Orazzo, Annagrazia (2013) Interfacial Instabilities of Two-Fluid Flows. [Tesi di dottorato]


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Tipologia del documento: Tesi di dottorato
Lingua: English
Titolo: Interfacial Instabilities of Two-Fluid Flows
Orazzo, Annagraziao.annagrazia@libero.it
Data: 29 Marzo 2013
Numero di pagine: 121
Istituzione: Università degli Studi di Napoli Federico II
Dipartimento: Ingegneria Industriale
Scuola di dottorato: Ingegneria industriale
Dottorato: Ingegneria aerospaziale, navale e della qualità
Ciclo di dottorato: 25
Coordinatore del Corso di dottorato:
De Luca, Luigideluca@unina.it
De Luca, Luigideluca@unina.it
Data: 29 Marzo 2013
Numero di pagine: 121
Parole chiave: gas-liquid flows; interfacial instability; VOF simulations; single Kelvin-Helmholtz wave; linear stability analysis; CAF; transient growth
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/06 - Fluidodinamica
Depositato il: 08 Apr 2013 15:01
Ultima modifica: 15 Lug 2014 13:26
URI: http://www.fedoa.unina.it/id/eprint/9234


Many flows in industry and in nature are two-fluid flows of immiscible fluids. Among them, the cases of parallel flowing gas-liquid layers in plane geometry and of oil-water arranged as Core-Annular Flow (CAF) in circular pipe are deeply investigated here. These are significant configurations, respectively, for the atomization of fuel in thermal engine and for the lubricated pipelining in petroleum industry. Two-fluid flows are often unstable. Aim of this thesis is to analyze the development of instability taking into account all the typical features associated with two-fluid flows: large density and viscosity ratios, surface tension acting on the interface separating the different fluids and the different spatial scales interested by the evolution of the interface. This analysis is very complex because it is necessary to distinguish between linear and nonlinear effects, normal mode and transient growth and delicate effects of viscosity even at high Reynolds numbers. Both linear stability theory and direct numerical simulations have been used. A linear stability analysis of these flows has been worked out only recently. It has let to verify that interfacial modes are the most unstable ones and represent the leading mechanism of primary instability. Numerical simulations, based on Volume of Fluid (VOF) method, have allowed to investigate the nonlinear development of this interfacial instability that, for gas-liquid flows characterizing the atomization process, is the main responsible of breakup and droplets formation. These simulations have displayed the formation and the evolution of a new type of instability: a nonlinear single wave Kelvin-Helmholtz instability. This structure has been extensively discussed and characterized. Two different codes have been used: an in-house modified version of SURFER and GERRIS flow solver, a new generation VOF code coupling classical VOF algorithms with the adaptive mesh refinement. Regarding CAF, in order to explain the disagreement between classic results of linear modal stability analysis and some experimental findings, a nonmodal analysis has been performed. It has highlighted how the instability of this flow is governed by transient energy amplification of infinitesimal three-dimensional disturbances that lead to particular flow patterns, such as emulsifications or water drops in oil, and play an important role in transition to turbulence.

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