. However, considering different hydraulic diameters of the constriction geometry, the best performance of the cell was achieved at a hydraulic diameter reduction of 60%, combined with a length of 4/7 of the total channel length. This optimal balance between pressure drops and reactants’ flow has enhanced the net power of the proposed geometry by 26.30 %, compared to the straight channel. The novel proposed single-channel configuration based on a simple constriction, is a low-cost design that has significantly improved cell performance compared to the straight one. Since operating conditions are crucial for cell performance, the effects of operating temperature and relative humidity were investigated, and the optimum conditions were determined.

Development of annular pump to enhance drilling distance and cuttings return in horizontal directional drilling

In many shallow horizontal directional drilling (HDD) projects, the annulus fluid pressure soon reaches the blowout or frac-out pressure of the soil formation. The pressure at single-source pump therefore must be limited. This limits the maximum drilling distance (borehole length), maximum attainable rate of penetration (ROP), and maximum distance over which cuttings can be returned. A novel concept involving a series of annular pumps distributed along the borehole length to aid in generation of distributed annulus pressure boost and cuttings transport in HDD was conceived. The success of the above concept, designated as distributed pumping system (DPS), relies on the ability to design and power an annular pump that can fit in the constricted annulus geometry of HDD boreholes while satisfying the performance requirements. We present the laboratory-scale development of three annular pumps based on the turbomachinery design principle of axial blade propellers. The pump prototypes were 3D printed and tested in the laboratory test rig flow loop built to simulate the annular flow configuration of HDD boreholes. The annular pump was powered using the axial propeller turbine housed inside the hub of each annular pump. The turbine converts hydraulic energy from the inner flow (analogous to drilling fluid flow within drill rod) to power the integrated setup of the axial inner turbine and annular pump, in short, designated as integrated turbine-pump (ITP). Results from the tests on three preliminary prototypes of the annular pump using water as the medium in the test rig yielded a maximum pressure boost of 44 kPa–enough to space the annular pumps 80 m apart when designed for a typical annular pressure drop rate of 0.54 kPa/m. The performance index (i) of the ITP, defined as the ratio of pressure boost provided by the annular pump in the outer (annular) flow to the pressure drop created by an inner turbine in the inner flow, reached a maximum value of 0.6. The results from the study promise a technology that can enable long-distance HDD cuttings return without the risk of blowout or the need for an external power source.

Interface Shapes in Microfluidic Porous Media: Conditions Allowing Steady, Simultaneous Two-Phase Flow

Microfluidic devices offer unique opportunities to directly observe multiphase flow in porous media. However, as a representation of flow in geological pore networks, conventional microfluidics face several challenges. One is whether steady simultaneous two-phase flow through a two-dimensional network is possible without fluctuating occupancy of the pore constrictions. Flow without fluctuations can occur only if the flow paths of the two phases can cross on the 2D network; this requires that wetting phase can form a bridge across the gap between grains at a pore constriction while non-wetting phase flows through the constriction. We consider the conditions under which this is possible as a function of the local capillary pressure and the geometry of the constriction. Using the Surface Evolver software, we determine conditions for stable interfaces in constricted geometries, the range of capillary pressures at which bridging can occur, and those where the wetting phase would re-invade the constriction to block the flow of the non-wetting phase (“snap-off”). If a constriction is long and either straight or uniformly curved, snap-off occurs at the same capillary pressure as bridging. For constrictions of concave shape, which we represent as constrictions between cylindrical grains, however, we find a range of capillary pressures at which bridging is stable; the range is greater the narrower the diameter of the cylinders (i.e. the more strongly concave the throat) relative to the width of the constriction. For smaller-diameter pillars, the phenomenon of “Roof” snap-off as non-wetting phase invades a downstream pore body, is predicted not to occur.

