## Publications

197 results found

Gorges C, Brömmer M, Velten C,
et al., 2024, Comparing two IBM implementations for the simulation of uniform packed beds, *Particuology*, Vol: 86, Pages: 1-12, ISSN: 1674-2001

Nowadays, the design of fixed packed bed reactors still relies on empirical correlations, which, especially for small tube to particle diameter ratios, are mostly too inaccurate because of the presence of wall effects. Therefore, the simulation of fixed packed bed reactors plays an important role to predict and control the flow and process parameters in such, nowadays and in the future. Because of its straightforward applicability to non-uniform packings with particles of arbitrary shapes, the immersed boundary method (IBM) has advantages over other numerical methods and is used more and more frequently. This paper compares two approaches of IBMs for the simulation of fixed bed reactors with spherical shaped particles. The classic, smooth approach is compared to the straightforward to implement blocked-off method for velocity fields above the fixed bed for particle Reynolds numbers of 300 and 500. Results from experimental inline PIV-measurements of the reactor to be simulated serve as a basis for comparison. Very good agreement with the experiment is found for both simulation methodologies with higher resolutions, considering the more stable flow at a particle Reynolds number of 300. Differences in the different IBM approaches occurred for the more unsteady flow at a particle Reynolds number of 500. Compared to the blocked-off method, the smooth IBM reflects the formation of additional jets and recirculation zones better right above the bed, though increasing the fluid mesh resolution improves the accuracy of the blocked-off method. Overall, a more diffusive behaviour is found for the blocked-off simulations due to the stairstep representation, which is avoided by using interpolation stencils as in the smooth IBM. With higher mesh refinement in the blocked-off IBM this effect can be reduced, but this also increases the computational effort.

Chéron V, Evrard F, van Wachem B, 2023, A hybrid immersed boundary method for dense particle-laden flows, *Computers and Fluids*, Vol: 259, ISSN: 0045-7930

A novel smooth immersed boundary method (IBM) based on a direct-forcing formulation is proposed to simulate incompressible dense particle-laden flows. This IBM relies on a regularization of the transfer function between the Eulerian grid points (to discretize the fluid governing equations) and Lagrangian markers (to represent the particle surface) to fulfill the no-slip condition at the surfaces of the particles, allowing both symmetrical and non-symmetrical interpolation and spreading supports to be used. This enables that local source term contributions to the Eulerian grid, accounting for the boundary condition enforced at a Lagrangian marker on the surface of a particle, can be present on the inside of the particle only when this is beneficial, for instance when the Lagrangian marker is near another particle surface or near a domain boundary. However, when the Lagrangian marker is not near another particle surface or a domain boundary, the interpolation and spreading operators are locally symmetrical, meaning a “classic” IBM scheme is adopted. This approach, named hybrid IBM (HyBM), is validated with a number of test-cases from the literature. These results show that the HyBM achieves more accurate results compared to a classical IBM framework, especially at coarser mesh resolutions, when there are Lagrangian markers close to a particle surface or a domain wall.

Jain A, Duill FF, Schulz F,
et al., 2023, Numerical Study on the Impact of Large Air Purifiers, Physical Distancing, and Mask Wearing in Classrooms, *Atmosphere*, Vol: 14

The risk of COVID-19 infection from virulent aerosols is particularly high indoors. This is especially true for classrooms, which often do not have pre-installed ventilation and are occupied by a large number of students at the same time. It has been found that precautionary measures, such as the use of air purifiers (AP), physical distancing, and the wearing of masks, can reduce the risk of infection. To quantify the actual effect of precautions, it is not possible in experimental studies to expose subjects to virulent aerosols. Therefore, in this study, we develop a computational fluid dynamics (CFD) model to evaluate the impact of applying the aforementioned precautions in classrooms on reducing aerosol concentration and potential exposure in the presence of index or infected patients. A CFD-coupled Wells–Riley model is used to quantify the infection probability (IP) in the presence of index patients. Different cases are simulated by varying the occupancy of the room (half/full), the volumetric flow rate of the AP, two different locations of the AP, and the effect of wearing masks. The results suggest that using an AP reduces the spread of virulent aerosols and thereby reduces the risk of infection. However, the risk of the person sitting adjacent to the index patient is only marginally reduced and can be avoided with the half capacity of the class (physical distancing method) or by wearing face masks of high efficiencies.

