L. Zhuang, S. M. Hassanizadeh, D. Bhatt, and C. van Duijn, “Spontaneous Imbibition and Drainage of Water in a Thin Porous Layer: Experiments and Modeling,”
Transport in Porous Media, vol. 139, no. 2, Art. no. 2, 2021, doi:
10.1007/s11242-021-01670-7.
Abstract
The typical characteristic of a thin porous layer is that its thickness is much smaller than its in-plane dimensions. This often leads to physical behaviors that are different from three-dimensional porous media. The classical Richards equation is insufficient to simulate many flow conditions in thin porous media. Here, we have provided an alternative approach by accounting for the dynamic capillarity effect. In this study, we have presented a set of one-dimensional in-plane imbibition and subsequent drainage experiments in a thin fibrous layer. The X-ray transmission method was used to measure saturation distributions along the fibrous sample. We simulated the experimental results using Richards equation either with classical capillary equation or with a so-called dynamic capillarity term. We have found that the standard Richards equation was not able to simulate the experimental results, and the dynamic capillarity effect should be taken into account in order to model the spontaneous imbibition. The experimental data presented here may also be used by other researchers to validate their models.J. Zeifang and A. Beck, “A Data-Driven High Order Sub-Cell Artificial Viscosity for the Discontinuous Galerkin Spectral Element Method,”
Journal of Computational Physics, vol. 441, p. Article 110475, 2021, doi:
10.1016/j.jcp.2021.110475.
Abstract
In this work, we present a novel higher-order smooth artificial viscosity method for the discontinuous Galerkin spectral element method and related high order methods. A neural network is used to detect the need for stabilization. Inspired by techniques from image edge detection, the neural network locates discontinuities inside mesh elements on a sub-cell level. Once the sub-cell positions of the shock fronts have been identified, the use of radial basis functions enables the construction of a high order smooth artificial viscosity field on quadrilateral meshes. We show the superiority of using higher order smooth artificial viscosity over piecewise linear approaches in particular on coarse meshes. The capabilities of the novel method are illustrated with typical applications.A. Yiotis, N. Karadimitriou, I. Zarikos, and H. Steeb, “Pore-scale effects during the transition from capillary-to viscosity-dominated flow dynamics within microfluidic porous-like domains,”
Scientific Reports, vol. 11, no. 1, Art. no. 1, 2021, doi:
10.1038/s41598-021-83065-8.
Abstract
We perform a numerical and experimental study of immiscible two-phase flows within predominantly 2D transparent PDMS microfluidic domains with disordered pillar-like obstacles, that effectively serve as artificial porous structures. Using a high sensitivity pressure sensor at the flow inlet, we capture experimentally the pressure dynamics under fixed flow rate conditions as the fluid–fluid interface advances within the porous domain, while also monitoring the corresponding phase distribution patterns using optical microscopy. Our experimental study covers 4 orders of magnitude with respect to the injection flow rate and highlights the characteristics of immiscible displacement processes during the transition from the capillarity-controlled interface displacement regime at lower flow rates, where the pores are invaded sequentially in the form of Haines jumps, to the viscosity-dominated regime, where multiple pores are invaded simultaneously. In the capillary regime, we recover a clear correlation between the recorded inlet pressure and the pore-throat diameter invaded by the interface that follows the Young–Laplace equation, while during the transition to the viscous regime such a correlation is no longer evident due to multiple pore-throats being invaded simultaneously (but also due to significant viscous pressure drop along the inlet and outlet channels, that effectively mask capillary effects). The performed experimental study serves for the validation of a robust Level-Set model capable of explicitly tracking interfacial dynamics at sub-pore scale resolutions under identical flow conditions. The numerical model is validated against both well-established theoretical flow models, that account for the effects of viscous and capillary forces on interfacial dynamics, and the experimental results obtained using the developed microfluidic setup over a wide range of capillary numbers. Our results show that the proposed numerical model recovers very well the experimentally observed flow dynamics in terms of phase distribution patterns and inlet pressures, but also the effects of viscous flow on the apparent (i.e. dynamic) contact angles in the vicinity of the pore walls. For the first time in the literature, this work clearly shows that the proposed numerical approach has an undoubtable strong potential to simulate multiphase flow in porous domains over a wide range of Capillary numbers.G. Yang
et al., “A superhydrophilic metal--organic framework thin film for enhancing capillary-driven boiling heat transfer,”
Journal of Materials Chemistry A, vol. 9, no. 45, Art. no. 45, 2021, doi:
10.1039/D1TA06826A.
