Flow Front Research Articles

Visualization Analysis on Reduction Effects of Bubble Breaking and Gas Cloud Generation at Flow Front by Counter Pressure Injection Molding with Low Pressure Level

Visualization Analysis on Reduction Effects of Bubble Breaking and Gas Cloud Generation at Flow Front by Counter Pressure Injection Molding with Low Pressure Level

Mould fill time sensitivity analysis using isothermal mould filling simulations for applications in liquid composite moulding processes

Development of effective injection strategy constituting injection gates and air vents to obtain reduced mould fill time without dry spots are the primary requirements to develop mould system for an application using Liquid Composite Moulding (LCM) processes. Recent days, virtual simulation becomes promising tool in comparison to trial and error experiments due to its costing factor and timely iterations. This work demonstrates the possibility of Ansys Fluent commercial simulator to perform porous flow isothermal mould filling simulations and to quantify the sensitivity of mould fill time and flow front progression with respect to variations in process and raw material parameters. A rectangular mould of dimension 1 m × 0.5 m with an injection strategy of single point gate with 10 mm diameter and single point air vent of diameter 10 mm was used as a domain for isothermal mould filling. The sensitivity of mould fill time and flow front pattern were evaluated by varying inlet velocity, resin viscosity and orthotropic reinforcement mat permeability. From the results, it was found that the mould fill time decreases with increasing velocity, lesser viscosities and more porous preforms. Also, in all case studies, the mould fill pattern ends at the air vent position ensuring there are no dry spots formed.

Direct numerical simulation of infusion and flow-front tracking in materials with heterogeneous permeability using a pressure mapping sensor

This paper explores the use of thin film piezoresistive pressure mapping sensors as a means to improve resin transfer moulding processes. The pressure mapping sensor was located between the preform and mould, giving information regarding the permeability map prior to infusion. The permeability map is used as an input to a direct numerical simulation of the infusion step of a highly variable reclaimed carbon fibre preform. The pressure sensor was also used to track the flow front position in-situ, due to a change in load sharing between the preform and liquid during the infusion experiment. Flow front tracking with the pressure mapping sensor was validated against conventional camera images taken through a transparent mould. The direct numerical simulation was able to account for local permeability variation in the preform, providing improved flow-front prediction than homogeneous permeability only, and could be part of a wider strategy to improve resin transfer moulding process robustness.

Real-time monitoring of resin infiltration process in vacuum assisted molding(VARI) of composites with carbon nanotube buckypaper sensor

In this study, a flexible and highly sensitive carbon nanotube buckypaper(BP) sensor was fabricated and validated which can be embedded in the composite reinforcement to real-time monitor flowing and infiltration of the resin for vacuum assisted liquid composite molding(VARI). The BP sensors were prepared by the spray-vacuum filtration method with mono-dispersion of multi-wall carbon nanotubes. It turns out that the excellent properties of the BP sensors can be obviously characterized; for example, conductivity, sensitivity and permeability, which were confirmed through contrasting the penetration of acetone and resin to the BP sensor. The utility of this novel technology about BP sensor monitoring resin flow front had been demonstrated through penetration evaluation experiment. The change in resistance with time obtained by BP sensors described the advancement of resin flow front with time, and comparing with the information recorded by a camera and a stopwatch, the results were basically the same. Additionally, because the permeability is an important process parameter during infiltration, affecting manufacturing cycle and product quality, four different conditions for the number of layers about the laid glass-fiber-reinforced were investigated. The comparison results of different permeability were gained from the information collected by BP sensors in real-time monitoring and combined with Darcy’s law. This paper not only reveals the ability of BP sensor as a new method in real-time monitoring, but also provides a new way to avoid defects and optimize production in VARI processes.

