Gravity-driven Flow Research Articles

Fungal brain infection modelled in a human-neurovascular-unit-on-a-chip with a functional blood-brain barrier.

The neurovascular unit, which consists of vascular cells surrounded by astrocytic end-feet and neurons, controls cerebral blood flow and the permeability of the blood-brain barrier (BBB) to maintain homeostasis in the neuronal milieu. Studying how some pathogens and drugs can penetrate the human BBB and disrupt neuronal homeostasis requires in vitro microphysiological models of the neurovascular unit. Here we show that the neurotropism of Cryptococcus neoformans-the most common pathogen causing fungal meningitis-and its ability to penetrate the BBB can be modelled by the co-culture of human neural stem cells, brain microvascular endothelial cells and brain vascular pericytes in a human-neurovascular-unit-on-a-chip maintained by a stepwise gravity-driven unidirectional flow and recapitulating the structural and functional features of the BBB. We found that the pathogen forms clusters of cells that penetrate the BBB without altering tight junctions, suggesting a transcytosis-mediated mechanism. The neurovascular-unit-on-a-chip may facilitate the study of the mechanisms of brain infection by pathogens, and the development of drugs for a range of brain diseases.

Experimental validation of numerical simulations of free-surface flow within casting mould cavities

PurposeThe paper targets on providing new experimental data for validation of the well-established mathematical models within the framework of the lattice Boltzmann method (LBM), which are applied to problems of casting processes in complex mould cavities.Design/methodology/approachAn experimental campaign aiming at the free-surface flow within a system of narrow channels is designed and executed under well-controlled laboratory conditions. An in-house lattice Boltzmann solver is implemented. Its algorithm is described in detail and its performance is tested thoroughly using both the newly recorded experimental data and well-known analytical benchmark tests.FindingsThe benchmark tests prove the ability of the implemented algorithm to provide a reliable solution when the surface tension effects become dominant. The convergence of the implemented method is assessed. The two new experimentally studied problems are resolved well by simulations using a coarse computational grid.Originality/valueA detailed set of original experimental data for validation of computational schemes for simulations of free-surface gravity-driven flow within a system of narrow channels is presented.

Irreversibility analysis in micropolar fluid film along an incline porous substrate with slip effects

The current article examined the semi-analytical solution for the rate of entropy generation in a steady, gravity-driven thin film flow of a micropolar fluid descending over a heated inclined substrate with slip constraints. A comprehensive semi-analytical approach (differential transformation method (DTM)) was instrumental in obtaining the solution of the nonlinear governing model. The characteristics of velocity, angular velocity, heat balance, entropy generation, skin-friction, wall couple stress, Nusselt number and Bejan number have been deliberated under the influence of involved flow controlling physical parameters. The outcomes of the present study shows that an incline in values of Microrotation parameter declines the velocity gradient and inclines thermal, dimensionless Bejan number and entropy generation profiles.

Hydroelastic response of a floating plate on the falling film: A stability analysis

The role of a floating elastic plate on the hydrodynamic instability of a gravity-driven flow down an inclined plane is examined in the linear and nonlinear regimes. Linear instability of the system with respect to infinitesimal disturbances is captured using normal-mode analysis. The critical conditions for instability are obtained analytically utilizing the small film aspect ratio. The bifurcation of the nonlinear evolution equation is analyzed using weakly nonlinear stability analysis. The time evolution of the surface elevation is analyzed using the nonlinear analysis. The Orr–Sommerfeld boundary value problem corresponding to the perturbed flow is derived, and it is solved numerically using the spectral collocation method. The behavior of the marginal stability curves and temporal growth of the unstable waves are portrayed for a range of dimensionless flow parameters. Moreover, the pressure acting on the surface are calculated and analyzed for various structural parameters, applicable to different flow configurations. The study reveals that the structural parameters such as rigidity and mass per unit length play a crucial role in suppressing and facilitating the unstable surface waves of the flow. However, the compressive force acting on the plate results in the flexural destabilization. Thus, the plate parameters are more efficient in damping the shorter wave disturbances. Numerical observations imply that the floating elastic plate assists in stabilizing the free-falling flow and dampens the high amplitude waves.