Zhang Y, Hodžić A, Evrard F,
et al., 2023, Phase proper orthogonal decomposition of non-stationary turbulent flow, *Physics of Fluids*, Vol: 35, ISSN: 1070-6631

A phase proper orthogonal decomposition (phase POD) method is demonstrated utilizing phase averaging for the decomposition of spatiotemporal behavior of statistically non-stationary turbulent flows in an optimized manner. The proposed phase POD method is herein applied to a periodically forced statistically non-stationary lid-driven cavity flow, implemented using the snapshot proper orthogonal decomposition algorithm. Space-phase modes are extracted to describe the dynamics of the chaotic flow, in which four central flow patterns are identified for describing the evolution of the energetic structures as a function of phase. The modal building blocks of the energy transport equation are demonstrated as a function of the phase. The triadic interaction term can here be interpreted as the convective transport of bi-modal interactions. Non-local energy transfer is observed as a result of the non-stationarity of the dynamical processes inducing triadic interactions spanning across a wide range of mode numbers.

Chéron V, Evrard F, Wachem BV, 2023, A hybrid immersed boundary method for dense particle-laden flows

A novel smooth immersed boundary method (IBM) based on a direct-forcingformulation is proposed to simulate incompressible dense particle-laden flows.This IBM relies on a regularization of the transfer function between theEulerian grid points (to discretise the fluid governing equations) andLagrangian markers (to represent the particle surface) to fulfill the no-slipcondition at the surfaces of the particles, allowing both symmetrical andnon-symmetrical interpolation and spreading supports to be used. This enablesthat local source term contributions to the Eulerian grid, accounting for theboundary condition enforced at a Lagrangian marker on the surface of aparticle, can be present on the inside of the particle only when this isbeneficial, for instance when the Lagrangian marker is near another particlesurface or near a domain boundary. However, when the Lagrangian marker is notnear another particle surface or a domain boundary, the interpolation andspreading operators are locally symmetrical, meaning a ``classic'' IBM schemeis adopted. This approach, named hybrid IBM (HyBM), is validated with a numberof test-cases from the literature. These results show that the HyBM achievesmore accurate results compared to a classical IBM framework, especially atcoarser mesh resolutions, when there are Lagrangian markers close to a particlesurface or a domain wall.

Reuter J, Elmestikawy H, Evrard F, et al., 2023, Graph Networks as Inductive Bias for Genetic Programming: Symbolic Models for Particle-Laden Flows, Pages: 36-51, ISSN: 0302-9743

High-resolution simulations of particle-laden flows are computationally limited to a scale of thousands of particles due to the complex interactions between particles and fluid. Some approaches to increase the number of particles in such simulations require information about the fluid-induced force on a particle, which is a major challenge in this research area. In this paper, we present an approach to develop symbolic models for the fluid-induced force. We use a graph network as inductive bias to model the underlying pairwise particle interactions. The internal parts of the network are then replaced by symbolic models using a genetic programming algorithm. We include prior problem knowledge in our algorithm. The resulting equations show an accuracy in the same order of magnitude as state-of-the-art approaches for different benchmark datasets. They are interpretable and deliver important building blocks. Our approach is a promising alternative to “black-box” models from the literature.

Hausmann M, Evrard F, van Wachem B, 2022, An efficient model for subgrid-scale velocity enrichment for large-eddy simulations of turbulent flows, *PHYSICS OF FLUIDS*, Vol: 34, ISSN: 1070-6631

Schenke S, Sewerin F, van Wachem B,
et al., 2022, Acoustic black hole analogy to analyze nonlinear acoustic wave dynamics in accelerating flow fields, *PHYSICS OF FLUIDS*, Vol: 34, ISSN: 1070-6631

Schiodt M, Hodzic A, Evrard F,
et al., 2022, Characterizing Lagrangian particle dynamics in decaying homogeneous isotropic turbulence using proper orthogonal decomposition, *PHYSICS OF FLUIDS*, Vol: 34, ISSN: 1070-6631

Schiødt M, Hodžić A, Evrard F,
et al., 2022, Characterizing Lagrangian particle dynamics in decaying homogeneous isotropic turbulence using proper orthogonal decomposition, *Physics of Fluids*, Vol: 34, Pages: 063303-063303, ISSN: 1070-6631