“Swimming Jellyfish“: Visualizing Jet-Like Internal Flow in Coalescing Droplets

Abstract Surface-tension-driven droplet coalescence has received significant attention due to its significant role in microfluidics, coalescence-induced droplet jumping, fluid mixing, and microscale heat and mass transfer. However, the study of internal flow characteristics of merging droplets remains a challenge due to visualization difficulty, limited droplet size control, poor droplet manipulation, and insufficient droplet front tracking. Here, in order to study droplet coalescence dynamics, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with diameters of ≈ 40 μm were dispensed onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. To track the internal flow front, an ethanol (20 wt %)-water mixture was used as the working fluid. Due to the unequal evaporation rate of water and ethanol, a density gradient was introduced at the liquid-gas interface of the droplets, resulting in a lower refractive index compared to that in the bulk liquid. The coalescence process of droplets with diameters of ≈ 232 μm and ≈ 1400 μm was imaged. We observed jet-like internal flow and vortex rings (“swimming jellyfish”) inside the merging droplet. It was shown that the jet-like flow underwent a significant deceleration while the vortex-ring experienced slow radial expansion. Our work not only presents a powerful platform capable of visualizing droplet coalescence hydrodynamics, but also provides insights into the internal flow dynamics during droplet coalescence.

Determination of flow front velocity and optimal injection pressures for better impregnation of E-glass with polyester in resin transfer mold

Flow front velocity of resin plays a key role in yielding better impregnation of fiber reinforcement in resin transfer molding to fabricate quality composites. A video camera was used to capture the patterns of resin flowing below the grid lines drawn on the acrylic sheet of resin transfer mold and also to accurately measure the flow front velocity using an image conversion tool. The effect of flow front velocity, permeability, Reynolds number and void content at five different injection pressures on chopped strand E-glass/polyester composites consisting of 4, 5 and 6 layers has been studied. On the basis of Reynolds number of resin flow and void content present in the composite, optimal injection pressures are suggested for better impregnation of preform. Composites processed at these injection pressures gave superior properties in tension and flexure. Fractured parts of the specimens were examined on Scanning Electron Microscope to explain the causes of superior mechanical properties.

Effects of dynamic fragmentation on the impact force exerted on rigid barrier: centrifuge modelling

Bi-dispersity is a prerequisite for grain-size segregation, which transports the largest particles to the flow front. These large and inertial particles can fragment upon impacting a barrier. The amount of fragmentation during impact strongly influences the force exerted on a rigid barrier. Centrifuge modelling was adopted to replicate the stresses for studying the effects of bi-dispersity in a granular assembly and dynamic fragmentation on the impact force exerted on a model rigid barrier. To study the effects of bi-dispersity, the ratio between the diameters of small and large particles (δs/δl), characterizing the particle-size distribution (PSD), was varied as 0.08, 0.26, and 0.56. The volume fraction of the large particles was kept constant. A δs/δl tending towards unity characterizes inertial flow that exerts sharp impulses, and a diminishing δs/δl characterizes the progressive attenuation of these sharp impulses by the small particles. Flows dominated by grain-contact stresses (δs/δl < 0.26), as characterized by the Savage number, are effective at attenuating dispersive stresses of the large particles, which are responsible for reducing dynamic fragmentation. By contrast, flows dominated by grain-inertial stresses (δs/δl > 0.26) exhibit up to 66% more impulses and 4.3 times more fragmentation. Dynamic fragmentation of bi-disperse flows impacting a rigid barrier can dissipate about 30% of the total flow energy.

Optimization of the injection molding process for the PC/ABS parts by integrating Taguchi approach and CAE simulation

This study was an application of the Taguchi method to optimize injection molding (IM) process parameters of the polycarbonate/acrylonitrile-butadiene styrene (PC/ABS) blends. A 4-factor, 3-level injection experiment was conducted using Taguchi L9 orthogonal array through the statistical design method. The parameters considered were material temperature, injection pressure, holding time, and mold temperature. The signal-to-noise (S/N) ratio was performed to obtain higher shear stress by defining the optimum injection parameters. The Taguchi method showed that injection pressure was the most influential parameter on the shear stress for the PC/ABS blends. The optimal injection parameter levels tested via Taguchi were verified via CAE simulation. SW plastics software was used to predict if the injected part contained injection defects through computation of key parameters such as the easy filling, the flow front central temperature, and the residual stress. The numerical simulation showed no injection defects observed on the injected PC/ABS parts. Therefore, the optimal combination parameters provided injected PC/ABS parts with a better shear stress and no injection defects. Such conditions would enhance the metallization process.

