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The present contribution is a follow-up of a recently conducted study to derive a data-driven model for the breakage of agglomerates by wall impacts. This time the collision-induced breakage of agglomerates and concurrently occurring particle agglomeration processes are considered in order to derive a model for Euler–Lagrange methods, in which agglomerates are represented by effective spheres. Although the physical problem is more challenging due to an increased number of influencing parameters, the strategy followed is very similar. In a first step extensive discrete element simulations are carried out to study a variety of binary inter-agglomerate collision scenarios. That includes different collision angles, collision velocities, agglomerate sizes and powders. The resulting extensive database accounts for back-bouncing, agglomeration and breakage events. Subsequently, the collision database is used for training artificial neural networks to predict the post-collision number of arising entities, their size distributions and their velocities. Finally, it is shown how the arising data-driven model can be incorporated into the Euler–Lagrange framework to be used in future studies for efficient computations of flows with high mass loadings. [Display omitted] • Data-driven model for the collision-induced breakage of agglomerates and agglomeration processes. • Extensive number of DEM simulations for studying collisions at a variety of collision scenarios. • Artificial neural network to predict the number of resulting fragments and their size distribution. • ANN for the post-breakage velocities of the arising fragments and possible agglomeration processes. • Model applicable in Euler-Lagrange simulations with (nearly) spherical agglomerates. [ABSTRACT FROM AUTHOR]

The present study is concerned with breakup models for agglomerates in turbulent flows. First, a brief literature review is provided describing the state of the art concerning the description and modeling of breakup of agglomerates with special emphasis on the role of fluid forces. That comprises turbulent and drag (inertia) stresses. Furthermore, the breakup by rotary stresses is taken into account. Based on the idea to describe these processes using first principles within an Eulerian–Lagrangian approach relying on the large-eddy simulation methodology, modeling ideas from the literature are revisited and extended in order to be applied in a four-way coupled simulation. Building on a deterministic collision and agglomeration model, the breakup process is tackled in a similar manner leading to a highly efficient procedure, which allows to track a huge number of agglomerates and particles but with the restriction to compact dry powder agglomerates. For this kind of agglomerates an enhanced evaluation of the strength is derived relying on an improved determination of the average porosities and coordination number. Furthermore, special emphasis is put on the post-breakup treatment, i.e., models for the sizes and velocities of the generated fragments. Finally, the proposed models are applied to a test case inspired by the experimental study of Weiler [ 1 ] for the deagglomeration in a dry powder disperser. The effect of the different physical mechanisms (turbulence, drag, rotation) is analyzed concerning breakup rates and resulting size distributions. Furthermore, the dominant breakup regions are identified separately. Since agglomeration processes are simultaneously taken into account, re-agglomeration of the fragments is found to play a non negligible role. Finally, the effect of the properties of the powder on the breakup and re-agglomeration processes is investigated. Unlabelled Image • Revisited and improved models for the breakup of compact dry powder agglomerates. • Breakup models for turbulent, drag and rotary stresses in turbulent flows. • Methodology within an Eulerian-Lagrangian approach relying on large-eddy simulations. • Enhanced evaluation of the strength of compact dry powder agglomerates. • Comparison with experimental investigation of deagglomeration in a disperser. [ABSTRACT FROM AUTHOR]

Highlights • A deterministic composite collision outcome model for Euler-Lagrange simulations. • Modeling and simulation of coalescence of surface-tension dominated droplets. • Efficient model for four-way coupled large-eddy simulations of droplet-laden flows. • Collision regimes: Bouncing, fast coalescence, reflexive and stretching separation. • Non-evaporating single diesel spray entering a quiescent nitrogen environment. Abstract The focus of the present study is on the modeling and simulation of coalescence of surface-tension dominated liquid droplets in a gaseous environment. In the framework of a four-way coupled Euler-Lagrange approach using the large-eddy simulation technique, an improved composite collision outcome model identifying the four regimes of binary collisions of such droplets (bouncing, fast coalescence, reflexive and stretching separation) is assembled based on well-known correlations. A specific feature is that a hard-sphere model in combination with a deterministic collision detection is applied and hence the stochastic parcel approach often used for spray simulations is obsolete. In this composite model the bounding curves between the regimes mentioned before are taken from available experiments and described by well-known analytic correlations. The main objective is to provide a compilation of the different modeling assumptions resulting in a consistent model for the entire regime. Furthermore, the composite model is improved by introducing an additional condition to consider the overlapping region of the stretching and reflexive separation regimes at high Weber numbers. The outcome of a binary collision is identified based on the collision Weber number, the dimensionless impact parameter and the droplet size ratio. The results of each collision is treated separately with regard to the collision regime found, whereas the formation of satellite droplets during stretching and reflexive separation is currently not taken into account. Additionally, a droplet injection model for spray systems is introduced to specify the injected mass, the position, the velocity and the diameters of the released droplets. In this model the initial droplet diameters mimicking the primary break-up (atomization process) of the jet are modeled by means of a number distribution function. The implementation of the composite collision outcome model is verified using the test case of an inter-impingement spray system consisting of two crossing conical water sprays. Besides correctly reproducing the experimental correlations the composite model predicts a coalescence rate which is within the experimental range found in the literature. Then, the composite collision outcome model is applied to simulate the injection process of a solid-cone non-evaporating diesel spray into a quiescent nitrogen environment and validated against experimental data. The validation study is carried out in terms of the spray tip penetration, since it is one of the most important characteristics of sprays. A detailed comparative study is carried out to investigate the effect of different simulation parameters on the spray tip penetration. The results clearly show that a four-way coupled simulation using the enhanced composite model leads to the best agreement with the experiment. [ABSTRACT FROM AUTHOR]

