Flow Front Research Articles

Cold Spray Metallization of Hybrid Thermoplastic - Thermoset Fiber Reinforced Composite

Surface metallization is amongst the recent trends in the polymer and polymer matrix composites (PMCs) research industries to improve the electrical and thermal properties and exploit the subsequent utilization in the aerospace sector. Specifically, polymer matrix composites have been subjected to the limitations in form of high temperature exposure and substrate deterioration. The present study encompasses a new strategy in the manufacturing and metallization process. The first stage in the manufacturing of hybrid thermoplastic-thermoset composite was the hot compaction which comprised of primary preform preparation enabling the partial impregnation of the thermoplastic resin through the fabric reinforcement layer. The subsequent stage entailed the preform vacuum bagging and conducting catalyzed thermoset resin impregnation. The vacuum resin infusion step included a cocuring cycle to generate a fiber reinforced composite comprising of thermoplastic and impregnated thermoset resin with improved adhesion. Resin flow front movement was analyzed during the resin infusion process. Composite metallization was achieved through cold spray (CS). CS process parameters influence on the coating quality and characterization of laminates through microstructural analysis and results have been reported. The hybrid composite with thermoset resin through thickness and in-plane impregnation was achieved with the intact adherent thermoplastic layer after the curing cycle. In the CS metallization, the effective operative window of stand-off distances (SoD) and temperature has been determined.

Forest destruction by ‘a‘ā lava flow during Etna's 2002–03 eruption: Mechanical, thermal, and environmental interactions

Forest destruction by ‘a‘ā lava flow is common. However, mechanical and thermal interactions between the invading lava and the invaded forest are poorly constrained. We complete mapping, thermal image and sample analyses of a channel-fed ‘a‘ā lava flow system that invaded forest on the NE flank of Mt. Etna (Italy) in 2002. These lava flows destroyed 231,000 trees, only 2% of which are still visible as felled trunks on the levees or at the channel-levee contact. The remaining 98% were first felled by the flow front, with the trunks then buried by the flow. Rare tree molds can be found at the rubble levee base where trees were buried by avalanching hot breccia and then burnt through, with a time scale for total combustion being a few days. Protruding trunks fell away from the flow, if felled by blocks avalanching down the levee flank, or became aligned with the flow if falling onto the moving stream. Estimated cooling rates (0.1–5.5 °C km−1) are normal for well-insulated ‘a‘ā flow, suggesting no thermal interaction. We find the highest phenocryst concentrations (of 50–60%, above an expected value of 30–40%) in low velocity (<0.5 m s−1) locations. These low velocity zones are also characterized by high trunk concentrations. Thus, the common factor behind crystal and trunk deposition is velocity. That is, when the lava slows down, crystal settling occurs and trunks are preferentially deposited. Thus, although we find no thermal or textural effects due to the presence of the forest, we do find mechanical and environmental interactions where the trees are consumed to become part of the flow.

Data-Driven Proxy Modeling of Water Front Propagation in Porous Media

In the water flooding process, determining the location of the injected water front is as one of the most critical variables, which is the basis of many subsequent predictions. Despite the importance and use of this parameter in a vast range of flooding-related assessments, there are no alternative methods to traditional analytical modeling or time-consuming numerical 3D simulation for its determination. This study introduces a data-driven proxy modeling approach based on two powerful deep learning algorithms for real-time determination of the injected water front location on the grid scale. The developed proxy models have realized the possibility of modeling the location of the flow front by minimally using the data extracted from the numerical simulators and only relying on commonly available field data. The proposed proxy models successfully simulated the breakthrough time in production wells and water arrival time in certain reservoir grids in new blind scenarios.

