Person: Pinelli, Alfredo
Universidad Complutense de Madrid
Faculty / Institute
Now showing 1 - 8 of 8
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. PublicationDynamics of the structures of near wall turbulence(Springer, 1999) Jiménez, J.; Pinelli, AlfredoNumerical 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. PublicationThe autonomous near-wall turbulent region(American Physical Society, 1998) Jiménez, J.; Pinelli, AlfredoThe 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. PublicationThe 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. PublicationLinear instability of a corrugated vortex sheet(2003) Kawahara, Genta; Jiménez, J.; Uhlmann, Markus; Pinelli, AlfredoThe 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. PublicationThe 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, AlfredoThe 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. PublicationThe autonomous near-wall turbulent cycle(American Physical Society, 1997) Jiménez, J.; Pinelli, AlfredoThe 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. PublicationTurbulent flow above a porous surface(American Physical Society, 1999) Jiménez, J.; Uhlmann, Markus; Kawahara, Genta; Pinelli, AlfredoWe 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.