In this study, an attempt is made to obtain convergent and stable solutions of the K-E turbulence model equations for non-reacting turbulent flows over an isothermal solid surface in regression. A physics based mathematical model is used to describe the flow and temperature field over the moving surface. The flow is assumed to be two-dimensional, unsteady, incompressible with boundary layer approximations. Parabolized form of the standard K-E equations is adopted to simulate turbulence in the flow. Regression of the solid surface causes the bounds of the solution domain to change with time, therefore a coordinate transformation is used in the vertical direction. The computational domain with fixed boundaries is discretized using an orthogonal grid system where a coordinate stretching is used in the vertical direction. A second order accurate, explicit finite difference technique is used for discretization of the governing equations. The final set of discretized equations is then solved using a solution algorithm specifically developed for this study. The verification of the solution algorithm includes a grid independence study, time increment study, and a comparison of the steady state results for the laminar and the turbulent flow cases. Finally, a parametric study is conducted using the proposed solution algorithm to test the stability of the numerical results for different Reynolds numbers, regression rates, and surface temperatures. It is concluded that the proposed numerical solution algorithm is capable of providing convergent and stable solutions of the two-equation turbulence model.

An experimental and numerical investigation of heat transfer enhancement in a three dimensional channel using wall mounted porous baffles was conducted. The module average heat transfer coefficients were measured in a uniformly heated rectangular channel with staggered positioned porous baffles. A numerical procedure was implemented, in conjunction with a commercially available Navier-Stokes solver, to model the turbulent flow in porous media. The Brinkman-Forchheimer-Extended Darcy model was used for modeling fluid flow through the porous baffles. Conventional, oneequation, and two-equation models were used for heat transfer modeling. The accuracy and characteristics of each model were investigated and discussed. The results were compared with experimental data. Baffles were mounted alternatively on the top and bottom walls. Heat transfer coefficients and pressure loss for periodically fully developed flow and heat transfer were obtained for different pore densities (10, 20, and 40 pores per inch (PPI)) with two different baffle heights ( / h h B D = 1/3 and 2/3), and two baffle thicknesses ( / t h B D = 1/3 and 1/12). The Reynolds number (Re) was varied from 20,000 to 50,000. To compare the effect of foam metal baffles, the data for conventional solid-type baffles was obtained for ( / t h B D =1/3). The maximum uncertainties associated with the module Nusselt number and friction factor were 5.8% and 4.3%, respectively. The experimental procedure was validated by comparing the data for the straight channel without baffles ( / h h B D = 0) with those in the literature. The use of porous baffles resulted in heat transfer enhancement as high as 300% compared to heat transfer in straight channels without baffles. However, the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Correlation equations were developed for the heat transfer enhancement ratio and the heat transfer enhancement per unit increase in pumping power in terms of Reynolds number. The conventional theoretical model, the dispersion conductivity model, and the modified two-phase model using the local thermal non-equilibrium theory were considered. The results from each model were compared against the experimental data, and compared to each other to investigate the efficiency of each model. Also, the characteristics of each model were discussed.

Study of passive dispersion of a neutral scalar in turbulentflows is highly important due to its numerous applications in the areas of turbulent flow visualization, turbulent heat transfer and transport of pollutants and other substances in the environment. Over the past few decades, many analytical, numerical, and experimental studies have been conducted on this topic to obtain a better understanding of the physical process. In the present work, Large Eddy Simulations (LES) of scalar dispersion in turbulent flow over smooth and rough channels is conducted to contribute to the further understanding of the relation between the turbulent velocity field and the concentration field. The LES results from the present work showed good agreement with a recently com-pleted experimental study(Rahman and Webster [2005]). An in-depth comparison of in-stantaneous concentration and velocity fields revealed thecorrelation between scalar dis-persion and coherent structures of the turbulent flow. Also,a three dimensional visual-ization of concentration iso-surfaces at different instants provided a good picture of the concentration structures transported as a result of hairpin vortices of turbulent flow, which is quite difficult to accomplish using experimental studies.

No description available.

This thesis presents a computational analysis of multi-element guard-heated sensors designed to overcome the most severe limitation of conventional thermal sensors for wall shear stress (WSS) measurement in turbulent flows –that of indirect heat conduction through the substrate. The objectives of this thesis are the study of guard-heated sensors {i} to quantify the reduction, over conventional single-element sensors, of substrate heat conduction losses and resultant errors over a range of applied shear and {ii} to examine a range of values of guard heater geometric parameters, in two common fluids, air and water and identify the best designs. Wall-turbulence, the turbulent flow in the vicinity of solid boundaries, has proved difficult to model accurately, due to the lack of accurate WSS measurements. Examples of areas of impact are drag force reduction on transport vehicles in land, sea, air, which today largely translate to reduced fossil fuel use and dependence; aerodynamic noise and control for flight and for wind energy conversion; atmospheric and oceanic transport studies for weather, climate and for pollutant transport; riverbank erosion. Constant-temperature anemometry with MEMS devices, flush-mounted hot-film thermal sensors, is non-intrusive, affords the best temporal resolution and is well-established. However, these hot-film probes suffer from unwanted heat transport to the fluid through the substrate, with errors and nonlinearity large enough to overwhelm quantitative utility of the data. Microfabrication techniques have enabled multi-element guard-heated prototypes to be fabricated. Our results show that errors in sensing-element signals, contributing to spectral distortion, are sensitive to sensor location within the guard heater. These errors can be reduced to below 1% of the signal with proper location of the sensor. Guard heating also reduces the large variation in spatial averaging due to substrate conduction. This makes them suitable for turbulent flows with a large range of fluctuations. / Graduate

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: caustics; turbulence; wave propagation; ultrasonic flow meter. Includes bibliographical references (p. 82-84).

