3D Experimental investigation of the hygro-mechanical behaviour of wood at cellular and sub-cellular scale: detection of local deformations

The swelling/shrinkage of spruce wood samples (Picea Abies) is documented with high resolution XRay Tomography and advanced image analysis tools. We report the reversible moisture-induced global and local deformations at the cellular and sub-cellular scales. In particular, we present sophisticated methods for detecting local deformations in the cell wall. Insight is given on the hygromechanical behaviour of wood cell material and on the role of ultra-cellular components in wood, such as bordered pits and rays

Segmentation of salt solution and hydrated salt crystals in porous limestone

This paper presents how simultaneous phase-and-amplitude retrieval during X-ray tomographic reconstruction allows a clear segmentation of sodium sulfate solution and hydrated sodium sulfate crystals in Savonnières limestone without the need for a dopant. This technique could be very useful to study salt crystallization processes at the pore scale, in order to improve the understanding and remediation of salt damage in porous media

Crystallization of hydrated and anhydrous salts in porous limestone resolved by synchrotron X-ray microtomography

ACLInternational audienceThe crystallization processes of two anhydrous salts (NaCl and Na 2SO4) and one hydrated (sodium sulfate) salt in the pore space of a natural building stone, Savonnières limestone, are studied. We imaged the salt solution distribution before and after crystallization and the solid crystal distribution in between repeated crystallization cycles using synchrotron X-ray microtomography. This technique proves to be very useful to study salt crystallization processes at the pore scale. The use of simultaneous phase-and-amplitude retrieval during X-ray tomographic reconstruction allows a clear segmentation of sodium sulfate solution and hydrated sodium sulfate crystals without the need for a dopant. Salt crystals can precipitate under unconfined as well as confined conditions in the multiple pore systems of Savonnières limestone, depending on their interconnection. Salt solution and salt crystals are located in mechanically weak zones of the limestone, which can be linked to damage patterns observed in this stone after repeated salt weathering cycles. The distribution and the process of pore filling by salt crystals that are revealed here advance the understanding of salt damage in porous media and may open ways to perform remediation. © 2014 Elsevier B.V. All rights reserved

Adaptive integration of element matrices in finite element moisture transfer simulations

While serving different purposes, numerical simulations of moisture and heat transfer in soils and in building components are very similar in methodology: in both cases, spatially and temporally discretised equations for transfer of moisture and heat in porous materials are solved subject to (atmospheric) boundary conditions. The strongly non-linear transfer equations and boundary conditions however render such hygrothermal simulations computationally very expensive, and an efficient numerical solution algorithm is required. Such increasingly efficient numerical solution schemes allow for more, larger, longer or more precise simulations, widening the application capabilities of hygrothermal simulations. The computational cost of hygrothermal simulations revolves around the serial iterative com-position and decomposition of the coefficient matrix of the system of algebraic equations de-scribing the discretised moisture and heat transfer, and is thus determined by the cost of one (de)composition, and the number of required (de)compositions. This article presents two op-timisation measures for simulations of moisture and heat transfer in building components un-der atmospherical excitation: adaptive integration and variations on the Newton-Raphson iterative scheme. Adaptive integration targets the cost of one (de)composition, while the varia-tions on Newton-Raphson aims at the number of required (de)compositions. While exempli-fied by building physical simulations, the presented optimisation measures are equally valid for simulations of moisture and heat transfer in soils. It will be demonstrated that the common preference for low-order numerical integration of the finite element matrices has an adverse effect on the required spatial discretisation: a fine dis-cretisation throughout is needed for accurate simulation of the moving moisture fronts typical of infiltration problems. Adaptive integration allows to merge low-order numerical integration with rougher spatial discretisations, reducing the number of required integration points and of discretisation nodes. A second section of the article investigates the efficiency of (variations on) the Newton-Raphson scheme. It will be demonstrated that appropriate application of Newton-Raphson on the boundary conditions, of modified iteration and of separate convergence criteria can drastically diminish the number of required (de) compositions.Presenters: name: Janssen, Hans affiliation: Laboratory for Building Physics, Faculty of Engineering, Katholieke Universiteit Leuve

Studying the impact of local urban heat islands on the space cooling demand of buildings using coupled CFD and building energy simulations

Surface as well as air temperatures are due to the urban heat island effect higher in urban compared to their surrounding rural areas. These increased temperatures have a strong impact on the building energy performance in urban environments and thermal comfort as well as health of inhabitants of these environments. At smaller scales, local heat islands are formed within urban environments, which have the same negative impacts. In this study, we investigate the influence of local heat islands on the space cooling demand of buildings. Commonly, climate information from one weather station is used for building performance simulations within a whole city. This climate information cannot take local hot spots into account, what can lead to inaccurate space cooling demand predictions. Here, we model the local urban climate with CFD (Computational Fluid Dynamics) simulations. With CFD also the local convective heat transfer coefficients (CHTC) for the building surfaces can be predicted. These local CHTC can strongly vary locally due to differences in local wind speeds. The commonly used coefficients are mostly based on measurements at facades of stand-alone buildings, where the local wind speeds are higher compared to urban areas. This study shows a dependency of the space cooling demands on the local urban climate. Space cooling demands are higher in areas with high local temperatures, where the winddriven ventilation is decreased. Additionally to the higher local temperatures also the local CHTC are lower leading to lower heat losses from the buildings. It can be concluded that it is important to account for the local microclimate to accurately predict the space cooling demand of buildings in urban environments

