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 - 6 of 6
  • Publication
    Turbulent shear flow over active and passive porous surfaces
    (Cambridge University Press, 2001) Jiménez, J.; Uhlmann, Markus; Pinelli, Alfredo; Kawahara, Genta
    The behaviour of turbulent shear flow over a mass-neutral permeable wall is studied numerically. The transpiration is assumed to be proportional to the local pressure fluctuations. It is first shown that the friction coefficient increases by up to 40% over passively porous walls, even for relatively small porosities. This is associated with the presence of large spanwise rollers, originating from a linear instability which is related both to the Kelvin–Helmholtz instability of shear layers, and to the neutral inviscid shear waves of the mean turbulent profile. It is shown that the rollers can be forced by patterned active transpiration through the wall, also leading to a large increase in friction when the phase velocity of the forcing resonates with the linear eigenfunctions mentioned above. Phase-lock averaging of the forced solutions is used to further clarify the flow mechanism. This study is motivated by the control of separation in boundary layers.
  • Publication
    Travelling waves in a straight square duct
    (Springer, 2009) Uhlmann, Markus; Kawahara, Genta; Pinelli, Alfredo
    Isothermal, incompressible flow in a straight duct with square cross-section is known to be linearly stable [1]. Direct numerical simulation, on the other hand, has revealed that turbulence in this geometry is self-sustained above a Reynolds number value of approximately 1100, based on the bulk velocity and the duct half-width [2]. Numerous non-linear equilibrium solutions have already been identified in plane Couette, plane Poiseuille and pipe flows [3, 4, 5], and their role in the transition process as well as their relevance to the statistics of turbulent flow have been investigated [6, 7, 8]. No non-linear travelling-wave solutions for the flow through a square duct have been published to date.
  • Publication
    Toward direct numerical simulation of reacting fluidized beds
    (Combustion Institute, 2003) Uhlmann, Markus; Pinelli, Alfredo; García Ybarra, P. L.
    Nowadays, the most used techniques to design dense gas-solid flow reactors rely upon numerical predictions obtained from hydrodynamic models, usually derived through some averaging processes of the complete conservation equations. The averaging process leads to unknown correlation terms that need further modeling for the final closure of the equations. Many of these terms represent complex interactions between phases and are usually modeled through semi-empirical relations. Direct Numerical Simulation (DNS) of idealized situations can help in grasping the basic mechanisms governing these systems, therefore fostering the development of improved models.
  • Publication
    Two-dimensional thermal convection flow with variable viscosity and embedded boundaries
    (2003) Uhlmann, Markus; Pinelli, Alfredo
    A two-dimensional model capable of simulating thermal convection flow in complex geometries has been implemented in a finite-difference setting and using a fictitious domain method of type “direct explicit forcing". The Boussinesq approximation is supposed to hold; the coupling between velocity and temperature fields is explicit; spatially varying viscosity is accounted for. The computation of a model for the thermally-induced flow in a three-chamber fuel tank reveals that the present method does not allow for sufficiently large time steps when the viscosity varies strongly.
  • Publication
    Numerical investigation of meso-scale structures using a two fluid model with non-Newtonian closure
    (American Physical Society, 2005) Pérez, J.; Pinelli, Alfredo
    The idea is based on identifying the physical roles of the solid and fluid stress tensors in the solid phase momentum equation. The tensors are reformulated as a sum of different terms. A comparison with the closure proposed by Marchioro et al. (Int. J. Multiphase flow. 27: 237-276, 2001), leads to a new non-Newtonian closure. The complete model has been tested with two different scenarios. First, we used an initial Taylor-Green base flow for the fluid phase with a highly diluted regime with mass fraction of order one. This case allows for a critical evaluation of the present formulation vs Saffman's 1962. We also considered a base channel flow with solid particles. Different regimes (solid fractions) have been considered. The results are compared vs Agrawal et al. (J. Fluid Mech. 445: 151-185, 2001) in terms of of meso-scale solid structures behaviours. The numerical discretization for both phases is based on a finite volume formulation using a Rusanov scheme for the hyperbolic part of the equations that preserves the positivity of the void fraction.
  • Publication
    Minimal requirements for self-sustained turbulence in a square duct: a numerical investigation
    (American Physical Society, 2005-11-22) Pinelli, Alfredo; Uhlmann, Markus; Kawahara, Genta
    Direct Numerical Simulations of unsteady square channel flows are performed at low to moderate Reynolds numbers diminishing in a systematic way the streamwise length of the computational domain. The basic motivation of the present study is twofold. On one hand we want to determine the minimal requirements for the self-sustainment of a turbulent flow (J. Jim\'enez \& P. Moin, JFM 225: 213-240, 1991). On the other hand, we wish to characterise the flow system on the verge of re-laminarization. Under this condition it is expected that the secondary corner vortices and the near-wall coherent structures collapse in terms of length scales leading to a global motion with a limited number of degrees of freedom. The eventual existence of this reduced basin of attraction may help in shedding some light on the generation mechanism of the secondary flow and on the mechanisms related with non-linear transitional regime. Another objective of the present work aims at establishing a detailed, highly resolved DNS description of this class of flow that received little attention in the past (S. Gavrilakis, JFM 244, 101-129, 1992, A. Huser \& S. Biringen, JFM 257, 65-95, 1993).