| 1. |
- Pettersson, Kaj, 1990, et al.
(författare)
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Contribution of dynamic capillary pressure to rainfall infiltration in thin homogeneous growth substrates
- 2021
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 603
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Tidskriftsartikel (refereegranskat)abstract
- The use of green roofs to help mitigate storm water contributions to urban flooding has been gaining popularity but is hindered by the limited data on the performance of such roofs with regard to storm water runoff mitigation. The underlying issue stems from the inherent complexity of modeling subsurface multiphase flow. Modeling of this phenomena requires calculating the contributions of substrate microstructure characteristics, the influence of the wetting and non-wetting phases upon each other, and the effect of the microstructure on the wetting phase. Previously we have observed how the microstructure can affect detention, however the quantification of this relationship is still missing. In the present paper we present numerical simulations of wetting phase infiltration of a thin monodisperse packed bed in order to understand and quantify the impact of microstructure geometry on storm water infiltration of a green roof substrate. For a slightly hydrophilic case, (θ=82°), we find that a dominant mechanism underlying this relationship is the microstructure-induced dynamic behavior of the capillary pressure. We determine that at larger packing ratios (ratio of packed bed depth to particle size), the influence of hydraulic head diminishes and behaves conversely for thinner layers, particularly when larger pores are present. Indeed, thin beds composed of large particles can exhibit high flow velocities that in turn affect the capillary pressure within the substrate. We observe that the capillary pressure can shift from negative values denoting capillary suction to positive ones which cause valve-like blocking effects on the flow; dependent upon the flow velocity as determined by the microstructure. In particular, we find that the capillary pressure depends on the value of the pore-scale gravity-induced flow velocity, quantified through a characteristic Capillary number. The provided quantification of this relationship can be invaluable from a design perspective to understand the behavior of capillary pressure of different substrates under a variety of flow rates prior to testing substrate candidates. In addition, a comparison of the behavior of the dynamic component of capillary pressure to other works is undertaken. Flow homogeneity is also found to be linked to the flow velocity, and consequently to the microstructure.
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| 2. |
- Pettersson, Kaj, 1990, et al.
(författare)
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On the impact of porous media microstructure on rainfall infiltration of thin homogeneous green roof growth substrates
- 2020
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 582
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Tidskriftsartikel (refereegranskat)abstract
- Green roofs are considered an attractive alternative to standard storm water management methods; however one of the primary issues hindering their proliferation is the lack of data regarding their ability to retain and reduce storm water under a variety of climatic conditions. This lack of data is partly due to the complexity of physical processes involved, namely the heterogeneous microscopic behavior that characterize flows in unsaturated porous media. Such an anomalous behavior is difficult to predict a priori, especially in the presence of layered structures. This paper examines water infiltration of a green roof at the pore-scale with the aim to evaluate the effect of the porous microstructure in thin substrate layers. In such layers, the thickness of the medium and the particle size are within the same order of magnitude and the effect of the packing arrangement on the flow dynamics can be pronounced. In this study, three packing arrangements and two different hydraulic heads, analogous to extreme rainfall events typical of Scandinavia, are investigated by means of direct numerical simulations based on the lattice Boltzmann method. The results show that a wider variability of pore sizes in a thin medium can be linked directly to flow pathing preference and consequently less homogenized flow in the primary flow direction. This situation corresponds to intermittent flow behavior at the pore-scale level and reduced macroscopic infiltration rates. This observation unveils the possibility of designing innovative green roof growth substrates: by tuning the particle size and thickness of the layers composing the medium the desired green roof detention time can be attained.
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| 3. |
- Abrahamsson, Per, 1985, et al.
(författare)
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CONTINUUM MODELING OF PARTICLE FLOWS IN HIGH SHEAR GRANULATION
- 2013
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Ingår i: 6th International Granulation Workshop.
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Konferensbidrag (refereegranskat)abstract
- High shear granulation (HSG) is a common process in the pharmaceutical industry. A better understanding of the flow conditions of powders and granulates in large-scale HSG equipment is crucial for constructing predictive models. The staggering amount of particles in the process makes the use of continuum flow models highly attractive. This article discusses the possibilities and problems in using continuum modelling in HSG systems and evaluates some of the available modelling approaches. We examine several dense granular flow models studying both the underlying theory and how they perform in practice. The studied models are the frictional model by Shaeffer [1], modifications to the transport coefficients that describe the solid phase stresses similar to those used in Khain and Meerson [2], and the framework developed by Jop et.al. [3] using a depth-averaged flow model for constant solid volume fraction flows.The model by Shaeffer has previously been used with the conclusion that the solid phase stresses are underestimated [4]. We show theoretically and in practice that this approach is not appropriate due to the strong resolution dependence of the model. The approach taken by Khain and Meerson, among others, to try to modify expressions from rapid granular flow to also be valid in the dense region is attractive from a theoretical point of view. Making use of the rigorous framework of kinetic theory, the applicability of a number of such models to HSG has been evaluated. The modelling framework developed by Jop et.al was used in disc impeller HSG equipment. The results show that the model can well predict the behaviour of the solid-phase viscosity of the dense granular flow. The model is nevertheless restricted to constant volume fraction flows and needs to be expanded to include a varying volume fraction. We conclude in this paper that continuum modelling of HSG has a promising outlook but there is a need to develop better models for the dense regions of the flow. We also give and evaluate some of the options available for treating these regions.
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| 4. |
- Abrahamsson, Per, 1985, et al.
