Person: Pinelli, Alfredo
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 10 of 56
PublicationThe autonomous cycle of near-wall turbulence(Cambridge University Press, 1999) Jiménez, Javier; Pinelli, AlfredoNumerical 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. PublicationComparison between large-eddy simulation and Reynolds-averaged Navier–Stokes computations for the MUST field experiment. Part I: Study of the flow for an incident wind directed perpendicularly to the front array of containers(Springer, 2010) Santiago, J. L.; Dejoan, A.; Martilli, A.; Martin, F.; Pinelli, AlfredoThe large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) methodologies are used to simulate the air flow inside the container's array geometry of the Mock Urban Setting Test (MUST) field experiment. Both tools are assessed and compared in a configuration for which the incident wind direction is perpendicular to the front array. The assessment is carried out against available wind-tunnel data. Effects of including small geometrical irregularities present in the experiments are analysed by considering LES and RANS calculations on two geometries: an idealized one with a perfect alignment and an identical shape of the containers, and a second one including the small irregularities considered in the experiment. These effects are assessed in terms of the local time-mean average and as well in terms of spatial average properties (relevant in atmospheric modelling) given for the velocity and turbulent fields. The structural flow properties obtained using LES and RANS are also compared. The inclusion of geometrical irregularities is found significant on the local time-mean flow properties, in particular the repeated flow patterns encountered in a perfect regular geometry is broken. LES and RANS provide close results for the local mean streamwise velocity profiles and shear-stress profiles, however the LES predictions are closer to the experimental values for the local vertical mean velocity. When considering the spatial average flow properties, the effects of geometrical irregularities are found insignificant and LES and RANS provide similar results. PublicationChebyshev 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. PublicationReynolds number dependence of mean flow structure in square duct turbulence - CORRIGENDUM(Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, Genta PublicationMarginally turbulent flow in a square duct(Cambridge University Press, 2007) Uhlmann, Markus; Pinelli, Alfredo; Kawahara, Genta; Sekimoto, AtshushiA direct numerical simulation of turbulent flow in a straight square duct was performed in order to determine the minimal requirements for self-sustaining turbulence. It was found that turbulence can be maintained for values of the bulk Reynolds number above approximately 1100, corresponding to a friction-velocity-based Reynolds number of 80. The minimum value for the streamwise period of the computational domain is around 190 wall units, roughly independently of the Reynolds number. We present a characterization of the flow state at marginal Reynolds numbers which substantially differs from the fully turbulent one: the marginal state exhibits a four-vortex secondary flow structure alternating in time whereas the fully turbulent one presents the usual eight-vortex pattern. It is shown that in the regime of marginal Reynolds numbers buffer-layer coherent structures play a crucial role in the appearance of secondary flow of Prandtl's second kind. PublicationLES and RANS simulations of the MUST experiment. Study of incident wind direction effects on the flow and plume dispersion(2007) Santiago, J. L.; Dejoan, A.; Martilli, A.; Martín , F.; Pinelli, AlfredoIn this study, we propose to assess and compare the performance of LES and RANS methodologies for the simulation of pollutant dispersion in an urban environment by making use of field and wind tunnel measurements of the MUST experiment configuration. First, the proposed analysis addresses the relevance of taking into account the small geometrical irregularities of the obstacle array in the computations. For this, local and spatial averaged time mean flow properties are compared for two geometries, one with a perfect alignment of the containers and another one including the irregularities present in the experiment. In both geometries the incident flow is orthogonal to the front array of obstacles. The second part of this study presents simulations with different approaching wind directions to analyse the effect of small changes in the incident wind direction on the flow and on the plume dispersion. In this second part, the mean concentration field is compared with the experimental data and an analysis that relates the channelling effects with the plume deflection is provided. PublicationImmersed boundary method for generalised finite volume and finite difference Navier-Stokes solvers(American Society of Mechanical Engineers, 2010) Pinelli, Alfredo; Naqavi, I.Z.; Piomelli, U.In Immersed Boundary Methods (IBM) the effect of complex geometries is introduced through the forces added in the Navier-Stokes solver at the grid points in the vicinity of the immersed boundaries. Most of the methods in the literature have been used with Cartesian grids. Moreover many of the methods developed in the literature do not satisfy some basic conservation properties (the conservation of torque, for instance) on non-uniform meshes. In this paper we will follow the RKPM method originated by Liu et al.  to build locally regularized functions that verify a number of integral conditions. These local approximants will be used both for interpolating the velocity field and for spreading the singular force field in the framework of a pressure correction scheme for the incompressible Navier-Stokes equations. We will also demonstrate the robustness and effectiveness of the scheme through various examples. PublicationReynolds number dependence of mean flow structure in square duct turbulence(Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, GentaWe have performed direct numerical simulations of turbulent flows in a square duct considering a range of Reynolds numbers spanning from a marginal state up to fully developed turbulent states at low Reynolds numbers. The main motivation stems from the relatively poor knowledge about the basic physical mechanisms that are responsible for one of the most outstanding features of this class of turbulent flows: Prandtl's secondary motion of the second kind. In particular, the focus is upon the role of flow structures in its generation and characterization when increasing the Reynolds number. We present a two-fold scenario. On the one hand, buffer layer structures determine the distribution of mean streamwise vorticity. On the other hand, the shape and the quantitative character of the mean secondary flow, defined through the mean cross-stream function, are influenced by motions taking place at larger scales. It is shown that high velocity streaks are preferentially located in the corner region (e.g. less than 50 wall units apart from a sidewall), flanked by low velocity ones. These locations are determined by the positioning of quasi-streamwise vortices with a preferential sign of rotation in agreement with the above described velocity streaks' positions. This preferential arrangement of the classical buffer layer structures determines the pattern of the mean streamwise vorticity that approaches the corners with increasing Reynolds number. On the other hand, the centre of the mean secondary flow, defined as the position of the extrema of the mean cross-stream function (computed using the mean streamwise vorticity), remains at a constant location departing from the mean streamwise vorticity field for larger Reynolds numbers, i.e. it scales in outer units. This paper also presents a detailed validation of the numerical technique including a comparison of the numerical results with data obtained from a companion experiment. PublicationTurbulent shear flow over active and passive porous surfaces(Cambridge University Press, 2001) Jiménez, J.; Uhlmann, Markus; Pinelli, Alfredo; Kawahara, GentaThe 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. PublicationAn 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.