Non-isothermal non-Newtonian three-dimensional flow simulation of fused filament fabrication

This work investigates three-dimensional simulation of fused filament fabrication using the Cross-WLF model for the non-isothermal and shear thinning behavior of the melt. To realistically simulate the deposition flow, the acceleration, viscosity evolution, and flow front tracking models have been included with the pressure gradient in the deposited road and boundary modeling of the melt and air interface. The results indicate that the non-isothermal and shear thinning behaviors greatly affect the geometry of the deposited roads including the flow front and trailing cross-section shapes. The thermal footprint of the interface between the deposited melt and the substrate is also predicted as a function of the thermal contact conductance. The pressure distribution within the deposited road is also modeled and is found to be not symmetric with respect to the nozzle center-line. Rather, the pressure peak shifts slightly downstream due to redirection of the melt around a stagnation point opposite the nozzle exit. Furthermore, a negative stress is observed downstream the exterior nozzle face associated with the free expansion of the melt as the extruded material climbs and releases from the exterior nozzle face. The developed simulation is verified by comparison with experimental results providing contact pressures ranging from 5 to 132 kPa.

A novel approach to simulate the resin infusion process by two phases coupling free and porous flows intra and inter fiber tows of liquid composite molding

In this work, a coupling Cahn-Hilliard-Navier-Stokes-Darcy two-phase flow model is developed to simulate the resin infusion process intra and inter fiber tow. Navier–Stokes equation is applied in the free flow region while two phases Darcy’s law governs flow in the porous medium. The refined “BJS” (Beavers-Joseph-Saffman) condition is included as the coupling interface condition to solve the two phases and two mediums coupling problems. Flow flux, normal stress and velocity conversations are also taken into consideration as the interface conditions. Transient resin flow front tracking is carried out by adopting Cahn-Hilliard equation in the free flow region based on this novel coupling method and results are compared with quasi realistic model to verify the accuracy of this novel method.

Formation of dry granular fronts and watery tails in debris flows

Debris flows are particle–fluid mixtures that pose a significant hazard to many communities throughout the world. Bouldery debris flows are often characterised by a deep dry granular flow front, which is followed by a progressively thinner and increasingly watery tail. The formation of highly destructive bouldery wave fronts is usually attributed to particle-size segregation. However, the moving-bed flume experiments of Davies (N. Z. J. Hydrol., vol. 29, 1990, pp. 18–46) show that discrete surges with dry fronts and watery tails also form in monodisperse particle–fluid mixtures. These observations motivate the development of a new depth-averaged mixture theory for debris flows, which explicitly takes account of the differing granular and phreatic surfaces, velocity shear, and relative motion between grains and fluid to explain these phenomena. The theory consists of four coupled conservation laws that describe the spatial and temporal evolution of the grain and water thicknesses and depth-averaged velocities. This system enables travelling wave solutions to be constructed that consist of (i) a large amplitude dry flow front that smoothly transitions to (ii) an undersaturated body, (iii) an oversaturated region and then (iv) a pure water tail. It is shown that these solutions are in good quantitative agreement with Davies’ experiments at high bed speeds and slope inclinations. At lower bed speeds and inclinations, the theory produces travelling wave solutions that connect to a steady-uniform upstream flow, and may or may not have a bulbous flow front, consistent with Davies’ observations.

Transition Indices of Sediment-Transport Modes on a Debris Flow Resulting from Changing Streambed Gradients

We conducted experiments using an experimental flume with two variable streambed gradients in the upstream and downstream parts with various debris flows, composition sizes, and supply flow rates. We investigated the transition processes of sediment transport modes along the longitudinal distances from the gradient change point using the transition mode indices, ICs¯x, Ih¯x, and IU¯x; these indices were calculated based on measurements of sediment transport concentrations, flow depths, and gravel migration velocities in the debris flow’s front in the downstream part. Using these indices, we postulated that after the debris flow passed the gradient change point, the transition of the sediment transport modes progressed by changing the measured parameters to those in the steady-state condition on the gradient of the downstream parts. In addition, these indices suggested that the gravel migration velocities in the flow front interior changed most rapidly after passing the gradient change point, and that flow depths tended to change most slowly. Finally, the indices suggested that as the debris flow material became finer and the supplied flow rates became larger, the longitudinal transition sections tended to be longer because the momentum needed to transport the material was less than the total debris flow momentum.

