Establishing material hygrothermal characteristics via long-term monitoring and best-fit numerical models

Numerical models of heat and moisture transfer for performance forecast of lightweight insulating assemblies require many inputs. These include exterior climate data (i.e. temperature, relative humidity, solar radiation), interior climate data or standard models, transfer coefficients, correct initial conditions, etc. Most importantly, one needs reliable material models. A material model includes porosity, density, heat capacity, but also non-constant properties, such as thermal conductivity, vapor/liquid water diffusivity, sorption curves. These are, in general, difficult to determine, and material database entries often are incomplete, or simply non-existent. However, if one performs long…

Argon bubble flow in liquid gallium in external magnetic field

MHT-X: Offline Multiple Hypothesis Tracking with Algorithm X

An efficient and versatile implementation of offline multiple hypothesis tracking with Algorithm X for optimal association search was developed using Python. The code is intended for scientific applications that do not require online processing. Directed graph framework is used and multiple scans with progressively increasing time window width are used for edge construction for maximum likelihood trajectories. The current version of the code was developed for applications in multiphase hydrodynamics, e.g. bubble and particle tracking, and is capable of resolving object motion, merges and splits. Feasible object associations and trajectory graph edge likelihoods are determined using weak mas…

Microwave heating of water in a rectangular waveguide: Validating EOF-Library against COMSOL multiphysics and existing numerical studies

The purpose of this work is two-fold: first, we successfully validate our open-source tool EOF-Library, which efficiently couples Elmer FEM and OpenFOAM, against COMSOL Multiphysics, a commercial simulation package; second, we inform about significant discrepancies between our results and the experimental and simulation data found in a series of research papers. We reproduce the previously published numerical simulations wherein microwaves are supplied to a water domain through a rectangular waveguide, inducing convective flow. This is a conjugate problem with weakly coupled electromagnetics, heat transfer and fluid dynamics. It involves effects such as permittivity dependence on temperatur…

Comparative Assessment of Mould Growth Risk in Lightweight Insulating Assemblies Via Analysis of Hygrothermal Data and In Situ Evaluation

In order to assess the sustainability of buildings with different types of insulating assemblies in the Latvian climate, a long-term test building monitoring experiment has been underway since 2013. There are a total of five test buildings on site with roughly six years’ worth of accumulated temperature and humidity readings in the key parts of assemblies. This study is meant to quantify the mould presence in building walls, floor and ceiling by performing laboratory tests, assessing the number of colony-forming units, and comparing the results with mould risk predictions due to the isopleth model developed by Sedlbauer, using both the hygrothermal data derived from the sensors within build…

Dynamic neutron imaging of argon bubble flow in liquid gallium in external magnetic field

This paper presents detailed results of neutron imaging of argon bubble flows in a rectangular liquid gallium vessel with and without the application of external horizontal magnetic field. The developed image processing algorithm is presented and its capability to extract physical information from images of low signal-to-noise ratio is demonstrated. Bubble parameters, velocity components, trajectories and relevant statistics were computed and analysed. A simpler version of the code was applied to the output of computational fluid dynamics simulations that reproduced the experiment. This work serves to further validate the neutron radiography as a suitable method for monitoring gas bubble fl…

Dynamic mode decomposition of magnetohydrodynamic bubble chain flow in a rectangular vessel

We demonstrate the first application of dynamic mode decomposition (DMD) to bubble flow with resolved dynamic liquid/gas boundaries. Specifically, we have applied DMD to the output of numerical simulations for a system where chains of bubbles ascend through a rectangular liquid metal vessel. Flow patterns have been investigated in the vessel and bubble reference frames. We show how gas flow rate and applied magnetic affect bubble wake flow and larger-scale flow structures within the liquid metal vessel by examining the velocity field mode statistics over trajectory time and total flow time as well as the computed mode velocity fields. The results of this proof-of-concept study indicate that…