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The distance required for flow entering a laboratory channel to become fully-developed and uniform can be substantial. Given the need to establish fully-developed uniform flow, if the length of a laboratory channel is not substantial then it likely that the flume cannot be used to conduct open-channel flow research. In laboratory studies where the channel bed is hydraulically rough, the noted problem can be lessened by minimizing the length over which the flow becomes fully-developed and uniform (Lunif). For this study it is hypothesized that if bed material with a roughness height (ks, ∆) is placed at the channel entrance and ks, ∆ is greater has the roughness height of bed material placed throughout the channel (ks, bed) then Lunif can be reduced. The length over which the larger bed material is referred to as the tripping zone length (∆). A second hypothesis for this study is that if ∆ is longer, then Lunif will be shorter. The primary objective of this study is to test the above mentioned hypothesis and to develop a relationship for predicting Lunif as a function of Δ. For this study, physical tests were performed in a rectangular Plexiglas flume with a variable slope. The flume was 6.1 m long, 45.7 cm wide, and 45.7 cm deep. The channel has smooth walls and the bed was lined with gravel (median particle size, d50 = 8.5 mm or 22 mm). Similarly tripping zone was lined with gravel of larger size (median particle size, d50 = 13 mm or 58 mm).Twelve tests were conducted for the study. For each test, longitudinal point velocity measurements (u) were made along the channel center, at five elevations (z), and at twelve longitudinal stations (x). An Acoustic Doppler Velocimeter was used to measure u. Lunif was determined by considering four indications of flow uniformity. Results indicate that having a tripping zone decreases Lunif and the magnitude of the decrease in Lunif was dependent on ∆. A function is presented for predicting Lunif /H = f (Rep, Fr, and Δ/H) where Rep is the Reynold's particle number, Fr is the Froude number and H is the flow depth.

In this PhD Thesis, we have studied several problems related to statistical properties of homogeneous, isotropic and turbulent flow of conducting fluid with direct numerical simulations (DNS) of equations of magnetohydrodynamics (MHD) and simplified shell models. The Thesis begins with an introductory overview of several statistical characterisation of fluid turbulence and MHD turbulence. Chapter-1 discusses various challenges in turbulence in MHD context. This chapter also describes specific problems that are attempted in this Thesis. The first problem, contained in Chapter 2, deals with dynamo action in a shell model for magnetohydrodynamic (MHD) turbulence. We have carried out systematic and high-resolution studies of dynamo action in a shell model over a wide range of the magnetic Prandtl number PrMand the magnetic Reynolds number ReM. Our study suggests that it is natural to think of dynamo onset as a nonequilibrium, first-order phase transition between two different turbulent, but statistically steady, states. The ratio of the magnetic and kinetic energies is a convenient order parameter for this transition. By using this order parameter, we obtain the stability diagram (or nonequilibrium phase diagram) for dynamo formation in our MHD shell model in the (PrM−1,ReM)plane. The dynamo boundary, which separates dynamo and no-dynamo regions, appears to have a fractal character. We obtain hysteretic behavior of the order parameter across this boundary and suggestions of nucleation-type phenomena. In Chapter 3 we present the results of our detailed pseudospectral direct numerical simulation (DNS) studies, with up to 10243 collocation points, of in-compressible, magnetohydrodynamic (MHD) turbulence in three dimensions, without a mean magnetic field. Our study concentrates on the dependence of various statistical properties of both decaying and statistically steady MHD turbulence on the magnetic Prandtl number PrMover a large range, namely, 0.01 ≤PrM≤10. We obtain data for a wide variety of statistical measures such as probability distribution functions (PDFs) of moduli of the vorticity and current density, the energy dissipation rates, and velocity and magnetic-field increments, energy and other spectra, velocity and magnetic-field structure func-tions, which we use to characterise intermittency, isosurfaces of quantities such as the moduli of the vorticity and current, and joint PDFs such as those of fluid and magnetic dissipation rates. Our systematic study uncovers in-teresting results that have not been noted hitherto. In particular, we find a crossover from larger intermittency in the magnetic field than in the velocity field, at large PrM, to smaller intermittency in the magnetic field than in the velocity field, at low PrM. Furthermore, a comparison of our results for decaying MHD turbulence and its forced, statistically steady analogue suggests that we have strong universality in the sense that, for a fixed value of PrM, multi-scaling exponent ratios agree, at least within our error bars, for both decaying and statistically steady homogeneous, isotropic MHD turbulence. Chapter 4 is devoted to pseudospectral direct numerical simulation (DNS) studies of the three-dimensional magnetohydrodynamic (MHD) equations (3DRFMHD) stirred by a stochastic force with zero mean and a variance ∼ k−3, where kis the wavevector, for magnetic Prandtl numbers PrM=0.1,1, and 10. We obtain velocity and magnetic-field structure functions and, from these, the multiscaling exponent ratios ζp/ζ3by using the extended self similarity (ESS) procedure. These exponent ratios lie within error bars of their counterparts for conventional three-dimensinal MHD turbulence (3DMHD). We carry out a systematic comparison of the statistical properties of 3DMHD and 3DRFMHD turbulence by examining various probability distribution functions (PDFs), joint PDFs, and isosurfaces of quantities such as the moduli of the vorticity and the cur-rent density. In Chapter 5 we present a study of the multiscaling of time-depedent velocity and magnetic-field structure functions in homogeneous, isotropic fluid turbulence. We first present a generalisation for magnetohydrodynamics of the formalisn that has been developed for analogous studies of time-dependent structure functions in fluid turbulence. We then carry out a detailed numerical study of such time-dependent structure functions in a shell model for MHD turbulence. From this study we extract both eqaul-time and dynamic multiscaling exponents; however, we have not so far been able to come up with the MHD analogues of the linear bridge relations that relate equal-time and dynamic multiscaling exponents in fluid turbulence; indeed, it is not clear whether such bridge relations should exist for MHD turbulence.

