The objective of this research is to assess the performance of two popularReynolds-averaged Navier-Stokes (RANS) models, standard k-E and k-w, andto suggest modifications to improve model predictions for high-speed flows. Numerical simulations of turbulent ow past a at plate are performed at M1 = 2:25; 7:2.The results from these two Mach number cases are compared with Direct NumericalSimulation (DNS) results from Pirozzoli et al. (2004) and experimental results fromHorstman & Owen (1975). The effect of the Boussinesq coefficient (Cu) and turbulenttransport coefficients (sigmak; sigmaE; sigma; sigma*) on the boundary layer ow is examined. Further,the performance of a new model with realizability-based correction to Cu and corresponding modifications to sigma; sigma* is examined. The modification to Cu is based oncontrolling the ratio of production to dissipation of kinetic energy (P/E=1). The firstchoice of P/E = 1 ensures that there is no accumulation of kinetic energy in stagnation or free-stream regions of the ow. The second choice of P/E= 1:6 holds underthe assumption of a homogeneous shear ow. It is observed that the new model'sperformance is similar to that of the existing RANS models, which is expected for asimple ow over a at plate. Finally, the role of turbulent Prandtl number (Prt) intemperature and density predictions is established. The results indicate that the k-wmodel's performance is better compared to that of the standard k-E model for highMach number flows. A modification to Cu must be accompanied with correspondingchanges to sigmak; sigmaE; sigma; sigma* for an accurate log-layer prediction. The results also indicate that a Prt variation is required across the boundary layer for improved temperatureand density predictions in high-speed flows.

The objective of this research is to assess the performance of two popularReynolds-averaged Navier-Stokes (RANS) models, standard k-E and k-w, andto suggest modifications to improve model predictions for high-speed flows. Numerical simulations of turbulent ow past a at plate are performed at M1 = 2:25; 7:2.The results from these two Mach number cases are compared with Direct NumericalSimulation (DNS) results from Pirozzoli et al. (2004) and experimental results fromHorstman & Owen (1975). The effect of the Boussinesq coefficient (Cu) and turbulenttransport coefficients (sigmak; sigmaE; sigma; sigma*) on the boundary layer ow is examined. Further,the performance of a new model with realizability-based correction to Cu and corresponding modifications to sigma; sigma* is examined. The modification to Cu is based oncontrolling the ratio of production to dissipation of kinetic energy (P/E=1). The firstchoice of P/E = 1 ensures that there is no accumulation of kinetic energy in stagnation or free-stream regions of the ow. The second choice of P/E= 1:6 holds underthe assumption of a homogeneous shear ow. It is observed that the new model'sperformance is similar to that of the existing RANS models, which is expected for asimple ow over a at plate. Finally, the role of turbulent Prandtl number (Prt) intemperature and density predictions is established. The results indicate that the k-wmodel's performance is better compared to that of the standard k-E model for highMach number flows. A modification to Cu must be accompanied with correspondingchanges to sigmak; sigmaE; sigma; sigma* for an accurate log-layer prediction. The results also indicate that a Prt variation is required across the boundary layer for improved temperatureand density predictions in high-speed flows.

