1,720,990 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
Development of a mixed control volume - finite element method for the advection-diffusion equation with spectral convergence
No full text
Numerical investigation of natural convection in inclinedparallel-plate channels partially filled with metal foams
No full textNumerical results of laminar fully developed natural convection in an inclined
composite channel are presented. The mathematical model relies on the
Method of Volume Averaging and thus a single domain approach is applied.
The Forchheimer correction and thermal dispersion effects are considered
while local thermal equilibrium between the porous matrix and the fluid is
assumed.
Velocity and temperature profiles are displayed and their rather weak
dependence upon porosity and fluid properties assessed. Forces acting on the
fluid are identified and discussed. Thermal performances of the composite
channel in laminar conditions are evaluated.
One of the main results of the present investigation is that when non-
Darcian effects and thermal dispersion are negligible, the Nusselt number
of the composite system in natural convection regime is independent of the
channel inclination
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
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
Direct Numerical Simulation of Turbulent Flow and 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, since the available experimental and
theoretical investigations did not clearly establish whether riblets, with different cross sections, produce a net variation in heat transfer in comparison to the flat plate value. In this paper, we report our preliminary numerical results for fully developed laminar and turbulent flow and heat transfer in a channel with triangular riblets. The study is performed by means of direct
numerical simulation at a Reynolds number, based on the wall-shear velocity, Re = 10 and Re = 180 for the laminar and turbulent case, respectively, for a fluid with a Prandtl number
Pr = 0.71. Two different ribbed channels are considered, under a constant heat flux boundary condition. The results obtained are in general agreement, for the flow and turbulent quantities, with past experimental and numerical studies, but the predicted heat transfer efficiency, in the turbulent regime, is significantly lower than some of the experimental studies
On the Wave-Induced Stokes Sublayer and Drag Reduction in the Turbulent Wind
No full textThe interactions of a turbulent wind with a water surface represents a very fundamental problem for many atmospheric processes. The momentum and heat exchanges across the interface with oceans abruptly affects the atmosphere and the understanding of the driving mechanisms would certainly improve weather predictions capabilities. We performed a Direct Numerical Simulation of the wind-wave interaction problem using realistic values of the fluid properties of air and water [3]. The simulation reveals that at low Reynolds numbers, an interesting wind-wave pattern propagating at an angle in the upstream direction is generated. This pattern is recognized to be at the basis of the generation of a spanwise oscillating Stokes sublayer that is responsible for a drag reduction mechanism in the turbulent wind. Despite the simulated flow conditions are far from the intense events occurring at the ocean-atmosphere interface, this basic flow phenomenon may actually explain the large scatter of the drag coefficient data in field measurements where swell waves of arbitrary directions are often present
Reynolds number effects in separating and reattaching flows with passive scalar transport
Full text linkA study of the physics of separating and reattaching flows around bodies with sharp edges is reported. Data from direct numerical simulations of the flow around a rectangular cylinder with aspect ratio 5 at different Reynolds numbers are used. The flow is decomposed into multiple interacting flow phenomena such as the laminar boundary layer in the front face, the separated shear layer, the flow impingement at reattachment, the reverse boundary layer within the recirculating bubble and the near-and far-wake flow. A detailed analysis of the physics of these phenomena is provided, including the slow modulation induced by large-scale instabilities related with vortex shedding. The entrainment phenomena acting along the separated shear layer and their unbalance between its inner and outer sides are recognised as fundamental mechanisms determining the tendency of the flow to reattach and the overall fluxes of momentum and heat. The behaviour of entrainment is found to be strictly related with the shear-layer velocity difference that in turn is determined by the behaviour of the reverse boundary layer and by its strength in counteract adverse pressure gradients. The physical understanding of the compound role played by these and all the other mechanisms composing the flow, poses the basis for the formulation of theoretical frameworks able to unify all these interacting phenomena. Finally, the present work provides access to high-fidelity flow statistics of relevance for benchmark activities on bluff bodies with sharp edges
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