(författare)
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On continuum modeling using kinetic-frictional models in high shear granulation
- 2014
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Ingår i: Particuology. - : Elsevier BV. - 2210-4291 .- 1674-2001. ; 13:1, s. 124-127
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Tidskriftsartikel (refereegranskat)abstract
- In this short communication we explain and demonstrate why extreme caution has to be taken when applying conventional kinetic-frictional closures to continuum modeling of high shear granulation (HSG). By conventional models, we refer to closure laws where the kinetic and frictional stresses are summed up in order to get the total stress field. In a simple dense sheared system of a Couette shear cell, we study how the lack of scale separation affects the model predictions, both quantitatively and qualitatively. We show here that the spatial resolution has a significant effect on the magnitude of the kinetic and frictional contributions to the solid phase stresses. With this new investigation and previous studies of HSG it is concluded that conventional kinetic-frictional models are inadequate for continuum modeling of HSG.
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| 5. |
- Abrahamsson, Per, 1985, et al.
(författare)
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On continuum modelling of dense inelastic granular flows of relevance for high shear granulation
- 2016
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 294, s. 323-329
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Tidskriftsartikel (refereegranskat)abstract
- This article investigates a number of possible formulations of a continuum description for modelling dense inelastic granular flows. The revised Enskog theory (RET) for expressing the granular temperature and formulation of transport coefficients has been used within the continuum framework. The framework assumes particles as inelastic spheres and can describe a granular system at a wide range of volume fractions. The transport coefficients depend on the volume fraction through a modified expression for the radial distribution function. The proposed radial distribution function is based on previous studies on the behaviour of the shear viscosity in which an earlier divergence of the latter compared to the other transport coefficients has been demonstrated for sheared dense granular systems. Our results show that the newly developed radial distribution function maintains the ability of RET to predict the occurrence of instabilities in a homogeneous cooling granular gas. The introduced function also improves predictions for the velocity and volume fraction profiles in a Couette cell dense shear flow. Thus the proposed formulation shows promising features in terms of improving predictions for volume fractions relevant in high shear granulators. We have also observed that a different formulation may be needed for the densest regions.
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| 6. |
- Abrahamsson, Per, 1985, et al.
(författare)
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On the continuum modeling of dense granular flow in high shear granulation
- 2014
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 268, s. 339-346
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Tidskriftsartikel (refereegranskat)abstract
- This article addresses the subject of continuum modeling of dense granular flows with an application in high shear granulation. The possible use of continuum models and their ability to reproduce correct dynamics of such flows has been a subject of debate for a long time in the literature, and no consensus has been achieved so far. In this paper, we examine and compare two ways for making it possible to study dense granular flows in a continuum framework: the one that considers the stress tensor of a particulate phase as a sum of frictional and kinetic-collisional terms and the one that is based on modification of transport coefficients of the kinetic theory of granular flow. The latter framework is based on an analogy with molecular systems and how they behave at the phase transition from a liquid to a crystalline state. We show here that the formulation proposed in this work is able to correctly capture the phase transition and coexistence of solid-like and fluid-like phases in dense granular flows. This is in contrast to the model with added friction where the stress-strain dependence is shown to give a qualitatively different behavior compared to experimental data.
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| 7. |
- Ahlberg, Charlotte, 1974-
(författare)
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An experimental study of fiber suspensions between counter-rotating discs
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The behavior of fibers suspended in a flow between two counter-rotating discs has been studied experimentally. This is inspired by the refining process in the papermaking process where cellulose fibers are ground between discs in order to change performance in the papermaking process and/or qualities of the final paper product. To study the fiber behavior in a counter-rotating flow, an experimental set-up with two glass discs was built. A CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the discs. Experiments were performed for gaps of 0.1-0.9 fiber lengths, and for equal absolute values of the angular velocities for the upper and lower disc. The aspect ratios of the fibers were 7, 14 and 28. Depending on the angular velocity of the discs and the gap between them, the fibers were found to organize themselves in fiber trains. A fiber train is a set of fibers positioned one after another in the tangential direction with a close to constant fiber-to-fiber distance. In the fiber trains, each individual fiber is aligned in the radial direction (i.e. normal to the main direction of the train). The experiments show that the number of fibers in a train increases as the gap between the discs decreases. Also, the distance between the fibers in a train decreases as the length of the train increases, and the results for short trains are in accordance with previous numerical results in two dimensions.Furthermore, the results of different aspect ratios imply that there are three-dimensional fiber end-effects that are important for the forming of fiber trains.
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| 9. |
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| 10. |
- Andric, Jelena, 1979, et al.
(författare)
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A particle-level rigid fiber model for high-Reynolds number flow, implemented in a general-purpose CFD code
- 2013
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Ingår i: 8th International Conference on Multiphase Flow ICMF 2013, Korea.
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Konferensbidrag (refereegranskat)abstract
- A particle-level rigid fiber model has been integrated into a general-purpose, open source computational fluid dynamics code to carry out detailed studies of fiber–flow interactions in realistic flow fields. The fibers are modeled as chains of cylindrical segments, and their translational and rotational degrees of freedom are considered. The equations of motion contain the contributions from hydrodynamic forces and torques, and the segment inertia is taken into account. The model is validated for the rotational motion of isolated fibers in simple shear flow, and the computed period of rotation is in good agreement with the one computed using Jeffery’s equation for a prolate spheroid with an equivalent aspect ratio. The model is applied by suspending a number of fibers in the swirling flow of a conical diffuser, resembling one stage in the dry-forming of pulp mats. The Reynolds-averaged Navier–Stokes equations with an eddy-viscosity turbulence model are employed to describe the fluid motion, and a one-way coupling between the fibers and the fluid phase is included. The dependence of the fiber motion on initial position and density is analyzed.
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