Abstract
Many engineering technologies such as electronic cooling and thermal desalination exemplify the enhancement of evaporation and boiling heat transfer by surface modification. Nevertheless, the core parameters of heat transfer such as critical heat flux and heat transfer coefficient are associated with surface wettability and morphology. Herein, for the first time, a metal–organic framework (MOF) film, viz. HKUST-1, was integrated into a metallic woven mesh (macroporous support) for enhancing liquid rewetting and capillary-driven evaporation and boiling heat transfer. Compared to bare copper mesh, this architecture was found to significantly increase the critical heat flux by 205% and the heat transfer coefficient by 90%. The complex coupled two-phase (liquid and gas) transport process involving capillary wicking, evaporation, adsorption and desorption were critically examined by analysing the dynamics of multiple interfaces during horizontal wicking. Relying upon visible colorimetric changes, HKUST-1 sustained on the copper woven mesh could expedite quantitative analysis of the coupled capillary evaporation process. In principle, this is primed to offer fundamental insights into the mechanisms of transport phenomena. Introduction of such previously unreported hierarchical porous structures could also potentially advance the state-of-the-art of passive thermal management technologies. In essence, a new route to elicit superhydrophilic surfaces emerges, paving new ways for understanding the intrinsic mechanisms of phase-change heat transfer.F. Weinhardt, H. Class, S. Vahid Dastjerdi, N. Karadimitriou, D. Lee, and H. Steeb, “Experimental Methods and Imaging for Enzymatically Induced Calcite Precipitation in a Microfluidic Cell,”
Water Resources Research, vol. 57, no. 3, Art. no. 3, 2021, doi:
10.1029/2020WR029361.
Abstract
Enzymatically induced calcite precipitation (EICP) in porous media can be used as an engineering option to achieve precipitation in the pore space, for example, aiming at a targeted sealing of existing flow paths. This is accomplished through a porosity and consequent permeability alteration. A major source of uncertainty in modeling EICP is in the quantitative description of permeability alteration due to precipitation. This report presents methods for investigating experimentally the time-resolved effects of growing precipitates on porosity and permeability on the pore scale, in a poly-di-methyl-siloxane microfluidic flow cell. These methods include the design and production of the microfluidic cells, the preparation and usage of the chemical solutions, the injection strategy, and the monitoring of pressure drops for given fluxes for the determination of permeability. EICP imaging methods are explained, including optical microscopy and X-ray microcomputed tomography (XRCT), and the corresponding image processing and analysis. We present and discuss a new experimental procedure using a microfluidic cell, as well as the general perspectives for further experimental and numerical simulation studies on induced calcite precipitation. The results of this study show the enormous benefits and insights achieved by combining both light microscopy and XRCT with hydraulic measurements in microfluidic chips. This allows for a quantitative analysis of the evolution of precipitates with respect to their size and shape, while monitoring their influence on permeability. We consider this to be an improvement of the existing methods in the literature regarding the interpretation of recorded data (pressure, flux, and visualization) during pore morphology alteration.W. Wang, G. Yang, C. Evrim, A. Terzis, R. Helmig, and X. Chu, “An assessment of turbulence transportation near
regular and random permeable interfaces,”
Physics of Fluids, vol. 33, p. 115103, 2021, doi:
10.1063/5.0069311.
Abstract
Turbulent channel flow with a porous wall is investigated using direct numerical simulation, where the porous media domain consists of
regular or random circular cylinder arrays. We compare the statistics and structure of the mean flow and turbulence in the channel flow with
a bulk Reynolds number of 2500 and two porosities (u ¼ 0:6 and 0.8) for the porous media. It is shown that the random interface
significantly affects the dynamics of turbulence and the time-averaged flow. More intense mixing is observed near the random interface due
to augmented form-induced shear stresses. Due to the strong dependence of induced flow direction on the interface geometry, we segmented
the flow field into two types of areas based on the slope angle formed by the top-layer cylinders: the windward area and leeward area. The
conditional average of turbulence kinematic energy budget over each type of area reveals their respective role in turbulence transportation
more explicitly. In addition, we use finite-time Lyapunov exponents to inspect the Lagrangian coherent structures in the flow fields, which
reveal the preferential fluid trajectories in the random porous medium geometry.W. Wang, X. Chu, A. Lozano-Durán, R. Helmig, and B. Weigand, “Information transfer between turbulent boundary layers and porous media,”
Journal of Fluid Mechanics, vol. 920, pp. A21--, 2021, doi:
DOI: 10.1017/jfm.2021.445.