<jats:p>Particle proper orthogonal decomposition (PPOD) is demonstrated as a method for extraction of temporal statistical information on dispersed (discrete) phases of multiphase flows. PPOD is an extension of the classical Eulerian POD, differentiating itself by its Lagrangian formulation and applicability to discrete phases in both stationary and non-stationary flows. The method is demonstrated on a test case of decaying homogeneous isotropic turbulence, where particle data are generated by one-way coupled simulations. Here, particle positions and velocities are integrated forward in time in a Lagrangian manner. The results demonstrate a proof of concept of the PPOD, and its potential for applicability. It is demonstrated that PPOD modes are able to capture both large scale temporal flow features as well as smaller scale variations. Additionally, particle trajectories/velocities are approximated using a subset of the PPOD basis where convergence is demonstrated. In the application of PPOD on multiple particle realizations, an increase in the convergence rate is observed as the initial particle separation is decreased. When decomposing both solid (rigid) and fluid particle velocities, the method provides the possibility of modal analysis of fluid–particle interactions in multiphase flows. For various configurations of rigid particle densities, the modal parallelity between the two phases is mapped, revealing a higher parallelity when the rigid particles are neutrally buoyant.</jats:p>

Denner F, Evrard F, van Wachem B, 2022, Breaching the capillary time-step constraint using a coupled VOF method with implicit surface tension, *JOURNAL OF COMPUTATIONAL PHYSICS*, Vol: 459, ISSN: 0021-9991

Gorges C, Evrard F, van Wachem B,
et al., 2022, Reducing volume and shape errors in front tracking by divergence-preserving velocity interpolation and parabolic fit vertex positioning, *JOURNAL OF COMPUTATIONAL PHYSICS*, Vol: 457, ISSN: 0021-9991

Schenke S, Sewerin F, van Wachem B,
et al., 2022, Explicit predictor-corrector method for nonlinear acoustic waves excited by a moving wave emitting boundary, *JOURNAL OF SOUND AND VIBRATION*, Vol: 527, ISSN: 0022-460X

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- Citations: 1

Tawfik M, Zhang X, Grigartzik L,
et al., 2021, Gene therapy with caspase-3 small interfering RNA-nanoparticles is neuroprotective after optic nerve damage, *NEURAL REGENERATION RESEARCH*, Vol: 16, Pages: 2534-+, ISSN: 1673-5374

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- Citations: 4

Denner F, Evrard F, Castrejon-Pita AA,
et al., 2021, Reversal and Inversion of Capillary Jet Breakup at Large Excitation Amplitudes, *FLOW TURBULENCE AND COMBUSTION*, Vol: 108, Pages: 843-863, ISSN: 1386-6184

Cerqueira RFL, Paladino EE, Evrard F,
et al., 2021, Multiscale modeling and validation of the flow around Taylor bubbles surrounded with small dispersed bubbles using a coupled VOF-DBM approach, *INTERNATIONAL JOURNAL OF MULTIPHASE FLOW*, Vol: 141, ISSN: 0301-9322

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- Citations: 10

Ren Z, Liu S, Tan BH,
et al., 2021, Strong shear flows release gaseous nuclei from surface micro- and nanobubbles, *PHYSICAL REVIEW FLUIDS*, Vol: 6, ISSN: 2469-990X

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- Citations: 1

Reichl U, Seidel-Morgenstern A, Sundmacher K,
et al., 2021, Research at the institute of process engineering at Otto von Guericke University Magdeburg, *Chemie Ingenieur Technik - CIT*, Vol: 93, Pages: 345-352, ISSN: 0009-286X

Seit der Gründung der Otto‐von‐Guericke‐Universität Magdeburg (OVGU) wurde der Forschungsbereich Verfahrenstechnik dort stetig ausgebaut. Heute umfasst die Fakultät für Verfahrens‐ und Systemtechnik der OVGU die vier Institute Verfahrenstechnik, Chemie, Strömungstechnik und Thermodynamik sowie Apparate‐ und Umwelttechnik. In diesem Beitrag stellen die fünf Lehrstühle des Instituts für Verfahrenstechnik (Bioprozesstechnik, Chemische Verfahrenstechnik, Systemverfahrenstechnik, Thermische Verfahrenstechnik und Mechanische Verfahrenstechnik) ihre Forschungsaktivitäten anhand ausgewählter Projekte vor.

Evrard F, Denner F, van Wachem B, 2021, Quantifying the errors of the particle-source-in-cell Euler-Lagrange method, *International Journal of Multiphase Flow*, Vol: 135, Pages: 1-6, ISSN: 0301-9322

