Heat transport by turbulent Rayleigh-B\'enard convection for $\Pra\ \simeq 0.8and and 3\times 10^{12} \alt \Ra\ \alt 10^{15}:Aspectratio: Aspect ratio \Gamma = 0.50$

We report experimental results for heat-transport measurements, in the form of the Nusselt number \Nu, by turbulent Rayleigh-B\'enard convection in a cylindrical sample of aspect ratio $\Gamma \equiv D/L = 0.50$ ($D = 1.12$ m is the diameter and $L = 2.24$ m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bars as the fluid. They are for the Rayleigh-number range 3\times 10^{12} \alt \Ra \alt 10^{15} and for Prandtl numbers \Pra\ between 0.79 and 0.86. For \Ra < \Ra^*_1 \simeq 1.4\times 10^{13} we find \Nu = N_0 \Ra^{\gamma_{eff}} with $\gamma_{eff} = 0.312 \pm 0.002$, consistent with classical turbulent Rayleigh-B\'enard convection in a system with laminar boundary layers below the top and above the bottom plate. For \Ra^*_1 < \Ra < \Ra^*_2 (with \Ra^*_2 \simeq 5\times 10^{14}) $\gamma_{eff}$ gradually increases up to $0.37\pm 0.01$. We argue that above \Ra^*_2 the system is in the ultimate state of convection where the boundary layers, both thermal and kinetic, are also turbulent. Several previous measurements for $\Gamma = 0.50$ are re-examined and compared with the present results.Comment: 44 pages, 18 figures, submitted to NJ

Contribution to the Physical Understanding of Supercritical Fluid Flows: A Computational Perspective

The modeling of fluids at supercritical and transcritical regimes is addressed at conditions characteristic of liquid propelled rocket engines, whose increasing performance demands have led to conditions in the combustion chambers to exceed the critical point of both fuels and oxidizers in the pursuit of higher specific impulses. In the present document, nitrogen is used as a surrogate for the commonly encountered oxygenhydrogen mixture so that turbulence mixing can be looked into without influences from combustion and chemically reacting effects. In contrast to the widespread use of compressible formulations in the literature, a distinct hypothesis is formulated and investigated, focusing on fluids’ incompressible but variabledensity behavior at supercritical and transcritical conditions. The incompressible but variabledensity hypothesis arose from the similarity of visualization data, namely measuring mixing efficiency through jet spreading rates. This document evaluates the capabilities and limitations of a computational method (Reynoldsaveraged NavierStokes) developed based on the incompressible variabledensity hypothesis when applied to supercritical and transcritical conditions. Based on the socalled ”thermal breakup mechanism concept” proposed in the literature, the mechanical description of supercritical jets is complemented, demonstrating that the amount of heat a jet receives inside the injector determines if a change from supercritical liquidtogaslike condition takes place, highlighting the importance of including the injector flow in the computations. Axial density and temperature decay rates of supercritical and transcritical jets are predicted for a wide range of conditions and geometries of increasing complexity, ranging from single species injection at supercritical and later transcritical conditions into quiescent environments to coaxial single and multispecies configurations. The results suggest that the incompressible but variable density hypothesis can sufficiently replicate the experimental data, rivaling the predictions of more sophisticated methods relying on large eddy simulation formulations. Moreover, the need to include the injector into the computations for an accurate flow description is demonstrated. Furthermore, the errors resulting from its absence are assessed and evaluated by comparing adiabatic and isothermal boundary conditions. Finally, the proposed solver has also demonstrated its capabilities in the temperature field predictions, making it one of the few solvers currently available to have been validated in terms of density and temperature.A modelação de fluidos em regimes supercríticos e transcríticos é levada a cabo em condições características daquelas encontradas em motores foguete de propelente líquido, nos quais a demanda por rendimentos mais elevados, para que sejam atingidos impulsos específicos superiores, faz com que as condições nas câmaras de combustão excedam o ponto crítico de combustíveis e oxidantes. No presente documento, azoto é utilizado como um substituto da mistura oxigéniohidrogénio para que o comportamento turbulento das misturas possa ser estudado sem as influências de efeitos de combustão e de reações químicas. Por contraste com o uso generalizado na literatura de ferramentas computacionais com formulações compressíveis, aqui uma hipótese distinta é formulada e investigada, focada no comportamento incompressível mas de massa volúmica variável de fluidos em condições supercríticas e transcríticas. A hipótese incompressível mas de massa volúmica variável surge da semelhança de visualização, nomeadamente da medição da eficiência da mistura através do grau de abertura dos jatos. Este documento tem como objetivo avaliar as capacidades e limitações de um método computacional (Reynoldsaveraged NavierStokes) desenvolvido com base na hipótese incompressível mas de massa volúmica variável, quando aplicado em condições transcríticas e supercríticas. Baseado no conceito de breakup térmico proposto na literatura, a descrição mecânica de um jato supercrítico é complementada, demonstrando que a quantidade de calor recebida pelo jato dentro do injetor determina a possibilidade de uma transição de liquidlike para uma condição de gaslike ter lugar. O decaimento axial da massa volúmica e da temperatura de jatos supercríticos e transcríticos é previsto para um leque de condições e geometrias de complexidade crescente, desde a injeção de uma espécie química, primeiro em regime supercrítico e depois transcrítico, num ambiente em repouso, até à injeção coaxial de uma e de várias espécies. Os resultados sugerem que a hipótese incompressível mas de massa volúmica variável é capaz de prever as condições experimentais com um bom grau de precisão, indo de encontro às previsões de métodos mais complexos baseados em large eddy simulation. Ademais, a necessidade de incluir o injetor nas simulações para uma descrição mais precisa do escoamento é demonstrada e os erros associados com a sua ausência avaliados através da comparação entre condições de fronteira adiabática e isotérmica. O método proposto demonstra também a sua capacidade em prever o campo de temperatura, sendo que se trata de um dos poucos métodos atualmente disponíveis validados em termos da massa volúmica e da temperatura