The paper is a follow-up of the recent study on the assessment of discrete wind gust parameters impacting a flexible lightweight structure as a first step towards the evaluation of the worst-case scenario caused by strong wind gusts (JWEIA 231, 105207, 2022). The present study goes beyond by suggesting an optimization framework which allows to determine the worst-case scenario automatically. For this purpose, a stochastic response surface algorithm with a surrogate model based on radial basis functions is chosen. The algorithm relies on costly evaluations of the objective function, which consist of CPU-time intensive fully coupled fluid–structure interaction (FSI) high-fidelity simulations including the pre- and post-processing of the results. Besides the parallelization of the coupled FSI solver, a parallel version of the optimization algorithm allows to carry out several costly evaluations simultaneously. The Metric Stochastic Response Surface algorithm determines the worst case fast. Then, it continues to explore the optimization space to ensure that the global extremum is reached. A sensitivity study on relevant parameters of the optimization algorithm is conducted. Typically, for the present FSI setup, an optimization run takes one week with 6 evaluations in parallel to compute 100 different configurations. The worst case is found after about one third of the evaluations. The increase of parallel evaluations drastically reduces the wall-clock time, but the worst case is found later after half of the evaluations. This later finding is due to the parallel nature of the algorithm. Finally, the various sources of uncertainties that arise throughout the entire procedure are assessed and discussed. • Methodology to find the worst-case scenario of flexible structures under gust load. • Surrogate-based optimization algorithm to minimize costly FSI simulations. • High-fidelity fluid and structure modeling to capture transient effects. • Deterministic discrete gust model for a total control of its shape. • Application on a air-inflated flexible membrane in a turbulent boundary layer. [ABSTRACT FROM AUTHOR]

The topic of the paper is the development of an enhanced film drainage model for the prediction of bubble coalescence in the context of the Euler–Lagrange approach relying on large-eddy simulations. The starting point is the coalescence model by Jeelani and Hartland (1991), which compared to other often used models has several benefits: (1) A temporally evolving contact surface is considered avoiding the strong simplification of a constant contact area. (2) For contaminated bubbles an initially inertia-dominated process followed by a viscous-controlled regime are distinguished. (3) The contact time of the bubbles results as a side product of the modeling assumptions and thus is consistent with the film drainage concept. The main reason why this improved coalescence model was not applied in the past is the specific circumstance that the implicit equation for the determination of the transition time between the two phases (inertial and viscous) cannot be determined analytically. This problem is eliminated in the present study by numerically solving this equation. However, to avoid a time-consuming procedure for each individual bubble collision, a regression function is set up for a pre-defined range of bubble diameters and relative collision velocities. This renders the coalescence model feasible for flows with a huge number of bubbles. In a first step, the new coalescence model is validated against the experiments of single bubble coalescence with a free surface by Zawala and Malysa (2011) and Kosior et al. (2014). For the different cases considered the coalescence model yields reasonable agreement with the experiments. Furthermore, it is demonstrated that the results are improved compared to more popular but simpler models available in the literature. Afterwards, the coalescence model is applied to four-way coupled Euler–Lagrange simulations of a bubble column with clean and contaminated bubbles considering two different sizes. Significant deviations are found between the different cases, which can be traced back to varying collision frequencies and the different coalescence mechanisms in effect. Thus, it is shown that on the one hand the enhanced coalescence model leads to reasonable results and on the other hand is highly efficient allowing to take a huge number of bubble collisions deterministically into account. [ABSTRACT FROM AUTHOR]

The paper is concerned with the development of a cost-efficient Langevin subgrid-scale model and the analysis of its influence on the dispersed phase of turbulent bubble–laden and particle–laden flows. For this purpose, the Langevin subgrid-scale model of [Pozorski, J., Apte, S. V., 2009. Filtered particle tracking in isotropic turbulence and stochastic modeling of subgrid-scale dispersion. Int. J. Multiphase Flow 35, 118–128.] is chosen as the starting point since it takes the temporal correlations of the subgrid-scale velocity fluctuations, the crossing-trajectory and the continuity effect into account. Based on the idea of [Minier, J.-P., Peirano, E., Chibbaro, S., 2004. PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow. Phys. Fluids 16, 2419–2431.] [Minier, J.-P., Chibbaro, S., Pope, S. B., 2014. Guidelines for the formulation of Lagrangian stochastic models for particle simulations of single-phase and dispersed two-phase turbulent flows. Phys. Fluids 26, 113303.] to formulate the drift and diffusion terms in matrix form, the model is extended for an arbitrary direction of the particle motion. Considering turbulent downward channel flows of different setups covering a large range of parameters, the influence of the subgrid-scale model is analyzed. After a detailed validation of the bubble–laden flow the Langevin model and a simple trivial model are applied to investigate the effect of the subgrid-scales. It is found that the Langevin subgrid-scale model only marginally changes the velocity statistics or the volume fraction of the bubbles, which can be attributed to the small magnitude of the subgrid-scale velocities obtained by the Langevin model. The model is able to estimate the correct level of the turbulent kinetic energy of the subgrid-scales. Similar results are found for the second setup consisting of solid particles of Stokes number St + = 1.67 . In this case the influence of the Langevin subgrid-scale model on the velocity statistics of the particles is found to be more pronounced. Furthermore, it is observed that the model leads to a strongly increased volume fraction of the particles at the walls and thus to a significant increase of particle-wall collisions. To further investigate this behavior and to analyze the impact of the particle inertia, additional simulations containing smaller particles ( St + = 1 and 0.1) are carried out. The results show that the influence of the Langevin subgrid-scale model on the velocity fluctuations and the volume fraction increases with decreasing Stokes number. Thus, for these cases the extended but nevertheless still cost-efficient Langevin model is a reasonable approach. [ABSTRACT FROM AUTHOR]