Three-dimensional simulation of droplet dynamics in a fractionally-wet constricted channel

This study presents droplet dynamics due to capillarity-wettability interaction through a constricted capillary channel. Previous research has investigated the droplet motion in a constricted capillary channel with a two-dimensional geometry and uniform wettability to determine the critical pressure necessary for the droplet to pass through the constriction, but did not consider the change of pressure as the droplet crosses through a constricted geometry. To explore the dynamic behavior of droplet motion across a constriction, a direct numerical simulation of a droplet in a three-dimensional fractionally-wet constricted channel is performed where we have two different contact angles on opposing faces of the constriction. The results show that the fractional wettability condition significantly affects the evolution of fluid-fluid and fluid-solid interfaces, pressure drop, and displacement patterns. We show that droplet dynamics in a fractionally-wet channel do not necessarily follow the same behavior as in a uniform capillary channel and cannot be predicted using uniform wettability surfaces depicted by an average contact angle. In particular, the pressure difference needed to push the droplet through the restriction with fractional wettability is lower than that for a uniform channel with a constant contact angle representing the less favorable wettability state. The fluid-fluid meniscus often forms saddle-shaped interfaces with curvature of opposite sign in orthogonal directions. Moreover, the effects of droplet size, capillary number, viscosity ratio, and constriction shape are elucidated. The pressure difference increases with droplet size and capillary number. Triangular cross-sections can be accessed at a lower pressure because of wetting layer flow.

Polymeric fiber sensors for insertion forces and trajectory determination of cochlear implants in hearing preservation

The level of hearing restoration in patients with severe to profound sensorineural hearing loss by means of cochlear implants (CIs) has drastically risen since the introduction of these neuroprosthetics. The proposed CI integrated with polymer optical fiber Bragg gratings (POFBGs) enables real-time evaluation of insertion forces and trajectory determination during implantation irrespective of the speed of insertion, as well as provides high signal quality, low stiffness levels, minimum induced stress even under forces of high magnitudes and exhibits significant reduction of the risk of fiber breakage inside the constricted cochlear geometry. As such, the proposed device opens new avenues towards atraumatic cochlear implantations and provides a direct route for the next generation of CIs with intraoperative insertion force assessment and path planning capacity crucial for surgical navigation. Hence, adaptation of this technology to clinical reality holds promising prospects for the hearing impaired.

Design of Electrochemical Microfluidic Detectors: Accurate Potential Measurement.

Potential drop due to the electrolyte conductivity between the reference electrode (RE) and the working electrode leads to measurement error. Because of the limited amount of electrolyte and constricted geometry in microfluidic systems, the total potential drop in a microfluidic system is confined within a small part of the cell. This makes the choice and placement of the RE an important consideration. In this article, we discuss ways to incorporate an RE in a microfluidic system and, through numerical modeling and experimental verification, present some design strategies for electrode placement to ensure accurate potential control.

Microstructure control of Hastelloy X by geometry-induced elevation of sample temperature during a laser powder bed fusion process

In this study, the solidification behaviors of Hastelloy X during laser powder bed fusion (LPBF) were deliberately changed by fabricating a sample with constricted geometry. In-situ temperature measurement by a high-speed thermographic camera and part-scale thermal analysis by the finite element method revealed that the constricted geometry suppressed thermal diffusion in the building sample, thereby increasing the sample temperature to over 1000 °C and maintaining the temperature at 700–1000 °C throughout the LPBF process. Multi-track thermal analyses also revealed that both the temperature gradient and solidification rate decreased by more than one digit, leading to a three-digit decrease in the cooling rate. Compared to the parts with the usual thermal history, the parts built at such high temperature had larger crystal grains elongated along the building direction, thicker cellular structures, more carbide precipitates, and a reduction of microhardness. From these results, a solidification map was created based on the correlations between the temperature gradient and solidification rate and the microstructures, and the effects of the elevated temperature on the epitaxial grain growth were also discussed. Consequently, it was demonstrated that the solidification behaviors could be controlled directly during the LPBF process without any additional heating mechanism.