A fully resolved SPH-DEM method for heterogeneous suspensions with arbitrary particle shape

The study on fluid-solid interaction in heterogeneous suspensions is essential for theoretical research and engineering practice. In this paper, a three-dimensional fully resolved SPH-DEM method is presented to model suspensions with non-Newtonian fluids and solid particles of arbitrary shape. The fluid phase is modelled using SPH with a regularized Bingham model, whereas aggregates with irregular shapes is modelled using a DEM method based on surface mesh representation. A simple unified hybrid contact method is proposed to model the SPH-boundary, SPH-DEM, DEM-DEM, and DEM-boundary interactions in a generalized fashion. Furthermore, GPU parallelization is employed to increase the numerical efficiency. The coupling method is validated using numerical examples including: (1) water-entry of a sphere, (2) Poiseuille flow of a Bingham material, (3) collapse of columns of irregular particles, and (4) gravity-driven heterogeneous flows with irregular particle shape. The simulation results indicate that the presented method is suitable for heterogeneous flows with irregular solid particles.

Numerical study on gravity-driven granular flow around tube out-wall: Effect of tube inclination on the heat transfer

Heat recovery from granular flow in moving bed heat exchangers has wide industrial applications. However, more knowledge is still necessary for the heat transfer enhancement. For the slow descending of random particles, the flow structure has crucial roles on the heat transfer by conduction and radiation. In this paper, the effect of tube inclination was studied by discrete element method. Influences of particle update, contact and radial migration were carefully discussed. Moreover, heat resistance analysis was conducted to explore the clear law. It was found that, the heat is obviously improved by tube inclination due to the increasing area, but there are uncertainties for the local heat transfer coefficients. At the tube bottom, particles become denser with tube inclination, which is beneficial to increase the heat transfer coefficient. But for the top or middle area, there are two opposite effects. Tube inclination enhances radial migration around tube wall, while simultaneously weakens local particle update. Finally, contact resistance firstly increases and then decreases with inclination angle, but there is a reversal tendency for the sensitivity of penetration resistance to the flow rate. In actual applications, penetration resistance is the major resistance, and the inclination angle is therefore suggested between 15° and 37.5°.

Numerical Investigation of Gravity-Driven Granular Flow around the Vertical Plate: Effect of Pin-Fin and Oscillation on the Heat Transfer

In this paper, the heat transfer of pin-fin plate unit (PFPU) under static and oscillating conditions are numerically studied using the discrete element method (DEM). The flow and heat transfer characteristics of the PFPU with sinusoidal oscillation are investigated under the conditions of oscillating frequency of 0–10 Hz, amplitude of 0–5 mm and oscillating direction of Y and Z. The contact number, contact time, porosity and heat transfer coefficient under the above conditions are analyzed and compared with the smooth plate. The results show that the particle far away from the plate can transfer heat with the pin-fin of PFPU, and the oscillating PFPU can significantly increase the contact number and enhance the temperature diffusion and heat transfer. The heat transfer coefficient of PFPU increases with the increase of oscillating frequency and amplitude. When the PFPU oscillates along the Y direction with the amplitude of 1 mm and the frequency of 10 Hz, the heat transfer coefficient of PFPU is increased by 28% compared with that of the smooth plate. Compared with the oscillation along the Z direction, the oscillation along the Y direction has a significant enhancement on the heat transfer of PFPU.

Effect of wall slip on vertical film drainage in presence of soluble surfactants

A mathematical model is established to investigate a vertical gravity-driven drainage flow containing a soluble surfactant when considering the effect of wall slip. The lubrication theory is employed to obtain the evolution equations describing film thickness, surface velocity, surfactant concentrations at the air–liquid, solid–liquid interface, and in the bulk. The influence of constant slip length bc and variable slip length bs varying with surfactant concentration on the drainage dynamics is investigated compared with the case of no-slip bo, and the mechanism of the film thinning and the backflow caused by wall slip is examined. Simulated results show that the wall slip has a significant impact on the dynamics of the film drainage compared with the no-slip case. For the case of constant slip length, the wall slip accelerates the film thinning in the early stage. At the middle stage, the wall slip enhances the Marangoni effect and surface velocity rapidly decreases, causing a surface backflow phenomenon at the film bottom; the higher the slip length, the more obvious surface backflow. In the late stage, surface backflow weakens, and the film thickness is less than that of bo. For the case of variable slip length, in the early stage, the film thickness and surface velocity are between those of bo and bc; at the middle stage, a weak surface backflow is evolved at the film bottom; in the late stage, the film thickness is close to that of bc, and the surfactant concentration is lower than those of bo and bc.

