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 - 5 of 5
  • Publication
    Reynolds number dependence of mean flow structure in square duct turbulence - CORRIGENDUM
    (Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, Genta
  • Publication
    Reynolds number dependence of mean flow structure in square duct turbulence
    (Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, Genta
    We 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.
  • Publication
    Turbulent puffs in a horizontal square duct under stable temperature stratification
    (Begell House, 2012) Otsuki, T.; Kawahara, Genta; Uhlmann, Markus; Pinelli, Alfredo
    Direct numerical simulations of streamwise-localized turbulent flow (turbulent puff) in a horizontal square duct heated from above are performed at Richardson numbers 0 ≤ Ri ≤ 0.58 to characterize differences between isothermal and non-isothermal puffs. It is found that the structure of the non-isothermal puff exhibits significantly different properties from that of the isothermal puff. In particular, differences are observed in wall shear stresses. In the upstream part of the non-isothermal puff, higher shear appears on the horizontal walls than the vertical ones. In the downstream part, meanwhile, higher shear appears on the vertical walls than the horizontal ones. The higher shear on the horizontal walls in the upstream part is interpreted in terms of coherent streamwise vortices induced by buoyancy in the corner regions. The higher shear on the vertical walls in the downstream part is considered to be a consequence of higher wall-normal (horizontal) turbulence intensity on the vertical walls under the weaker constraint of the stable stratification.
  • Publication
    Direct numerical simulation of vertical particulate channel flow in the turbulent regime
    (Springer, 2010) Uhlmann, Markus; Pinelli, Alfredo
    We have conducted a DNS study of dilute turbulent particulate flow in a vertical plane channel, considering up to 8192 finite-size rigid particles with numerically resolved phase interfaces. The particle diameter corresponds to approximately 9 wall units and their terminal Reynolds number is set to 136. The fluid flow with bulk Reynolds number 2700 is directed upward, which maintains the particles suspended upon average. Two different density ratios were simulated, varying by a factor of 4.5. The corresponding Stokes numbers of the two particles were O(10) in the near-wall region and O(1) in the outer flow. We have observed the formation of large-scale elongated streak-like structures with streamwise dimensions of the order of 8 channel half-widths and cross-stream dimensions of the order of one half-width. At the same time, we have found no evidence of significant formation of particle clusters, which suggests that the large structures are due to an mtxinsic instability of the flow, triggered by the presence of the particles. It was found that the mean flow velocity profile tends towards a concave shape, and the turbulence intensity as well as the normal stress anisotropy are strongly increased. The effect of varying the Stokes number while keeping the buoyancy, particle size and volume fraction constant was relatively weak. More details about part of this work can be found in (2008).
  • Publication
    A fast Lagrangian tracking method capturing finite-size effects in particulate flows
    (American Physical Society, 2011) Wu, M.; Favier, J.; Pinelli, Alfredo
    We present a new method to capture the finite-size effects induced by particles transported by a fluid flow, with a low computational cost compared to available fully-resolved methods, thus allowing to tackle configurations with high volume fractions of particles. The basic idea consists in tracking a source/sink of momentum occurring within a compact support of the mesh, centered on the particle and taking the form of a mollified Dirac kernel, or blob. In the spirit of the immersed boundary method, the shape and the intensity of the kernel are found by imposing appropriate reproducing conditions on the blob (to model accurately a Dirac pulse) and spreading on the mesh cells a volume force determined by the desired boundary condition. The particles occupy a finite-size volume of fluid, therefore introducing a two-way coupled behavior, for the computational cost of only one Lagrangian point. To build the blobs, we will either spread a zero-velocity condition at the blob center, or spread a zero-velocity condition averaged on the fluid parcel enclosed within the support. Both methods are discussed and validated by comparing with free falling fully-resolved particles, in 2D and 3D.