Thomas K. Thiis

Large scale studies of development of snowdrifts around buildings

Abstract Snowdrift formation around buildings should be considered in snowy and windy areas to ensure proper accessibility and safety of buildings. Tools to model snow accumulation include wind tunnels and numerical simulation. These tools need to be carefully validated before they can be used for planning purposes. Important understanding for use in such validation is gathered in outdoor full-scale experiments. This study presents snowdrift accumulation data for two building geometries in two wind episodes. The effect of different wind velocities and snowdrifting intensities is documented. The rate of deposition of a leeward snowdrift was sensitive to the vertical snow concentration profile in that considerably more snow was deposited in such snowdrifts during wind episodes with higher transport rate from suspension mode. The deposition rate of lateral and upwind snowdrifts seems to be dependent on the presence of already deposited snowdrifts. Existing snowdrifts seem to smoothen the wind pattern around a building increasing the shear stress on the ground and decreasing the deposition rate.

Model for Snow Loading on Gable Roofs

AbstractA particularly large database containing simultaneous measurements of roof and ground snow loads were analyzed for the purpose of establishing a simple empirical model for gable roof snow loads. Due to the nature of the database, the model was developed specifically for unheated structures with nonslippery roof surfaces. As such, it is expected to provide conservative load estimates for heated structures and structures with slippery roof surfaces. The model results are compared to corresponding provisions for balanced and unbalanced snow loads in various United States and European codes and load standards.

Large-scale measurements of snowdrifts around flat-roofed and single-pitch-roofed buildings

Abstract Snowdrifts around buildings can cause serious problems when formed on undesirable places. The formation of snowdrifts is highly connected to the wind pattern around the building, and the wind pattern is again dependent on the building design. The snowdrifts around three different model buildings are investigated. The buildings have the same floor space, 2500×2500 mm but different rooftops. The buildings were placed in a valley 3 km wide and 20 km long on Spitsbergen, Norway. The wind in this valley is blowing in the same direction approximately 90% of the time during winter and the site is well suited for studies of snowdrifts and snowdrifting. The wind direction and vertical wind profile is measured. The snowdrift height around each building were surveyed in approximately 200 points and contour maps produced. The different roof designs proved to have a significant influence on the size of the snowdrifts produced. A flat roof gives larger snowdrifts than a single pitch roof, and a single pitch roof tilted upstream the wind gives a larger snowdrift than a single pitch roof tilted downstream the wind. The results can be used to improve building design in areas with snowdrifting conditions.

Near Infrared Hyperspectral Imaging in Transmission Mode: Assessing the Weathering of Thin Wood Samples:

Untreated wooden surfaces degrade when exposed to natural weathering. In this study thin wood samples were studied for weather degradation effects utilising a hyperspectral camera in the near infrared wavelength range in transmission mode. Several sets of samples were exposed outdoors for time intervals from 0 days to 21 days, and one set of samples was exposed to ultraviolet (UV) radiation in a laboratory chamber. Spectra of earlywood and latewood were extracted from the hyperspectral image cubes using a principal component analysis-based masking algorithm. The degradation was modelled as a function of UV solar radiation with four regression techniques, partial least squares, principal component regression, Ridge regression and Tikhonov regression. It was found that all the techniques yielded robust prediction models on this dataset. The result from the study is a first step towards a weather dose model determined by temperature and moisture content on the wooden surface in addition to the solar radiation.

Hyperspectral imaging of weathered wood samples in transmission mode

Surfaces are the most vulnerable part of structures due to continuous exposure to variable climatic conditions. Even if weathering mainly affects the aestethic appearance of wooden facades it may lead to more advanced degradation such as wood cracking, checks and consequently penetration of the wood-decaying agents into the material. The goal of this research was to investigate the kinetics of the degradation rate of wooden samples. The experimental specimens were weathered at 15 locations in Europe for a period of 1 month. Hyperspectal imaging was used for evaluation of earlywood and latewood degradation. Two approaches for image analysis were presented highlighting their advantages and constraints regarding the evaluation of weathered samples. The proposed technique was able to scrutinise differences in degradation of earlywood and latewood, therefore provided new understanding for the kinetic of the weathering process.

Hyperspectral near Infrared Imaging of Wooden Surfaces Performed Outdoors and Indoors

Hyperspectral near infrared imaging has been applied in a field study of fungal growth on a variety of wood substrates exposed in an outdoor environment over a six-month period. This study was performed as a follow up to a hyperspectral examination study of fungal growth on wood surfaces in a laboratory setting. Hyperspectral measurements were carried out both outdoors and indoors in order to explore the influence of the different light conditions. Segmentation of the mould growth on the wood surfaces was carried out using principal component analysis, spectral angle mapper and partial least squares-discriminant analysis. Growth curves showing the fungal growth over time were obtained for all the samples from the measurements performed outdoors. However, there are some challenges connected to studies of wood surfaces due to structures caused by growth rings, knots and sometimes cracks. These wood properties will cause a great variation in the spectra from the wood and also cause natural variation in the fungal growth. Determining specific classes in a classification model such as partial least squares-discriminant analysis is proposed as a way to overcome these issues. Moreover, the wood substrates exposed in an outdoor environment will have a colour change due to photodegradation of lignin, wetting/leaching of the upper layer of the wood surface and growth of a variety of wood discolouring fungi. Hyperspectral technology is a promising technique to study wood properties and we plan to carry out a study to be able to separate and model the different effects on colour degradation on wood surfaces.

