1,721,137 research outputs found
Flow control design inspired by linear stability analysis
Full text linkIn 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
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-
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-
Large-eddy simulation of the flow around a forward-swept wing at high angle of attack
No full textPreliminary results of large-eddy simulations of the flow around a forward-swept wing are presented and compared to experimental data obtained in the wind tun- nel of the Dipartimento di Ingegneria Aerospaziale (Pisa). The numerical solver is based on a mixed finite-volume/finite-element formulation. A modified Roe scheme is employed for convective terms, together with the MUSCL method to increase the order of accuracy. An upwinding parameter, , directly controls the numerical diffusion, which is formed of fourth-order space derivatives. The eddy- viscosity Smagorinsky model and its dynamic version are used for the subgrid scale (SGS) terms in the LES equations
Feedback control by low-order modelling of the laminar flow past a bluff body
No full textIn this work a two-dimensional laminar flow past a square cylinder is considered.
Actuators placed on the cylinder enable active control by blowing and suction.
Proportional feedback control is then applied using velocity measurements taken
in the cylinder wake. Projection onto an empirical subspace is combined with a
calibration technique to build a low-order model of the incompressible Navier–Stokes
equations. This model is used within an optimization method to determine a set of
feedback gains which reduces the unsteadiness of the wake at Re =150. The resulting
controlled flows are further characterized by computing the critical Reynolds numbers
for the onset of the vortex shedding instability
Generalized energies for stability analysis and application to a low-order model of the plane Poiseuille flow
No full textAtti del Dipartimento di Ingegneria Aerospaziale di Pisa, ADIA
2006-
Large-eddy simulation of the flow around a triangular prism with moderate aspect ratio
No full textThe flow around a triangular prism with moderate aspect-ratio placed vertically on a plane is studied numerically using the Large-Eddy Simulation approach. The results are compared with available and new experimental data in order to obtain clues on the physical origin of the velocity fluctuations inside and around the wake. Both the numerical and the experimental velocity signals are analysed through time-frequency techniques based on the wavelet and Hilbert transforms in order to characterize the time variation of the amplitude and frequency of the different detectable fluctuating components. It is shown that the numerical simulation, even if relating to a Reynolds number that is one order of magnitude lower than in the experiments, provides values of the rms wake fluctuations that are in good agreement with those obtained from the hot-wire measurements. Furthermore, it allows the shape of the upper near wake to be defined, and gives useful indications on the dynamics of the vorticity structures originating from the free-end of the body and on their possible connection with the wake fluctuating flow field
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