Abstract
The interaction between the flow above and below a permeable wall is a central topic in the study of porous media. While previous investigations have provided compelling evidence of the strong coupling between the two regions, few studies have quantitatively measured the directionality, i.e. cause-and-effect relations, of this interaction. To shed light on the problem, interface-resolved direct numerical simulations of channel flow over a cylinder array for porosity $=0.4$–$0.9$ are performed, and the friction Reynolds number of the top smooth wall is controlled to be $Re_180$. We use transfer entropy as a marker to evaluate the causal interaction between the free turbulent flow and the porous medium. Correlation analysis and linear coherence spectra are also leveraged to complete the study. Our results show that the permeability of the porous medium has a profound impact on the intensity, time scale and spatial extent of surface–subsurface interactions. The interaction of the free flow and porous medium is further decomposed into two coupling directions, namely, top-down and bottom-up. For low-porosity cases, top-down and bottom-up interactions are strongly asymmetric, the former being mostly influenced by small near-wall eddies, and the latter reflecting the onset of Kelvin–Helmholtz type instabilities due to the perturbation from the porous medium. As the porosity increases, both top-down and bottom-up interactions are dominated by shear-flow instabilities.A. Wagner
et al., “Permeability Estimation of Regular Porous Structures: A Benchmark for Comparison of Methods,”
Transport in Porous Media, vol. 138, no. 1, Art. no. 1, 2021, doi:
10.1007/s11242-021-01586-2.
Abstract
The intrinsic permeability is a crucial parameter to characterise and quantify fluid flow through porous media. However, this parameter is typically uncertain, even if the geometry of the pore structure is available. In this paper, we perform a comparative study of experimental, semi-analytical and numerical methods to calculate the permeability of a regular porous structure. In particular, we use the Kozeny–Carman relation, different homogenisation approaches (3D, 2D, very thin porous media and pseudo 2D/3D), pore-scale simulations (lattice Boltzmann method, Smoothed Particle Hydrodynamics and finite-element method) and pore-scale experiments (microfluidics). A conceptual design of a periodic porous structure with regularly positioned solid cylinders is set up as a benchmark problem and treated with all considered methods. The results are discussed with regard to the individual strengths and limitations of the used methods. The applicable homogenisation approaches as well as all considered pore-scale models prove their ability to predict the permeability of the benchmark problem. The underestimation obtained by the microfluidic experiments is analysed in detail using the lattice Boltzmann method, which makes it possible to quantify the influence of experimental setup restrictions.J. Steigerwald, M. Ibach, J. Reutzsch, and B. Weigand, “Towards the Numerical Determination of the Splashing Threshold of Two-component Drop Film Interactions,” in
High Performance Computing in Science and Engineering ’20, in High Performance Computing in Science and Engineering ’20. Springer, 2021, pp. 261--279. doi:
10.1007/978-3-030-80602-6_17.
Abstract
The scenario of an impacting drop onto a film is highly relevant in many natural and technical systems. A fundamental and often required parameter of these interactions is the so called splashing threshold above which secondary droplets are generated. For interactions with differing liquid properties for the film and the impacting drop a general splashing threshold is, however, still unknown because an experimental determination is difficult to achieve. For this reason, we investigate the suitability of a numerical determination of this threshold by means of direct numerical simulation using the multiphase flow solver Free Surface 3D (FS3D). Simulations across an already existing splashing threshold are performed stemming from an empirical correlation. In order to determine the necessary grid resolution for accurately reproducing the corresponding impact regime, all interactions are simulated for several grids. A detailed grid study shows that only by using very high grid resolutions the threshold can be reproduced with a sufficient accuracy, whereas the use of coarser resolutions leads to a significant underestimation of the threshold. Additionally, simulations of highly resolved impact phenomena on thin films depend heavily on the efficient solution of the problem with most of the computational costs affiliated to solving the Pressure Poisson Equation within the FS3D framework. Therefore, the implemented multigrid solver was optimized employing advanced tree structured communication during coarsening and refinement on the levels during the solution cycle. A performance analysis of FS3D using the original and the improved multigrid solver shows that the implemented tree structured communication leads to a remarkable speed-up.A. Schlaich, D. Jin, L. Bocquet, and B. Coasne, “Electronic screening using a virtual Thomas--Fermi fluid for predicting wetting and phase transitions of ionic liquids at metal surfaces,”
Nature Materials, Nov. 2021, doi:
10.1038/s41563-021-01121-0.
Abstract
Of relevance to energy storage, electrochemistry and catalysis, ionic and dipolar liquids display unexpected behaviours---especially in confinement. Beyond adsorption, over-screening and crowding effects, experiments have highlighted novel phenomena, such as unconventional screening and the impact of the electronic nature---metallic versus insulating---of the confining surface. Such behaviours, which challenge existing frameworks, highlight the need for tools to fully embrace the properties of confined liquids. Here we introduce a novel approach that involves electronic screening while capturing molecular aspects of interfacial fluids. Although available strategies consider perfect metal or insulator surfaces, we build on the Thomas--Fermi formalism to develop an effective approach that deals with any imperfect metal between these asymptotes. Our approach describes electrostatic interactions within the metal through a `virtual' Thomas--Fermi fluid of charged particles, whose Debye length sets the screening length $łambda$. We show that this method captures the electrostatic interaction decay and electrochemical behaviour on varying $łambda$. By applying this strategy to an ionic liquid, we unveil a wetting transition on switching from insulating to metallic conditions.C. Rohde and H. Tang, “On the stochastic Dullin--Gottwald--Holm equation: global existence and wave-breaking phenomena,”
Nonlinear Differential Equations and Applications NoDEA, vol. 28, no. 5, Art. no. 5, 2021, doi:
10.1007/s00030-020-00661-9.