Enhanced weakly-compressible MPS method for immersed granular flows

We develop and validate a three-dimensional particle method based on an Enhanced Weakly-Compressible MPS approach for modeling immersed dense granular flows. For this purpose, we adopt a generalized rheological model, using a regularized visco-inertial rheology, for all regimes of multiphase granular flow. Moreover, we propose a new consistent formulation to estimate the effective pressure of the solid skeleton based on the continuity equation of the pore-water. To improve the accuracy of the multiphase particle methods, especially near boundaries and interfaces, we introduce a modified high-order diffusive term by employing the convergent form of the Laplacian operator. The effectiveness of the new diffusive term is particularly demonstrated by modeling the hydrostatic pressure of two fluid phases. Further, coupling the generalized rheology model with the flow equations, we investigate the gravity-driven granular flows in the immersed granular collapse and slide in three dimensions. As a part of this study, we represent the experiment on the immersed granular collapse to validate the model. The evolution and runout length of the granular bulk are compared with those from experiments confirming good compatibility. Overall, the qualitative and quantitative results justify the proposed developments shown to be essential for predicting different states of the immersed granular flows.

A coupled SPH-DEM-FEM model for fluid-particle-structure interaction and a case study of Wenjia gully debris flow impact estimation

Debris flows are rapid gravity-driven unsteady flows of highly concentrated mixtures of water and solid particle material, destroying numerous mountain building structures and traffic facilities. The investigation of debris flows is thus of significance to hazard prevention and mitigation. This paper aims to provide a numerical model capable of reproducing debris flow impact estimation by accounting for the complicated fluid-particle-structure interaction (FPSI) using a coupled smoothed particles hydrodynamics (SPH), discrete element method (DEM), and finite element method (FEM) approach. The fluid phase is represented by SPH. The solid phase consists of physical particle(s) and is represented by the DEM, and the deformable structure is represented by the FEM. The interaction forces among the fluid, solid particles, and structure are computed using the penalty function method. The proposed model is capable of simultaneously simulating a fluid-particle interaction (FPI), particle-structure interaction (PSI), and fluid-structure interaction (FSI), with good agreement between the complicated hybrid numerical and experimental results. Finally, a Wenjia gully debris flow is carried out to demonstrate the capability of the coupled model in simulating the FPSI as an application of debris flow impact simulations. When compared with the actual situation, the propagation of the debris flow and destruction of the structures were predicted accurately. Additionally, the Wenjia gully debris flow was treated, and the treatment measure was analyzed from the impact evolution (e.g., interception by dams, impact force, and destruction of dams). The developed method will contribute to a better understanding of the FPSI and is a promising tool for hazard analysis and mitigation.

Groundwater quality index as a hydrochemical tool for monitoring saltwater intrusion into coastal freshwater aquifer of Eastern Dahomey Basin, Southwestern Nigeria

