3,531 research outputs found
Flow control design inspired by linear stability analysis
No full textIn the recent literature, a growing number of research papers have been dedicated to applying the techniques of global stability and sensitivity analysis to the design of flow controls. The controls that are designed in this way are mainly passive or open-loop controls. Among those, we consider here controls that are aimed at linearly stabilizing flow configurations which would be otherwise globally unstable. In particular, a review of the literature on flow controls designed on the basis of stability and sensitivity analysis is presented. The mentioned methods can be rigorously applied to relatively simple flow regimes, typically observed at low values of the Reynolds number. In this respect, the recent literature also demonstrates a large interest in the application of the same methods for the control of coherent large-scale flow structures in turbulent flows, as, for instance, the quasiperiodic shedding of vortices in turbulent wakes. The papers dedicated to this subject are also reviewed here. Finally, all the described methods imply the solution of eigenvalue problems which are at the state-of-the-art for computational complexity. On the one hand, there are attempts to reduce the complexity of the involved computational problems by applying local stability analysis, and some examples are illustrated. On the other hand, recent advances in numerical methods, also concisely reviewed here, allow the manipulation of large eigenvalue problems and greatly simplify the development of numerical tools for stability and sensitivity analysis of complex flow models, often built using existing fluid dynamics codes
Effect of confinement on three-dimensional stability in the wake of a circular cylinder
No full textThis paper investigates the three-dimensional stability of the wake behind a symmetrically confined circular cylinder by a linear stability analysis. Emphasis has been placed on discussing analogies and differences with the unconfined case to highlight the role of the inversion of the von Kármán street in the nature of the three-dimensional transition. Indeed, in this flow, the vortices of opposite sign that are alternately shed from the body into the wake cross the symmetry line further downstream and they assume a final configuration which is inverted with respect to the unconfined case. It is shown that the transition to a three-dimensional state has the same space–time symmetries of the unconfined case, although the shape of the linearly unstable modes is affected by the inversion of the wake vortices. A possible interpretation of this result is given here
A spectral code for the numerical simulation of transition in the plane Pouiseuille flow
No full textAtti del
Dipartimento di Ingegneria Aerospaziale di Pisa, ADIA 2005-
Further generalized energies for the application of an energy criterion of conditional stability
No full textA previously proposed energy criterion for predicting the conditional stability of incompressible flows, which is based on properly defined generalized energies, is considered. An extension of the procedure for the construction of generalized energies is proposed here, leading to the definition of a broader class of generalized energies which may depend on several free parameters. The optimal free parameters are computed by searching the value for which the energetic stability criterion predicts the maximum critical L 2 energy. The capabilities of the resulting stability criterion are appraised through the application to low-dimensional non-linear quadratic models, mimicking the subcritical instability behavior of particular incompressible flows
Stokes eigenfunctions and Galerkin projection of the disturbance equations in plane Poiseuille flow: a systematic analytical approach
No full textIn the present paper, the L2-normalized
Stokes eigenfunctions for plane Poiseuille flow,
which form an orthonormal functional basis for the
space of disturbances, are written in a general exponential
form. Then, the evolution equations for the
disturbances areGalerkin-projected on the considered
basis functions, and all the terms of the resulting
dynamical system are expressed systematically
in analytical form. Finally, a numerical example is
given in which the proposed basis functions are
used for the simulation of the time evolution of
the critical disturbance predicted by the energetic
stability theory
Blended RANS/LES simulations of massively separated flows
No full textA hybrid RANS/LES approach, based on the Limited Numerical Scales concept, is applied to the numerical simu- lation of the flow around a square cylinder and of a jet in cross-flow. The key feature of this approach is a blending between two eddy-viscosities, one given by the k − ε RANS model and the other by the Smagorinsky LES closure. A mixed finite-element/finite-volume formulation on unstructured grids is used for the numerical discretization. For the square cylinder flow, it is shown that the LNS recovers the LES results when the grid is sufficiently refined and it leads to an improvement of the results with respect to LES and RANS for coarser grids. Conversely, for the simulation of the jet in cross-flow, the results are not completely satisfactory. Indeed, the model works in RANS mode, as wished, in the attached boundary layers but also in the shear layer near the jet orifice and this inhibits the formation of the ring-like vortices, leading to significant errors in the shape and in the mixing characteristics of the jet
On the approximate treatment of wall-boundary conditions in large-eddy simulation
No full textAtti del
Dipartimento di Ingegneria Aerospaziale di Pisa, ADIA 2002-
Structural sensitivity of the secondary instability in the wake of a circular cylinder
No full textThe stability properties of the flow past an infinitely long circular cylinder are studied in the context of linear theory. An immersed-boundary technique is used to represent the cylinder surface on a Cartesian mesh. The characteristics of both direct and adjoint perturbation modes are studied and the regions of the flow more sensitive to momentum forcing and mass injection are identified. The analysis shows that the maximum of the perturbation envelope amplitude is reached far downstream of the separation bubble, where as the highest receptivity is attained in the near wake of the cylinder, close to the body surface. The large difference between the spatial structure of the two-dimensional direct and adjoint modes suggests that the instability mechanism cannot be identified from the study of either eigenfunctions separately. For this reason a structural stability analysis of the problem is used to analyse the process which gives rise to the self-sustained mode. In particular, the region of maximum coupling among the velocity components is localized by inspecting the spatial distribution of the product between the direct and adjoint modes. Results show that the instability mechanism is located in two lobes placed symmetrically across the separation bubble, confirming the qualitative results obtained through a locally plane-wave analysis. The relevance of this novel technique to the development of effective control strategies for vortex shedding behind bluff bodies is illustrated by comparing the theoretical predictions based on the structural perturbation analysis with the experimental data of Strykowski & Sreenivasan (J. Fluid Mech. vol. 218, 1990, p. 71)
Analisi di un modello di data assimilation per il flusso in scie turbolente di corpi tozzi
No full textL’obiettivo di questo lavoro di tesi è validare il modello di data assimilation, sviluppato in ambiente FreeFem++, proposto da Camarri et al. (2017) e testare le potenzialità di autoconsistenza del codice applicato a flussi in scie turbolente di corpi tozzi.
Lo studio è stato condotto utilizzando sia dati numerici che sperimentali e ha richiesto, l’uso integrato di diversi tool numerici.
La fase preliminare ha previsto la creazione di esperimenti numerici in ambiente OpenFoam, in particolare è stato riprodotto numericamente il test eseguito in galleria del vento riportato in Camarri et al. (2017) allo scopo di ottenere un riferimento numerico, oltre che sperimentale, per la validazione del modello.
Nella fase successiva, sono stati utilizzati i database disponibili per eseguire la procedura di validazione del codice tramite la ricostruzione delle informazioni dei campi numerici e sperimentali. Sono stati eseguiti ulteriori test per individuare le informazioni fondamentali per il funzionamento del codice.
Infine, sono state testate le capacità di data assimilation del modello riducendo le informazioni di input, in particolare l’informazione relativa ai Reynolds stress. Sono stati sviluppati appositi algoritmi che sfruttano le tecniche di stabilità per ottenere in output lo stesso numero di informazioni, valide dal punto di vista quantitativo e qualitativo
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