The present study is concerned with the development of a computational model for predicting particle–wall adhesion (deposition) of aerosol particles in turbulent particle-laden flows. Particularly, the standard hard-sphere model is extended to include the adhesion during sticking or sliding particle–wall collisions. For both impact types the deposition condition of a particle on bounding walls is determined. The strategy of the proposed model is based on the momentum-based agglomeration model by Breuer and Almohammed (2015). The main advantage of the proposed model compared to the state-of-the-art in the literature is that the adhesive impulse is determined more reasonably taking the time intervals of the compression and the restitution phase into account. Furthermore, if the deposition condition is not satisfied, the treatment of the particle motion after the impact including adhesion depends on the particle–wall type (sticking or sliding). To examine the effect of the particle–wall adhesion, the developed model is first evaluated using simple test cases including oblique particle–wall collisions with friction. In the second step, the performance of the adhesion model is validated based on a horizontal turbulent channel flow against existing experimental data of Kvasnak et al. (1993) and numerical results by Fan and Ahmadi (1993)based on an energy-based deposition model. The predictions of the present model agree very well with the experiments and the numerical results chosen for the validation study as well as the empirical relation by Wood (1981). Third, the adhesion model is employed to investigate the influence of different simulation parameters (normal restitution coefficient for particle–wall collisions and particle diameter) on the particle–wall adhesion and deposition process in a vertical turbulent channel flow laden with a huge number of primary particles. The results show that the inclusion of the adhesion significantly reduces the number of particle–wall collisions, whereas the number of particle–particle collisions is only slightly reduced. Furthermore, the number of deposited particles is higher for small particles than for large particles, since the adhesive impulse is inversely proportional to the diameter of the primary particles. The proposed adhesion model is developed and tested in the context of large-eddy simulation, but it can also be applied in DNS or RANS predictions. [ABSTRACT FROM AUTHOR]

In the present work, the particle agglomeration in shear flows is investigated in the framework of a hard-sphere model with deterministic collision detection employing two different modeling approaches: the energy-based and the momentum-based agglomeration models. The former is further improved concerning the agglomeration conditions. Moreover, the application area of both models is extended towards fully three-dimensional turbulent flows applying the large-eddy simulation technique. Here, the particles are assumed to be rigid, dry and electrostatically neutral and hence only the cohesion due to the van-der-Waals forces is considered. First, the energy-based and the momentum-based agglomeration models are successfully validated based on theoretical results using an existing laminar test case. The numerical results are found to be in close agreement with the theory. Then, both agglomeration approaches are used to investigate the dynamic process of the particle agglomeration in a vertical fully developed turbulent channel flow. A detailed comparison of the results obtained using both agglomeration models is reported. Additionally, the performance of both techniques is examined under the influence of varying normal restitution coefficients of the inter-particle collisions. Furthermore, the influence of the inclusion of three sub-models (the feedback effect of the particles on the fluid, the lift forces and the subgrid-scale model for the particles) on the agglomeration process is studied. The results show that a significantly lower agglomeration rate is observed if the sub-models are considered. Next, the agglomeration models are applied to evaluate the effect of the diameter of the primary particles and the wall roughness on the agglomeration behavior. The reduction of the diameter of the particles leads to a stronger cohesive impulse and hence to a higher agglomeration rate. The wall roughness enhances the particle–particle collisions and slightly increases the number of agglomeration processes leading to higher agglomeration rates. The comparative study clearly indicates that both models predict similar trends of the physical behavior of the agglomeration process, but their results deviate slightly from each other. The most important reasons for the differences observed between the numerical results of both models are discussed. Based on the advantages and drawbacks of both models highlighted in this study, it can be concluded that owing to the reduced necessity of empirical parameters and the slightly more accurate results the momentum-based agglomeration model is superior to the energy-based model. [ABSTRACT FROM AUTHOR]