Landscape evolution associated with the 2014–2015 Holuhraun eruption in Iceland

The 2014–2015 Holuhraun eruption in Iceland developed between the outlet glacier Dyngjujökull and the Askja central volcano and extruded a bulk lava volume of over 1 km3 onto the floodplain of the Jökulsá á Fjöllum river, making it the largest effusive eruption in Iceland during the past 230 years. Time-series monitoring using a combination of traditional aerial imaging, unmanned aerial systems, and field-based geodetic surveys, established an unprecedented record of the hydrological response of the river system to this lava flow. We observed: (1) the formation of lava-dammed lakes and channels produced during dam-breaching events; (2) percolation of glacial meltwater into the porous and permeable lava, forming an ephemeral hydrothermal system that included hot pools and hot springs that emerged from the lava flow front; and (3) the formation of new seepage channels caused by upwelling of water around the periphery of the lava flow. The observations show that lava flows, like the one produced by the 2014–2015 Holuhraun eruption, can cause significant hydrological changes that continue for several years after the lava is emplaced. Documenting these processes is therefore crucial for our interpretation of volcanic landscapes and processes of lava–water interaction on both Earth and Mars.

Load-attenuation mechanisms of flexible barrier subjected to bouldery debris flow impact

The impulse load of boulders at the front of debris flows is critical to the design of structural defense measures, which are commonly constructed on hillsides to mitigate landslide risk. Field evidences have demonstrated the capability of some steel flexible barriers in intercepting debris flows with bouldery inclusions. However, there is still a lack of fundamental understanding of the load-attenuation mechanisms of flexible barriers, especially under bouldery debris flow impact. In this study, systematic tests of mono-disperse and bi-disperse bouldery flows impacting an instrumented flexible barrier were conducted using a geotechnical centrifuge. The impact kinematics and barrier responses, such as mobilized structural forces and elongation of cables, were recorded synchronously. The results reveal that the load-attenuation mechanism of flexible barriers for the frontal impact originates from the barrier deflections and extended interaction duration. Only 30% of the frontal momentum is transferred to the flexible barrier. The performance of the flexible barrier is compared with that of a rigid barrier model under identical testing conditions. It is found that the boulder impulse loads on flexible barrier are significantly attenuated, resulting in a “plateau” pattern of the impact time history. The practical implication is that the design of flexible barriers may not demand separate considerations of the bulk debris and individual boulder impact loads. Detailed examination of the state of debris deposited behind the flexible barrier indicates that the static dry debris is close to the active failure state due to the large barrier deflection.

Permeability of untreated and atmospheric plasma treated coconut fiber mats

Composites manufactured by resin transfer molding depend on appropriate processing parameters to ensure adequate reinforcement-matrix adhesion. Permeability predicts the fluid flow resistance through reinforcement. The purpose of this work is to evaluate the influence of atmospheric plasma treatment on the permeability of coconut fiber mats. Glycerin solution simulated matrix impregnation of untreated and treated mats. Data from scanning electron microscopy and a decrease in contact angle from 96° to 61° for treated fibers explained the difference in permeability due to etching phenomenon that caused a decrease in permeability value. Kozeny-Carman ratified changes in the permeability of coconut fiber mats. Atmospheric plasma treatment turns fibers more hydrophilic enabling better fluid impregnation in addition to a more regular and slower flow front.

A unique ~12 ka subaerial record of rift-transform triple-junction tectonics, NE Iceland

In northern Iceland the European-North American plate boundary is broad and complex but includes a remarkable subaerial triple-junction intersection between the Husavik-Flatey Fault (HFF) dextral transform and rifting in the Northern Volcanic Zone. Fortuitously, the triple junction occurs in a sheet of ~12 ka pahoehoe lavas; a tabula rasa recording innumerable fault features displayed in exquisite detail. High-resolution drone imagery, coupled with 120 field measurements of fault slip directions and opening amounts, made possible the mapping and analysis of this detail and, importantly, enabled recognition and exclusion of potentially misleading primary deformation features associated with emplacement of the lavas. Rift-transform interactions in this natural laboratory have remained spatially stable throughout post-glacial time, although with transform-affinity faults reactivated to accommodate rift extension and transform ‘encroachment’ into the rift domain. First-order en-echelon Riedel fault complexes are recognised, linked by transpressional faulting and compressional strike-slip relay ramps, as well as second-order R shears, R’ and P shears, and previously undescribed R’ Riedel-in-Riedel relationships. A pahoehoe flow front offset along a first-order Riedel fault complex records slip at ~3.8 mm a−1, which may be consistent with the published GPS-based current slip-rate estimate of ~6.8 mm a−1 across the HFF as a whole.

