## Person: Pinelli, Alfredo

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##### First Name

Alfredo

##### Last Name

Pinelli

##### Affiliation

Universidad Complutense de Madrid

##### Faculty / Institute

Ciencias Matemáticas

##### Department

##### Area

Matemática Aplicada

##### Identifiers

36 results

## Search Results

Now showing 1 - 10 of 36

Publication The autonomous cycle of near-wall turbulence(Cambridge University Press, 1999) Jiménez, Javier; Pinelli, AlfredoNumerical experiments on modified turbulent channels at moderate Reynolds numbers are used to differentiate between several possible regeneration cycles for the turbulent fluctuations in wall-bounded flows. It is shown that a 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 laminarization. The presence of the wall seems to be only necessary to maintain the mean shear. The generation of secondary vorticity at the wall is shown to be of little importance in turbulence generation under natural circumstances. Inhibiting its production increases turbulence intensity and drag.Publication Chebyshev collocation method and multidomain decomposition for the incompressible Navier‐Stokes equations(Wiley, 1994) Pinelli, Alfredo; Vacca, A.The two-dimensional incompressible Navier-Stokes equations in primitive variables have been solved by a pseudospectral Chebyshev method using a semi-implicit fractional step scheme. The latter has been adapted to the particular features of spectral collocation methods to develop the monodomain algorithm. In particular, pressure and velocity collocated on the same nodes are sought in a polynomial space of the same order; the cascade of scalar elliptic problems arising after the spatial collocation is solved using finite difference preconditioning. With the present procedure spurious pressure modes do not pollute the pressure field. As a natural development of the present work a multidomain extent was devised and tested. The original domain is divided into a union of patching sub-rectangles. Each scalar problem obtained after spatial collocation is solved by iterating by subdomains. For steady problems a C1 solution is recovered at the interfaces upon convergence, ensuring a spectrally accurate solution. A number of test cases have been solved to validate the algorithm in both its single-block and multidomain configurations. The preliminary results achieved indicate that collocation methods in multidomain configurations might become a viable alternative to the spectral element technique for accurate flow prediction.Publication Reynolds number dependence of mean flow structure in square duct turbulence - CORRIGENDUM(Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, GentaPublication Marginally turbulent flow in a square duct(Cambridge University Press, 2007) Uhlmann, Markus; Pinelli, Alfredo; Kawahara, Genta; Sekimoto, AtshushiA 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 Immersed boundary method for generalised finite volume and finite difference Navier-Stokes solvers(American Society of Mechanical Engineers, 2010) Pinelli, Alfredo; Naqavi, I.Z.; Piomelli, U.In Immersed Boundary Methods (IBM) the effect of complex geometries is introduced through the forces added in the Navier-Stokes solver at the grid points in the vicinity of the immersed boundaries. Most of the methods in the literature have been used with Cartesian grids. Moreover many of the methods developed in the literature do not satisfy some basic conservation properties (the conservation of torque, for instance) on non-uniform meshes. In this paper we will follow the RKPM method originated by Liu et al. [1] to build locally regularized functions that verify a number of integral conditions. These local approximants will be used both for interpolating the velocity field and for spreading the singular force field in the framework of a pressure correction scheme for the incompressible Navier-Stokes equations. We will also demonstrate the robustness and effectiveness of the scheme through various examples.Publication Reynolds number dependence of mean flow structure in square duct turbulence(Cambridge University Press, 2010) Pinelli, Alfredo; Uhlmann, Markus; Sekimoto, Atshushi; Kawahara, GentaWe 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 Dynamics 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.Publication Coherent structures in marginally turbulent square duct flow(Springer, 2008) Uhlmann, Markus; Pinelli, Alfredo; Sekimoto, Atshushi; Kawahara, Genta; Kaneda, YukioDirect numerical simulation of fully developed 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 measures around 190 wall units, roughly independently of the Reynolds number. Furthermore, we present a characterization of the marginal state, where coherent structures are found to have significant relevance to the appearance of secondary flow of Prandtl’s second kind.Publication The eﬀect of coherent structures on the secondary ﬂow in a square duct(Springer, 2009) Sekimoto, Atshushi; Pinelli, Alfredo; Uhlmann, Markus; Kawahara, Genta; Eckhardt, BrunoThe appearance of secondary flow of Prandtl’s second kind is a well-known phenomenon in fully developed turbulent rectangular duct flow. The intensity of the secondary flow is two orders of magnitude smaller than that of the mean streamwise velocity; however, it plays an important role in the crossstreamwise momentum, heat and mass transfer. Our recent study [1] revealed that the mean secondary flow is a statistical footprint of the turbulent flow structures, i.e. streamwise vortices and streaks which are observed in the nearwall region, whose cross-sectional positions are constrained by the presence of the side walls at marginal Reynolds number (approximately 1100, based on the bulk velocity and the duct half width, corresponding to a friction Reynolds number of about 80). In this marginal case, one low-speed streak associated with a pair of counter-rotating streamwise vortices can exist over each wall and they are self-sustained [2]. When considering the higher Reynolds numbers, the increment of duct width in wall unit allows the simultaneous presence of multiple low velocity streaks and pairs of streamwise vortices upon the wall.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, AlfredoDirect 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.