The paper is a step towards the evaluation of the worst-case scenario caused by strong wind gusts impacting civil engineering air-inflated lightweight structures. These extreme events with short durations but high strengths are responsible for short-term highly instantaneous loads endangering the structural integrity of the design. For this purpose, a generic test case is defined which includes a discrete wind gust model, the approaching turbulent boundary layer and a flexible structure exposed to the resulting fluid flow. The simulation framework relies on a partitioned coupled solver for fluid–structure interaction extended by two source-term formulations which allow to inject the wind gusts as well as the background turbulence. To save CPU time, a part of the investigations is conducted for the rigid case as a physical meta-model. The particularly critical cases found in this way were examined for the case of the flexible structure. Under varying system parameters such as the strength, length and position of the gust the following objective functions are evaluated: Force coefficients, maximal deflections and local inner stresses. The worst case occurs for maximal gust strength and length, when the gust hits the membrane at half height. Furthermore, the effect of the superposition of the discrete gust with background turbulence is analyzed for two scenarios. The gust is first superimposed to different inflow turbulences of the same intensity leading to non-negligible deviations of force coefficients and deflections. Second, the level of the turbulence intensity is successively increased up to a factor of five showing only a minor effect on the flexible structure not generating a new worst case. • Towards the worst-case scenario of material failure caused by discrete wind gusts. • Civil engineering application based on an air-inflated lightweight structure. • Simulation framework based on a partitioned coupled LES-solver for FSI. • Assessment of discrete wind gust parameters (strength, length, position). • Evaluation of force coefficients, maximal deflections and local inner stresses. [ABSTRACT FROM AUTHOR]

The paper is concerned with a novel approach to generate horizontal wind gusts in classical wind tunnels. Assuming an open test section of an Eiffel- or Göttingen-type wind tunnel, the new wind gust generator denoted "The Paddle" can be easily retrofitted to such experimental setups at low costs. The device is constructed based on commercially available components including a programmable software tool that allows to adjust all relevant kinematic parameters. Five predefined motion patterns of the paddle are investigated which differ according to the achieved blocking ratio of the wind tunnel nozzle as well as the velocity and acceleration of the downward and upward motion of the paddle. It is shown that the shape and intensity of the generated gusts can be fully controlled by these parameters. That guarantees a strong individual adjustability and customization of the induced gusts. Furthermore, a synchronization of the wind gust generator with measurement devices such as laser-Doppler anemometer or particle-image velocimetry is easy to implement. An extensive measurement campaign has shown that the generated horizontal gusts are highly reproducible for the presently investigated laminar boundary layer and vary in the core area of the gust measurements solely according to free-stream turbulence of the wind tunnel. • New type of retrofittable wind gust generator (WGG) denoted the paddle. • A novel approach for artificially generating wind gusts in a wind tunnel. • Generation of horizontal wind gusts with large gust ratios GR > 1. • Individual adjustability and customization of the gust shapes. • Possibility to generate a complex sequence of differently shaped gusts. [ABSTRACT FROM AUTHOR]

The present paper is concerned with the modeling and simulation of particle agglomeration of rigid, dry and electrostatically neutral particles in turbulent gas flows. Based on a deterministic collision model in the framework of an Euler–Lagrange approach for the description of disperse particle-laden flows, the original agglomeration model of Kosinski and Hoffmann (2010) is improved regarding the determination of the collision time. The application area of the agglomeration model is extended towards fully three-dimensional turbulent flows simulated by the large-eddy simulation technique. Additionally, the model is enhanced by introducing three different concepts to model the structure of the arising agglomerate, namely the volume-equivalent sphere model, the inertia-equivalent sphere model and the closely-packed sphere model. Furthermore, the resulting simulation strategy for turbulent particle-laden shear flows is first analyzed and evaluated concerning the three structure models. Then, the performance of the extended agglomeration model is tested by investigating the influence of various simulation parameters such as the restitution and friction coefficients of the particles, the inclusion of the two-way coupling, the subgrid-scale model for the particles, the lift forces, the wall roughness and the particle mass loading on the agglomeration process in a turbulent particle-laden vertical channel flow. In the parameter study solely the closely-packed sphere model is adopted, since it takes the interstitial space between the agglomerated particles into account and additionally satisfies the conservation of mass and angular momentum. Based on the results, it can be concluded that the enhanced agglomeration model using the closely-packed sphere model for the arising agglomerate realistically predicts the physical behavior of the agglomeration process within particle-laden flows. [ABSTRACT FROM AUTHOR]

• Wall-impact breakage model for agglomerates based on artificial neural networks. • Extensive DEM simulations of wall-impacts for normal/oblique and shear impacts. • Prediction of the onset of breakage and the particle size distribution. • Prediction of the post-breakage velocities of the arising fragments. • Model applicable in Euler-Lagrange simulations with (nearly) spherical agglomerates. The present study proposes a novel modeling approach for predicting the wall-impact breakage of agglomerates in wall-bounded particle-laden turbulent flows based on artificial neutral networks (ANN). The suggested model is especially useful for efficient Euler-Lagrange simulation methods relying on the hard-sphere approach and the equivalent-sphere model for the agglomerate structure, allowing LES predictions of high mass loadings. Based on the impact conditions, i.e., the impact velocity, the impact angle, the number of included primary particles and the diameter of the primary particles, the outcomes of the breakage events are forecasted using two pre-trained artificial neural networks. The first network is concerned with the prediction of the possibility of breakage and the resulting fragment size distribution, whereas the second network predicts the post-breakage velocities of the fragments. The supervised training of the employed networks relies on a database obtained by extensive DEM simulations of agglomerate wall-impacts covering wide ranges of impact conditions, which were partially reported in Khalifa and Breuer (2020, 2021) for developing a breakage model based on a dimensional analysis and regression techniques. In the present contribution, the database mainly comprising the normal or oblique impact case is extended by adding results for the practically relevant shear impact case of wall-bounded particle-laden flows at extremely small impact angles, i.e., 3 ° and practically flat (0.2 °). In addition, the breakage behavior of agglomerates containing very small numbers of particles are investigated under different impact angles and primary particle sizes. The ANN model is employed in Euler-Lagrange LES predictions of duct flows taking three Reynolds numbers and agglomerates of two powders distinguished by the size of the primary particles into account. The results obtained are compared with those based on a previous regression model (Khalifa and Breuer, 2021). In general, a good agreement between the results is found. However, the new ANN model is more widely applicable since the shear impact case is taken into account, which leads to subtle differences enhancing the reliability of the predictions. [ABSTRACT FROM AUTHOR]