Simulation of liquid composite moulding using a finite volume scheme and the level-set method

In this paper, a 2D numerical framework for the simulation of flows in porous media that are characterised by a sharp transition between the saturated and unsaturated zone is presented. Using a finite volume scheme and the level-set method, the framework is derived based on a conventional, implicit solution of Darcys law for the pressure field, while the level-set function is advected explicitly locally at the flow front. With the main application to liquid composite moulding (LCM) in mind, the numerical framework is verified against analytical solutions, experiments, and other benchmark cases for a variety of situations that occur in this composite manufacturing process. The cases include local changes in permeability related to edge-effects, merging of flow fronts, and continuous manufacturing processes. It is highlighted that the numerical framework achieves convergence with a spatial accuracy between 1st and 2nd-order; the level-set operations only take about 20% of the total CPU time; and the propagation of the resin front can be achieved with CFL-sized time steps without overfilling cells and without applying any artificial smoothing.

Out–of–plane capillary pressure of technical textiles

Textile permeability characterization often follows an unsaturated approach, i.e. a dry textile is impregnated by a fluid and flow front movement is monitored. For permeability calculation based on Darcy‘s law, a pressure-driven injection is commonly applied. Depending on flow velocity, capillary pressure can also contribute to flow movement, which would lead to a calculation error if not considered. This again requires capillary pressure determination. In the present work, a novel measurement system for out-of-plane capillary pressure based on weight increase was developed and applied to characterize the out–of–plane capillary pressure of a random mat, a twill 2/2 woven fabric, and a biaxial non-crimp fabric (all glass fiber) at three different fiber volume contents. Silicone oil was used as measurement fluid. Capillary pressures ranged between 150 and 225 Pa.

The influence of the type of polypropylene and the length of the flow path on the structure and properties of injection molded parts with the weld lines

Weld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. In the weld line area, the V‐shaped notch is formed and its shape and size depend on injection molding conditions and properties of processed polymer. Addition of the foaming agent to the polymer can be one of the way to improve conditions of melt streams welding due to the higher velocity of the colliding streams of unfilled polypropylene (PP) and PP filled with talc. The examinations of mechanical properties showed, however, lower tensile strength of porous parts compared to solid ones, but in the microscopic observation and measurements of the geometric structure of moldings, in the weld lines area, better surface conditions were achieved for samples made of the foamed polypropylene. The size of V‐notch, determined by the total height of the raw profile Pt, depends also on the length of polymer flow path from the gate to the weld line area. The values of Pt parameter increase with the length of the flow path, but this increase is smaller for foamed polypropylene. POLYM. ENG. SCI., 59:1710–1718 2019. © 2019 Society of Plastics Engineers

Filling kinetics of Zr-based amorphous supercooled liquid in micro filling process with longer unsteady flow

The hot-embossing tests of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1) bulk metallic glass (BMG) have been performed under various experimental parameters. The filling features of prepared micro channels, such as filling height, aspect ratio and surface quality, were observed by scanning electron microscope and laser scanning confocal microscopy. It is suggested that the velocity gradient induced by friction force produces an arc-shaped flow front on the BMG micro channel. The filling heights of micro channels increase with increasing processing temperature, embossing pressure and cavity width. The aspect ratio decreases sharply on increasing the cavity width from 20 μm to 50 μm and remains approximate constant when the cavity width exceeds 200 μm. Considering the BMG supercooled liquid should not be within steady flow state at the early stage of the hot-embossing experiment, which is inconsistent with the common assumption of Hagen-Poiseuille equation, an universal equation has been developed by introducing the contribution of linear loading process to filling behavior. Comparison results prove that the theoretical calculations are in better agreement with the hot-embossing test data, thus verifying the validity and accuracy of the new kinetic equation.

