Fluid Dynamics

New submissions

• New submissions
• Cross-lists

New submissions for Wed, 5 Jun 24

[1]  arXiv:2406.01634 [pdf, ps, other]

Title: Fluids flow in granular aggregate packings reconstructed by high-energy X-ray computed tomography and lattice Boltzmann method

Authors: Qifeng Lyu, Anguo Chen, Jie Jia, Amardeep Singh, Pengfei Dai

Comments: This article is published on this https URL

Journal-ref: Computers and Fluids, 2023, 253: 105787

Subjects: Fluid Dynamics (physics.flu-dyn)

Properties of fluids flow in granular aggregates are important for the design of pervious infrastructures used to alleviate urban water-logging problems. Here in this work, five groups of aggregates packing with similar average porosities but varying particle sizes were scanned by a high-energy X-ray computed tomography (X-CT) facility. The structures of the packings were reconstructed. Porosities were calculated and compared with those measured by the volume and mass of infilled water in the packing. Then pore networks were extracted and analyzed. Simulations of fluids flow in the packings were performed by using a lattice Boltzmann method (LBM) with BGK (Bhatnagar-Gross-Krook) collision model in the pore-network domain of the packings. Results showed wall effect on the porosity of aggregates packing was significant and the influence increased with the aggregate sizes. In addition, Poisson law and power law can be used to fit the coordination number and coordination volume of the packing's pore network, respectively. Moreover, the mass flow rates of fluids in the aggregates were affected by the porosities. On the two-dimensional slices, the mass flow rate decreased when the slice porosity increased. But for the three-dimensional blocks, the average mass flow rate increased with the volume porosity. And the permeability of the aggregates packing showed correlating change trend with the average pore diameter and fitting parameters of coordination volumes, when the sizes of aggregates changed. Though the limitation of merging interfaces causing fluctuation and discontinuity on micro parameters of fluid flow existed, the methods and results here may provide knowledge and insights for numerical simulations and optimal design of aggregate-based materials.

[2]  arXiv:2406.01711 [pdf, other]

Title: Flow stability and permeability in a nonrandom porous medium analog

Authors: Tairone Paiva Leão

Comments: 10 pages, 6 figures

Subjects: Fluid Dynamics (physics.flu-dyn)

The estimation of the permeability of porous media to fluids is of fundamental importance in fields as diverse as oil and gas industry, agriculture, hydrology and medicine. Despite more than 150 years since the publication of Darcy's linear law for flow in porous media, several questions remain regarding the range of validity of this law, the constancy of the permeability coefficient and how to define the transition from Darcy flow to other flow regimes. This study is a numerical investigation of the permeability and flow stability in a nonrandom quasi-tridimensional porous medium analog. The effect of increasing pressure gradient on the velocity field and on the estimation of Darcy and Darcy-Forchheimer coefficients is investigated for three different obstacle's radiuses. The transition from Darcy flow to nonlinear behavior is associated with the formation of jets in the outlet of the porous medium and development of flow instabilities. Different representations of the Reynolds number proved adequate to detect deviation from the linear law. The instantaneous permeability calculated at each pressure gradient was sensitive to flow velocity, in agreement with previous studies stating that permeability cannot be conceptualized as a constant for real flows.

[3]  arXiv:2406.01728 [pdf, other]

Title: Predictive Model and Optimization of Micromixers Geometry using Gaussian Process with Uncertainty Quantification and Genetic Algorithm

Authors: Daniela de Oliveira Maionchi, Neil Diogo Silva Coimbra, Junior Gonçalves da Silva, Fabio Pereira dos Santos

Comments: 13 pages, 21 figures

Subjects: Fluid Dynamics (physics.flu-dyn)

Microfluidic devices are gaining attention for their small size and ability to handle tiny fluid volumes. Mixing fluids efficiently at this scale, known as micromixing, is crucial. This article builds upon previous research by introducing a novel optimization approach in microfluidics, combining Computational Fluid Dynamics (CFD) with Machine Learning (ML) techniques. The research focuses on improving global optimization while reducing computational expenses. It draws inspiration from a Y-type micromixer, initially featuring cylindrical grooves on the main channel's surface and internal obstructions. Simulations, conducted using OpenFOAM software, evaluate the impact of circular obstructions on mixing percentage and pressure drop, considering variations in obstruction diameter and offset. A Gaussian Process (GP) was utilized to model the data, providing model uncertainty. Thus, this study optimizes geometries by using genetic algorithm (GA) and least-square optimization based on the reduced order model provided by GP. Results align with previous research, showing that medium-sized obstructions (137 mm diameter, 10 mm offset) near the channel wall are optimal. This approach not only provides efficient microfluidic optimization with uncertainty quantification but also highlights the effectiveness of combining CFD and ML techniques in achieving desired outcomes.