Abstract: The present paper is concerned with numerical simulations of pneumatic conveying in pipes of circular cross-section. Based on an Euler–Lagrange approach relying on the large-eddy simulation technique for the fluid flow and a particle tracking scheme accounting for all relevant elementary processes (particle rotation, transverse lift forces, inter-particle collisions, particle–wall collisions with smooth and rough walls, coupling between phases) several cases are analyzed in detail to elucidate the origin of secondary flow structures in the pipe cross-section. A smooth glass pipe and a rough steel pipe are taken into account at two different mass loadings considering a polydisperse size distribution with a number mean diameter of about 40μm mimicking the corresponding reference experiment. After a detailed validation of the single-phase as well as the two-phase flow based on experimental and DNS data, the secondary flow structures are analyzed qualitatively and quantitatively. That confirms recently published experimental results that the secondary flow observed is of second kind. Finally, to prove that for another particle size distribution numerically investigated in the literature the secondary flow is still of second kind, rather large monodisperse inertial particles (134μm) hitting the pipe walls with two different roughnesses are additionally simulated. The strength of the secondary flow is found to be strongly reduced for these cases compared with the polydisperse smaller particles, but the mechanism responsible for the secondary flow is the same. [Copyright &y& Elsevier]

Abstract: In the present contribution an eddy-resolving scheme (large-eddy simulation) is combined with an efficient particle tracking algorithm for individual particles and a deterministic collision model. The purpose is to set-up a reliable methodology for the prediction of complex particle-laden two-phase flows at high mass loadings. The objectives are two-fold. On the one hand the suitability of the entire method to tackle practically relevant turbulent flows should be proven. On the other hand the influence of the fluid-particle interaction (two-way coupling) as well as particle–particle collisions (four-way coupling) is investigated in detail. For both purposes this numerical study is aligned to the experimental investigation of the bluff-body configuration by Borée et al. [J. Borée, T. Ishima, I. Flour, The effect of mass loading and inter-particle collisions on the development of the polydisperse two-phase flow downstream of a confined bluff body, J. Fluid Mech. 443 (2001) 129–165]. In this set-up a fully developed pipe flow laden with polydisperse glass beads enters a cylindrical chamber with an outer annular confined flow without swirl. In contrast to previous numerical studies both mass loadings and 110%) and thus also inter-particle collisions are taken into account. Contrary to the experimental investigation the predictions allow to artificially isolate different physical effects in order to clarify their importance. Especially for the high mass loading case interesting new results about the role of fluid-particle interactions and particle-particle collisions are enlightened. [Copyright &y& Elsevier]

Abstract: Wall roughness is known to have a significant influence on particle-laden wall-bounded flows directly affecting the particulate and the continuous phase. For sufficiently high mass loading the fluid flow is also indirectly altered by the particles subjected to collisions with rough walls. The paper is concerned with the question how the effect of rough walls on the particulate phase can be modeled taking a minimum of measured or empirically determined physical quantities into account. Following Nikuradse’s idea, a sandgrain roughness model is proposed for the dispersed phase in which the wall is covered by a densely packed layer of sand grains idealized by mono-disperse spheres. Based on geometric considerations relying on generally used roughness parameters such as R z or R q the local inclination of the wall is determined in order to predict the inelastic collision of the particles with the wall including friction. The sandgrain model also takes the shadow effect into account leading to asymmetric probability density functions of the wall inclination angles, where the mean normal vector is turned towards the incoming particle trajectory. The wall model applicable in 3-D is evaluated in the context of four-way coupled large-eddy simulations for turbulent plane channel flow but is also applicable in direct numerical simulations or Reynolds-averaged Navier–Stokes predictions. A variety of test scenarios were considered including varying wall roughness values, several mass loadings and different particle sizes. [Copyright &y& Elsevier]

