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 - 3 of 3
  • Publication
    Marginally turbulent flow in a square duct
    (Cambridge University Press, 2007) Uhlmann, Markus; Pinelli, Alfredo; Kawahara, Genta; Sekimoto, Atshushi
    A 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.
  • Publication
    Turbulence-and buoyancy-driven secondary flow in a horizontal square duct heated from below
    (American Institute of Physics, 2011) Sekimoto, Atshushi; Kawahara, Genta; Sekiyama, K.; Uhlmann, Markus; Pinelli, Alfredo
    Direct numerical simulations of fully developed turbulent flows in a horizontal square duct heated from below are performed at bulk Reynolds numbers Re(b) = 3000 and 4400 (based on duct width H) and bulk Richardson numbers 0 <= Ri <= 1.03. The primary objective of the numerical simulations concerns the characterization of the mean secondary flow that develops in this class of flows. On one hand, it is known that turbulent isothermal flow in a square duct presents secondary mean motions of Prandtl's second kind that finds its origin in the behavior of turbulence structures. On the other hand, thermal convection drives a mean secondary motion of Prandtl's first kind directly induced by buoyancy. As far as the mean structure of the cross-stream motion is concerned, it is found that different types of secondary flow regimes take place when increasing the value of the Richardson number. The mean secondary flow in the range 0.025 less than or similar to Ri less than or similar to 0.25 is characterized by a single large-scale thermal convection roll and four turbulence-driven corner vortices of the opposite sense of rotation to the roll, as contrasted with the classical scenario of the eight-vortex secondary flow pattern typical of isothermal turbulent square-duct flow. This remarkable structural difference in the corner regions can be interpreted in terms of combined effects, on instantaneous streamwise vortices, of the large-scale circulation and of the geometrical constraint by the duct corner. When further increasing the Richardson number, i.e., Ri greater than or similar to 0.25, the structure of the mean secondary flow is solely determined by the large-scale circulation induced by the buoyancy force. In this regime, the additional mean cross-stream motion is characterized by the presence of two distinct buoyancy-driven vortices of opposite sense of rotation to the circulation only in two of the four corner regions. With increasing Ri, the large-scale circulation is found to enhance the wall skin friction and heat transfer. In the significant-buoyancy regime Ri greater than or similar to 0.25, the mean cross-stream motion and its rms fluctuations are found to scale, respectively, with the buoyancy-induced velocity u(g)=root g beta Delta TH (g, beta, and Delta T being the gravity acceleration, the volumetric coefficient of thermal expansion, and the temperature difference across the duct, respectively) and with the mixed velocity scale root(nu/H)u(g) (nu being the kinematic viscosity). It is suggested that the probable scalings for the rms of streamwise velocity component and of temperature fluctuation are related with the friction velocity u(tau) and friction temperature T(tau) according to the magnitudes u(tau)(2)/ and T(tau)u(tau)/root(nu/H)u(g), respectively.
  • Publication
    Characterisation of marginally turbulent square duct flow
    (Springer, 2007) Uhlmann, Markus; Pinelli, Alfredo; Sekimoto, Atshushi; Kawahara, Genta; Palma, J.L.M.L.; Silva Lopes, A.