The particle-source-in-cell Euler-Lagrange (PSIC-EL) method is widely used to simulate flows laden with particles. Its accuracy, however, is known to deteriorate as the ratio between the particle diameter ( d p ) and the mesh spacing ( h) increases, due to the impact of the momentum that is fed back to the flow by the Lagrangian particles. Although the community typically recommends particle diameters to be at least an order of magnitude smaller than the mesh spacing, the errors corresponding to a given d p /h ratio and/or flow regime have not been systematically studied. In this paper, we provide an expression to estimate the magnitude of the flow velocity disturbance resulting from the transport of a particle in the PSIC-EL framework, based on the d p /h ratio and the particle Reynolds number, Re p . This, in turn, directly relates to the error in the estimation of the undisturbed velocity, and therefore to the error in the prediction of the particle motion. We show that the upper bound of the relative error in the estimation of the undisturbed velocity, for all particle Reynolds numbers, is approximated by (6 / 5 ) d p /h . Moreover, for all cases where d p /h 1 / 2 , the expression we provide accurately estimates the value of the errors across a range of particle Reynolds numbers that are relevant to most gas-solid flow applications ( Re p < 500 ).

van Wachem B, Thalberg K, Duy N,
et al., 2020, Analysis, modelling and simulation of the fragmentation of agglomerates, *CHEMICAL ENGINEERING SCIENCE*, Vol: 227, ISSN: 0009-2509

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- Citations: 12

Evrard F, Denner F, van Wachem B, 2020, Euler-Lagrange modelling of dilute particle-laden flows with arbitrary particle-size to mesh-spacing ratio, *Journal of Computational Physics: X*, Vol: 8

This paper addresses the two-way coupled Euler-Lagrange modelling of dilute particle-laden flows, with arbitrary particle-size to mesh-spacing ratio. Two-way coupled Euler-Lagrange methods classically require particles to be much smaller than the computational mesh cells for them to be accurately tracked. Particles that do not satisfy this requirement can be considered by introducing a source term regularisation operator that typically consists in convoluting the point-wise particle momentum sources with a smooth kernel. Particles that are larger than the mesh cells, however, generate a significant local flow disturbance, which, in turn, results in poor estimates of the fluid forces acting on them. To circumvent this issue, this paper proposes a new framework to recover the local undisturbed velocity at the location of a given particle, that is the local flow velocity from which the disturbance due to the presence of the particle is subtracted. It relies upon the solution of the Stokes flow through a regularised momentum source and is extended to finite Reynolds numbers based on the Oseen flow solution. Owing to the polynomial nature of the regularisation kernel considered in this paper, a correction for the averaged local flow disturbance can be analytically derived, allowing to filter out scales of the flow motion that are smaller than the particle, which should not be taken into account to compute the interaction/drag forces acting on the particle. The proposed correction scheme is applied to the simulation of a particle settling under the influence of gravity, for varying particle-size to mesh-spacing ratios and varying Reynolds numbers. The method is shown to nearly eliminate any impact of the underlying mesh resolution on the modelling of a particle's trajectory. Finally, optimal values for the scale of the regularisation kernel are provided and their impact on the flow is discussed.

Liu D, Song J, Ma J,
et al., 2020, Gas flow distribution and solid dynamics in a thin rectangular pressurized fluidized bed using CFD-DEM simulation, *POWDER TECHNOLOGY*, Vol: 373, Pages: 369-383, ISSN: 0032-5910

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- Citations: 11

Shen L, Denner F, Morgan N,
et al., 2020, Transient structures in rupturing thin-films: Marangoni-induced symmetry-breaking pattern formation in viscous fluids, *Science Advances*, Vol: 6, ISSN: 2375-2548

In the minutes immediately preceeding the rupture of a soap bubble,distinctive and repeatable patterns can be observed. These quasi-stabletransient structures are associated with the instabilities of the complexMarangoni flows on the curved thin film in the presence of a surfactantsolution. Here, we report a generalised Cahn-Hilliard-Swift-Hohenberg modelderived using asymptotic theory which describes the quasi-elastic wrinklingpattern formation and the consequent coarsening dynamics in a curvedsurfactant-laden thin film. By testing the theory against experiments on soapbubbles, we find quantitative agreement with the analytical predictions of thenucleation and the early coarsening phases associated with the patterns. Ourfindings provide fundamental physical understanding that can be used to(de-)stabilise thin films in the presence of surfactants and have importantimplications for both natural and industrial contexts, such as the productionof thin coating films, foams, emulsions and sprays.

Evrard F, Denner F, van Wachem B, 2020, Height-function curvature estimation with arbitrary order on non-uniform Cartesian grids, *Journal of Computational Physics: X*, Vol: 7

This paper proposes a height-function algorithm to estimate the curvature of two-dimensional curves and three-dimensional surfaces that are defined implicitly on two- and three-dimensional non-uniform Cartesian grids. It relies on the reconstruction of local heights, onto which polynomial height-functions are fitted. The algorithm produces curvature estimates of order N−1 anywhere in a stencil of (N+1)d−1 heights computed from the volume-fraction data available on a d-dimensional non-uniform Cartesian grid. These estimates are of order N at the centre of the stencil when it is symmetric about its main axis. This is confirmed by a comprehensive convergence analysis conducted on the errors associated with the application of the algorithm to a fabricated test-curve and test-surface.