Saltwater intrusion into coastal freshwater aquifers is a threat to groundwater quality globally. This study aims to determine the extent of saltwater intrusion into the coastal freshwater aquifer of the Eastern Dahomey Basin (EDB), Nigeria. Groundwater chemistry was sampled and analysed for ionic ratios and interpreted using a hydrochemical facie evolution diagram (HFE-D), the saltwater mixing index (SMI) and the Groundwater quality index for saltwater intrusion (GQIswi). High EC and TDS and the concentration of dissolved ions showed increased salinity as a result of seawater intrusion in wells located around communities in Seme, Lekki, Eleko, Okun-Ajah, Ode-Mahin and Igbokoda. Correlation of ions in the wet season also suggests higher salinities which originate partly from industrial and municipal effluents especially from wells which are close to river channels, while dry season groundwater shows the dominant influence of seawater intrusion. HFE-D revealed that mixed groundwater of Na + Ca–HCO3, Na–Cl and Ca–Cl dominate the area due to gravity-driven flow leading to groundwater freshening inland from the coastline towards the northern part of the basin. The groundwater quality index from SMI and GQIswi shows areas within 3 km from the coastline that are more sensitive to saltwater intrusion based on abstraction rate and depth of the wells. The present study provides information of value to planners and policy-makers for the sustainable management and protection of coastal groundwater resources in the Eastern Dahomey Basin.

Dilatancy-driven secondary flows in dense granular materials

Abstract

Regional hydrogeology of the Arab-D aquifer in Central and Eastern Saudi Arabia with highlighting its environmental and energy potential

The Jurassic Arab-D reservoir hosts major oil-producing zone in Eastern Saudi Arabia with production from individual wells exceeding 25000 B/D. From a water supply perspective, Saudi water authorities consider it a minor aquifer with small yields of relatively high salinity; hence, it did not receive much attention in previous studies of aquifers. The present paper describes regional hydrogeological characteristics of the Arab-D in terms of groundwater flow, salinity and temperature, and discusses geological factors controlling these characteristics. Although high groundwater salinity in the Arab-D precludes it from being a potential water supply resource, studying the hydrogeology of Arab-D provides a hydrodynamic tool to oil exploration and development project. It also sets the stage for detailed studies of Arab-D aquifer potential use for industrial effluent disposal and as new geothermal energy source. Most of groundwater flow in Arab-D occurs in the porous and permeable grainy-textured upper zones. The top limit of the groundwater flow domain in Arab-D is the overlying massive Arab-D anhydrite unit. Trends in Arab-D hydraulic head, water salinity and temperature show gravity-driven flow towards the coastal region of Eastern Arabia. Major anomalies in these trends exist near the Ghawar, Summan, Jauf and Yabrin north-south faults. These anomalies are results of fault zone architecture that consists of sealing fault core zones surrounded by conductive fault damage zones. Depth, salinity, and temperature of the Arab-D between north Ghawar and Berri fields show that it has potential uses for industrial effluent disposal and geothermal energy projects.

Experimental study of the dynamic behavior and segregation of density-bidisperse granular sliding masses at the laboratory scale

From the understanding of dynamics and processes of rapid granular flows and the granular-segregation mechanism in gravity-driven flow, we can clarify the particle-composition structure in the downstream areas of avalanches in geophysical contexts, such as landslides, rock falls, and snow-slab avalanches. Such dynamics also provide a basis for geophysical studies. This study experimentally investigates the dynamic behavior and segregation phenomena of a density-bidisperse, rapid, granular flow down a quasi-2D, rough, inclined rectangular chute. Particles with two density ratios are used to investigate the mechanism of density-induced segregation, and four chute-inclination angles are tested to examine the influence of driving forces. The dynamics of the mixture flow—which includes the flow-depth evolution, stream-wise and depth-wise velocity profiles, shear rate, and granular temperature in the upper high-shear band of the flow—are obtained from particle image velocimetry (PIV) measurements. The two-dimensional concentration distributions of the particles in the stream-wise direction are also obtained using 2D image processing to determine the segregation state. In the upstream region, the variation in the concentration of heavier particles is defined as the strength of the density-induced segregation state, Sd. Our results indicate that the mixture-flow parameter—particularly the shear rate and the granular temperature in the upper high-shear band—crucially influence the strength of particle segregation in granular avalanches. In the upstream region, a higher shear rate and a higher granular temperature in the upper high-velocity band result in a smaller drag force in the mixture flow, causing stronger density-induced particle segregation. These results well describe the entire processes of dense granular flows, from upstream initiation to the downstream steady state. Therefore, they reveal the structure of the mixed flow in the depth direction and are expected to explain various gravity-driven mixture granular flows.