Abstract: Our numerical study focuses on convection in a rotating spherical shell with the objective to model combined thermal and compositional convection as proposed for the Earth’s core. Since the core is cooling, a thermal gradient is established, which can drive thermal convection. Simultaneously, due to the solidification of the inner core latent heat is released at the freezing front and the concentration of the light constituents of the liquid phase increases thus providing a source for compositional buoyancy. Typically, the molecular diffusivities of both driving components differ by some orders of magnitude. To account for this difference it is indicated to adopt a double-diffusive convection model in treating Earth’s core dynamics. As opposed to purely thermal or purely compositional convection the double-diffusive system is controlled by two Rayleigh numbers associated with the respective buoyancy sources. Using the Rayleigh numbers as control parameters neutral curves of the linear onset of convection in the rotating shell are determined for different Ekman numbers and diffusivity ratios. It is found that the neutral curves depend significantly on the system parameters. By comparison with the analytical solutions of the rotating cylindrical annulus it is shown that the neutral curves represent a superposition of curves associated with solutions for different azimuthal wave numbers. Furthermore, fully non-linear simulations are presented in order to elucidate the effect of isochemical and fixed chemical flux boundary conditions on the convection. We consider three driving scenarios with varying thermo-chemical forcing ratios. Both the forcing ratio and the chemical boundary condition have distinct effects on the system that are discussed separately. [Copyright &y& Elsevier]

Abstract: Background: There are only a few reports about combined heart-liver transplantations. The surgical techniques differ widely, ranging from sequential implantation of the organs to simultaneous transplantations. We report our experience with simultaneous, combined heart-liver transplantations without using a veno-venous bypass demonstrating that this is a feasible surgical technique. Methods: Since 2005, we performed 4 combined heart-liver transplantations by implanting the liver during the reperfusion period of the newly implanted heart. We retrospectively reviewed patient clinical data and outcomes. Results: The mean operative time was 534 ± 247 minutes and the ischemia times for heart and liver were 190 ± 72 minutes (cold ischemia time for the heart), 98 ± 96 minutes (warm ischemia time for the heart), 349 ± 101 minutes (cold ischemia time for the liver), and 36.25 ± 3.5 minutes (warm ischemia time for the liver). Three patients were discharged from the hospital after an uneventful clinical course. One patient died due to multi-organ failure during the intensive care unit stay on the 23rd postoperative day. Conclusion: We suggest that combined, simultaneous heart-liver transplantation without veno-venous bypass is a feasible surgical technique. [Copyright &y& Elsevier]

Abstract: Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy-carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks. In this study optimal angles of attack of the delta-winglets are investigated based on the Pareto optimal strategy. The optimization process combines a CFD analysis, genetic algorithms and the response surface methodology. The angle of attack of a pair a delta-winglet-type VGs mounted behind each tube is varied between β =−90° and +90°. Three circular tube rows with inline and staggered tube arrangements are investigated for Reynolds numbers from 200 to 1200 (based on the inlet height and inlet velocity). The flow structure and heat transfer behavior is analyzed in detail for certain cases and the staggered and the inline tube arrangements are compared. Finally, for each of these arrangements the optimal sets of angles of attack for different Reynolds numbers are presented. [ABSTRACT FROM AUTHOR]

Abstract: The combination of turbulent buoyant flow with a free surface (Rayleigh–Bénard–Marangoni convection) and rotation is hardly investigated in detail, especially for low Prandtl number fluids, although it can be found in several applications such as Czochralski (Cz) crystal growth. Therefore, a Direct Numerical Simulation (DNS) of such a Cz case with an idealized cylindrical crucible geometry of 170mm radius and a rotating crystal of 50mm radius was conducted applying realistic boundary conditions, which lead to the dimensionless numbers of , and . The computational grid contained ca. 8.4 million control volumes to resolve all turbulent scales based on a finite-volume scheme for curvilinear block-structured grids and an explicit time discretization. The resulting velocity and temperature fields show fully developed three-dimensional turbulence and are characterized by thermal buoyant plumes rising from the bottom of the crucible to the free surface, surface tension effects, and the strong impact of the counterrotating crystal. The analysis of the instantaneous flow revealed that in the rotating melt a large, slowly moving spiral vortex evolves. The averaged data show the formation of Bénard cell-like structures. Below the crystal, along the free surface, and especially at the corner of the crystal, the turbulence intensity is strongest. The DNS results were generated and analyzed in detail in order to serve as a reference and will also be made available to the public for further investigations. Within an ongoing study these data will be used to validate computations for practical applications employing the Large-Eddy Simulation (LES) technique, which is used to model the turbulent flow and temperature field in order to save computational time compared to DNS. [Copyright &y& Elsevier]

Abstract: A 6-kDa phase-related peptide (PRP) was recently identified from the hemolymph of the desert locust Schistocerca gregaria. Its presence in much higher concentrations in the crowd-reared (gregarious) phase than in the isolated-reared (solitarious) one suggests a role in phase polyphenism. However, when tested in a variety of classical bioassays, no activity could be found. We hoped that uncovering its site(s) of synthesis might yield hints as to possible functions. An antiserum was raised against the C-terminal 16 aa part of PRP for use in immunocytochemistry. No immunoreactivity was recorded in the fat body, midgut, or Malpighian tubules. The strongest positive immunostaining was observed in the follicle cells of the ovary and in the seminal vesicle tubes of the male accessory gland complex. Also, positive were a pair of large neurosecretory cells in the subesophageal ganglion, the storage part of the corpora cardiaca and some nerve fibers in the brain- and abdominal regions. An additional mass spectrometric analysis was successfully done in combination with a BLAST search to detect possible false positive staining. This confirmed the presence of genuine PRP in most of the immunopositive tissues. Additional experiments are needed to unravel the role of PRP. [Copyright &y& Elsevier]