[4]  arXiv:2406.01751 [pdf, ps, other]

Title: Stable Boundary Layers with Subsidence: Scaling and Similarity of the Truly Steady State

Authors: Thijs Bon, Raúl Bayoán Cal, Johan Meyers

Subjects: Fluid Dynamics (physics.flu-dyn); Atmospheric and Oceanic Physics (physics.ao-ph)

The stable boundary layer (SBL) subjected to large-scale subsidence is studied through large-eddy simulations (LESs) with fixed surface temperature and a linear subsidence velocity profile. These boundary layers reach a truly steady state, where thermal equilibrium is established by a balance between surface cooling and subsidence-induced heating. We identify three governing dimensionless groups by scaling the governing equations with the geostrophic wind and Coriolis frequency, and systematically investigate the impact of these external parameters on global flow properties and mean profiles in the steady state. The SBL depth, low-level jet, and the magnitude of the turbulent momentum flux are reduced when the subsidence rate or Buoyancy number increases, while surface heat flux is enhanced. The shape of normalized mean profiles of temperature and heat flux is mainly determined by the subsidence rate, while they collapse for varying buoyancy and surface Rossby numbers. We develop empirical correlations for the stability parameter $h_{\theta}/L_O$ and a thermal shape factor, and propose a new unidirectional geostrophic drag law, to form a closed set of equations that estimates relevant flow properties from external parameters. The estimation errors compared to the LES data are less than 5% for friction velocity and surface heat flux, and at most 10% for the SBL depth $h_{\theta}$. Within the surface layer, dimensionless velocity and temperature gradients in the steady SBL with subsidence show acceptable agreement to Monin-Obukhov similarity theory, while the collapse is improved when a recently proposed mixed scaling parameter, that includes $h_{\theta}/L_O$, is used.

[5]  arXiv:2406.01785 [pdf, other]

Title: Tumbling elimination induced by permeability: an experimental approach

Authors: Jesús Sánchez-Rodríguez, François Gallaire

Subjects: Fluid Dynamics (physics.flu-dyn)

Archetypal falling behaviors of impervious objects are classified into four modes: fluttering, tumbling, steady descent and chaotic motion. The classical scenario predicts these behaviors to be affected by two dimensionless quantities: dimensionless inertia and Reynolds number. In this article we explore experimentally the effect of permeability and porosity on the falling regimes of porous plates. By drilling several hole distributions in rectangular plates, both permeability and porosity are varied systematically. We discover that the introduction of porosity affects the stability of the falling regimes eliminating tumbling. Using a phenomenological model we show that a decrease in circulation induced by the introduction of holes is the primary mechanism for stabilizing the plates' trajectories.

[6]  arXiv:2406.01989 [pdf, other]

Title: Life beyond Fritz: On the detachment of electrolytic bubbles

Authors: Çayan Demirkır, Jeffery A. Wood, Detlef Lohse, Dominik Krug

Comments: 11 figures

Subjects: Fluid Dynamics (physics.flu-dyn); Chemical Physics (physics.chem-ph)

We present an experimental study on detachment characteristics of hydrogen bubbles during electrolysis. Using a transparent (Pt or Ni) electrode enables us to directly observe the bubble contact line and bubble size. Based on these quantities we determine other parameters such as the contact angle and volume through solutions of the Young-Laplace equation. We observe bubbles without ('pinned bubbles') and with ('spreading bubbles') contact line spreading, and find that the latter mode becomes more prevalent if the concentration of HClO4 is greater than or equal to 0.1 M. The departure radius for spreading bubbles is found to drastically exceed the value predicted by the well-known formula of W. Fritz (Physik. Zeitschr. 1935, 36, 379-384) for this case. We show that this is related to the contact line hysteresis, which leads to pinning of the contact line after an initial spreading phase at the receding contact angle. The departure mode is then similar to a pinned bubble and occurs once the contact angle reaches the advancing contact angle of the surface. A prediction for the departure radius based on these findings is found to be consistent with the experimental data.

