Experimental Investigation of Structure Function and Flow Circulatin of the Velocity Field in Turbulent Thermal Convection PDF Download
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Author: Pengfei Qi
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 69
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Book Description
Author: Pengfei Qi
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 69
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Book Description
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 495
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The turbulence structure of convective beat transfer was studied experimentally in complex three-dimensional and separating turbulent boundary layers. Three test cases whose fluid dynamics have been well documented were examined. In case 1, time- and spatially-resolved surface heat transfer was measured in the nose region of a wing-body junction formed by a wing and a flat plate. Both the wing and the endwall were heated and held at a constant uniform temperature 20 C above ambient temperature. Heat flux rates were increased up to a factor of 3 over the heat flux rates in the approach boundary layer. The rms of the heat flux fluctuations were as high as 25% of the mean heat flux in the vortex-dominated nose region. Away from the wing, upstream of the time-averaged vortex center, augmentation in the heat flux is due to increased turbulent mixing caused by large-scale unsteadiness of the vortex. Adjacent to the wing the augmentation in heat flux is due to a change in the mean velocity field. In case 2, simultaneous surface heat flux and temperature profiles were measured at 8 locations in the spatially-developing pressure-driven three-dimensional turbulent boundary layer upstream of a wing-body junction. Mean heat transfer was decreased 10% by three-dimensionality. The turbulent Prandtl number in the near-wall region of logarithmic temperature variation was approximately 0.9 at all measurement locations in the three-dimensional boundary layer. Profiles of the skewness factor of temperature fluctuations and conditionally-averaged temperature signals during a sweep/ejection event suggest that the strength of ejections of hot fluid from the near-wall region are decreased by three-dimensionality.
Author: Syed Junaid Kamran Bukhari
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Present dissertation describes the turbulent convection in horizontal layers of air and water separated by an evaporative shear-free water surface during natural convection. A detailed analysis of the hydrodynamic turbulent structure on water and air sides is presented in this study. The major portion of the research comprised of experimental investigation in addition to simulation of simultaneous airside and waterside velocity fields. The experimental measurements were made using non-intrusive state-of-the-art techniques, namely particle image velocimetry (PIV) and laser induced fluorescence (LIF) for instantaneous measurements of velocity and temperature fields, respectively. The numerical study shows organized bulk vortical motions on air and water sides. The results also show that for unstable thermal stratification the magnitude of the airside velocities is an order of magnitude higher than the waterside velocities. The experimental results show that the water and air flow fields undergo three-dimensional flow interactions which form complex flow patterns. A detailed spectral analysis has been performed to investigate the temporal and spatial scales of turbulent motions induced during natural convection. The wavenumber and frequency spectra showed the existence of two distinct power law regimes. In low wavenumber (100 rad/m
Author:
Publisher:
ISBN:
Category : Aeronautics
Languages : en
Pages : 994
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Author: Claudia Zimmermann
Publisher: Cuvillier Verlag
ISBN: 3736980612
Category : Technology & Engineering
Languages : en
Pages : 358
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Die vorliegende Arbeit beschäftigt sich mit der numerischen und experimentellen Modellierung turbulenter thermisch angetriebener Luftströmungen in unterschiedlichen Test-Konfigurationen. Diese so genannten Konvektionsströmungen stellen eines der fundamentalen Probleme der Strömungsmechanik dar. Sie ziehen ein großes wissenschaftliches Interesse auf sich, da sie in vielen Bereichen, wie z. B. in der Meteorologie oder auch in industriellen Anwendungen, eine wesentliche Rolle spielen. Vor allem vor dem Hintergrund atmosphärischer Konvektionsströmungen, die durch eine durch die Erdbewegung hervorgerufene Corioliskraft beeinflusst werden und sich zu Wirbelstürmen ausbilden können, ist ein Verständnis dieser Strömungen sehr wichtig. Die Konvektionsströmung wird in jeder einzelnen Test-Konfiguration immer zwischen zwei sich gegenüberliegenden, isotherm beheizten Wänden erzeugt. Neben der Validierung des gewählten numerischen Modells einer Large-Eddy-Simulation (LES), geben die erzielten Ergebnisse der Arbeit Aufschluss über die Strömungsdynamik, die entstehenden Turbulenzstrukturen sowie die Wärmeübertragung in der jeweiligen Test-Konfiguration. Hierbei steht vor allem die Auflösung der thermalen Wandgrenzschicht im Mittelpunkt. Hervorzuheben ist die numerische sowie auch experimentelle Untersuchung eines sogenannten Rayleigh-Bénard Problems, das unter Einfluss einer zusätzlich wirkenden Corioliskraft steht. Das hierfür realisierte Experiment in einer Großzentrifuge und die zugehörige numerische Studie geben Aufschluss über die Auswirkung der zusätzlich wirkenden Relativbeschleunigungen auf die Turbulenzproduktion und die Wärmeübertragung im Fluid.
