Influences on the drying behavior of a concrete ceiling below a cold attic

The article describes the current state of a project examining the influences on the moisture distribution in cold attics above concrete ceilings of residential buildings. Considerable research has been done on moisture damages in cold attics, especially in Scandinavia and North America, focussing on spaces above wooden ceilings. The project (ongoing until Sept 2021) underlying the article deals with cold attics above concrete ceilings resting on masonry walls, a frequent variant in Austria. Research was triggered by a regional Austrian building industry association to shed light onto recent detrimental moisture accumulation in the wooden wall plate (= bearing for the rafters along the eaves) and in the two EPS insulation layers on top of the ceiling. Suspected reasons for the moisture problems and for the local moisture distribution are 1) a too small diffusion resistance of the vapour retarder covering the ceiling, 2) insufficient (natural) attic ventilation and 3) convection, e. g. in the gap between the polystyrene blocks. In order to rank these potential causes by influence and also to find a practical solution a two stage experimental approach was chosen: 1) A handy small scale replica (order of dimension: 1m) of the situation was exposed to the according indoor and outdoor climate in a climate chamber. Different vapour retarders on top of the ceiling were chosen. 2) A larger 1:1 replica has been erected as well but not yet delivered monitoring data. In parallel, a hygrothermic model taking convection into account was established and simulations carried out. The project will deliver a contribution to the Austrian standard on moisture safety 8110-2 on how to judge the moisture safety of joints via simulation.

Numerical Investigation of Potential Cause of Instabilities in a Hydrogen Micromix Injector Array

Hydrogen micromix combustion is a promising concept to reduce the environmental impact of both aero and land-based gas turbines by delivering carbon-free and ultra-low-NOx combustion. The high-reactivity and wide flammability limits of hydrogen in a micromix combustor can produce short and small diffusion flames at lean overall equivalence ratios. There is limited published information on the instabilities of such hydrogen micromix combustors. Diffusion flames are less prone to flashback and autoignition problems than premixed flames as well as combustion dynamics issues. However, with the high laminar flame speed of hydrogen, lean fuel air ratio (FAR) and very compact flames, the risk of combustion dynamics for micromix flames should not be neglected. In addition, the multi-segment array arrangement of the injectors could result in both potential causes and possible solutions to the instabilities within the combustor. This paper employs numerical simulations to investigate potential sources of instabilities in micromix flames by modelling an extended array of injectors, represented by either single or multiple injectors with appropriate boundary conditions at elevated pressure and temperature. Both RANS and LES simulations were performed and used to derive the Flame Transfer Function (FTF) of the micromix flames to inform lower order thermoacoustic modelling of micromix combustion. LES simulations indicate that the gain of the FTF is lower than predicted from the RANS simulations indicating a lower risk of high frequency thermoacoustic issues than suggested by RANS. When LES simulations are conducted for certain representative configurations it is observed that there are persistent high-frequency instabilities due to the interaction of the flames. This phenomenon is not observed when only a single injector is modelled. LES simulations for two injectors are conducted with various geometries and radial boundary conditions to identify the cause of the instabilities. It is concluded that the observed high-frequency instabilities are related to aerodynamic jet instabilities enhanced by both aerodynamic and acoustic feedback and key geometric features affecting the occurrence of the instabilities are identified. Only transient simulations such as LES are able to capture such effects and RANS simulations typically used in early stage design will not identify this issue.

Toward the Next Generation of Passive Micromixers: A Novel 3-D Design Approach

Passive micromixers are miniaturized instruments that are used to mix fluids in microfluidic systems. In microchannels, combination of laminar flows and small diffusion constants of mixing liquids produce a difficult mixing environment. In particular, in very low Reynolds number flows, e.g., Re < 10, diffusive mixing cannot be promoted unless a large interfacial area is formed between the fluids to be mixed. Therefore, the mixing distance increases substantially due to a slow diffusion process that governs fluid mixing. In this article, a novel 3-D passive micromixer design is developed to improve fluid mixing over a short distance. Computational Fluid Dynamics (CFD) simulations are used to investigate the performance of the micromixer numerically. The circular-shaped fluid overlapping (CSFO) micromixer design proposed is examined in several fluid flow, diffusivity, and injection conditions. The outcomes show that the CSFO geometry develops a large interfacial area between the fluid bodies. Thus, fluid mixing is accelerated in vertical and/or horizontal directions depending on the injection type applied. For the smallest molecular diffusion constant tested, the CSFO micromixer design provides more than 90% mixing efficiency in a distance between 260 and 470 µm. The maximum pressure drop in the micromixer is found to be less than 1.4 kPa in the highest flow conditioned examined.

