Physics-based turbulence models for large-eddy simulation:Theory and application to rotating turbulent flows

Fluid flows are everywhere. Consider, for example, rivers, the flow of air in the atmosphere and the blood that is flowing through our veins. Most fluid flows are very chaotic, or turbulent, and the prediction of their behavior is essential for many applications, including the design of cars, boats and airplanes. However, accurately predicting turbulent flows is very challenging because computers do not have enough memory to store all flow details. In this thesis, we, therefore, apply a method called large-eddy simulation. With this method, the large eddies, or vortices, in flows are directly computed, whereas small eddies have to be described using turbulence models. The question we consider is: how to create physics-based turbulence models, which respect the physical and mathematical properties of flows? To answer this question, we follow a systematic approach. We thereby obtain a framework of constraints for the construction of physics-based turbulence models. Using this framework, we show that existing turbulence models do not respect all properties of flows. We also illustrate how new physics-based turbulence models with desired properties can be created systematically. We then apply the framework of constraints to develop a new turbulence model for rotating flows. We show that this model respects many properties of flows and provides outstanding predictions of rotating flows. We also demonstrate that these predictions are as good as, or much better than, predictions from existing turbulence models. Our work can, thus, aid in improving predictions of both rotating and nonrotating turbulent flows

The Legal Needs of Farmers: An Analysis of the Family Farm Legal Needs Survey

The Legal Needs of Farmer

Mixed modeling for large-eddy simulation: The single-layer and two-layer minimum-dissipation-Bardina models

Predicting the behavior of turbulent flows using large-eddy simulation requires modeling of the subgrid-scale stress tensor. This tensor can be approximated using mixed models, which combine the dissipative nature of functional models with the capability of structural models to approximate out-of-equilibrium effects. We propose a mathematical basis to mix (functional) eddy-viscosity models with the (structural) Bardina model. By taking an anisotropic minimum-dissipation (AMD) model for the eddy viscosity, we obtain the (single-layer) AMD-Bardina model. In order to also obtain a physics-conforming model for wall-bounded flows, we further develop this mixed model into a two-layer approach: the near-wall region is parameterized with the AMD-Bardina model, whereas the outer region is computed with the Bardina model. The single-layer and two-layer AMD-Bardina models are tested in turbulent channel flows at various Reynolds numbers, and improved predictions are obtained when the mixed models are applied in comparison to the computations with the AMD and Bardina models alone. The results obtained with the two-layer AMD-Bardina model are particularly remarkable: both first- and second-order statistics are extremely well predicted and even the inflection of the mean velocity in the channel center is captured. Hence, a very promising model is obtained for large-eddy simulations of wall-bounded turbulent flows at moderate and high Reynolds numbers.Comment: 29 pages, 14 figures, 3 tables; revised, accepted manuscrip

De agrarische sector in Nederland naar 2020 : perspectieven en onzekerheden

Dit rapport beantwoordt vooral de vraag: wat zijn de perspectieven van de agrarische sector in Nederland op middellange termijn, tot 2020? Met welke ontwikkelingen en onzekerheden moet rekening worden gehouden en welke aandachtspunten voor het beleid vloeien daar uitvoort? Het rapport is gebaseerd op een combinatie van expertkennis en modelberekeningen. De vraagontwikkeling wordt mede bepaald door de tot 2020 met 1% per jaar doorgaande groei van de wereldbevolking. Deze groei, die lager is dan in het vorige decennium, manifesteert zich vooral buiten de EU, met name in Azië. Voor het aanbod van producten is, naast de beschikbare arbeid, kapitaal en grond, de technologische vernieuwing een belangrijke factor. Het aantal in de agrosector werkzame mensen zal vooral blijven afnemen door de technologische vernieuwing, die een verhoging van de productiviteit mogelijk maakt. Dit kan knelpunten opleveren in arbeidsintensieve deelsectoren, zoals in de tuinbouw. Nieuwe technologische doorbraken, zoals robotisering, kunnen hier een oplossing biede