Experimental characterization of turbulent superstructures in large aspect ratio Rayleigh-Bénard convection

Die Untersuchung von thermisch induzierten Strömungen hat in den letzten Jahrzehnten eine enorme Aufmerksamkeit erfahren, um geophysikalische und astrophysikalische Systeme besser verstehen zu können. Hierfür hat sich das sogenannte Rayleigh-Bénard Modell als eines der meist untersuchten fluidmechanischen Systeme etabliert, da es die kaum abzubildende Komplexität von natürlichen Systemen in ihrer Mannigfaltigkeit auf ein Fluidvolumen reduziert, welches von unten isotherm erwärmt und von oben isotherm gekühlt wird. Trotz dieser Reduzierung an Komplexität können mit diesem Modell die wesentlichen Eigenschaften von thermischer Konvektion abgebildet werden. Die Strömung in einem solchen System, welche als Rayleigh-Bénard Konvektion bekannt ist, weist Strömungsstrukturen auf unterschiedlichsten Längenskalen auf. In der vorliegenden Arbeit werden die sogenannten Superstrukturen untersucht. Diese sich in horizontaler Richtung weit erstreckenden Strukturen treten in Erscheinung, wenn die horizontale Dimension der Fluidschicht wesentlich größer als der vertikale Abstand zwischen der erwärmten Unterseite und der gekühlten Oberseite ist. Da die Superstrukturen bisher im Wesentlichen anhand von numerischen Simulationen untersucht wurden, soll in dieser Arbeit erstmals vom experimentellen Standpunkt ein besserer Eindruck gewonnen werden. Zur Untersuchung der Superstrukturen wird eine Rayleigh-Bénard Zelle mit den Abmessungen l × w × h = 700 mm × 700 mm × 28 mm und folglich mit einem Aspektverhältnis von Γ = l/h =25 aufgebaut. Bei allen Experimenten wird diese Zelle mit Wasser als Arbeitsmedium befüllt. Um die Rayleigh-Bénard Strömung zu untersuchen, werden thermochrome Flüssigkristalle als Impfpartikel der Strömung beigefügt, sodass simultane Messungen des Temperatur- und Geschwindigkeitsfeldes in horizontalen Ebenen der Zelle vorgenommen werden können. Während das Geschwindigkeitsfeld mittels der Bewegung der thermochromen Flüssigkristalle im zeitlichen Verlauf anhand der etablierten Partikelbild-Geschwindigkeitsmessung (Particle Image Velocimetry) bestimmt wird, basiert die Messung des Temperaturfelds auf der farblichen Erscheinung der thermochromen Flüssigkristalle, welche unter der Beleuchtung von Weißlicht temperaturabhängig ist. Im Hinblick auf die genaue Bestimmung der Temperatur wird diese Messtechnik umfänglich charakterisiert, wobei die wesentlichen Einflussfaktoren auf die Messunsicherheit diskutiert werden. Da die Untersuchung der turbulenten Superstrukturen mittels dieser Messtechnik den optischen Zugang zur flachen Rayleigh-Bénard Zelle erfordert, ist der Aufbau speziell konstruiert und ermöglicht die Beobachtung der Strömung durch eine transparente Kühlplatte. Der Entwicklungsprozess wird in der Arbeit aus ingenieurstechnischer Sicht genauestens erklärt. Bei der Auswertung der Messungen kommen die großskaligen Strukturen sowohl im Temperaturfeld als auch im Geschwindigkeitsfeld zum Vorschein. Die Größe der Superstrukturen wird untersucht in Abhängigkeit der Rayleigh-Zahl Ra, welche den thermischen Antrieb der Strömung beschreibt und in der vorliegenden Arbeit etwa im Bereich 2 × 10^5 ≤ Ra ≤ 2 × 10^6 variiert wird. Auf der Basis dieser Messungen, welche jeweils einen großen Zeitraum abdecken, wird das Langzeitverhalten der Superstrukturen analysiert, womit deren langsam voranschreitende Umstrukturierung gezeigt wird. Da die kombinierte Messung des Temperatur- und Geschwindigkeitsfeldes in den horizontalen Messebenen die Berechnung