Surface and deep ocean connectivity inferred from robust extraction of coherent sets in ocean flow using models and sparse, scattered, and incomplete float data with transfer operator and dynamic Laplacian methods.

&lt;p&gt;Transport and mixing properties of the ocean's circulation is crucial to dynamical analyses, and often have to be carried out with limited observed information. Finite-time coherent sets are regions of the ocean that minimally mix (in the presence of small diffusion) with the rest of the ocean domain over the finite period of time considered. In the purely advective setting (in the zero diffusion limit) this is equivalent to identifying regions whose boundary interfaces remain small throughout their finite-time evolution. Finite-time coherent sets thus provide a skeleton of distinct regions around which more turbulent flow occurs. Well known manifestations of finite-time coherent sets in geophysical systems include rotational objects like ocean eddies, ocean gyres, and atmospheric vortices. In real-world settings, often observational data is scattered and sparse, which makes the difficult problem of coherent set identification and tracking challenging. I will describe mesh-based numerical methods [3] to efficiently approximate the recently defined dynamic Laplace operator [1,2], and rapidly and reliably extract finite-time coherent sets from models or scattered, possibly sparse, and possibly incomplete observed data. From these results we can infer new chemical and physical ocean connectivities at global and intra-basin scales (at the surface and at depth), track series of eddies, and determine new oceanic barriers.&lt;/p&gt;&lt;p&gt;[1] G. Froyland. Dynamic isoperimetry and the geometry of Lagrangian coherent structures. &lt;em&gt;Nonlinearity&lt;/em&gt;, 28:3587-3622, 2015&lt;/p&gt;&lt;p&gt;[2] G. Froyland and E. Kwok. A dynamic Laplacian for identifying Lagrangian coherent structures on weighted Riemannian manifolds. &lt;em&gt;Journal of Nonlinear Science&lt;/em&gt;, 30:1889&amp;#8211;1971, 2020.&lt;/p&gt;&lt;p&gt;[3] Gary Froyland and Oliver Junge. Robust FEM-based extraction of finite-time coherent sets using scattered, sparse, and incomplete trajectories. &lt;em&gt;SIAM J. Applied Dynamical Systems&lt;/em&gt;, 17:1891&amp;#8211;1924, 2018.&lt;/p&gt;

Convective instability in a diffusive predator–prey system

It is well known that biological pattern formation is the Turing mechanism, in which a homogeneous steady state is destabilized by the addition of diffusion, though it is stable in the kinetic ODEs. However, steady states that are unstable in the kinetic ODEs are rarely mentioned. This paper concerns a reaction diffusion advection system under Neumann boundary conditions, where steady states that are unstable in the kinetic ODEs. Our results provide a stabilization strategy for the same steady state, the combination of large advection rate and small diffusion rate can stabilize the homogeneous equilibrium. Moreover, we investigate the existence and stability of nonconstant positive steady states to the system through rigorous bifurcation analysis.

Concentration phenomenon of the endemic equilibrium of a reaction-diffusion-advection SIS epidemic model with spontaneous infection

<p style='text-indent:20px;'>In this paper, we investigate the effect of spontaneous infection and advection for a susceptible-infected-susceptible epidemic reaction-diffusion-advection model in a heterogeneous environment. The existence of the endemic equilibrium is proved, and the asymptotic behaviors of the endemic equilibrium in three cases (large advection; small diffusion of the susceptible population; small diffusion of the infected population) are established. Our results suggest that the advection can cause the concentration of the susceptible and infected populations at the downstream, and the spontaneous infection can enhance the persistence of infectious disease in the entire habitat.</p>

Investigation of the Diffusion Couple Ductile Cast Iron / Iron

Forging of ductile cast iron with pure iron by the Damascus technique, results in a new composite material. The combination of cast iron and pure iron is unusual because of its rather different properties. After forging these two materials a small diffusion zone of about 150 µm was observed. Various heat treatments at 900 °C for 2, 4 or 20 hours and 950 °C for 4 h were performed to increase the diffusion zone up to 2.4 mm. At 900 °C carbon solubility in austenite is about 1.2 wt. % and at 950 °C 1.4 wt. %. During the heat treatment carbon diffuses from cast iron into the pure iron and the diffusion gradient grows with time and temperature. Furthermore, the samples were air cooled or water quenched. In the ductile cast iron, graphite nodules are surrounded by ferrite. During the heat treatment graphite is dissolved and pores are observed. In the diffusion gradient layer, a broad range of microstructures observed in hyper- and hypoeutectoid steels could be found. The microstructures were revealed by different etchants and moreover, hardness measurements were performed.