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
    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
    The influence of wall-porosity upon the near-wall turbulence dynamics
    (Center for Numerical Methods in Engineering, 2000) Uhlmann, Markus; Pinelli, Alfredo; Jiménez, J.; Kawahara, Genta; Dopazo, C.
  • Publication
    Linear instability of a corrugated vortex sheet
    (2003) Kawahara, Genta; Jiménez, J.; Uhlmann, Markus; Pinelli, Alfredo
    The linear inviscid instability of an infinitely thin vortex sheet, periodically corrugated with finite amplitude along the spanwise direction, is investigated analytically. Two types of corrugations are studied, one of which includes the presence of an impermeable wall. Exact eigensolutions are found in the limit of a very long wavelength. The sheets are unstable to both sinuous and varicose disturbances. The former is generally found to be more unstable, although the difference only appears for finite wavelengths. The effect of the corrugation is shown to be stabilizing, although in the wall-bounded sheet the effect is partly compensated by the increase in the distance from the wall. The instability is traced to a pair of oblique Kelvin-Helmholtz waves in the flat-sheet limit.
  • Publication
    The instability of streaks and the generation mechanism of streamwise vorticity in near-wall turbulence
    (Japan Society of Mechanical Engineers, 2000) Kawahara, Genta; Jiménez, J.; Uhlmann, Markus; Pinelli, Alfredo
    The linear stability analysis has been performed at Re.TAU.=180 for a turbulent-channel-type base flow with a periodic undulation in the spanwise direction in order to elucidate the generation mechanism of streamwise vorticity through the instability of streaks in near-wall turbulence. It is found that there appear three different instability modes depending on the spanwise wavenumber of streaks. In the case of the streak with around 100 wall-unit spanwise wavelength its critical velocity amplitude lies at .DELTA.Uc+.IMAGE.3, above which streaky flow is unstable to an infinitesimal sinuous disturbance, i. e. a bending mode along the streamwise direction. The instability is identified to originate from inflection points, i. e. wake-like instability, in the spanwise variation of the streaky flow. In this case, unstable eigenmodes take a form that is inclined towards the streamwise direction from the wall-normal direction, and they directly induce the streamwise vorticity on low- and high-speed streaks. In addition, the streamwise vorticity is secondarily produced pricipally through tilting of the wall-normal disturbance vorticity by the base flow shear across the wall-normal direction.