Material flow influence on the quality of molded parts

This paper focuses on a new occurrence mechanism of the flow marks, which appear on the surface of injection-molded parts, through characterization of the part surface structure. Injection-molded parts involving long flow lengths can exhibit visible flow defects characterized by alternating glossy and dull bands on the polymer surface, which have approximately the same direction with the material flow front. These flow defects are considered to have strong negative effect on the appearance of the molded component and thus on the perceived quality. The reasons for the appearance of these stripes are not entirely clear. Defect flow marks (shiny areas) were cut from the molded part and analyzed using optical microscopy, Fourier transform infrared spectroscopy, laser Raman spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray (SEM/EDX) spectroscopy. Raman spectra detected differences between spectra of defect area and of the reference one. The EDX analysis revealed that the defect area contains slightly more quantity of talc (Mg and Si). This happened because of material flow morphology in the areas where the molded part presents ribs. The flow of melted resin through an enlargement (rib) results in a decrease in flow velocity, and consequently, a pressure rise will occur and it also involves a vortex development. Therefore, the shear stress in this area is higher than upstream and downstream ones, and it will cause a delay of the flow stream, a premature solidification of the melted resin and ribs flow marks occurrence.

A two-layer numerical model for simulating the frontal plowing phenomenon of flow-like landslides

Many flow-like landslides entrain material from their paths during motion. At the flow front and along the lateral margins the sliding mass can plow into the path material pushing or entraining the existing soil. Although plowing can be a dominant mechanism for landslide mobility, little attention has been paid to this phenomenon in comparison with other entraining mechanism such as basal scouring. Therefore, establishing a suitable mathematical description is still a challenge. In this paper, a two-layer finite difference model is proposed to simulate frontal plowing. The frontal erodible mass and the sliding mass are simplified as two separate layers based on the assumption that they are immiscible in their propagation processes. The interaction (i.e., thrusting and shear) between the two layers is simulated by the normal force and shear force acting on the two-layer interface. The governing equations for the two-layer model are deduced from the mass and momentum conservations of a soil column and transformed into a finite difference form for numerical solving. Then the proposed model is tested in the back analysis of the Ximiaodian landslide which is a typical loess flow-like landslide located at the south bank of the Jing River, China. The modeling results show that frontal plowing has significant influence on the propagation of this landslide, especially on the final topography of the deposit. Without considering this effect, the thickness of the final deposit tends to be underestimated, while the propagation duration, area and distance are likely to be overestimated. The proposed model can provide more accurate and reliable simulations for rapid flow-like landslides with frontal plowing phenomenon.

Finite Volume Method Study on Contact Line Jump Phenomena and Dynamic Contact Angle of Underfill Flow in Flip-Chip of Various Bump Pitches

Three distinct flip-chip underfill cases were considered in this work, with each possess a bump pitch of 0.8 mm, 1.0 mm and 1.2 mm. Contact line jump (CLJ) phenomenon during the underfill flow were successfully visualized with the aided of finite volume method (FVM) based simulation. The current simulated underfill flow fronts were qualitatively validated with the existing experimental data. Generally, the attachment process occurs much faster than the detachment process, by a factor of 14. Within the investigated pitches range, shorter bump pitch would yield a slower underfill flow but a faster-completion underfill process. The voiding mechanism during the underfill process were presented. Furthermore, the dynamic contact angle of underfill meniscus were analytically computed from the numerical data. The contact angle is found to be varies sinusoidally with the filling time, while the impact of bump pitch is minimal. Lastly, it is found that the bump pitch of flip-chip does affect the void formation and propagation.

Load model for designing flexible steel barriers for debris flow mitigation

Light-weight flexible steel net barriers catch coarse debris, but let some of the fine material and water pass through the net. They are difficult to design so that they can withstand the impact pressures of both boulder-laden granular and water-saturated debris flows. Using results from laboratory and full-scale field tests, a debris flow load model has been developed for flexible barriers in torrent channels. The model accounts for the forces of initial impact as well as the filling process discretized stepwise over time (barriers in the field and laboratory fill continuously). Laboratory tests with fast debris flow front velocities revealed a run-up behaviour that was not observed in the field (“pile-up”). The load model divides the flow forces into a hydrostatic component and a dynamic part depending on a pressure coefficient, the flow velocity, and the density of the flow. This dynamic part, which is more complex to quantify, accounts for the wide-ranging debris flow characteristics from watery and muddy debris floods to granular friction-dominated mass flows.