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 - 4 of 4
  • 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
    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
    The autonomous near-wall turbulent cycle
    (American Physical Society, 1997) Jiménez, J.; Pinelli, Alfredo
    The regeneration cycle of near-wall turbulence is investigated by numerical experiments in which different possibly-important effects are selectively removed either from the equations of motions or from the boundary conditions. A candidate is considered important to the cycle if its removal results in substantial damping or decay of the turbulence intensity. Other effects, although probably present, are considered secondary. It is shown in this way that neither the presence of a turbulent core nor the generation of secondary vorticity at the wall are important in the generation of turbulence, while the presence of low-velocity streaks is crucial. Damping the streaks, even if the quasi-streamwise vortices are not directly modified, leads to full relaminarisation. Interrupting the process by which streaks are created by advection of the mean profile by the quasi-streamwise vortices has the same effect. This suggests that drag control strategies could be equally directed to damp the streaks or, as is more common, to damp directly the quasi-streamwise vortices.
  • Publication
    Turbulent flow above a porous surface
    (American Physical Society, 1999) Jiménez, J.; Uhlmann, Markus; Kawahara, Genta; Pinelli, Alfredo
    We have investigated a fully turbulent, low-Reynolds number plane channel with one impermeable and one porous wall. The latter is assumed to obey Darcy's law and to be mass-neutral in the mean. Our DNS data show an enhancement of wall friction up to 40%, along with an increase in overall turbulence intensity. Most strikingly, a strong spanwise organization of the flow is observed due to the presence of large-scale, roller-like structures. Their effect is to alternatively energize and almost laminarize the buffer zone and the sublayer streaks. Linear analysis of the turbulent mean profile over a porous surface shows it to be unstable, with spanwise two-dimensional eigenfunctions whose shape and phase velocity are similar to those of the observed rollers. Finally, we show that the above effects of passive porosity can be duplicated by means of active two-dimensional wall transpiration, with the right shape and advection velocity. Interestingly, active injection with the wrong advection velocity has no effect on the average skin friction.