Slāpekļa vakances centru relaksācijas laika noteikšana mākslīgā dimanta kristālā

Zināms, ka spinu gareniskās relaksācijas laiks T_1 izteikti korelē ar NV centru koncenrtāciju mērījumu vietā, tomēr nav skaidra precīza atkarība. Tika veikti divi neatkarīgie eksperimenti (Latvijā, LU FMF Lāzeru centrā un ASV, Berklijā), kuru rezultātā iegūtie dati ir pretrunā. Darba mērķis ir noteikt slāpekļa-vakances jeb NV centru T_1 atkarību no to lokālas koncentrācijas mākslīgā dimanta kristālā. Tika izveidota jaunā iekārta lāzeru inducētās fluorescences (LIF) iegūšanai un detektēšanai, kurā pirmo reizi Lāzeru Centra praksē tika izmantots augstās izšķirtspējas eļļas imersijas fluorescences objektīvs. Izmantojot optiski detektētās magnētiskās rezonanses (ODMR) spektrus un programmatūru …

Resolving gas bubbles ascending in liquid metal from low-SNR neutron radiography images

We demonstrate a new image processing methodology for resolving gas bubbles travelling through liquid metal from dynamic neutron radiography images with an intrinsically low signal-to-noise ratio. Image pre-processing, denoising and bubble segmentation are described in detail, with practical recommendations. Experimental validation is presented—stationary and moving reference bodies with neutron-transparent cavities are radiographed with imaging conditions representative of the cases with bubbles in liquid metal. The new methods are applied to our experimental data from previous and recent imaging campaigns, and the performance of the methods proposed in this paper is compared against our p…

Modelling of thermal stresses in bearing steel structure generated by electrical current impulses

This work is the study of one particular candidate for white etching crack (WEC) initiation mechanism in wind turbine gearbox bearings: discharge current impulses flowing through bearing steel with associated thermal stresses and material fatigue. Using data/results from previously published works, the authors develop a series of models that are utilized to simulate these processes under various conditions/local microstructure configurations, as well as to verify the results of the previous numerical studies. Presented models show that the resulting stresses are several orders of magnitude below the fatigue limit/yield strength for the parameters used herein. Results and analysis of models …

Phase boundary dynamics of bubble flow in a thick liquid metal layer under an applied magnetic field

Dynamic neutron radiography is used to observe the effect of a transverse magnetic field on argon bubbles rising through a thick layer of liquid gallium without interactions with the container walls.

Balto kodināšanas plaisu intensīvi slogotos gultņos elektromagnētiskās izcelsmes hipotēzes pārbaude

Šajā kvalifikācijas darbā tiek aprakstīti analītiskie un skaitliskie modeļi, kuri tika izveidoti, lai aprakstītu elektromagnētiskus procesus, kuri saskaņā ar 2013./14. gados izvirzītu hipotēzi, norīt vēja turbīnu ātrumkārbju gultņos, un varētu būt atbildīgi par anomāli ātru gultņu sabrukšanu, pie kam gultņi iet bojā diezgan īpatnējā veidā – to šķērsgriezumu analīze parāda, ka gultņu tēraudā izveidojas tā saucāmo balto kodināšanas plaisu tīkli. Izpēti motivē strauji augoša vēja enerģētikas industrija, kas cieš no šis parādības visvairāk un visizteiktāk. Nopietni pētījumi šajā jomā pasaulē ilgst jau vairāk kā 20 gadus, bet baltās kodināšanas plaisu izveides mehānismu fizikālā aina joprojām na…

Design Optimization Automation for Luminaire Reflectors Using COMSOL Multiphysics and Performance Comparison Against Zemax Opticstudio

This work showcases the complete design pipeline based on COMSOL Multiphysics for one of the luminaire models manufactured by Vizulo. Generation of optimization targets, ray tracing models and utilized optimization algorithms are described. The authors also evaluate the performance of COMSOL against Zemax OpticStudio Premium, a specialized optics design suite widely used by the optics industry. Out-of-the-box version of both packages are tested: ray tracing and optimization performance are compared. It is found that, while OpticStudio' ray tracer is by far superior, OpticStudio is greatly outperformed by COMSOL in optimization tasks for considered problems. Other aspects of both packages ar…

Numerical models for long-term performance assessment of lightweight insulating assemblies

Abstract This paper presents the results of numerical modelling of hygrothermal processes in test buildings situated in the Botanical garden of the University of Latvia. Long-term performance of wall, floor and ceiling insulating assemblies for five different buildings was simulated using WUFI Pro 6.3. On the experimental side, measurements of temperature and relative humidity at key points of assembles have been accumulated over a period of roughly six years and material samples were taken from building envelope materials for biological analysis. We found that our models rather successfully reproduce the experientially observed temperature and relative humidity dynamics. In the process, we…