Zugl.: Braunschweig, Techn. Univ., Diss., 2007

This thesis investigates three problems relating to thermoconvective stability in porous media. These are (i) the stability of an inclined boundary layer flow to vortex type instability, (ii) front propagation in the Darcy-B´enard problem and (iii) the onset of Prantdl-Darcy convection in a horizontal porous layer subject to a horizontal pressure gradient. The nonlinear, elliptic governing equations for the inclined boundary layer flow are discretised using finite differences and solved using an implicit, MultiGrid Full Approximation Scheme. In addition to the basic steady state three configurations are examined: (i) unforced disturbances, (ii) global forced disturbances, and (iii) leading edge forced disturbances. The unforced inclined boundary layer is shown to be convectively unstable to vortex-type instabilities. The forced vortex system is found to produce critical distances in good agreement with parabolic simulations. The speed of propagation and the pattern formed behind a propagating front in the Darcy-B´enard problem are examined using weakly nonlinear analysis and through numerical solution of the fully nonlinear governing equations for both two and three dimensional flows. The unifying theory of Ebert and van Saarloos (Ebert and van Saarloos (1998)) for pulled fronts is found to describe the behaviour well in two dimensions, but the situation in three dimensions is more complex with combinations of transverse and longitudinal rolls occurring. A linear perturbation analysis of the onset of Prandtl-Darcy convection in a horizontal porous layer subject to a horizontal pressure gradient indicates that the flow becomes more stable as the underlying flow increases, and that the wavelength of the most dangerous disturbances also increases with the strength of the underlying flow. Asymptotic analyses for small and large underlying flow and large Prandtl number are carried out and results compared to those of the linear perturbation analysis.

Invasive aquatic species such as Hydrilla verticillata (hydrilla) have become a pervasive and nearly ineradicable part of the waterways of the American south. Hydrilla is an aggressive colonizer; grows rapidly and rapidly blocks flow areas, which greatly reduces the capacity of water supply canals. Hydrilla grows up through the water column and is present throughout flow zones that are typically assumed to be free flowing and without resistance, other than that transmitted via the mechanics of a Newtonian fluid. Hydrilla is highly flexible and its morphology in the flow field is dependent on many parameters, including flow, growth stage, cross-section geometry and substrate. Traditional methods of calculating canal flow capacities assume that resistance to flow originates at the boundary of the channel. These methods typically attempt to account for vegetation by increasing resistance coefficients, which are associated with the boundary of the canal. A combination of field studies and experimentation in three separate laboratory channels was used to characterize the behavior of hydrilla and its impacts on open-channel flow. This work developed relationships for energy losses of flow within the vegetation, as well as velocity gradients within the vegetation and through the vegetation water interface to the open water. The information developed in this investigation was used to develop a model of the cross-section of flow with vegetation growing in the center of the channel. The model is based on the Prandtlvon Kármán universal-velocity-distribution law; and uses modifications to the method of calculating the hydraulic radius, to account for the increased frictional elements and reduced flow areas in the canal cross-section. A simple function was developed to estimate the remaining flow capacity in a canal as a function of the remaining unblocked area. The Prandtl-von Kármán universal-velocity-distribution law, together with modifications to the method for calculating the hydraulic radius, can improve estimates of the flow in channels impacted by submerged aquatic vegetation. The effects of a broad range of parameters can thus be represented by a relatively simple function, which was developed in this project.

A numerical study is performed to model the 2-D axisymmetric turbulent flow of supercritical water flowing upward in vertical pipes with constant wall heat fluxes, using ANSYS CFX v14.5. This study was aimed to use CFD in analyzing supercritical flow instability and heat transfer. Two types of flow instabilities are analyzed and results are compared with 1-D non-linear code solutions. Also, conditions for approximating the thresholds of instabilities based on steady-state results are assessed. It is determined that the results of instability thresholds obtained using the k-ɛ and the SST models are similar. Also the results of CFD and 1-D codes are different mainly due to the difference in the pressure drop predictions. Moreover, approximating the flow instability threshold by the conditions proposed holds true for a CFD solution. Results also indicate that Prt does not have a noticeable effect on the instability threshold for the cases examined.

Zugl.: Aachen, Techn. Hochsch., Diss., 2007.

Zugl.: Aachen, Techn. Hochsch., Diss., 2007

Ziel der Arbeit ist es, auf der Basis von Messungen ein rheologisches Materialmodell für technische Gummiwerkstoffe zu erstellen, welches deren Eigenschaften nachbildet, insbesondere vorhandene komplexe Zusammenhänge zwischen Relaxation, Erholung, Versuchsgeschwindigkeit und Belastungsamplitude. Dabei wird sich auf die Simulation von großen einfachen, aber beliebigen Scherverformungen beschränkt, woraus ein skalarwertiges Modell resultiert. Anwendung finden generalisierte Maxwell-Elemente und generalisierte kontinuierliche Prandtl-Elemente. Verschiedene Modellvarianten werden diskutiert. Es wird ein Berechnungsprogramm unter MATLAB erstellt.