Physical and chemical structure of the Serpens filament: Fast formation and gravity-driven accretion

Context. The Serpens filament, a prominent elongated structure in a relatively nearby molecular cloud, is believed to be at an early evolutionary stage, so studying its physical and chemical properties can shed light on filament formation and early evolution. Aims. The main goal is to address the physical and chemical properties as well as the dynamical state of the Serpens filament at a spatial resolution of ~0.07 pc and a spectral resolution of ≲0.1 km s−1. Methods. We performed 13CO (1–0), C18O (1–0), C17O (1–0), 13CO (2–1), C18O (2–1), and C17O (2–1) imaging observations toward the Serpens filament with the Institut de Radioastronomie Millimétrique 30-m and Atacama Pathfinder EXperiment telescopes. Results. Widespread narrow 13CO (2–1) self-absorption is observed in this filament, causing the 13CO morphology to be different from the filamentary structure traced by C18O and C17O. Our excitation analysis suggests that the opacities of C18O transitions become higher than unity in most regions, and this analysis confirms the presence of widespread CO depletion. Further we show that the local velocity gradients have a tendency to be perpendicular to the filament’s long axis in the outskirts and parallel to the large-scale magnetic field direction. The magnitudes of the local velocity gradients decrease toward the filament’s crest. The observed velocity structure can be a result of gravity-driven accretion flows. The isochronic evolutionary track of the C18O freeze-out process indicates the filament is young with an age of ≲2 Myr. Conclusions. We propose that the Serpens filament is a newly-formed slightly-supercritical structure which appears to be actively accreting material from its ambient gas.

Experimental study of critical heat flux in pool boiling using visible-ray optics

Liquid, vapor and their dynamics on boiling surfaces were directly observed by a simple visible-ray optics with a sub-millisecond time scale. The observation results revealed that the critical heat flux (CHF) was triggered by a localized dry region. The fast and dense nucleation of vapor bubbles, and liquid supplied to the dry regions by bubbles, afford the boiling process with extremely high heat transport capability. However, the intensive nucleation of the bubbles in a narrow region, and their coalescence, generated a large, irregularly shaped dry region at high heat flux that functioned to impede heat transfer between the boiling surface and fluid. Sequential images showing the phase distribution revealed that the gravity-driven liquid flow was obstructed by the excessive evaporation occurring around the local dry region that triggered the CHF condition. We propose a CHF triggering mechanism based on gravity and vapor recoil force balance. Under the conditions of saturated water boiling under 1 atm of pressure, these forces balance were balanced when the temperature of the dry region was approximately 130°C. The temperature limit corresponding to the non-wetting of a local dry spot was found to be in agreement with previous studies and our simulation results for transient thermal conduction on a boiling surface.

Experimental investigation on the interaction between rapid dry gravity-driven debris flow and array of obstacles

Arrays of obstacles are a potentially effective measure to manage a channel landslide run-out deposit. In this research, three kinds of debris sands with different particle sizes, 0.25–0.5 mm, 1–2 mm and 2–5 mm respectively, are investigated in a 4.28-m-long chute with a fixed incline angle of 40°. Structure from motion (SfM) and other novel image analysis techniques are proposed to analyse the deposits’ characteristics, including time histories. These allow measuring accurately the run-out distance, width, 3D topography and area of the deposits which are used to assess the effectiveness of the obstacles to manage run-out deposits efficiently. Experimental results reveal that particle size has a significant impact on the final deposition because they effectively behave as three different rheology characteristics: viscous (fine), frictional (medium) and collisional (coarse). The observations show emerging shape properties that are characterised, as well as surprising behaviours when considering time histories such as the non-monotonic area growth in fine, or viscous, landslides. Other phenomena such as airborne particle jets are observed for the coarse particle size, representing a collisional-type flow. In practical terms, the experiments show that when designing protection barriers, a compromise is needed between length, width and depth of deposition and that this can only be decided based on the structure to be protected.