An Experimental Model of Unconfined Bubbly Lava Flows: Importance of Localized Bubble Distribution

AbstractMost lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle‐bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid‐rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho’eho’e.

Analytical Analysis on the Capillary Pressure of Underfill Flow Meniscus During the Flip-Chip Encapsulation Process

In the conventional flip-chip underfill encapsulation process, the underfill fluid is driven into the gap beneath the chip and substrate through capillary action. Particularly, the capillary pressure of the flow front determined the strength of overall capillary flow advancement through the bump array, hence the filling time and subsequently the manufacturing productivity. This paper analytically studied the capillary pressure of the flow front. Accordingly, the variation effects of bump contact angle and bump pitch on the flow front capillary pressure were presented. It was found that the variation trend of capillary pressure along the flow displacement between the region confined by two adjacent bumps is sinusoidal. The magnitude of capillary pressure is positive near bump entrant and acting along the flow direction before the capillary pressure gradually decreasing to oppose the flow advancement. The increasing of contact angle shifted the capillary pressure trend leftward with the peak being lowered, whereas the magnitude of capillary pressure decreases with the increases in bump pitch. Additionally, by studying the capillary pressure of flow meniscus, a new dynamic-based contact line jump criterion was proposed by considered the peak capillary pressure along the underfill advancement. These findings are practically useful for the design optimization of flip-chip package to promote the longevity of electronic package and reduce electronic wastes.

Residual-based adaptivity for two-phase flow simulation in porous media using Physics-informed Neural Networks

This paper aims to provide a machine learning framework to simulate two-phase flow in porous media. The proposed algorithm is based on Physics-informed neural networks (PINN). A novel residual-based adaptive PINN is developed and compared with the residual-based adaptive refinement (RAR) method and with PINN with fixed collocation points. The proposed algorithm is expected to have great potential to be applied to different fields where adaptivity is needed. In this paper, we focus on the two-phase flow in porous media problem. We provide two numerical examples to show the effectiveness of the new algorithm. It is found that adaptivity is essential to capture moving flow fronts. We show how the results obtained through this approach are more accurate than using RAR method or PINN with fixed collocation points, while having a comparable computational cost.

Effects of the Interlayer Toughening Agent Structure on the Flow Behavior during the z-RTM Process.

In this paper, interlayer toughening composites were prepared by the z-directional injection RTM process (z-RTM), which has the advantage of increasing the interlaminar toughness and shortening the filling time and completely impregnating the fibers. The nonwoven fabrics and dot matrix structure material were used as ex situ interlayer toughening agents. The effect of the interlayer toughening agent structure on the resin flow behavior during the z-RTM process was investigated. The macro-flowing and micro-infiltration behaviors of the resin inside the preforms were deduced. The permeability of the fabric preforms with different toughening agents was investigated. The results show that the introduction of the nonwoven structure toughening agent makes the macro flow slow, and the flow front more uniform. The toughening agent with a dot matrix structure promotes the resin macro flow in the preforms, and shortens the injection time. The z-directional permeability of the preform with a dot matrix structural toughening agent is one order of magnitude lower than that of the non-toughened preform, while being higher than the preform toughened by the nonwoven fabric preforms, which is helpful for the further applicability of the z-RTM process. Furthermore, the mode II interlaminar fracture toughness of composites was evaluated.

Does middle-lower crustal flow exist in the eastern Tibetan Plateau? Insights from finite-element modeling and geodetic observations