[pt] Neste trabalho foi estudado o atrito em escala nanométrica em materiais atomicamente finos, como o grafeno e os dicalcogenetos de metais de transição (TMD) como o dissulfeto de molibdênio (MoS2) e o dissulfeto de tungstênio (WS2). Para tanto, foi utilizado um microscópio de força atômica (AFM), de modo que uma ponta de nitreto de silício suportada por uma haste (cantiléver) é deslizada sob a superfície do material em análise, e o atrito é quantificado de acordo com a deformação lateral da haste. Diferentes parâmetros foram alterados durante a varredura para verificar suas influências, tais como a força normal aplicada durante a varredura e a velocidade relativa em que o sistema ponta-amostra desliza. Parâmetros relativos às superfícies, como número de camadas, rugosidade e adesão também foram investigados. Com a variação da velocidade de deslizamneto, verificamos uma dependência linear com o logaritmo da velocidade, até um ponto de saturação. Esta dependência é amplificada de acordo com o número de camadas do grafeno, de modo que numa monocamada essa inclinação é mais acentuada do que nas demais camadas. Usando o modelo de Prandtl-Tomlinson termicamente ativo, conseguiu-se determinar o potencial de interação entre a ponta do AFM e a superfície analisada, as forças críticas em que a saturação do atrito ocorre e a frequência estipulada com que os eventos de superação da barreira de pontecial acontecem. Com a variação da força normal aplicada, os resultados mostram que grafeno e MoS2 seguem o modelo Johnson-Kendall-Roberts (JKR) de mecânica de contato, enquanto o WS2 segue o modelo Derjaguin-Muller-Toporov (DMT). Para explicar tal diferença, uma hipótese associada ao efeito piezoelétrico é estipulada. Ademais, foi observado que a contaminação das superfícies de grafeno por adsorção de hidrocarbonetos pela exposição ao ar aumenta o atrito medido, e altera sua relação à carga aplicada. Os estágios iniciais da contaminação foram observados, e notou-se que esta se propaga da monocamada para as demais camadas da folha de grafeno, com diferentes taxas de área contaminada por tempo. / [en] In this work, the friction mechanism at the nanoscale of atomically thin materials such as graphene, transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), and muscovite mica was studied with the use of an atomic force microscope (AFM). The AFM scans these materials surfaces with a silicon nitride tip which is attached at the end of a cantilever. The tips slides through the surface and friction is measured by the torsional deflection of the cantilever. Parameters such as applied normal load and sliding speed were varied in order to verify their influences. Surfaces properties such as number of layers, roughness and tip-sample adhesion were also analyzed. The sliding speed experiment shows a linear dependence with the logarithm of the scanning velocity, until friction reaches a saturation point, where it remains the same even at higher velocities. Such dependence is amplified with the number of graphene layers, as a monolayer presents a steeper curve than few layers graphene. The data was fitted using the thermally active Prandtl-Tomlinson model and the tip-sample interaction potential was estimated, as well as the critical forces at which friction saturation occurs and the hop frequency at which a potential barrier is surpassed. In the applied normal load experiment, results shows that both graphene and MoS2 follow the Johnson-Kendall-Roberts (JKR) model, while WS2 and mica follows the Derjaguin-Muller-Toporov (DMT) model. In order to explain the different behavior in both TMDs samples, a hypothesis associated with the piezoelectric effect is proposed. Furthermore, the influence of airborne contamination in the friction of graphene was studied. Results shows that the contact mechanics is altered due to adsorbed hydrocarbon molecules on the graphene flakes. Initial stages of contamination shows that it propagates from the monolayer to subsequent layers, with a different contaminated area over time rate.