Abstract
We consider a class of stochastic evolution equations that include in particular the stochastic Camassa–Holm equation. For the initial value problem on a torus, we first establish the local existence and uniqueness of pathwise solutions in the Sobolev spaces Hs with s>3/2. Then we show that strong enough nonlinear noise can prevent blow-up almost surely. To analyze the effects of weaker noise, we consider a linearly multiplicative noise with non-autonomous pre-factor. Then, we formulate precise conditions on the initial data that lead to global existence of strong solutions or to blow-up. The blow-up occurs as wave breaking. For blow-up with positive probability, we derive lower bounds for these probabilities. Finally, the blow-up rate of these solutions is precisely analyzed.M. Osorno, M. Schirwon, N. Kijanski, R. Sivanesapillai, H. Steeb, and D. Göddeke, “A cross-platform, high-performance SPH toolkit for image-based flow simulations on the pore scale of porous media,”
Computer Physics Communications, vol. 267, no. 108059, Art. no. 108059, Oct. 2021, doi:
10.1016/j.cpc.2021.108059.
Y. Liu, A. Geppert, X. Chu, B. Heine, and B. Weigand, “Simulation of an annular liquid jet with a coaxial supersonic gas jet in a medical inhaler,”
Atomization and Sprays, vol. 31, no. 9, Art. no. 9, 2021, doi:
10.1615/AtomizSpr.2021037223.
Abstract
Medical inhalers have been used for the treatment of a wide range of respiratory diseases including COVID-19. In this study, we investigate the annular liquid jet breakup with a coaxial supersonic gas jet using large eddy simulations. This contributes to a further understanding and improvement of medical inhaler designs. The liquid is sucked in by the low pressure as a result of the high velocity gas jet and breaks up due to the interaction with the gas jet. This type of spray nozzle configuration is commonly used in medical inhalers. Two different gas nozzle diameters are studied. The simulated liquid structure is compared with preliminary, qualitative experimental results. The gas jet pressure and radial velocity of the liquid are found to be coupled and the interaction between them plays an important role in formation of the liquid structure. The effect of gas nozzle diameter on flow rate, mean radius of the liquid, and mean radial velocity, as well as its oscillation behavior has been investigated. The power development of dominate frequencies of averaged radial liquid velocity along the flow direction is shown. The growth of the instabilities can be observed from these results.S. Konangi, N. K. Palakurthi, N. K. Karadimitriou, K. Comer, and U. Ghia, “Comparison of pore-scale capillary pressure to macroscale capillary pressure using direct numerical simulations of drainage under dynamic and quasi-static conditions,”
Advances in Water Resources, vol. 147, p. 103792, 2021, doi:
10.1016/j.advwatres.2020.103792.
Abstract
Conventional macroscale two-phase flow equations for porous media (such as Darcy's law and Richards Equation) require a constitutive relation for capillary pressure (Pc). The capillary pressure relation significantly impacts the behavior and prediction of fluid flow in porous media, and needs to accurately characterize the capillary forces. In a typical laboratory experiment, a functional macroscopic capillary pressure-saturation (Pc-Sw) relationship is measured as the difference between the pressures of the non-wetting-phase reservoir at the inlet (Pnw) and wetting-phase reservoir at the outlet (Pw) of a porous medium. It is well-known that this traditional macroscopic capillary pressure definition is valid only at equilibrium conditions and if the phases are connected. Under non-equilibrium (dynamic) conditions, when the fluids are moving, the macroscopic capillary pressure measured in experiments implicitly includes the pressure head caused by viscous effects.
The goal of the present effort is to understand how well the traditional macroscopic capillary pressure definition represents the pore-scale capillary forces under different flow conditions. Using direct numerical simulations (DNS) of two-phase flow in a porous medium, we evaluate the capillary pressure at the pore-scale, and compare it to the macroscopic capillary pressure, Pc(Sw), that is typically measured in experiments using pressure transducers. The pore-scale capillary pressure is the pressure difference across the interface between two fluids as the fluids move through a porous medium; the interface pressure differences at fluid-fluid invasion front are averaged across all the pores of the porous medium to yield a representative pore-scale capillary pressure curve, referred to as the interface capillary pressure. The pore-scale interface capillary pressure represents the “true” capillary forces in the system, since depends only on the pore morphology (shape) and interfacial energy of the two fluids, and does not account for the viscous dissipation. In experiments it is difficult to measure the interface capillary pressure jump without accounting for the viscous pressure head, which is at least an order of magnitude larger. Upscaling the pore-scale capillary pressure is an essential step for complete characterization of capillary-dominant two-phase flow in a porous medium at the laboratory scale.