Continental shelf morphology controlled by bottom currents, mud diapirism, and submarine slumping to the east of the Gaoping Canyon, off SW Taiwan

The Gaoping River-Gaoping Submarine Canyon (GPC) is one of the dispersal river systems in the world. Tens of tons of terrestrial sediments discharge per year at the mouth of the river, where gravity-driven flow can damage the telecommunication cable along the 260-km-long GPC; to the east of the GPC, the continental shelf has been eroded. To understand this geological phenomenon, high-resolution sparker seismic data around the continental shelf as well as parallel and across the GPC were collected. Deep-towed sub-bottom profiler (SBP) data and data from shipboard acoustic Doppler current profiler were also collected. Three major system tracts off southwest Taiwan were identified, based on which the coastline of the last glacial maximum was identified at approximately 138 m below the current sea level. Analysis of the deep-towed SBP profiles suggested sandy sediment overflows through breakages on the east bank of the GPC. A convergence of bottom currents with a speed of up to 1.5 m/s flowing southeastward has eroded the continental shelf intensively, forming the Xiaoliuchiu channel. Although a mud diapir is active to the east of the GPC and forms the Xiaoliuchiu islet, it has been eroded at the Xiaoliuchiu channel. The average erosion rate on the east bank of the GPC was estimated to be approximately 609 cm/ka in the past 8.2 ka; between the GPC and the Xiaoliuchiu islet, the continental shelf is wasted due to severe seabed slumping.

Free-surface behaviour of shallow turbulent flows

Over the last two decades, interest in the free-surface behaviour of gravity-driven shallow turbulent flows has increased considerably. It is believed that observation of free-surface behaviour can provide useful information about the hydrodynamic characteristics of the flow and enable remote retrieval of these characteristics to non-invasively and rapidly monitor river flows. At the current state the literature presents scattered knowledge and also exhibits non-uniformity in the terminology used. This paper is a review of the state-of-art of this area of research and was created with two objectives: to gather the information relevant to understand the linkages between the free-surface behaviour and underpinning hydrodynamic processes while using a uniform terminology, and to analyse the gaps in our knowledge of this critical topic.

On-chip perivascular niche supporting stemness of patient-derived glioma cells in a serum-free, flowable culture.

Glioblastoma multiforme (GBM) is the most common and the most aggressive type of primary brain malignancy. Glioblastoma stem-like cells (GSCs) can migrate in vascular niches within or away from the tumour mass, increasing tumour resistance to treatments and contributing to relapses. To study individual GSC migration and their interactions with the perivasculature of the tumour microenvironment, there is a need to develop a human organotypic in vitro model. Herein, we demonstrated a perivascular niche-on-a-chip, in a serum-free condition with gravity-driven flow, that supported the stemness of patient-derived GSCs and foetal neural stem cells grown in a three-dimensional environment (3D). Endothelial cells from three organ origins, (i) human brain microvascular endothelial cells (hCMEC/D3), (ii) human umbilical vein endothelial cells (HUVECs) and, (iii) human lung microvascular endothelial cells (HMVEC-L) formed rounded microvessels within the extracellular-matrix integrated microfluidic chip. By optimising cell extraction protocols, systematic studies were performed to evaluate the effects of serum-free media, 3D cell cultures, and the application of gravity-driven flow on the characteristics of endothelial cells and their co-culture with GSCs. Our results showed the maintenance of adherent and tight junction markers of hCMEC/D3 in the serum-free culture and that gravity-driven flow was essential to support adequate viability of both the microvessel and the GSCs in co-culture (>80% viability at day 3). Endpoint biological assays showed upregulation of neovascularization-related genes (e.g., angiopoietins, vascular endothelial growth factor receptors) in endothelial cells co-cultured with GSCs in contrast to the neural stem cell reference that showed insignificant changes. The on-chip platform further permitted live-cell imaging of GSC – microvessel interaction, enabling quantitative analysis of GSC polarization and migration. Overall, our comparative genotypic (i.e. qPCR) and phenotypic (i.e. vessel permeability and GSC migration) studies showed that organotypic (brain cancer cells–brain endothelial microvessel) interactions differed from those within non-tissue specific vascular niches of human origin. The development and optimization of this on-chip perivascular niche, in a serum-free flowable culture, could provide the next level of complexity of an in vitro system to study the influence of glioma stem cells on brain endothelium.