Denner F, Evrard F, van Wachem B, 2020, Modeling Acoustic Cavitation Using a Pressure-Based Algorithm for Polytropic Fluids, *FLUIDS*, Vol: 5

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- Citations: 6

Lukas E, Roloff C, van Wachem B,
et al., 2020, Experimental investigation of the grade efficiency of a zigzag separator, *Powder Technology*, Vol: 369, Pages: 38-52, ISSN: 0032-5910

An experimental study is conducted on a pilot-scale zigzag air separator (ZZS) to study the effects of varying the solid feed mass stream, the mean channel air velocity, and the number of channel segments onto the grade efficiency. Spherical glass beads are classified. A straight pipe separator model (PSM) is modified for the ZZS and fitted to the experimental data to estimate the relative cut-point settling velocity, the separation sharpness, the relative rise velocity, the diffusion coefficient, and the particle loading. The proposed model is thoroughly investigated with regard to all important parameters, e.g. the estimated particle loading is shown to be more precise than the ratio of the solid and air mass stream, used in many publications. Finally, the relative rise velocity is shown to be only a function of the particle loading, making the experimental results within the model collapse.

Denner F, Evrard F, van Wachem BGM, 2020, Conservative finite-volume framework and pressure-based algorithm for flows of incompressible, ideal-gas and real-gas fluids at all speeds, *JOURNAL OF COMPUTATIONAL PHYSICS*, Vol: 409, ISSN: 0021-9991

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- Citations: 21

van Wachem B, Curran T, Evrard F, 2020, Fully Correlated Stochastic Inter-Particle Collision Model for Euler-Lagrange Gas-Solid Flows, *FLOW TURBULENCE AND COMBUSTION*, Vol: 105, Pages: 935-963, ISSN: 1386-6184

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- Citations: 2

Lukas E, Roloff C, Mann H, et al., 2020, Experimental study and modelling of particle behaviour in a multi-stage zigzag air classifier, Dynamic Flowsheet Simulation of Solids Processes, Pages: 391-410, ISBN: 9783030451677

In most industrial solid processing operations, the classification of particles is important and designed based on the terminal settling velocity as the main control parameter. This settling velocity is dependent on characteristic particle properties like size, density, and shape. Turbulent particle diffusion is the other key property controlling the efficiency of the separation. In this project, multi-stage separation experiments of a variety ofmaterials have been performed using different flow velocities, mass loadings of the air, number of stages. Separation has been investigated separately concerning particle size, particle density, and particle shape. Continuous operation in terms of solid material and airflow has been mostly considered. However, variations in mass loading and pulsating operation of the fan have been investigated as well. The performance has been analyzed and discussed with respect to the separation functions, for instance regarding separation sharpness. Severalmodelling approaches have been checked and/or developed to describe theoretically the corresponding observations. After fitting the free model parameters, a very good agreement has been obtained compared to experimental measurements. Finally, the reduced model has been implemented into the central software DYSSOL.

Knight C, O'Sullivan C, Dini D,
et al., 2020, Computing drag and interactions between fluid and polydisperse particles in saturated granular materials, *Computers and Geotechnics*, Vol: 117, Pages: 1-16, ISSN: 0266-352X

Fundamental numerical studies of seepage induced geotechnical instabilities and filtration processes depends on accurate prediction of the forces imparted on the soil grains by the permeating fluid. Hitherto coupled Discrete Element Method (DEM) simulations documented in geomechanics have most often simulated the fluid flow using computational fluid dynamics (CFD) models employing fluid cells that contain a number of particles. Empirical drag models are used to predict the fluid-particle interaction forces using the flow Reynolds number and fluid cell porosity. Experimental verification of the forces predicted by these models at the particle-scale is non-trivial. This contribution uses a high resolution immersed boundary method to model the fluid flow within individual voids in polydisperse samples of spheres to accurately determine the fluid-particle interaction forces. The existing drag models are shown to poorly capture the forces on individual particles in the samples for flow with low Reynolds number values. An alternative approach is proposed in which a radical Voronoi tesselation is applied to estimate a local solids volume fraction for each particle; this local solids fraction can be adopted in combination with existing expressions to estimate the drag force. This tessellation-based approach gives a more accurate prediction of the fluid particle interaction forces.

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