Drainage is simulated under equilibrium (quasi-static) and non-equilibrium (dynamic) conditions for various capillary numbers. The Navier–Stokes (NS) equations are solved in the pore space using the open-source finite-volume computational fluid dynamics (CFD) code, OpenFOAM. The Volume-of-Fluid (VOF) method is employed to track the evolution of the fluid–fluid interfaces, and a contact angle is used to account for the effect of wall adhesion. The simulations are first validated with published experimental data for dynamic and quasi-static drainage in a micromodel. From the microscale simulations, the interface capillary pressure is determined, and compared to the macroscopic capillary pressure under equilibrium and non-equilibrium conditions. Our results show the traditionally-measured macroscopic capillary pressure curves exhibit a strong dependence on the capillary number under dynamic flow conditions. In contrast, the interface capillary pressure-saturation relation, which relies on pore-scale pressure differences at the invasion front, is almost invariant of flow conditions (dynamic and quasi-static).T. Koch
et al., “DuMux 3--an open-source simulator for solving flow and transport problems in porous media with a focus on model coupling,”
Computers & Mathematics with Applications, vol. 81, pp. 423--443, 2021, doi:
10.1016/j.camwa.2020.02.012.
Abstract
We present version 3 of the open-source simulator for flow and transport processes in porous media DuMux. DuMux is based on the modular C++ framework Dune (Distributed and Unified Numerics Environment) and is developed as a research code with a focus on modularity and reusability. We describe recent efforts in improving the transparency and efficiency of the development process and community-building, as well as efforts towards quality assurance and reproducible research. In addition to a major redesign of many simulation components in order to facilitate setting up complex simulations in DuMux, version 3 introduces a more consistent abstraction of finite volume schemes. Finally, the new framework for multi-domain simulations is described, and three numerical examples demonstrate its flexibility.M. Ibach
et al., “Direct Numerical Simulations of Grouping Effects in Droplet Streams Using Different Boundary Conditions,” in
International Conference on Liquid Atomization and Spray Systems (ICLASS), in International Conference on Liquid Atomization and Spray Systems (ICLASS), vol. 1. 2021. doi:
10.2218/iclass.2021.5815.
T. Hitz, S. Jöns, M. Heinen, J. Vrabec, and C.-D. Munz, “Comparison of macro-and microscopic solutions of the Riemann problem II. Two-phase shock tube,”
Journal of Computational Physics, vol. 429, p. 110027, 2021, doi:
10.1016/j.jcp.2020.110027.
Abstract
The Riemann problem is one of the basic building blocks for numerical methods in computational fluid mechanics. Nonetheless, there are still open questions and gaps in theory and modeling for situations with complex thermodynamic behavior. In this series, we compare numerical solutions of the macroscopic flow equations with molecular dynamics simulation data. To enable molecular dynamics for sufficiently large scales in time and space, we selected the truncated and shifted Lennard-Jones potential, for which also highly accurate equations of state are available. A comparison of a two-phase Riemann problem is shown, which involves a liquid and a vapor phase, with an undergoing phase transition. The loss of hyperbolicity allows for the occurrence of anomalous wave structures. We successfully compare the molecular dynamics data with two macroscopic numerical solutions obtained by either assuming local phase equilibrium or by imposing a kinetic relation and allowing for metastable states.H. Gao, A. B. Tatomir, N. K. Karadimitriou, H. Steeb, and M. Sauter, “A two-phase, pore-scale reactive transport model for the kinetic interface-sensitive tracer,”
Water Resources Research, vol. 57, no. 6, Art. no. 6, 2021, doi:
10.1029/2020WR028572.
Abstract
Previous laboratory experiments with the kinetic interface sensitive (KIS) tracers have shown promising results with respect to the quantification of the fluid-fluid interfacial area (IFA) under dynamic, two-phase flow conditions. However, pore-scale effects relevant to two-phase flow (e.g., the formation of hydrodynamically stagnant/immobile zones) are not yet fully understood, and quantitative information about how far these effects influence the transport of the tracer reaction products is not yet available. Therefore, a pore-scale numerical model that includes two-phase, reactive flow, and transport of the KIS tracer at the fluid-fluid interface is developed. We propose a new method to quantitatively analyze how the mass of the KIS-tracer reaction product in the flowing water is affected by the presence of the immobile zones. The model employs the phase field method (PFM) and a new continuous mass transfer formulation, consistent with the PFM. We verify the model with the analytical solutions of transport involving advection, reaction and diffusion processes. The model is tested for two-phase flow conditions in a conceptual 2D slit. The applicability of the model is demonstrated in NAPL/water drainage scenarios in a conceptual porous domain, comparing the results in terms of the spatial distribution of the phases and solute concentration. Furthermore, we distinguish the mobile and immobile zones based on the local Péclet number, and the corresponding IFA, and solute mass in these two zones is quantified. Finally, we show that the solute mass in flowing water can be employed to selectively determine the mobile part of the IFA.H. Gao, A. Tatomir, N. Karadimitriou, H. Steeb, and M. Sauter, “Effects of surface roughness on the kinetic interface-sensitive tracer transport during drainage processes,”
Advances in Water Resources, vol. 157, p. 104044, 2021, doi:
10.1016/j.advwatres.2021.104044.
