CREA Lab | Non-ideal compressible-fluid dynamics: developing a combined perspective on modeling, numerics and experiments
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Non-ideal compressible-fluid dynamics: developing a combined perspective on modeling, numerics and experiments

Non-Ideal Compressible-Fluid Dynamics (NICFD) is the branch of fluid mechanics devoted to the study of compressible flows whose behavior deviates from the one predicted by the ideal-gas model. This behavior is typical of molecular complex compounds in the vapor phase, namely when the operating thermodynamics conditions are close to the liquid-vapour saturation curve and critical point. NICFD flows are characterized by peculiar features which are not physically admissible under the ideal flow assumption, like for instance a non-ideal increase of the speed of sound upon isentropic expansion. Complex thermodynamic models were devised and applied to the theoretical analysis and numerical simulation of non-ideal flows. Nevertheless, only few experimental data in the NICFD regime were available up to date. Nowadays, the NICFD community is calling for the development of reliable and predictive numerical tools. To do so, the aim is to rely on and mix results from experiments, computations and theory. Since NICFD is a quite unexplored field, the objective is not only to numerically reproduce an observed phenomenon with a high level of fidelity, but also to develop tools which might be reasonably used to predict the reality in situations for which they were not specifically validated nor tested. To this extend, reliable numerical predictions require the further development of sophisticated physical models as well as a systematic treatment of calibration and validation procedure. This dissertation, moving from the first-ever observations of non-ideal compressible flows, develops a combined perspective on modeling, numerics and experiments regarding NICFD flows and it provides advancements in all the mentioned branches. In this context, this work is a preliminary attempt to answer the craving of the NICFD community for a substantial improvement of the currently available numerical prediction tools and physical models. Moreover, results from experiment, computation and theory are combined using an uncertainty quantification framework, paving the way for a future systematic and comprehensive treatment of the inherent uncertainties. The findings reported in the thesis reveal the existence of an unprecedented non-ideal effect. Namely, it is predicted that an overall Mach number increase may occur upon flow compression across steady oblique shock waves in fluids belonging to a special family. For these fluids, it is shown that there exists a particular subspace of the non-ideal region for which this unprecedented phenomenon is admitted. Moreover, it is also shown that, in a non-ideal regime, the flow maximum turning angle and the shock wave slope, w.r.t.\ the upstream flow direction, strictly depend on the whole pre-shock state. This oppose the physics ruling out dilute gas flows, for which the variation of fluid properties across the shock solely depend on the pre-shock Mach number value.



Gori G.




PhD Thesis, Politecnico di Milano

PhD Thesis