Effect of double cross sectional ratio on performance characteristics of pulsating heat pipes

Effect of double cross sectional ratio on performance characteristics of pulsating heat pipes is experimentally investigated. The cross sectional ratios in the evaporator and condenser sides are different from each other, and this novel gradually constricted geometry provides to boost flow circulation and to maintain a stable performance regardless of orientation. Tests are performed for different values of inclination angle (0°, 30°, 60° and 90°) and filling ratio (20, 40 and 60%). Also, results of originally structured closed loop flat plate pulsating heat pipe (novel design, ND) are compared with those of the conventional pulsating one with uniform cross section (conventional design, CD). Measurements are supported by high speed flow visualization. It is shown that the ND shows better thermal performance than the CD, and when appropriate filling ratio (40%) is provided, the ND presents orientation-independent running. A turn of the ND is divided into three different zones, and a pressure-balance equation is presented following a detailed flow analysis. Rapid bubble growth instability and annular flow pattern characterized by thin film evaporation are two important characteristics observed for the ND.

Numerical simulation of intracellular drug delivery via rapid squeezing.

Intracellular drug delivery by rapid squeezing is one of the most recent and simple cell membrane disruption-mediated drug encapsulation approaches. In this method, cell membranes are perforated in a microfluidic setup due to rapid cell deformation during squeezing through constricted channels. While squeezing-based drug loading has been successful in loading drug molecules into various cell types, such as immune cells, cancer cells, and other primary cells, there is so far no comprehensive understanding of the pore opening mechanism on the cell membrane and the systematic analysis on how different channel geometries and squeezing speed influence drug loading. This article aims to develop a three-dimensional computational model to study the intracellular delivery for compound cells squeezing through microfluidic channels. The Lattice Boltzmann method, as the flow solver, integrated with a spring-connected network via frictional coupling, is employed to capture compound capsule dynamics over fast squeezing. The pore size is proportional to the local areal strain of triangular patches on the compound cell through mathematical correlations derived from molecular dynamics and coarse-grained molecular dynamics simulations. We quantify the drug concentration inside the cell cytoplasm by introducing a new mathematical model for passive diffusion after squeezing. Compared to the existing models, the proposed model does not have any empirical parameters that depend on operating conditions and device geometry. Since the compound cell model is new, it is validated by simulating a nucleated cell under a simple shear flow at different capillary numbers and comparing the results with other numerical models reported in literature. The cell deformation during squeezing is also compared with the pattern found from our compound cell squeezing experiment. Afterward, compound cell squeezing is modeled for different cell squeezing velocities, constriction lengths, and constriction widths. We reported the instantaneous cell center velocity, variations of axial and vertical cell dimensions, cell porosity, and normalized drug concentration to shed light on the underlying physics in fast squeezing-based drug delivery. Consistent with experimental findings in the literature, the numerical results confirm that constriction width reduction, constriction length enlargement, and average cell velocity promote intracellular drug delivery. The results show that the existence of the nucleus increases cell porosity and loaded drug concentration after squeezing. Given geometrical parameters and cell average velocity, the maximum porosity is achieved at three different locations: constriction entrance, constriction middle part, and outside the constriction. Our numerical results provide reasonable justifications for experimental findings on the influences of constriction geometry and cell velocity on the performance of cell-squeezing delivery. We expect this model can help design and optimize squeezing-based cargo delivery.

Topological analysis of single-stranded DNA with an alpha-hederin nanopore

Nanopores have been emerged as a powerful tool for analyzing the structural information and interactional properties of a range of biomolecules. The spatial resolution of nanopore is determined by the diameter and effective thickness of its constriction region, but the presence of vestibule or stem structure in protein-based nanopore could negatively affect the sensitivity of the nanopore when applied for genome sequencing and topological analysis of DNA. Recently, alpha-hederin (Ah) has been reported to form a sub-nanometer scale pore structure in lipid membrane. With the simple structure and extremely small effective thickness, the Ah nanopore was shown to discriminate four different types of nucleotides. However, identification of a certain nucleotide in a strand of DNA, which is essential for genome sequencing, remains challenging. Here, we investigated the resolving capability of Ah nanopore to discriminate few nucleotides in a strand of single-stranded DNA, and the factors determining the sensitivity of Ah nanopore. The Ah nanopore was shown to be able to identify as few as three adenosine nucleotides in a strand of poly cytidine, in which the dwell time of the additional current blockade that represents the adenosine residue was in good agreement with their physical length. We also found that the lateral tension and chain pressure generated around the nanopore were influenced by pore's diameter and played as a dependent variables to determine the geometry of nanopore's constriction as well as the spatial resolution of the Ah nanopore.