It remains unknown whether middle-lower crustal flow exists in the eastern Tibetan Plateau. More and more geodetic data have been acquired during the past several tens of years, especially after the 2008 Mw 7.9 Wenchuan earthquake. Some important questions are proposed: can we use the geodetic data to determine if middle-lower crustal flow exists in the eastern Tibetan Plateau, and can we estimate the patterns and magnitudes of the crustal flow in this region if it exists? To investigate these questions, we construct a three-dimensional viscoelastoplastic finite-element model to simulate regional crustal deformation during the seismic cycle of 2008 Wenchuan earthquake in cases with and without middle-lower crustal flow, and analyze and compare these model results with geodetic observations. From modeling the co- and post-seismic deformation, we have suggested a steady-state (Maxwell) viscosity of 1.1 × 1019 Pa s within a 40-km-thick middle-lower crust beneath the eastern Tibetan Plateau. Through inter-seismic deformation, we find that the eastern Tibetan Plateau, in crustal shortening model, has undergone pure shear deformation, leading to an even-distributed surface uplift rate of 0.5 mm/yr. It is significantly lower than the average uplift rate of 2.2 mm/yr from the observed leveling data. On the other hand, the hybrid crustal flow model can predict a faster surface uplift rate with the fastest uplift rate localized at the flow front, which is more consistent with the observed leveling data. Based on the model results and the leveling data, we infer that middle-lower flow may exist within a wide area beneath the eastern Tibetan Plateau.

A Comparison of Numerical and Actual Measurements of Large-Scale Rib-Structured Pallet Flatness Using Recycled Polypropylene in Injection Molding.

Many challenges are associated with the injection molding process for forming a rib-structured pallet (1100 mm × 1100 mm × 140 mm, length × width × height) because greater flowing resistance through the rib channels within the pallet can induce insufficient filling. Essentially, multi-gate filling involves a sequential valve gate system, which helps to spread the filling front with fewer weld lines. Based on the presetting of the sequential scheme of the valve gates, actual measurements of pallet flatness using the ATOS scan system were compared to numerical warpage measurements of a pallet derived by Moldex3D 2020. In this study, we propose a sequential scheme by actuating the valve gates to open once the flow front spreads towards them; then, actual warpage measurements of a pallet are compared with numerical measurements. The results show that the warpage of the top surface of the pallet is 5.144 mm in actual measurements and 5.729 mm in simulation. The results all indicated small warpage with respect to the pallet size. The simulation and actual measurements of flatness are in excellent agreement; the difference in top flatness between the simulated and actual pallet is 0.59 mm, while the bottom flatness difference is 0.035 mm. By adjusting the cooling water temperature, increasing the mold temperature, and decreasing the material temperature, overall flatness and warpage displacement can be reduced.

The Interaction between In-Cylinder Turbulent Flow and Flame Front Propagation in an Optical SI Engine Measured by High-Speed PIV

The relationship between the flow field and flame propagation is essential in determining the dynamics and effects of turbulent flow in an optical SI engine. In this study, high turbulence flow at stable operations was achieved using 12,000 rpm engine speed, 60 kPa absolute intake pressure, 14.7 A/F, and 15 deg. BTDC spark timing. The turbulent flow field and flame propagation interplay were analyzed through the simultaneous high-speed PIV measurements of the in-cylinder flow and flame front propagation under firing conditions. The intensity of the seeder used was optimized by changing the crank angle. Successful simultaneous detection of the flame front and turbulent flow was demonstrated. Strong turbulence was produced at the flame front simultaneously with the flame movement. After ignition timing, the flame accelerated in the unburned region, and a vital turbulence region occurred.

Vacuum Filling Simulation with Combined Lagranian and VOF Method

Jetting succeeded by accumulation is the characteristic of the vacuum filling, which is different from the conventional pressure-driven flow. In order to simulate this kind of flow, a three-dimensional theoretical model in terms of incompressible and viscous flow is established, and an iterative method combined with finite element method (FEM) is proposed to solve the flow problem. The Lagranian-VOF method is constructed to trace the jetting and accumulated flow fronts. Based on the proposed model and algorithm, a simulation program is developed to predict the velocity, pressure, temperature, and advancement progress. To validate the model and algorithm, a visual experimental equipment for vacuum filling is designed and construted. The vacuum filling experiments with different viscous materials and negative pressures were conducted and compared the corresponding simulations. The results show the flow front shape closely depends on the fluid viscosity and less relates to the vacuum pressure.