Abstract
Porous media surface roughness strongly influences the transport of solutes during drainage due to the formation of thick water films (capillary condensation) on the surface of the porous medium. For interfacially-reacted, water-based solutes, these water films increase both the solute production at the fluid-fluid interface, due to the increased number of fluid-fluid interfaces, and the loss of the solute due to retention in the water films. This study applied a pore-scale, direct numerical simulation with the phase-field method-based continuous solute transport model to simulate the reactive transport of the kinetic interfacial sensitive tracer. The study is implemented during primary drainage in a 2D slit with rough solid walls, where the fractal geometries of the solid surfaces were generated numerically. The moving interfacial area is found to be changing non-monotonically with the root mean square of surface roughness. With increasing root mean square roughness, the average film thickness increases linearly, whereas the film-associated interfacial area per smooth surface area converges to a value slightly larger than one. The retention of the solute mass produced by the moving meniscus in the water film is observed, and this is described by a film-associated mobile mass retention term. An implicit relation between the mobile interfacial area and the solute mass in flowing zones is found. Finally, it is found that the film-associated mobile mass retention term is linearly related to the root mean square roughness.B. Gao, E. Coltman, J. Farnsworth, R. Helmig, and K. M. Smits, “Determination of Vapor and Momentum Roughness Lengths Above an Undulating Soil Surface Based on PIV-Measured Velocity Profiles,”
Water Resources Research, vol. 57, no. 7, Art. no. 7, 2021, doi:
https://doi.org/10.1029/2021WR029578.
Abstract
Abstract Accurately predicting bare-soil evaporation requires the proper characterization of the near-surface atmospheric conditions. These conditions, dependent on factors such as surface microtopography and wind velocity, vary greatly and therefore require high-resolution datasets to be fully incorporated into evaporation models. These factors are oftentimes parameterized in models through the aerodynamic resistance (ra), in which the vapor roughness length (z0v) and the momentum roughness length (z0m) are two crucial parameters that describe the transport near the soil-atmosphere interface. Typically, when evaluating bare-soil evaporation, these two characteristic lengths are assumed equal, although differences are likely to occur especially in turbulent flows over undulating surfaces. Thus, this study aims to investigate the relationship between z0v and z0m above undulating surfaces to ultimately improve accuracy in estimating evaporation rate. To achieve this goal, four uniquely designed wind tunnel—soil tank experiments were conducted considering different wind speeds and undulation spacings. Particle image velocimetry (PIV) was used to measure the velocity field above the undulating surface in high resolution. Using the high-fidelity data set, the logarithmic ratio of z0v to z0m is determined and used to estimate ra. Results confirm that these lengths differ significantly, with the logarithmic ratio roughly ranging from −15 to −5 under the conditions tested. PIV-measured results demonstrate this ratio is closely tied to the mass and momentum transport behaviors influenced by surface undulations. Using the data-integrated formulation of ra, predictions of evaporation rate were prepared for both the laboratory and lysimeter experiments, demonstrating the efficacy of the proposed approach in this study.C. Evrim, X. Chu, F. E. Silber, A. Isaev, S. Weihe, and E. Laurien, “Flow features and thermal stress evaluation in turbulent mixing flows,” vol. 178, p. 121605, Oct. 2021, doi:
10.1016/j.ijheatmasstransfer.2021.121605.
J. Dürrwächter, M. Kurz, P. Kopper, D. Kempf, C.-D. Munz, and A. Beck, “An efficient sliding mesh interface method for high-order discontinuous Galerkin schemes,”
Computers & Fluids, vol. 217, p. 104825, Mar. 2021, doi:
10.1016/j.compfluid.2020.104825.
C. Dingler, H. Müller, M. Wieland, D. Fauser, H. Steeb, and S. Ludwigs, “Actuators: From Understanding Mechanical Behavior to Curvature Prediction of Humidity-Triggered Bilayer Actuators (Adv. Mater. 9/2021),”
Advanced Materials, vol. 33, no. 9, Art. no. 9, 2021, doi:
10.1002/adma.202170067.