Surfactant-Controlled Mobility of Oil Droplets in Mineral Nanopores.

Polymer flooding is one of the widely used enhanced oil recovery (EOR) methods. However, tuning polymer properties to achieve improved performance in porous mineral rocks of diverse oil reservoirs remains one of the challenges of EOR processes. Here, we use molecular dynamics (MD) simulations to examine decane/water mixtures with surfactant additives in calcite and kaolinite mineral nanopores and characterize surfactant properties associated with increased fluid mobility and improved wettability in planar and constricted nanopore geometries. Cetyltrimethylammonium chloride (CTAC) and sodium dodecyl sulfate (SDS) surfactants are found to modulate the contact angles of decane droplets and reduce the decane density on mineral surfaces. CTAC can enhance and unblock the flow of decane droplets through narrowing nanopores with constricted geometries while aiding in decane droplet shape deformation, whereas SDS leads to decane droplets stalling in front of constrictions in nanopores. We hypothesize that the inability of the cationic CTAC headgroup to form hydrogen bonds is one of the key factors leading to enhanced CTAC-coated decane flow through constricted nanopores. The obtained molecular view of equilibrium and dynamic properties of complex fluids typical of oil reservoirs can provide a basis for the future design of new molecules for EOR processes.

Recapitulating atherogenic flow disturbances and vascular inflammation in a perfusable 3D stenosis model

Blood vessel narrowing and arterial occlusion are pathological hallmarks of atherosclerosis, which involves a complex interplay of perturbed hemodynamics, endothelial dysfunction and inflammatory cascade. Herein, we report a novel circular microfluidic stenosis model that recapitulates atherogenic flow-mediated endothelial dysfunction and blood-endothelial cell (EC) interactions in vitro. 2D and 3D stenosis microchannels with different constriction geometries were fabricated using 3D printing to study flow disturbances under varying severity of occlusion and wall shear stresses (100 to 2000 dynecm−2). Experimental and fluid simulation results confirmed the presence of pathological shear stresses in the stenosis region, and recirculation flow post stenosis. The resultant pathological flow profile induced pro-inflammatory and pro-thrombotic EC state as demonstrated by orthogonal EC alignment, enhanced platelet adhesion at the stenosis, and aberrant leukocyte-EC interactions post stenosis. Clinical utility of the vascular model was further investigated by testing anti-thrombotic and immunomodulatory efficacy of aspirin and metformin, respectively. Overall, the platform enables multi-factorial analysis of critical atherogenic events including endothelial dysfunction, platelets and leukocyte adhesion, and can be further developed into a liquid biopsy tool for cardiovascular risk stratification.

Effect of Subsurface Microseisms on the Motion of Dispersed Droplets in Pores

AbstractHuman‐induced seismicity has drawn substantial attention in recent years. The effect of seismicity on subsurface structures has been studied extensively. However, the effect of seismicity, especially microseismicity, on surrounding immiscible fluids is rarely investigated. In porous media with two or more immiscible fluids, different amplitudes of vibration induced by seismicity have distinct effects on the dynamic behavior of the fluids. Three types of pore‐scale models are prevalent for analyzing the motion of immiscible droplets. The underlying assumptions and accuracy of these models are compared in this study in both the frequency and time domains. The frequency domain analysis shows that resonance can be addressed in all three of the models; the frequency response curves, however, present significant differences, which are attributed to the missing physics considered in some models. Time domain analysis is performed in both small‐amplitude and large‐amplitude oscillation. The nonlinearity in large‐amplitude oscillation is attributed to the constricted geometry of the capillary tube. Although the term “large‐amplitude” is used in this study, the corresponding amplitude is still within the amplitude range of microseisms. The momentum balance model is identified as the most accurate oscillatory model so far in comparison with computational fluid dynamics simulations. In addition, the potential approach to incorporate this pore‐scale model in seismic wave attenuation analysis is found to be possible. The frequency correction function and structural factor are calculated to embed the momentum balance model into Biot's poroelastic model. The resonance of the dispersed phase can also be addressed theoretically in porous media of random packed spheres.

