Pinelli, Alfredo

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Universidad Complutense de Madrid
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Ciencias Matemáticas
Matemática Aplicada
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Now showing 1 - 10 of 19
  • Publication
    The autonomous cycle of near-wall turbulence
    (Cambridge University Press, 1999) Jiménez, Javier; Pinelli, Alfredo
    Numerical experiments on modified turbulent channels at moderate Reynolds numbers are used to differentiate between several possible regeneration cycles for the turbulent fluctuations in wall-bounded flows. It is shown that a cycle exists which is local to the near-wall region and does not depend on the outer flow. It involves the formation of velocity streaks from the advection of the mean profile by streamwise vortices, and the generation of the vortices from the instability of the streaks. Interrupting any of those processes leads to laminarization. The presence of the wall seems to be only necessary to maintain the mean shear. The generation of secondary vorticity at the wall is shown to be of little importance in turbulence generation under natural circumstances. Inhibiting its production increases turbulence intensity and drag.
  • Publication
    Chebyshev collocation method and multidomain decomposition for the incompressible Navier‐Stokes equations
    (Wiley, 1994) Pinelli, Alfredo; Vacca, A.
    The two-dimensional incompressible Navier-Stokes equations in primitive variables have been solved by a pseudospectral Chebyshev method using a semi-implicit fractional step scheme. The latter has been adapted to the particular features of spectral collocation methods to develop the monodomain algorithm. In particular, pressure and velocity collocated on the same nodes are sought in a polynomial space of the same order; the cascade of scalar elliptic problems arising after the spatial collocation is solved using finite difference preconditioning. With the present procedure spurious pressure modes do not pollute the pressure field. As a natural development of the present work a multidomain extent was devised and tested. The original domain is divided into a union of patching sub-rectangles. Each scalar problem obtained after spatial collocation is solved by iterating by subdomains. For steady problems a C1 solution is recovered at the interfaces upon convergence, ensuring a spectrally accurate solution. A number of test cases have been solved to validate the algorithm in both its single-block and multidomain configurations. The preliminary results achieved indicate that collocation methods in multidomain configurations might become a viable alternative to the spectral element technique for accurate flow prediction.
  • Publication
    An efficient iterative solution method for the Chebyshev collocation of advection-dominated transport problems
    (Society for Industrial and Applied Mathematics, 1996) Pinelli, Alfredo; Couzy, W.; Deville, M. O.; Benocci, C.
    A new Chebyshev collocation algorithm is proposed for the iterative solution of advection-diffusion problems. The main features of the method lie in the original way in which a finite-difference preconditioner is built and in the fact that the solution is collocated on a set of nodes matching the standard Gauss-Lobatto-Chebyshev set only in the case of pure diffusion problems. The key point of the algorithm is the capability of the preconditioner to represent the high-frequency modes when dealing with advection-dominated problems. The basic idea is developed for a one-dimensional case and is extended to two-dimensional problems. A series of numerical experiments is carried out to demonstrate the efficiency of the algorithm. The proposed algorithm can also be used in the context of the incompressible Navier-Stokes equations.
  • Publication
    Dynamics of the structures of near wall turbulence
    (Springer, 1999) Jiménez, J.; Pinelli, Alfredo
    Numerical experiments on modified turbulent channels are used to differentiate between possible turbulence generation mechanisms in wall bounded flows. It is shown that a regeneration cycle exists which is local to the near-wall region and does not depend on the outer flow. It involves the formation of velocity streaks from the advection of the mean profile by streamwise vortices, and the generation of the vortices from the instability of the streaks. Interrupting any of those processes leads to laminarisation of the wall. The production of secondary vorticity at the wall is not important in turbulence generation.
  • Publication
    A spectral multidomain method for the numerical simulation of turbulent flows
    (Elsevier, 1997) Pinelli, Alfredo; Vacca, A.; Quarteroni, A.
    The primitive variable formulation of the unsteady incompressible Navier-Stokes equations in three space dimensions is discretized with a combined Fourier-Legendre spectral method. A semi-implicit pressure correction scheme is applied to decouple the velocity from the pressure. The arising elliptic scaler problems are first diagonalized in the periodic Fourier direction and then solved by a multidomain Legendre collocation method in the two remaining space coordinates. In particular, both an iterative and a direct version of the so-called projection decomposition method (PDM) are introduced to separate the equations for the internal nodes from the ones governing the interface unknowns. The PDM method, first introduced by V. Agoshkov and E. Ovtchinnikov and later applied to spectral methods by P. Gervasio, E. Ovtchinnikov, and A. Quarteroni is a domain decomposition technique for elliptic boundary value problems, which is based on a Galerkin approximation of the Steklov-Poincare equation for the unknown variables associated to the grid points lying on the interface between subdomains. After having shown the exponential convergence of the proposed discretization technique, some issues on the efficient implementation of the method are given. Finally, as an illustration of the potentialities of the algorithm for the numerical simulation of turbulent flows, the results of a direct numerical simulation (DNS) of a fully turbulent plane channel flow are presented.
  • Publication
    The role of coherent structure interactions in the regeneration of wall turbulence
    (Springer, 1998) Jiménez, Javier; Pinelli, Alfredo
    The near-wall region is characterized by the presence of organized structures [1], but the way in which they interact to maintain the local dynamics and to extract energy from the mean flow is still controversial. The two dominant structures are the streamwise velocity streaks and the quasi-streamwise vortices. The former consist of long (x + ~ 1000) wavy arrays of alternating streamwise jets with an average spanwise wavelength of z + ~ 100. The latter are vortical structures almost aligned with the mean flow but slightly tilted away from the wall. While it is generally accepted that the vortices induce streaks by transferring mean streamwise momentum towards and away from the wall [2], there is less consensus on the generation mechanism of the streamwise vortices. Two conceptual models, widely discussed in the literature, are illustrated in Fig. 1: A) the streamwise vortices are a consequence of the breakdown of the streaks, probably due to inflectional instabilities of the local velocity profiles; B) the streamwise vortices are formed directly, or at least triggered, by the amplification of outer flow perturbations. Hypothesis A assumes that an autonomous wall cycle exists, while B predicts that wall turbulence depends on the existence of an outer flow.
  • Publication
    The autonomous near-wall turbulent region
    (American Physical Society, 1998) Jiménez, J.; Pinelli, Alfredo
    The near-wall region is the only place in zero-pressure-gradient boundary layer where the production of turbulent energy exceeds dissipation. The excess energy helps maintain turbulence in the core region, where the opposite is true. It is shown that it is possible to maintain turbulence in the region below y^+≈ 60 without any input from the outer flow. In the numerical experiment all the fluctuations in a plane channel are artificially damped by increasing viscosity with height, and the outer flow is laminar above that level. The near-wall region nevertheless survives indefinitely, suggesting that wall turbulence can be studied in terms of modular units, with the near-wall and the logarithmic and outer layers as interacting but distinct phenomena. The cycle responsible for maintaining near-wall turbulence is shown to involve low-velocity streaks and streamwise vortices, but essentially no hairpins. The intensity of the near-wall longitudinal velocity fluctuations agrees well with those in fully developed flows, but the wall-normal fluctuations are weaker, in agreement with the Reynolds number behaviour found experimentally for those quantities. The reason is explored using higher Reynolds number simulations.
  • Publication
    Large Eddy simulation of turbulence on an alliant FX computer
    (ASFRA, 1990) Benocci, C.; Pinelli, Alfredo; Abba, A.