Characterising turbulent ship wakes from an environmental impact perspective

The world’s oceans, especially coastal areas, are intensively trafficked by ships. All these ships exert pressure on the marine environment, through emission to the atmosphere, discharges of pollutants to the water, and physical disturbance through energy input. Of these impacts, energy pollution from shipping has received the least attention. Especially the impact of ship-induced turbulence in the wake, which is induced by the hull friction and propeller, and remains for up to 15 minutes. The turbulent wake can impact the spread of contaminants, affect air-sea gas exchange, physically disturb plankton, and potentially impact local biogeochemistry through increased entrainment and vertical mixing. To assess these impacts, an understanding of the turbulent wake development and interaction with surface ocean stratification, is essential. However, characterisation of the turbulent wake development in time and space, especially in stratified conditions, is challenging and requires an interdisciplinary approach.\ua0\ua0 The aim of this thesis is to advance the understanding of turbulent wake development from an environmental impact perspective. The intensity and spatiotemporal extent of the turbulent wake, and its impact, have been investigated through a combination of in situ and ex situ observations, and Computational Fluid Dynamic (CFD) modelling of ships in full-scale. The unique dataset of several hundred in situ turbulent wake observations, showed large variation in spatiotemporal extent and intensity. Wake depths can reach down to 30 m, and the turbulent intensities in the near wake are 1–3 orders of magnitude higher than generally observed in the upper ocean surface layer. In addition, during stratified conditions ship-induced turbulence entrain water from below the pycnocline, with implications for local nutrient input and primary production in the ocean surface layer. In addition, ship-passages were observed to frequently trigger large methane emissions in an estuarine shipping lane. The results highlight the importance of addressing ship-induced turbulence in marine environmental management. Intensively trafficked coastal areas should be considered anthropogenically impacted, even unnatural, with respect to turbulence. The interdisciplinary approach applied in this thesis, is a first step towards a holistic assessment of the environmental impact of the turbulent wake

Development and Validation of a Numerical Model of the CO2 Dry-ice Blasting Process for Aircraft Engine Cleaning Applications

On-wing cleaning of engine compressors for commercial aircraft is a required maintenance task which results in greater operating efficiency and lower emission rates. It is typically carried out by injection of water and detergents into the intake of an engine while the engine is being cranked by the starter. Two drawbacks of this process are the risk of icing in cold weather and the collection and treatment of the water effluent. The dry-ice blasting process, a cleaning system which uses pressurized air and CO2 dry-ice particles as cleaning agent, has been proposed as an alternative method which does not suffer the above drawbacks but is potentially capable of efficient cleaning. In this context, such a cleaning system is currently being developed by Lufthansa Technik in association with Hochschule Darmstadt and DIT. This work focuses on the development and validation of a numerical model of this process, which can be used to improve the understanding of the complex multiphase flow phenomena involved and to assess the cleaning physics. Appropriate multiphase flow set-ups and new particle breakup and erosion models are developed. These new models will facilitate the numerical prediction of particle behaviour and defouling erosion rates during the defouling process. An appropriate simulation set-up for the particle laden injection system flow simulations using the Euler-Lagrange method is investigated. Three possible injection systems with various air flow velocities and particle loading densities are considered. These systems are investigated by means of high-speed camera (HSC) experiments and the predicted results are compared to the experimental in order to find the best numerical set-up. An improvement to the particle drag force formulation is proposed for highly pressurized air-flows. A new particle breakup model for dry-ice in Euler-Lagrange simulations is developed. This model is theoretically derived from an energy balance and un-derpinned with data from HSC experiments. It includes velocity, impact angle and target temperature as factors determining breakup behaviour of dry-ice particles impinging solid walls. A new defouling erosion model utilizing an energy balance approach and based on a range of experiments with several types of actual and artificial fouling material is developed and tested. The particle breakup and the erosion model are implemented into the commercial CFD code Ansys CFX. Verification and validation studies of both new models are presented. The validation of the new models uses data acquired in a specially-designed wind-tunnel experiment. All main findings and models are used in a final application case study where the new dry-ice based cleaning procedure is applied to a GE-CF6-50 test engine. Comparison of numerical results to data from air-flow, particle tracking and defouling experiments is also presented for this case