Author: Xiaozheng Zhao
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 114
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Author: Guy Amir
Publisher:
ISBN:
Category :
Languages : en
Pages : 30
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"In this study we investigate the phenomenon of turbulent thermal diffusion of inertial and non-inertial particles in stably stratified turbulent flow. The phenomenon of turbulent thermal diffusion was predicted theoretically 20 years ago and since then was detected in laboratory and numerical experiments and confirmed by atmospheric observations. The essence of this phenomenon of turbulent thermal diffusion is the appearance of a nondiffusive mean flux of particles in the direction of the mean heat flux that results in a accumulation of particles in the vicinity of temperature minima. The theory of this effect has been previously developed only for small temperature gradients and small Stokes numbers [Elperin et al. (1996a)]. In the course of the research we conducted comprehensive measurements of spatial distributions of particles in oscillating grid turbulence and turbulent velocity distributions using Particle Image Velocimetry for various flow conditions. The temperature distribution was measured using a temperature probe that includes an array of twelve E-thermocouples." -- From the abstract.
Author: Mohsen Sayed Mohamed
Publisher:
ISBN:
Category :
Languages : en
Pages : 858
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Author: Noushin Amini
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Turbulent flows and coherent structures emerging within turbulent flow fields have been extensively studied for the past few decades and a wide variety of experimental and numerical techniques have been developed for measurement and analysis of turbulent flows. The complex nature of turbulence requires methods that can accurately estimate its highly chaotic spatial and temporal behavior. Some of the classical cases of turbulent flows with simpler geometries have been well characterized by means of the existing experimental techniques and numerical models. Nevertheless, since most turbulent fields are of complex geometries; there is an increasing interest in the study of turbulent flows through models with more complicated geometries. In this dissertation, characteristics of turbulent flows through two different facilities with complex geometries are studied applying two different experimental methods. The first study involves the investigation of turbulent impinging jets through a staggered array of rods with or without crossflow. Such flows are crucial in various engineering disciplines. This experiment aimed at modeling the coolant flow behavior and mixing phenomena within the lower plenum of a Very High Temperature Reactor (VHTR). Dynamic Particle Image Velocimetry (PIV) and Matched Index of Refraction (MIR) techniques were applied to acquire the turbulent velocity fields within the model. Some key flow features that may significantly enhance the flow mixing within the test section or actively affect some of the structural components were identified in the velocity fields. The evolution of coherent structures within the flow field is further investigated using a Snapshot Proper Orthogonal Decomposition (POD) technique. Furthermore, a comparative POD method is proposed and successfully implemented for identification of the smaller but highly influential coherent structures which may not be captured in the full-field POD analysis. The second experimental study portrays the coolant flow through the core of an annular pebble bed VHTR. The complex geometry of the core and the highly turbulent nature of the coolant flow passing through the gaps of fuel pebbles make this case quite challenging. In this experiment, a high frequency Hot Wire Anemometry (HWA) system is applied for velocity measurements and investigation of the bypass flow phenomena within the near wall gaps of the core. The velocity profiles within the gaps verify the presence of an area of increased velocity close to the outer reflector wall; however, the characteristics of the coolant flow profile is highly dependent on the gap geometry and to a less extent on the Reynolds number of the flow. The time histories of the velocity are further analyzed using a Power Spectra Density (PSD) technique to acquire information about the energy content and energy transfer between eddies of different sizes at each point within the gaps.
Author: Siu Lung Lui
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 278
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