des lokalen Wärmestroms ermöglicht, wird diese Möglichkeit ebenfalls demonstriert. Um die experimentellen Ergebnisse dieser Arbeit bewerten zu können, werden jene mit den Resultaten aus numerischen Simulationen verglichen.Aiming at a better understanding of geophysical and astrophysical settings, the investigation of thermally driven fluid flows has attracted great attention in the last decades. In this context, the so-called Rayleigh-Bénard model has established as one of the most studied fluid-mechanical systems, since this reduces the hardly representable complexity of the natural environment with its enormous diversity to a fluid volume, which is uniformly heated from below and cooled from above. Despite this reduction of the complexity, this model is capable of representing the main characteristics of thermal convection. The flow in such a system, well-known as the Rayleigh-Bénard convection, exhibits flow structures on a large range of length scales. In this work, the so-called turbulent superstructures are studied. These horizontally stretched structures, which appear when the fluid layer has a much larger horizontal extent compared to the vertical distance between the hot and cold boundary, have mainly been investigated with numerical simulations so far. Therefore, the aim of the present work is to get a better impression of the turbulent superstructures from the experimental point of view for the first time. For the investigation of the superstructures a Rayleigh-Bénard cell with dimensions of l × w × h = 700 mm × 700 mm × 28 mm, thus having an aspect ratio of Γ = l/h =25, is set up. Here, water is used as the working fluid in the cell for all the experiments. In order to analyze the Rayleigh-Bénard flow, thermochromic liquid crystals are applied as tracer particles in the flow, which allows to perform simultaneous measurements of the temperature and velocity field in horizontal planes of the cell. While the velocity field is measured via the temporal displacement of the thermochromic liquid crystals using the established Particle Image Velocimetry, the temperature field is determined by evaluating their color shade, which depends on the temperature upon illumination with white light. With regard to the accurate determination of the temperature, this measuring technique is extensively characterized and the main influencing factors on the measurement uncertainty are discussed. Since the investigation of the turbulent superstructures with this optical measuring technique requires optical access to the flat Rayleigh-Bénard cell, the setup is specially designed and allows to observe the flow through a transparent cooling plate. The design process from the engineering point of view is thoroughly explained. In the evaluation of the measurements the large-scale structures are uncovered in both the temperature and the velocity field. The size of the turbulent superstructures is investigated in dependency of the Rayleigh number Ra, which characterizes the thermal driving force of the flow and is here approximately varied in the range 2 × 10^5 ≤ Ra ≤ 2 × 10^6. On the basis of the measurements conducted over extended time intervals, the long-term behavior of the superstructures is analyzed, thereby demonstrating their gradual reorganization. Since the combined measurement of the temperature and of the velocity field in the horizontal planes enables to estimate the local heat flux, this possibility is presented as well. In order to assess the experimental results of this work, these are compared to the outcomes of numerical simulations