A Comparative Analysis of Mixing Performance of Power-Law Fluid in Cylindrical Microchannels With Sudden Contraction/Expansion

Abstract This paper focuses on the comparative electrokinetic micromixing of non-Newtonian fluid in cylindrical microchannels with surface potential heterogeneity due to sudden constriction/expansion. In numerical simulations, the rheology of the aqueous solution is considered to follow power-law characteristic. Based on the Poisson–Nernst–Planck model, the simulations are performed to investigate the mixing efficiency and pressure drop for constricted and expanded configurations over a wide range of the flow behavior index, potential patch strength, and geometric parameters. The results show that, irrespective of geometric configurations, the mixing efficiency can be improved significantly by increasing the flow behavior index, geometric parameters, and the overpotential patch strength. In addition, it is also revealed that the constricted geometry yields better mixing as compared to the other configuration, but the average pressure drop shows reverse characteristics. Thus, a parametric relationship is tried to be established between mixing efficiency and pressure drop for both these configurations to propose an effective and efficient micromixer, which can produce maximum possible mixing efficiency with minimum pressure drop.

Solute release from an elastic capsule flowing through a microfluidic channel constriction

In recent years, microfluidic channels with narrow constrictions are extensively proposed as a new but excellent possibility for advanced delivery technologies based on either natural or artificial capsules. To better design and optimize these technologies, it is essential and helpful to fully understand the releasing behavior of the encapsulated solute from capsules under various flow conditions which, however, remains an unsolved fundamental problem due to its complexity. To facilitate studies in this area, we develop a numerical methodology for the simulation of solute release from an elastic capsule flowing through a microfluidic channel constriction, in which the tension-dependent permeability of the membrane is appropriately modeled. Using this model, we find that the release of the encapsulated solute during the capsule’s passage through the constriction is enhanced with the increase in the capillary number and constriction length or the decrease in the constriction width. On the other hand, a large variation in the channel height, which is generally larger than the capsule diameter, generates little effect on the released amount of the solute. We reveal that the effects of the capillary number and constriction geometry on the solute release are generally attributed to their influence on the capsule deformation. Our numerical results provide a reasonable explanation for previous experimental observations on the effects of constriction geometry and flow rate on the delivery efficiency of cell-squeezing delivery systems. Therefore, we believe these new insights and our numerical methodology could be useful for the design and optimization of microfluidic devices for capsule-squeezing delivery technologies.

Comparison between 2D Shallow-Water Simulations and Energy-Momentum Computations for Transcritical Flow Past Channel Contractions

Multidimensional simulators of channel and river flow are widely used in industry and academia, raising the question about whether the classical one-dimensional theory of open-channel flow remains relevant in hydraulic engineering. Channel contractions that induce transcritical flow are interesting scenarios to test the classical 1D theory against multidimensional simulations, because supercritical flow in channels of variable width leads to multidimensional flow structures. Transcritical flows are important in practice, because the ensuing hydraulic jumps and regions of supercritical flow may damage hydraulic structures that otherwise operate under tranquil conditions. We compare well-resolved simulations of the 2D shallow-water Equations (SWE) with 1D energy-momentum calculations for flow past symmetric channel contractions. We analyze the accuracy of the classical energy-momentum gradually-varied flow theory to predict the onset of regime transitions and the location of hydraulic jumps. We test the relative performance of the 1D theory for different constriction geometries, and identify the flow mechanisms behind the discrepancies between the 1D and 2D predictions. The grid resolution used in the 2D SWE plays an important role in these predictions, because coarse-grid 2D simulations yield essentially quasi-1D results. Considering its simplicity and negligible computational cost compared with the 2D SWE simulations, the classical 1D theory performs remarkably well for a wide range of flow conditions and contraction geometries. In contrast, we observe large deviations between the 1D and 2D models in flow past abrupt contractions with a large width ratio, as expected. Only modified versions of the 1D theory, taking into account intense local head losses and the propagation of spatial flow structures downstream from the contraction, can succeed at describing these flow scenarios.