Abstract: Aiming at the better prediction of pharmaceutical aerosol deposition in extrathoracic airways, a simpler test case, namely a bend flow (tubular cross-section) laden with monodisperse particles, is adopted here and studied numerically. The continuous phase is calculated using a large-eddy simulation technique along with a finite-volume method for block-structured curvilinear grids. The particulate phase is simulated using a Lagrangian approach where hundred thousands of individual monodisperse particles with varying particle diameters are released and tracked throughout the computational domain. To allow such a large number of particles, a highly efficient tracking algorithm is applied, where particle paths are predicted in an orthogonal computational domain, avoiding time-consuming search algorithm, normally required when particles are tracked in the actual physical domain of a curvilinear body-fitted block-structured grid. Both simulation algorithms, for the continuous and particulate phases, are completely parallelized using domain decomposition. Additionally, the in-house code applied supports vector processing allowing efficient usage of nearly all kinds of high-performance architectures. Two different Reynolds numbers are considered where is based on the bend diameter and mean flow velocity. The first case is within the laminar regime at and serves for the purpose of verification and validation. The second, more challenging case comprises the turbulent regime at , which is the intrinsic objective of the present study. Depending on the Stokes number of the particles, , and the releasing locations at the entrance of the bend, the particles will either deposit on the wall or penetrate and exit the computational domain. Simulation results of aerosol deposition efficiency, over the entire range of particle diameters considered here, show an excellent agreement when compared to experimental values obtained by Pui, Romay-Novas, and Liu [(1987). Experimental study of particle deposition in bends of circular cross-section. Aerosol Science and Technology, 7, 301]. [Copyright &y& Elsevier]

• Efficient model for the breakage of agglomerates by wall impact. • Derivation of a data-driven model for the post-breakage kinetics of fragments. • Huge number of DEM simulations for a wide range of influencing parameters. • Description by the reflection angle, the spreading angle and a velocity ratio. • Model applicable in Euler-Lagrange simulations relying on the hardsphere approach. The present study completes the development of a model for predicting the effect of wall impacts on agglomerates in turbulent flows. Relying on an Euler-Lagrange hard-sphere approach this physical phenomenon is described in an efficient manner allowing practically relevant multiphase flow simulations at high mass loadings. In a recent study (Khalifa and Breuer, 2020) conditions for the onset of breakage and the resulting fragment size distribution were derived. In the present investigation a data-driven description of the post-breakage kinetics of the fragments is developed based on extensive DEM simulations taking a variety of impact conditions (impact velocity, impact angle, agglomerate size) into account. The description relates the velocity vectors of the fragments after breakage to three parameters: The reflection angle, the spreading angle and a velocity ratio of the magnitude of the fragment velocity to the impact velocity of the agglomerate. Relying on the DEM results Weibull distribution functions are used to describe the parameters of the wall-impact model. The shape and scale parameters of the Weibull distributions are found to mainly depend on the impact angle of the agglomerate. Consequently, relationships between the shape and the scale parameters and the impact angle are established for each of the three parameters based on a fourth-order regression. This allows to determine the velocity vectors of the fragments randomly based on the corresponding Weibull distributions of the reflection angle, the spreading angle and the fragment velocity ratio. The devised model is evaluated in a turbulent duct flow at five Reynolds numbers and three agglomerate strengths given by powders consisting of primary particles of different size. The analysis first concentrates on the pure wall-impact breakage but then also includes agglomerate breakup due to turbulence, drag forces and rotation allowing to determine the shares of the different physical phenomena. It is found that with increasing Stokes number the wall-impact breakage occurs less effectively due to the reduced responsiveness of the agglomerates to the secondary flow motions in the duct. However, in the very high range of St + other mechanisms such as the turbophoresis and the lift force augment the breakage at walls. Comparing the contributions of the different breakage mechanism reveals that the wall impact is dominant at the lowest Reynolds numbers, whereas the drag stress prevails at high Re. [ABSTRACT FROM AUTHOR]

The objective of the present paper is to develop a methodology to inject strong wind gusts into the computational domain in order to efficiently simulate their effect on the fluid flow. The design of the methodology based on a source-term formulation takes the feedback effect of the resulting turbulent flow (and, if present, the impacted structure) on the wind gust itself into account. Since the injection of the wind gusts can be carried out close to the region of main interest, CPU-time intensive methods to ensure a proper transport of the gust through the flow field can be avoided. The methodology is mainly intended for the application within eddy-resolving simulations (e.g., LES), but it is not restricted to this class of simulation approaches. For the description of the gusts classical shape functions such as the Extreme Coherent Gust (ECG) and the Extreme Operating Gust (EOG) as well as a newly derived C 2 -"1-cosine" shape are applied. Two scenarios are taken into account to assess the proposed gust injection technique. On the one hand a (laminar) undisturbed flow field is considered and the effect of different time and length scales of the gusts on their evolution and propagation through the flow field is studied in detail. On the other hand a turbulent background flow is assumed demonstrating that the methodology suggested is also applicable for practically relevant turbulent flows. • Efficient methodology to inject strong wind gusts into the computational domain. • Source-term formulation taking the feedback effect on the wind gust into account. • Modifications of the advective fluxes or the transient term not required. • Applicable to eddy-resolving simulations but not restricted to this approach. • Gust injection method applicable to laminar and turbulent flows. [ABSTRACT FROM AUTHOR]