On the application of neural networks for temperature field measurements using thermochromic liquid crystals

This study presents an investigation regarding the applicability of neural networks for temperature measurements using thermochromic liquid crystals (TLCs) and discusses advantages as well as disadvantages of common calibration approaches. For the characterization of the measurement technique, the dependency of the color of the TLCs on the temperature as well as on the observation angle and, therefore, on the position within the field of view of a color camera is analyzed in detail. In order to consider the influence of the position within the field of view on the color, neural networks are applied for the calibration of the temperature measurements. In particular, the focus of this study is on analysis of the error of temperature measurement for different network configurations as well as training methods, yielding a mean absolute deviation and a mean standard deviation in the range of 0.1 K for instantaneous measurements. On the basis of a comparison of this standard deviation to that of two further calibration approaches, it is shown that neural networks are suited for temperature measurements via the color of TLCs. Finally, the applicability of this measurement technique is illustrated at an exemplary temperature measurement in a horizontal plane of a Rayleigh-Bénard cell with large aspect ratio, which clearly shows the emergence of convective flow patterns by means of the temperature field

Long-time experimental investigation of turbulent superstructures in Rayleigh-Bénard convection by noninvasive simultaneous measurements of temperature and velocity fields

Large-scale mean patterns in Rayleigh-Bénard convection, also referred to as turbulent superstructures, have mainly been studied by means of numerical simulations so far, but experimental investigations are still rare. However, the analysis of turbulent superstructures, which are of great importance due to their effect on the local transport of heat and momentum, require both numerical and experimental data. Therefore, within the scope of this study measurements were performed in the horizontal mid plane and in a horizontal plane closer to the top of a Rayleigh-Bénard cell with an aspect ratio of [Gamma]=l/h=25, thereby showing the initial formation of turbulent superstructures and their long-time rearrangement. The turbulent superstructures are investigated experimentally by noninvasive simultaneous measurements of temperature and velocity fields, using the color signal of thermochromic liquid crystals (TLCs) for the evaluation of the temperature and their temporal displacement for the determination of all three velocity components in the measurement planes via stereoscopic particle image velocimetry (stereo-PIV). Applying this measuring technique it is demonstrated that the time-averaging of instantaneous temperature and velocity fields uncovers the turbulent superstructures in both fields. Furthermore, the combination of the temperature and velocity data is used to characterize the local heat flux quantified by the local Nusselt number, which confirms that the turbulent superstructures strongly enhance the heat transfer in Rayleigh-Bénard convection

How does the use of Experiential Futures as Design Help Facilitate Difficult Conversations?

This research project studies the intersection of design and futures to uncover how both practices amplify each other’s potential to facilitate difficult conversations. This project is part horizonscan of current design and futures practices to better understand the professional landscape and part evocative autoethnographic study of my personal design practices throughout the last decade to evaluate a series of tools and frameworks could be useful to both fields by selecting those that intuitively made sense . Through my immersion into the field of futures and foresight as a professional designer, I wanted to first understand how designers and futurists are adopting each other’s practices and at what level of maturity they are at with the adoption and integration of the shared knowledge. Both practices have similar goals, adoption barriers, and areas for malpractice. At the intersection of design and futures both disciplines seem diluted. On one side designers have little understanding of futures studies but great creative capacity to do design and run ethnographic research with humans. On the other side, futurists that have adopted design practices lack the rigorous understanding of design and the complexity of the process, and they therefore run shallow design sprints with unknown outcomes. Many futurists use design as a medium of delivery for experiential futures and scenarios that they have built when they could be leveraging design’s ability to find problems/solutions, unearthing deep human insight from personas of the futures, and using scenarios as the starting point to understand problems and constraints that may emerge in the futures by mapping out context specific futures

Combined particle image velocimetry and thermometry of turbulent superstructures in thermal convection

Turbulent superstructures in horizontally extended three-dimensional Rayleigh-B\'enard convection flows are investigated in controlled laboratory experiments in water at Prandtl number $Pr = 7$. A Rayleigh-B\'enard cell with square cross-section, aspect ratio $\Gamma = l/h = 25$, side length $l$ and height $h$ is used. Three different Rayleigh numbers in the range $10^5 < Ra < 10^6$ are considered. The cell is accessible optically, such that thermochromic liquid crystals can be seeded as tracer particles to monitor simultaneously temperature and velocity fields in a large section of the horizontal mid-plane for long time periods of up to 6 h, corresponding to approximately $10^4$ convective free-fall time units. The joint application of stereoscopic particle image velocimetry and thermometry opens the possibility to assess the local convective heat flux fields in the bulk of the convection cell and thus to analyse the characteristic large-scale transport patterns in the flow. A direct comparison with existing direct numerical simulation data in the same parameter range of $Pr, Ra$ and $\Gamma$ reveals the same superstructure patterns and global turbulent heat transfer scaling $Nu(Ra)$. Slight quantitative differences can be traced back to violations of the isothermal boundary condition at the extended water-cooled glass plate at the top. The characteristic scales of the patterns fall into the same size range, but are systematically larger. It is confirmed experimentally that the superstructure patterns are an important backbone of the heat transfer. The present experiments enable, furthermore, the study of the gradual evolution of the large-scale patterns in time, which is challenging in simulations of large-aspect-ratio turbulent convection.Comment: 25 pages, 11 figure