Dynamic induced softening in frictional granular material investigated by DEM simulation

A granular system composed of frictional glass beads is simulated using the Discrete Element Method. The inter-grain forces are based on the Hertz contact law in the normal direction with frictional tangential force. The damping due to collision is also accounted for. Systems are loaded at various stresses and their quasi-static elastic moduli are characterized. Each system is subjected to an extensive dynamic testing protocol by measuring the resonant response to a broad range of AC drive amplitudes and frequencies via a set of diagnostic strains. The system, linear at small AC drive amplitudes has resonance frequencies that shift downward (i.e., modulus softening) with increased AC drive amplitude. Detailed testing shows that the slipping contact ratio does not contribute significantly to this dynamic modulus softening, but the coordination number is strongly correlated to this reduction. This suggests that the softening arises from the extended structural change via break and remake of contacts during the rearrangement of bead positions driven by the AC amplitude.Comment: acoustics, nonlinearity, granular medi

Influence of the underneath cavity on buoyant-forced cooling of the integrated photovoltaic panels in building roof: a thermography study

Airflow around building-integrated photovoltaics (BIPV) has a significant impact on their hygrothermal behavior and degradation. The potential of reducing the temperature of BIPV using an underneath cavity is experimentally and numerically investigated in literature. Most of the models are oversimplified in terms of modeling the impact of 3D flow over/underneath of PV modules, which can result in a non-uniform surface temperature and consequently a non-homogenous thermal degradation. Moreover, the simultaneous presence of radiation and convection related to upstream wind, in addition to the combined impact of back-ventilation and surface convection, is barely addressed in literature. However, these simplifications can result in the unrealistic loading climate conditions. This paper aims to present a unique experimental setup to provide more realistic climate conditions for investigating the ventilation potential of the underneath. The setup consists of a solar simulator and a building prototype with installed PV, placed inside an atmospheric wind tunnel to control upstream wind velocity. Thermography is performed using an infrared camera to monitor the surface temperature of the BIPV. The potential of an underneath cavity with various cavity heights and PV arrangement is further investigated in this paper. The outcome would be eventually useful in the development of practical guidelines for BIPV installation. Copyright © 2013 John Wiley & Sons, Ltd

Dynamical Modeling of Stochastic Wind Flow in Street Canyons

For decades, the borders of building studies were restricted up to the exterior walls. With better understanding the side effect of urbanization on human health and with excessive progress in new investigation tools, the area of building studies were enlarged to neighborhood environment where mass and air transport vividly interacts with the buildings. Unlike the indoor studies, one can emphasize the significance of the stochastic wind in outdoor studies. To verify the contribution of the wind velocity over/within street canyons/buildings many experimental and simulation studies have been conducted. These works were mainly designed to observe the relations between wind flow and natural ventilation, pollution dispersion, pedestrian thermal/wind comfort, and drag resistance of the buildings. All these efforts continuously proved the significance of vortices caused by upstream wind as they interact on physical phenomena within the street canyons. The main drawback of these studies is however not attributed to the extracted results, but the way in which wind, an inherently transient and stochastic phenomenon, is presented by a steady state consideration as applied through the boundary conditions. For example, upstream wind in terms of direction and magnitude is widely considered as a constant and steady state profile induced through the street canyons. Nonetheless, due to the transient and stochastic nature of encountered wind in the urban environment, these assumptions fail to fully capture the physics of the problems. Thus, the main point of concern in street canyon modeling should be described as constant and steady state reflection of stochastic and transient phenomena through the boundary conditions. Although there are existing studies related to the application of transient boundary conditions, a practical procedure to model the dynamic and stochastic behavior of wind direction is barely addressed in literature. In light of the lack of stochastic wind modeling, this study intends to introduce for the first time an approach in order to generate dynamic wind. This is followed by a brief discussion of existing approaches in urban-scale wind modeling and their major shortcomings. For this purpose, a computational Fluid dynamics (CFD) model is developed considering a novel cylindrical computational domain. An analytical and a parametric study have been also conducted to obtain proper sizes for the domain geometries. Moreover, the advantages of the proposed model comparing to the traditional approaches are depicted using a case study of cuboids’ array. It is also more feasible to investigate the dynamic impact of pollution, moisture, and air transport on pedestrian comfort and health. It should be mentioned that this technique can be applied and expanded to other wind engineering subject areas

Deformation and damage due to drying-induced salt crystallization in porous limestone

ACLInternational audienceThis paper presents a computational model coupling heat, water and salt ion transport, salt crystallization, deformation and damage in porous materials. We focus on crystallization-induced damage. The theory of poromechanics is employed to relate stress, induced by crystallization processes or hygro-thermal origin, to the material's mechanical response. A non-local formulation is developed to describe the crystallization kinetics. The model performance is illustrated by simulating the damage caused by sodium chloride crystallization in a porous limestone. The results are compared with experimental observations based on neutron and X-ray imaging. The simulation results suggest that the crystallization kinetics in porous materials have to be accurately understood in order to be able to control salt damage. The results show that the effective stress caused by salt crystallization depends not only on the crystallization pressure but also on the amount of salt crystals, which is determined by the spreading of crystals in the porous material and the crystallization kinetics. © 2013 Elsevier Ltd