Abstract
In article number 2007982, Holger Steeb, Sabine Ludwigs, and co-workers present humidity-triggered bending actuators based on simple polymer bilayers and their in-depth mechanical characterization. A loop consisting of the mechanical characterization and a simple analytical model allows the prediction of deformations of in principle any complex geometry and material combination, for example, for moving parts and curvatures in soft robotics.D. de Winter
et al., “The complexity of porous media flow characterized in a microfluidic model based on confocal laser scanning microscopy and micro-piv,”
Transport in Porous Media, vol. 136, no. 1, Art. no. 1, 2021, doi:
10.1007/s11242-020-01515-9.
Abstract
In this study, the complexity of a steady-state flow through porous media is revealed using confocal laser scanning microscopy (CLSM). Micro-particle image velocimetry (micro-PIV) is applied to construct movies of colloidal particles. The calculated velocity vector fields from images are further utilized to obtain laminar flow streamlines. Fluid flow through a single straight channel is used to confirm that quantitative CLSM measurements can be conducted. Next, the coupling between the flow in a channel and the movement within an intersecting dead-end region is studied. Quantitative CLSM measurements confirm the numerically determined coupling parameter from earlier work of the authors. The fluid flow complexity is demonstrated using a porous medium consisting of a regular grid of pores in contact with a flowing fluid channel. The porous media structure was further used as the simulation domain for numerical modeling. Both the simulation, based on solving Stokes equations, and the experimental data show presence of non-trivial streamline trajectories across the pore structures. In view of the results, we argue that the hydrodynamic mixing is a combination of non-trivial streamline routing and Brownian motion by pore-scale diffusion. The results provide insight into challenges in upscaling hydrodynamic dispersion from pore scale to representative elementary volume (REV) scale. Furthermore, the successful quantitative validation of CLSM-based data from a microfluidic model fed by an electrical syringe pump provided a valuable benchmark for qualitative validation of computer simulation results.X. Chu, W. Wang, G. Yang, A. Terzis, R. Helmig, and B. Weigand, “Transport of Turbulence Across Permeable Interface in a Turbulent Channel Flow: Interface-Resolved Direct Numerical Simulation,”
Transport in Porous Media, vol. 136, no. 1, Art. no. 1, 2021, doi:
10.1007/s11242-020-01506-w.
Abstract
Turbulence transportation across permeable interfaces is investigated using direct numerical simulation, and the connection between the turbulent surface flow and the pore flow is explored. The porous media domain is constructed with an in-line arranged circular cylinder array. The effects of Reynolds number and porosity are also investigated by comparing cases with two Reynolds numbers (Re≈3000,6000) and two porosities (φ=0.5,0.8). It was found that the change of porosity leads to the variation of flow motions near the interface region, which further affect turbulence transportation below the interface. The turbulent kinetic energy (TKE) budget shows that turbulent diffusion and pressure transportation work as energy sink and source alternatively, which suggests a possible route for turbulence transferring into porous region. Further analysis on the spectral TKE budget reveals the role of modes of different wavelengths. A major finding is that mean convection not only affects the distribution of TKE in spatial space, but also in scale space. The permeability of the wall also have an major impact on the occurrence ratio between blow and suction events as well as their corresponding flow structures, which can be related to the change of the Kármán constant of the mean velocity profile.X. Chu, W. Wang, J. Müller, H. V. Schöning, Y. Liu, and B. Weigand, “Turbulence Modulation and Energy Transfer in Turbulent Channel Flow Coupled with One-Side Porous Media,” in
High Performance Computing in Science and Engineering’20, in High Performance Computing in Science and Engineering’20. Springer, 2021, pp. 373--386. doi:
10.1007/978-3-030-80602-6_24.
Abstract
The microscopic structure of porous walls modulates the turbulent flow above. The standard approach, the volume-averaged modelling of the porous wall, does not resolve the pore structure. To systematically link geometric characteristics with flow properites, direct numerical simulations are conducted which are fully-resolving the microscopic structure. A high-order spectral/hp element solver is adopted to solve the incompressible Navier-Stokes equations. Resolving the full energy-spectra relies on a zonal polynomial refinement based on a conforming mesh. A low and a high porosity case with in-line arrays of cylinders are analysed for two Reynolds numbers. The peak in the streamwise energy spectra is shifted towards the pore unit length for both cases. Proper Orthogonal Decomposition (POD) shows that the fluctuations in the porous wall are linked to the structures above. Q2 structures are linked with blowing events and Q4 structures with suction events in the first pore row. The numerical solver Nektar exhibits an excellent scalability up to 96k cores on “Hazel Hen” where a slightly improved performance is observed on the brand new HPE “Hawk” system. Strong scaling tests indicate an efficiency of 70% with around 5, 000 mesh-nodes per core, which indicates a high potential for an adequate use of a HPC platform to investigate turbulent flows above porous walls while resolving the pore structure.Y. Chen
et al., “Nonuniqueness of hydrodynamic dispersion revealed using fast 4D synchrotron x-ray imaging,”
Science advances, vol. 7, no. 52, Art. no. 52, 2021, doi:
10.1126/sciadv.abj0960.