Delay Sensitivity Classification of Cloud Gaming Content

Cloud Gaming is an emerging service that catches growing interest in the research community as well as industry. While the paradigm shift from a game execution on clients to streaming games from the cloud offers a variety of benefits, the new services also require a highly reliable and low latency network to achieve a satisfying Quality of Experience (QoE) for its users. Using a cloud gaming service with high latency would harm the interaction of the user with the game, leading to a decrease in playing performance and thus frustration of players. However, the negative effect of delay on gaming QoE depends strongly on the game content. At a certain level of delay, a slow-paced card game is typically not as delay sensitive as a shooting game. For optimal resource allocation and quality estimation, it is highly important for cloud providers, game developers, and network planners to consider the impact of the game content. This paper contributes to a better understanding of the delay impact on QoE for cloud gaming applications by identifying game characteristics influencing the delay perception of users. In addition, an expert evaluation methodology to quantify these characteristics, as well as a delay sensitivity classification based on a decision tree is presented. The ratings of 14 experts for the quantification indicated an excellent level of agreement which demonstrates the reliability of the proposed method. Additionally, the decision tree reached an accuracy of 86.6 % on determining the delay sensitivity classes which were derived from a large dataset of subjective input quality ratings during a series of experiments.Comment: Accepted In International Workshop on Immersive Mixed and Virtual Environment Systems 2020. ACM, Istanbul, Turke

Real-time affect detection in virtual reality: a technique based on a three-dimensional model of affect and EEG signals

This manuscript explores the development of a technique for detecting the affective states of Virtual Reality (VR) users in real-time. The technique was tested with data from an experiment where 18 participants observed 16 videos with emotional content inside a VR home theater, while their electroencephalography (EEG) signals were recorded. Participants evaluated their affective response toward the videos in terms of a three-dimensional model of affect. Two variants of the technique were analyzed. The difference between both variants was the method used for feature selection. In the first variant, features extracted from the EEG signals were selected using Linear Mixed-Effects (LME) models. In the second variant, features were selected using Recursive Feature Elimination with Cross Validation (RFECV). Random forest was used in both variants to build the classification models. Accuracy, precision, recall and F1 scores were obtained by cross-validation. An ANOVA was conducted to compare the accuracy of the models built in each variant. The results indicate that the feature selection method does not have a significant effect on the accuracy of the classification models. Therefore, both variations (LME and RFECV) seem equally reliable for detecting affective states of VR users. The mean accuracy of the classification models was between 87% and 93%

Influence of the illumination spectrum and observation angle on temperature measurements using thermochromic liquid crystals

As measurements of velocity and temperature fields are of paramount importance for analyzing heat transfer problems, the development and characterization of measuring techniques is an ongoing challenge. In this respect, optical measurements have become a powerful tool, as both quantities can be measured noninvasively. For instance, combining particle image velocimetry (PIV) and particle image thermometry (PIT) using thermochromic liquid crystals (TLCs) as tracer particles allows for a simultaneous measurement of velocity and temperature fields with low uncertainty. However, the temperature dependency of the color appearance of TLCs, which is used for the temperature measurements, is affected by several experimental parameters. In particular, the spectrum of the white light source, necessary for the illumination of TLCs, shows a greater influence on the range of color play with temperature of TLCs. Therefore, two different spectral distributions of the white light illumination have been tested. The results clearly indicate that a spectrum with reduced intensities in the blue range and increased intensities in the red range leads to a higher sensitivity for temperature measurements, which decreases the measurement uncertainty. Furthermore, the influence of the angle between illumination and observation of TLCs has been studied in detail. It is shown that the temperature measurement range of TLCs drastically decreases with an increasing angle between illumination and observation. A high sensitivity is obtained for angles in between and , promising temperature measurements with a very low uncertainty within this range. Finally, a new calibration approach for temperature measurements via the color of TLCs is presented. Based on linear interpolation of the temperature dependent value of hue, uncertainties in the range of 0.1 K are possible, offering the possibility to measure very small temperature differences. The potential of the developed approach is shown at the example of simultaneous measurements of velocity and temperature fields in Rayleigh–Bénard convection