Investigating Instantaneous Wind-Driven Infiltration Rates using the CO2 Concentration Decay Method

Abstract Carbon dioxide has already been recognized as a potential tracer gas for estimating the mean air exchange rates of a room or building. The wind direction and mean wind velocity have also been identified as critical factors that affect the air infiltration. In this paper, the indoor CO2 concentration has been logged at three specific points in an office room for seven selected measurement-periods. The decay method was used to estimate the infiltration rates (ac/h). In parallel, an ultrasonic anemometer was used outdoors for monitoring wind characteristics such as the direction angle and the instantaneous velocity components. The results of the calculated ac/h varied from 0.32 h-1 to 0.75 h-1 from measurement to measurement and thus an investigation was carried out from the perspective of unsteady wind conditions. The wind direction angle and the relevant velocity components seem inadequate to entirely explain the differences in ac/h and thus the wind turbulence intensity was calculated and a spectral analysis of the wind measurements was applied. The corresponding power spectra Syy(f) were correlated to the mean ac/h of the room, giving a better understanding of wind-driven infiltration and depicting the role of the wind fluctuation frequency. In addition, a hypothesis of using the indoor spatial distribution of CO2 concentration as an airflow pattern tracer is presented respective to the location of the leakages. Finally, the local CO2 fluctuations are discussed and the respective standard deviations are presented as indicators of leakage detection.

Urban surfaces studied by VIS/NIR imaging from UAV: possibilities and limitations

The present research approach aims at analyzing the relation between material properties and their thermal behavior using airborne multispectral imaging in VIS/NIR and IR with sensors mounted on Unmanned Aerial Vehicle (UAV). As a follow up to a pilot study from spring 2016, a survey including several flights spanned over three days, from early morning before sunrise until late evening after sunset, was carried out in Athens in June 2017. The camera specifications for the survey in 2017 were different than the ones used in 2016. The performance of the cameras was evaluated, taking into account atmospheric correction. The images have been combined to form maps of surface temperature distribution and material physical properties. The VIS/NIR images were used to classify the different surface materials, to compute a map of estimated albedo, and to construct a 3D-model of the area. By combining thermal maps with material classification, albedo information and local weather data, thermal material properties could be characterized for the various materials. The derived properties from this dataset yield valuable information for improved simulation models of urban climate.

CLASSIFICATION OF URBAN BLUE GREEN STRUCTURES WITH AERIAL MEASUREMENTS

The development of climate-responsive design has social and environmental impacts, as the adverse effects of climate change are particularly relevant for urban areas. Green and blue infrastructure has been identified as best practice for achieving greater urban sustainability and resilience. The climatic improvements from use of blue-green infrastructure are generally related to the ability to moderate the impacts of extreme precipitation and temperature. However, the challenges and barriers to implementation of climate adaptation plans focusing on the use of blue-green spaces have not been analysed extensively to date. The present work describes a novel methodology to measure and classify urban surface parameters, which are important for the understanding and simulation of urban flooding. An aerial survey with multispectral sensors in VIS/NIR (Visible and Near Infrared) and IR (Infrared) wavelengths on a UAV (Unmanned Airborne Vehicle) has been carried out at the campus of the Norwegian University of Life Sciences in Ås, Norway. The area covers various types of surface such as asphalt, concrete, gravel, vegetation and water. The Normalized Difference Vegetation Index (NDVI) derived from the VIS/ NIR images have been used to study the spatial distribution and physical characteristics of the vegetation. Multivariate statistical tools have further been utilized to classify the different terrain materials according to their reflectance spectral properties from the multispectral VIS/NIR/IR data cubes. These materials have been linked to roughness and infiltration properties that are commonly used in water analysis simulation tools. Photogrammetry was applied to compute the Digital Surface Map (DSM), which was used to determine drainage lines and water accumulation areas in the surveyed area. The applied method provides data with high spatial resolution that can simplify and improve simulation of urban flooding.

Assessment and monitoring of aesthetic appearance of building biomaterials during the service life

Bio-based building materials offer a wide range of outlooks, from traditional rustic to modern design products. Recent development in the science of materials significantly improves their functional performance. However, when considering the use of bio-materials in outdoor environments, materials will deteriorate due to processes like weathering, oxidation, biodegradation, wear, and decay. Consequentially, biomaterials may lose visual appeal, leading to a perceived need for replacement even if the material is far from reaching functional failure. Visual assessment is the most direct method for evaluation of the aesthetic appearance of materials. However, it possesses a high degree of subjectivity when performed by an untrained person. On the contrary, measurement of surface properties with dedicated sensors provides objective values that might be related to the current state of the material in use. Recent developments in field of optics and electronics opens a new possibility to perform measurements in-situ. Colour-, gloss-, or spectro-photo-meters allow non-destructive measurements without particular sample preparation. Since all of the above techniques provide complementary information, the multi-sensor approach is more frequently suggested for applied research. The material state can be assessed regularly during service life. In this case, such measurement turns into monitoring. The paper illustrates examples of assessment and monitoring of biomaterials’ degradation due to weathering. Direct implementation of various sensors is demonstrated. A proposal for the approach of combining data provided by various sensing techniques with data mining is also presented.