• Numerical LES investigations on the fluid–structure interaction of a wing. • Elastically mounted wing with two degrees of freedom, i.e., pitch and heave. • Detailed validation based on complementary measurements in a wind tunnel. • Consideration of two configurations with different flutter stabilities. • Small and large-amplitude oscillations as well as flutter observed depending on Re. The paper is the numerical counterpart of the experimental investigation on the fluid–structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave. Wood et al. (2020) has provided the experimental basis by studying the flutter stability of an elastically mounted straight wing (NACA 0012 airfoil) in a wind tunnel considering the transitional Reynolds number regime. Three different configurations with varying distances between the fixed elastic axis and the variable center of gravity were considered. Additional free-oscillation tests in still air were carried out in order to make the mechanical properties of the setup available for the simulations. The present contribution describes the numerical methodology applied consisting of a partitioned coupled solver combining eddy-resolving large-eddy simulations on the fluid side with a solver for the governing equations of the translation and rotation of the rigid wing. In order to prove the parameters provided by the experiment and to determine the pure material damping coefficients not available from the measurements, simulations of 1-DOF free-oscillation tests in still air are carried out and analyzed. For validation purposes the corresponding 2-DOF free-oscillation tests in still air are assessed and a good agreement with the experimental data is achieved. Finally, the wing exposed to a constant free-stream of varying strength is analyzed leading to the characterization of complex instantaneous FSI phenomena such as limit-cycle oscillations and flutter. Under full utilization of the supplementary measurements the predictions are evaluated in detail. Contrary to the experiments the simulations provide the entire fluid data and unique data for the translatory and rotatory movement allowing to investigate the causes of the observed phenomena. Both limit-cycle oscillations and flutter can be reproduced by the coupled FSI predictions. [ABSTRACT FROM AUTHOR]

• A deterministic and viable breakup model for microbubble-laden turbulent flows. • Transfer from the Euler-Euler method to the Euler-Lagrange approach. • Deterministic estimation of the size of arising daughter bubbles. • Physically motivated model for the separation axis and separation velocity. • Time lag between two successive breakup processes proposed. The present study is concerned with breakup models for microbubbles in turbulent flows. Analyzing the different physical mechanisms responsible for breakup based on a literature review, breakage due to turbulent fluctuations in the inertial subrange is identified as the most important one. Widely used breakup models for this mechanism are discussed concerning their advantages and drawbacks with special emphasis on thoughts how these models developed in the Euler-Euler context can be transferred into the Euler-Lagrange approach favored in this study. The most promising model is chosen as a basis and then implemented in an efficient bubble tracking scheme relying on the large-eddy simulation technique. The size of the daughter bubbles is deterministically estimated based on the breakup mechanism. Furthermore, a physically motivated model for the axis along which bubbles separate and for the separation velocity of the daughter bubbles is developed. Lastly, an estimate of the time lag between two successive breakup processes is provided. The simulation methodology is validated against an experimental study by Martínez-Bazán et al. (1999) investigating bubble breakup within a turbulent jet flow. The predicted results are found to be in reasonable agreement with the measurements. Furthermore, the effect of coalescence and other properties of the bubbles on the breakup behavior is investigated. [ABSTRACT FROM AUTHOR]

This experimental study is concerned with gas–liquid heat exchangers relying on cross-corrugated tubes. The experiments are carried out with air flow on the inner side of the tube, cooled by a secondary outer water flow. Overall 18 different geometrical configurations are investigated. In contrast to Harleß et al. (2016) the emphasis of the present investigation is put on the cross-corrugated variant. The dimensionless corrugation pitch p / d i varies between 0.283 and 1.117, the dimensionless corrugation height e / d i between 0.024 and 0.087, and the corrugation angle φ between 14.7° and 48.8°. The examined Reynolds number range is 5000 < Re < 23 , 000 , to match a typical range of gas–liquid heat exchangers. Nusselt number and friction factor are plotted for all tubes and are correlated with a power law of the corrugation dimensions. Finally, the thermal performance of the tubes is investigated in terms of the R 3 criterion. Within this study the highest thermal performance is obtained by a cross-corrugated tube with p / d i = 0.769 and e / d i = 0.081 . [ABSTRACT FROM AUTHOR]

18 helically corrugated tubes with different geometrical configurations for exhaust gas heat exchangers are investigated in this study. Experiments are carried out with air flow on the tube side, cooled by a secondary water flow. The dimensionless corrugation pitch p / d i varies from 0.27 to 1.53, the dimensionless corrugation height e / d i from 0.02 to 0.056, and the corrugation angle φ from 9.2° to 37.0°. To match the typical range of exhaust gas heat exchangers, the examined Reynolds number range is 5000 < Re < 23,000 . Nusselt number and friction factor are correlated with a power law of the corrugation dimensions. Furthermore, the thermal performance of the tubes is examined. It is shown that the highest thermal performance is achieved by a single-start corrugated tube with p / d i = 0.517 and e / d i = 0.040 . Finally, suggestions are made, how to handle different manufacturing constraints for additional optimization purposes. [ABSTRACT FROM AUTHOR]