The validity of Darcy’s law at very low Reynolds numbers is discussed controversially in literature, as some authors propose a pre–Darcy flow regime below some critical Reynolds number. The scope of this work is to investigate this problem experimentally. Therefore, a packing of glass spheres is perfused by different glycerin–water solutions. A linear behaviour between the flow velocity and the pressure drop through the packed spheres is found in the complete investigated range of Reynolds number $${Re}_{d'}$$ Re d ′ , based on the mean-pore diameter $$d'$$ d ′ and mean-pore velocity $$v'$$ v ′ with $$10^{-9} \le {Re}_{d'} \le 10^{-1}$$ 10 - 9 ≤ Re d ′ ≤ 10 - 1 . This contradicts the results of different authors like Fand et al. (1987) or Kececioglu and Jiang (1994), postulating a pre–Darcy regime for $${Re}_{d'} \le 2.8 \cdot 10^{-6}$$ Re d ′ ≤ 2.8 · 10 - 6 or $${Re}_{d'} \le 0.13$$ Re d ′ ≤ 0.13 , respectively. Graphical abstract
To examine the filling process in a lithium-ion battery, a numerical model to characterize the displacing flow of a liquid in air-filled pores of thin heterogeneous porous materials is elaborated. The investigation is based on the volume-averaged Navier-Stokes equations for small Reynolds numbers, using a volume-of-fluid method to cover the multiphase flow. The flow is investigated with respect to the wall effect and to capillary action within the porous matrix. On the one hand, model experiments with similar particle-size distributions as in the battery layers are conducted to extract the porosity as function of the wall distance. On the other hand, experiments with the three different porous layers of the battery are performed to receive mean values for the most important properties related to the two-phase flow. Results for the displacement flow in parts of the battery are presented and discussed, showing a considerable influence of the modeled effects onto the flow characteristics.
Security is an important and difficult topic in today's complex computer systems. Cloud-based systems adopting microservice architectures complicate that analysis by introducing additional layers. In the test system analyzed, base layers are combined into three groups (compute provider, encapsulation technology, and deployment) and possible security risks introduced by technologies used in these layers are analyzed. The application layer focuses on security concerns that concern authorization and authentication. The analysis is based on a microservice-based rewritten version of the seat reservation system of the Deutsche Bahn using technologies such as Amazon Web Services, Docker, and Kubernetes. The comparison concludes that the security of communication in the test system could be significantly improved with little effort. If security is not considered as an integral part from the beginning of a project, it can easily be neglected and be expensive to add later on.
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