139 research outputs found
A fast algorithm for Direct Numerical Simulation of turbulent convection with immersed boundaries
No full textA parallel algorithm is presented for the Direct Numerical Simulation of convection flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. The method presented includes a triperiodic Poisson solver employed irrespective of the actual boundary shape and a second order accuracy for the computational domain, including the near wall regions, when walls are defined as immersed boundaries. The numerical solution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for laminar and turbulent pipe flow, and for the case of flow around an array of heated cylindrical rods arranged in a triangular lattice. The formal accuracy of the method is demonstrated in laminar flow conditions, and DNS results in turbulent conditions are compared to available literature data, thus confirming the favorable qualities of the method
Direct Numerical Simulation of Heat Transfer Over Riblets
No full textRiblets are well-known as a passive mean for drag reduction in turbulent flow conditions, but their effectiveness for heat transfer is quite controversial. In this paper we present the numerical results for fully developed laminar and turbulent flow and heat transfer in a channel with triangular riblets. The turbulent study is performed by means of direct numerical simulation at a Reynolds number Re_\tau =180 based on the wall-shear velocity, for a fluid with a Prandtl number Pr=0.71. Four different ribbed channels are considered, under a constant heat flux boundary condition, and correspond to ridge angle a \alpha = 45 and 60 degrees, and riblet spacing s^+ = 20 and s^+ = 40. The results obtained, for the flow and turbulent quantities, are in good agreement with past experimental and numerical studies, and correctly reproduce drag reduction over the smaller s^+ = 20 riblets and drag increase over the larger s^+ = 40 riblets. The predicted heat transfer efficiency of riblets do not agree with some experimental results, and is below that of a flat plate for all the configurations. The conditions for heat transfer enhancement are discussed
Finite Volume Compact Schemes on Staggered Grids
No full textCompact finite-difference schemes have been recently used in several Direct Numerical Simulations of turbulent
flows, since they can achieve high-order accuracy and high resolution without exceedingly increasing the size of the
computational stencil. The development of compact finite-volume schemes is more involved, due to the appearance of
surface and volume integrals. While Pereira et al. [J. Comput. Phys. 167 (2001)] and Smirnov et al. [AIAA Paper, 2546,
2001] focused on collocated grids, in this paper we use the staggered grid arrangement. Compact schemes can be tuned
to achieve very high resolution for a given formal order of accuracy. We develop and test high-resolution schemes by
following a procedure proposed by Lele [J. Comput. Phys. 103 (1992)] which, to the best of our knowledge, has not yet
been applied to compact finite-volume methods on staggered grids. Results from several one- and two-dimensional
simulations for the scalar transport and Navier–Stokes equations are presented, showing that the proposed method is
capable to accurately reproduce complex steady and unsteady flows
Direct numerical simulation of turbulence in the wake of a metal foam
No full textA Direct Numerical Simulation is carried out to study a turbulent wake. The flow configuration is typical of grid turbulence investigations, but in place of a regular grid or fractal grid, the initially uniform flow passes through a three-dimensional, irregular yet statistically isotropic porous matrix. A synthetic, periodic, open cell metal foam of porosity ε = 0.92 is the geometry selected. The flow is at a Reynolds number based on the mean pore diameter dp and the freestream velocity U∞ of Redp = 4000. An approximation to homogeneous and isotropic decaying turbulence is achieved in the lee of the porous layer. Statistics reported include isotropy indicators, skewness, flatness, velocity autocorrelations, the integral scale of turbulence and compensated spectra. Dissipation of turbulent kinetic energy is calculated from its definition and from some known approximations based on different hypotheses, results extracted provide practical advice for experimentalists and give an insight in the isotropic features of the flow
A Phase-Field Approach for Liquid-Liquid Flow Simulations
No full textA phase-field approach is presented for the numerical simulation of two-phase forced flow in channels. The difference in physical properties of the two components is handled following a quasi-incompressible approach (Lowengrub and Truskinovsky [16]). The axisymmetric form of the Navier − Stokes and Cahn − Hilliard equations system is solved for a narrow pipe of radius R = 1 mm, where buoyancy effects are neglected. Results of three simulations for the evolution of spherical and elongated bubbles are reported
Experimental investigation of three phase oil/water/air flow
No full textThree-phase flow of oil, water and air is investigated in a horizontal pipe of inner diameter 40 mm. The mineral oil used in the experiments is similar to crude oil transported in industrial pipelines with a viscosity of μo = 0.919 Pa s and ro = 889 kg/m3 at 20◦C. Careful measurements of pressure drop are performed at different flow rates of the liquids and with the air superficial velocity being varied in steps between Jg = 0.0 m/s and Jg ≈ 6.5 m/s. The pressure drop behaviour depends upon the initial two-phase flow regime of the two liquids, and in most cases it shows a progressive increase caused by the injection of air. A wide variety of flow patterns is observed: pictures of flow configurations obtained by starting with the two liquids in core-annular and wavy-annular regimes are provided together with full descriptions
Buoyancy-driven turbulent convection in a bundle of vertical heated cylinders
No full textBackground Buoyant, turbulent convective heat transfer around cylindrical rods arranged in bundles is a technically relevant heat transfer configuration which finds application in steam generators, cooling of reactor core fuel assemblies and heat exchangers in general. Most of the research performed so far considered forced convection conditions on vertical rod bundles, corresponding for example to the configuration of a nuclear reactor primary loop. Fewer works have focused on the effect of buoyancy, with or without an external source of momentum. In their experimental investigation, Hallinan and Viskanta [4] employed a thermosyphon loop to determine the average heat transfer coefficients for water under natural circulation conditions in a tube bundle containing twenty-one tubes; their work is mainly focused on the favorable effect of grid spacers on heat transfer enhancement. El Genk et al. performed experiments of upflow- and downflow-forced turbulent and laminar convection, natural convection and buoyancy-assisted combined convection of water in a uniformly heated square lattice of seven [2] and nine [3] rod bundles with variable pitch-to-diameter ratio, Reynolds and Rayleigh number. They proposed heat transfer correlations and concluded that the rod arrangement only negligibly affects the overall Nusselt number in both forced and natural convection regimes. Concerning the numerical modeling of this class of flows, only very recent works resort to the Large Eddy Simulation [5], and even less frequently, to the Direct Numerical Simulation [7]. This is largely due to the geometric complexity of the flow domain and the difficulties related to the adoption of numerical techniques allowing for sufficiently accurate results
Development of a mixed control volume - finite element method for the advection-diffusion equation with spectral convergence
No full text
Luigi Stalio - verso
No full textBotanico: Stalio, Luigi (1799-1882).
Professore di nautica nella scuola di marina mercantile in Venezia; raccoglitore e conoscitore di piante e animali dalmati.
Titolo manoscritto sul recto, dove compare anche la nota: Ripr. dall’album Nardo 1899.
Montata su cartoncino 103 x 66 mm.
1 fotografia : aristotipo ; 84 x 56 mm.
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