Note on semi‐analytical nonstationary solution for the rivulet flows of non‐Newtonian fluids

We have presented in this analytical research the revisiting of approach for mathematical modeling the rivulet flows on inclined surface in terms of viscous‐plastic theory of two‐dimensional movements in a frame of (x, y)‐plane in Cartesian coordinates. A semi‐analytical solution has been obtained here for nonstationary creeping approximation of plane‐parallel flow slowly moving on inclined surface. The main motivation of the current development is that each additional enhancement of knowledge in regard to this phenomenon should obviously improve the accuracy of calculations for possible applications in technological or engineering areas of fluid dynamics researches. Even in such simple formulation, equations of motion that govern the dynamics of rivulet flows of non‐Newtonian fluids are hard to be solved analytically. Nevertheless, we have succeeded in obtaining analytical expressions for the components of velocity in {Ox, Oy}‐directions of motion for slowly moving the advancing front of rivulet flow (where Ox‐axis coincides to the initial main direction of slowly moving rivulet flow). Restrictions to the form of flow (stemming from the continuity equation) have been used accordingly.

In situ 3D observations of capillary-driven flows in parallel arrangements of rigid fibres using X-ray microtomography

Capillary-driven impregnation phenomena occurring in fibre bundles during the processing of composite materials are complex and still not fully understood. Hence, synchrotron X-ray microtomography was used to characterise the longitudinal propagation of the flow front within a parallel arrangement of rigid fibres. The analysis of 3D images enabled the fluid-air interface curvatures, triple line lengths, and local contact angles to be quantified during wetting and dewetting experiments. The results showed that even in quasi-static situations, local contact angles exhibited significant variations along the fibres. These variations also depended on the wetting/dewetting state. The transverse capillary forces measured at the fibre scale from the analysis of 3D images were shown to be of the same order of magnitude as the longitudinal capillary forces. Local curvatures of the fluid-air interface and the resulting estimate of the average capillary pressure proved the relevance of a mesoscale capillary pressure model adapted for fibre bundles.

Advanced process simulation of compression molded carbon fiber sheet molding compound (C-SMC) parts in automotive series applications

The following paper focuses on the development of a 3D virtual process chain (VPC) for parts made from Carbon Fiber Sheet Molding Compounds (C-SMC). The VPC consists of two modules addressing the compression molding and warpage simulation of parts made from two different high fiber volume fraction C-SMC materials. The press simulation module applies a solid mechanics material modelling approach within the Coupled Eulerian-Lagrangian (CEL) simulation framework available in ABAQUS/Explicit. Through a combination of flow characterization experiments and numerical simulations, local mechanical properties and fiber orientation information can be predicted from numerically simulated local flow strains. A warpage simulation module has also been developed in ABAQUS/Standard together with a data processing/transfer interface implemented in ANSA. With these highly integrated numerical tools, process characteristics such as press forces, flow fronts and weld-line development based on material amount, insert geometry and placement position in the mold can all be predicted and analyzed.

The influence of fabric architecture on impregnation behavior and void formation: Artificial neural network and statistical‐based analysis

AbstractThis work proposes an approach combining artificial neural networks (ANN) with statistical models to predict injection processing conditions for four reinforcement architectures: plain weave, bidirectional noncrimp fabrics, unidirectional fabrics (Uni) and random fiber mats (Random). Key results allow evaluating the velocity of the flow front by combining processing parameters and creating a three‐dimensional response surface based on a properly trained ANN. This investigation is based on a large number of experimental results. The key role played by some physical parameters was associated with predicting the impregnation behavior (velocity of the flow front) during resin injection. The main outcome aims to provide a better control of void content in terms of size and position to the four fibrous reinforcements considered.

Flow Marks in Injection Molding of PP

Abstract Flow marks in injection moulding is quite often encountered on different type of polymers. This defects is known as successive and periodic bright and dull bands oriented perpendicular to the flow direction. Their appearances meanly depend on processing and observation conditions. This study was lead on two industrial polypropylene polymers with a simplified polished rectangular mould geometry. We show that this defect appears during the filling stage and comes from a local change of morphological and molecular orientation along the filling direction on the surface. An instability of the flow front due to a limited sliding at the contact combined to the strong changed of molecular orientation before and after the stagnation point are invoked to explain this instability.