Abstract
Experimental and field studies reported a significant discrepancy between the cleanup and contamination time scales, while its cause is not yet addressed. Using high-resolution fast synchrotron x-ray computed tomography, we characterized the solute transport in a fully saturated sand packing for both contamination and cleanup processes at similar hydrodynamic conditions. The discrepancy in the time scales has been demonstrated by the nonuniqueness of hydrodynamic dispersion coefficient versus injection rate (Péclet number). Observations show that in the mixed advection-diffusion regime, the hydrodynamic dispersion coefficient of cleanup is significantly larger than that of the contamination process. This nonuniqueness has been attributed to the concentration-dependent diffusion coefficient during the cocurrent and countercurrent advection and diffusion, present in contamination and cleanup processes. The new findings enhance our fundamental understanding of transport processes and improve our capability to estimate the transport time scales of chemicals or pollution in geological and engineering systems.A. Beck and M. Kurz, “A perspective on machine learning methods in turbulence modeling,”
GAMM-Mitteilungen, vol. 44, no. 1, Art. no. 1, Mar. 2021, doi:
10.1002/gamm.202100002.
A. Beck et al., “Increasing the flexibility of the high order discontinuous Galerkin framework FLEXI towards large scale industrial applications,” in High Performance Computing in Science and Engineering ’20, W. E. Nagel, D. H. Kröner, and M. M. Resch, Eds., in High Performance Computing in Science and Engineering ’20. Cham: Springer International Publishing, 2021.
H. Aslannejad, S. Loginov, B. van der Hoek, E. Schoonderwoerd, H. Gerritsen, and S. Hassanizadeh, “Liquid droplet imbibition into a thin coating layer: direct pore-scale modeling and experimental observations,”
Progress in Organic Coatings, vol. 151, p. 106054, 2021, doi:
10.1016/j.porgcoat.2020.106054.
Abstract
In order to control ink droplet movement into the printing-paper layer, a set of pore-scale two-phase flow simulations were performed. The high-resolution three-dimensional pore space of the paper was obtained using focused ion beam scanning electron microscopy (FIB-SEM). Solving Navier-Stokes equations yielded details about dynamic movement of a droplet into the layer. To evaluate simulation results, for the first time, confocal laser microscopy imaging technique was integrated into a FIB-SEM chamber. Doing so, high resolution imaging of the droplet penetration inside paper was conducted and computed volume of penetrated ink at final stage was compared to the imaged volume. The ink penetration and spreading extent showed a good agreement with simulation results. Therefore, the developed simulation case was further investigated to study impact of liquid contact angle, real ink properties, and droplet arrival velocity on paper surface on final print quality. A faster penetration into the paper coating was observed for smaller equilibrium contact angles; meanwhile, more radial wicking was observed. In case of velocity of impact, higher velocity caused creation of irregular shapes of the ink footprint on paper surface. In addition to that, higher velocity caused ink splash which consequently created satellite droplets and lowered the print quality. Comparing ink-like liquid (representing real ink liquid properties) with water, water moves faster than ink-like liquid into the paper. This is mainly due to the higher viscosity and lower surface tension of the ink-like liquid.A. Arad, D. Katoshevski, V. Vaikuntanathan, M. Ibach, J. B. Greenberg, and B. Weigand, “Longitudinal and Lateral Grouping in Droplet Streams using the Eulerian-Lagrangian Approach,” Dec. 2021.
D. Alonso-Orán, C. Rohde, and H. Tang, “A Local-in-Time Theory for Singular SDEs with Applications to Fluid Models with Transport Noise,”
Journal of Nonlinear Science, pp. 1–32, 2021, doi:
10.1007/s00332-021-09755-9.
Abstract
We establish a local theory, i.e., existence, uniqueness and blow-up criterion, for a general family of singular SDEs in Hilbert spaces. The key requirement relies on an approximation property that allows us to embed the singular drift and diffusion mappings into a hierarchy of regular mappings that are invariant with respect to the Hilbert space and enjoy a cancellation property. Various nonlinear models in fluid dynamics with transport noise belong to this type of singular SDEs. By establishing a cancellation estimate for certain differential operators of order one with suitable coefficients, we give the detailed constructions of such regular approximations for certain examples. In particular, we show novel local-in-time results for the stochastic two-component Camassa–Holm system and for the stochastic Córdoba–Córdoba–Fontelos model.