Orientador: Erick de Moraes Franklin / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-27T18:01:24Z (GMT). No. of bitstreams: 1 CardonaFlorez_JorgeEduar_M.pdf: 8246209 bytes, checksum: 9860a8456aed83e182b9580704940549 (MD5) Previous issue date: 2015 / Resumo: Na natureza e na indústria é comum encontrar fluidos em movimento transportando grãos quando escoam sobre superfícies granulares. Por exemplo, na natureza o transporte de areia por rios e mares quando o fluido é um liquido, ou na formação e migração de dunas nos desertos quando o fluido é um gás, são casos bem conhecidos. Na indústria apresenta-se por exemplo, na extração e transporte de petróleo nos oleodutos, assim como em variados processos químico-industriais envol- vidos com sedimentação, ou simplesmente por utilização de matérias-primas impuras. O fenômeno apresenta-se porque o fluido em escoamento transfere parte da quantidade de movimento à superfí- cie granular, e alguns grãos são postos em movimento mantendo contato com o fundo se a força de arrasto devida ao movimento do fluido supera levemente à força de resistência devida ao peso dos grãos. Os grãos em movimento formam uma camada móvel (leito móvel ou bed-load em inglês), tornando-se instável em alguns casos, gerando rugas e/ou dunas que interagem com o escoamento. Na indústria este fenômeno está associado principalmente a problemas de perda de carga, e a tran- sientes de pressão e vazão, quando fluidos misturados com material granular são transportados por dutos. O presente trabalho faz um estudo experimental da formação e migração deste tipo de ins- tabilidades quando escoamentos turbulentos de água a diferentes velocidades são impostos sobre leitos granulares inicialmente planos de diferentes granulometrias. Os experimentos foram feitos em um canal retangular de material transparente. A evolução do leito foi filmada por uma câmera de alta definição. O comprimento de onda e a celeridade de rugas formadas foram determinados mediante o desenvolvimento de um código numérico em Matlab, utilizado para o tratamento au- tomático das imagens obtidas nos experimentos. Os resultados são comparados com os de outros trabalhos publicados na literatura / Abstract: In nature and industry, it is common to find moving fluids carrying grains, when they flow over granular surfaces. For instance, in nature it can be found in the sand transported by rivers and ocean currents, or on the formation and migration of sand dunes in deserts by air; and in industry, it is common to find them in the extraction and transport of oil through pipelines, as well as in several chemical processes involving granular material. This phenomenon arises because part of the momentum is transferred from the fluid flow to the granular surface. Therefore, if the drag force, resulting from the movement of the fluid, exceeds slightly the resistance force occasioned by the weight of the grains, some of these particles are set in motion keeping in touch with the bottom. Consequently, the moving grains form a mobile layer (bed-load) that becomes unstable in some cases producing ripples and/or dunes, which interact with the flow. In industry, this is mainly associated with problems of pressure loss in closed conduits. This work presents an experimental study on the formation and migration of ripples in a turbulent flow. In the experiments, fully-developed turbulent water flows were imposed over a granular bed of known granulometry in a transparent closed-conduit. For different grain sizes and flow rates, a high-definition camera filmed the growth and migration of bedforms. The wavelength and the celerity of bedforms were determined from the acquired images through a numerical code developed in Matlab. The obtained results are compared with other published works in the area / Mestrado / Termica e Fluidos / Mestre em Engenharia Mecânica