[7]  arXiv:2406.02389 [pdf, other]

Title: Role of surfactants on droplet formation in piezoacoustic inkjet printing across microsecond-to-second timescales

Authors: Maaike Rump, Christian Diddens, Uddalok Sen, Michel Versluis, Detlef Lohse, Tim Segers

Comments: 13 pages, 8 figures

Subjects: Fluid Dynamics (physics.flu-dyn); Soft Condensed Matter (cond-mat.soft)

In piezo acoustic drop-on-demand inkjet printing a single droplet is produced for each piezo driving pulse. This droplet is typically multicomponent, including surfactants to control the spreading and drying of the droplet on the substrate. However, the role of these surfactants on the droplet formation process remains rather elusive. Surfactant concentration gradients may manifest across microsecond-to-second timescales, spanning both the rapid ejection of ink from the nozzle exit and the comparatively slower idling timescale governing the firing of successive droplets. In the present work, we study the influence of surfactants on droplet formation across 6 orders of magnitude in time. To this end, we visualize the microsecond droplet formation process using stroboscopic laser-induced fluorescence microscopy while we vary the nozzle idle time. Our results show that increasing the idle time up to O(1) s affects only the break-up dynamics of the inkjet but not its velocity. By contrast, for idle times $>$ O(1) s, both the break-up dynamics are altered and the velocity of the inkjet increases. We show that the increased velocity results from a decreased surface tension of the ejected droplet, which we extracted from the observed shape oscillations of the jetted droplets in flight. The measured decrease in surface tension is surprising as the $\mu$s timescale of droplet formation is much faster than the typical ms-to-s timescale of surfactant adsorption. By varying the bulk surfactant concentration, we show that the fast decrease in surface tension results from a local surfactant concentration increase to more than 200 times the CMC. Our results suggest that a local high concentration of surfactant allows for surfactant adsorption to the interface of an inkjet at the $\mu$s-to-ms timescale, which is much faster than the typical ms-to-s timescale of surfactant adsorption.

Cross-lists for Wed, 5 Jun 24

[8]  arXiv:2406.01936 (cross-list from cs.GR) [pdf, ps, other]

Title: Fluid Implicit Particles on Coadjoint Orbits

Authors: Mohammad Sina Nabizadeh, Ritoban Roy-Chowdhury, Hang Yin, Ravi Ramamoorthi, Albert Chern

Subjects: Graphics (cs.GR); Fluid Dynamics (physics.flu-dyn)

We propose Coadjoint Orbit FLIP (CO-FLIP), a high order accurate, structure preserving fluid simulation method in the hybrid Eulerian-Lagrangian framework. We start with a Hamiltonian formulation of the incompressible Euler Equations, and then, using a local, explicit, and high order divergence free interpolation, construct a modified Hamiltonian system that governs our discrete Euler flow. The resulting discretization, when paired with a geometric time integration scheme, is energy and circulation preserving (formally the flow evolves on a coadjoint orbit) and is similar to the Fluid Implicit Particle (FLIP) method. CO-FLIP enjoys multiple additional properties including that the pressure projection is exact in the weak sense, and the particle-to-grid transfer is an exact inverse of the grid-to-particle interpolation. The method is demonstrated numerically with outstanding stability, energy, and Casimir preservation. We show that the method produces benchmarks and turbulent visual effects even at low grid resolutions.

[9]  arXiv:2406.02303 (cross-list from cond-mat.soft) [pdf, other]

Title: When does the elastic regime begin in viscoelastic pinch-off?

Authors: A. Gaillard, M. A. Herrada, A. Deblais, C. van Poelgeest, L. Laruelle, J. Eggers, D. Bonn

Comments: 41 pages, 14 figures

Subjects: Soft Condensed Matter (cond-mat.soft); Fluid Dynamics (physics.flu-dyn)

In this experimental and numerical study, we revisit the question of the onset of the elastic regime in viscoelastic pinch-off. This is relevant for all modern filament thinning techniques which aim at measuring the extensional properties of low-viscosity polymer solutions such as the Slow Retraction Method (SRM) in Capillary Breakup Extensional Rheometry (CaBER) as well as the dripping method where a drop detaches from a nozzle. In these techniques, a stable liquid bridge is slowly brought to its stability threshold where capillary-driven thinning starts, slowing down dramatically at a critical radius $h_1$ marking the onset of the elastic regime where the bridge becomes a filament with elasto-capillary thinning dynamics. While a theoretical scaling for this transition radius exists for the classical step-strain CaBER protocol, where polymer chains stretch without relaxing during the fast plate separation, we show that it is not necessarily valid for a slow protocol such as in SRM since polymer chains only start stretching (beyond their equilibrium coiled configuration) when the bridge thinning rate becomes comparable to the inverse of their relaxation time. We derive a universal scaling for $h_1$ valid for both low and high-viscosity polymer solution which is validated by both CaBER (SRM) experiments with different polymer solutions, plate diameters and sample volumes and by numerical simulations using the FENE-P model.

• New submissions
• Cross-lists