Statistical Breakdown of Numerically Simulated Shear-Free Flows

Alberti, Luca (2020) Statistical Breakdown of Numerically Simulated Shear-Free Flows. [Laurea magistrale], Università di Bologna, Corso di Studio in Aerospace engineering / ingegneria aerospaziale [LM-DM270] - Forli', Documento full-text non disponibile
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A statistical analysis of several DNSs of incompressible turbulent shear-free flows (with Reλ ranging from ≈ 86 to ≈ 245) was performed, aimed to study the small scale processes that promote turbulent entrainment at the TNTI. After a certain transient, the shearless flows reached a statistically steady state. From that time instant, the main features of the various flow configurations were statistically investigated by assessing the classical and conditional mean profiles of total kinetic energy (K) and anisotropy ratio (urms/ξ), the classical and conditional mean profiles of the terms of the total kinetic energy budget (advection C, pressure strain Π, viscous diffusion Dv, dissipation ε and power injection Pi), the classical PDFs of the velocity components (u, v and w), the classical and conditional mean profiles of the invariants of the velocity gradient (R and Q), rate-of-strain (RS and QS) and rate-of-rotation (QW) tensors and the classical and conditional J PDFs of the same invariants. Conditional statistical quantities were computed in relation to the distance from the TNTI. To avoid the accidental mixing of turbulent and non-turbulent points in the conditional statistics, all the nodes belonging to irrotational bubbles and turbulent islands were neglected. All the conditional mean profiles are characterized by very sharp gradients at the TNTI, meanwhile in the classical mean profiles the variations are distributed on a much wider portion of the domain, indicating how the conditional averages are able to successfully discern turbulent and irrotational regions, avoiding the smoothening of the mean profiles via large scale intermittency. The similarity between the conditional statistics obtained here for shearless flows, and those extracted from temporal jets in previous works, is very interesting, since it suggests some sort of universality in the TNTI dynamics, as well asin the turbulent entrainment processes.

Tipologia del documento
Tesi di laurea (Laurea magistrale)
Autore della tesi
Alberti, Luca
Relatore della tesi
Corso di studio
Ordinamento Cds
Parole chiave
Statistical breakdown, statistical analysis, numerical simulation, shear-free flows, TNTI, conditional statistics, kinetic energy budget, turbulent entrainment
Data di discussione della Tesi
8 Ottobre 2020

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