1,720,998 research outputs found
Direct numerical simulation of low-Prandtl number turbulent convection above a wavy wall
Full text linkTurbulent forced convection is investigated by Direct Numerical Simulation in a channel with one sinusoidal wavy wall and one flat wall. Fluid flow and heat transfer are periodically fully developed, the simulated Reynolds number of the bulk velocity and the hydraulic diameter is Re = 18, 880 while three Prandtl numbers are considered, i.e. Pr = 0.025, Pr = 0.2, and Pr = 0.71. The fluid flow is characterized by separation, reattachment and a shear layer downstream the wave peak, these are conditions relevant for turbulent heat transfer and passive scalar transport applications. In the range of Péclet numbers investigated, the most important heat transfer mechanism is by mean flow advection. Accordingly, the peak heat transfer region is in the upslope part of the domain. The separation bubble instead acts as a barrier to convection and the heat transfer rate is minimum close to separation. An a priori analysis is performed in order to assess the accuracy of turbulent heat transfer models based on the Generalized Gradient Diffusion Hypothesis
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
Compact finite volume schemes on boundary-fitted grids
No full textThe paper focuses on the development of a framework for high-order compact finite volume discretization of the three dimensional scalar advection–diffusion equation. In order to deal with irregular domains, a coordinate transformation is applied between a curvilinear, non-orthogonal grid in the physical space and the computational space. Advective fluxes are computed by the fifth-order upwind scheme introduced by Pirozzoli [S. Pirozzoli, Conservative hybrid compact-WENO schemes for shock turbulence interaction, J. Comp. Phys. 178 (2002) 81] while the Coupled Derivative scheme [M.H. Kobayashi, On a class of Pade ́ finite volume methods, J. Comp. Phys. 156 (1999) 137] is used for the discretization of the diffusive fluxes. Numerical tests include unsteady diffusion over a distorted grid, linear free-surface gravity waves in a irregular domain and the advection of a scalar field. The proposed methodology attains high-order formal accuracy and shows very favorable resolution characteristics for the simulation of problems with a wide range of length scales
Numerical investigation of natural convection in inclined parallel-plate channels partly 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 turbulent forced convection in a wavy channel at low and order one Prandtl number
No full textTurbulent forced convection in a channel with one planar wall and one wall of sinusoidal shape is investigated by Direct Numerical Simulation. The flow is fully developed and the Reynolds number based on the mean bulk velocity and the average hydraulic diameter is Re ≈ 18,900; in this weakly turbulent flow regime three different Prandtl number values are investigated, Pr = 0.025, 0.20, 0.71. The fluid is in contact with the colder channel walls at an equal, uniform temperature. The main statistical quantities, like the root-mean-square of temperature fluctuations and the turbulent heat fluxes, the local heat transfer coefficient and turbulent Prandtl number values are reported. Effects of flow separation and reattachment on the local heat transfer rate and turbulent Prandtl number distribution are also presented and discussed. An a priori analysis of the behavior of the simple gradient diffusion model of turbulent heat fluxes is performed in the low Prandtl number, separated flow conditions of the present work. While the low Prandtl number effect can be accounted for by an appropriate selection of the turbulent Prandtl number value to be provided to the model, deviations form the expected behavior of turbulent heat fluxes are seen to occur in the flow separation region and downstream reattachment
Development of a mixed control volume – Finite element method for the advection–diffusion equation with spectral convergence
No full textIn this paper we attack the problem of devising a finite volume method for computational fluid dynamics and related phenomena which can deal with complex geometries while attaining high-orders of accuracy and spectral convergence at a reasonable computational cost. As a first step towards this end, we propose a control volume finite element method for the solution of the advection–diffusion equation. The numerical method and its implementation are carefully tested in the paper where h- and p-convergence are checked by comparing numerical results against analytical solutions in several relevant test-cases. The numerical efficiency of a selected set of operations implemented is estimated by operation counts, ill conditioning of coefficient matrices is avoided by using an appropriate distribution of interpolation points and control-volume edges
Numerical simulation of weakly turbulent heat transfer over cavities at low Prandtl numbers
No full textHeat transfer investigations at low Prandtl number values are of interest for different applications, but reliable physicalmodels for turbulent convection in these fluids are still missing. In the present study forced convective heat transfer isinvestigated numerically for weakly turbulent flow in a streamwise periodic channel with cavities. Simulations are performedfor different values of the Prandtl number, with the focus on the case with Prandtl number equal to 0:025, which representsliquid lead-bismuth eutectic (LBE). In the paper the main features of the mean and instantaneous flow field are presentedtogether with first order statistics. The influence of flow separation mechanisms and turbulent mixing on local and globalheat transfer rates are discussed, for both isoflux and isothermal boundary conditions (BCs hereafter). The numericalcode used for the simulations is based on a second order, finite volume algorithm, implemented over structured, curvilinearmesh. This work aims to contribute to the development of physical model of turbulent convection for low Prandtl numberfluid flows, by providing information for both velocity and temperature fields
Direct Numerical Simulation of turbulent mixed convection around a bundle of heated rods at low-Prandtl number
No full textThe present work reports an overview of the results of Direct Numerical Simulations performed on the case of fully-developed, mixed convection flow of a liquid metal around a uniformly heated bundle of vertical rods. Finite-Volume computations are performed by an original discretization technique based on the representation of arbitrarily-shaped cylindrical boundaries on a non-uniform Cartesian grid. A domain consisting of four subchannels of a triangular lattice of rods with a pitch-to-diameter ratio P/D = 1.4 is considered as the reference geometry. A single friction Reynolds number value is simulated, namely Reτ = 550. Both forced and mixed convection regimes are investigated, buoyancy effects being introduced by imposing a Richardson number value of Ri = 0.25. A Prandtl number Pr = 0.031 is chosen to represent LBE as the working fluid. Instantaneous snapshots and relevant statistics of the velocity and thermal fields are reported here for the considered case, and integral results are compared against available literature data
Numerical simulation of forced convection over steps at low Prandtl number
No full textConvective heat transfer in laminar conditions is investigated numerically for a Prandtl number Pr = 0.025, representative of liquid lead-bismuth eutectic (LBE). The geometry selected is a periodic channel with a set of backward and forward steps. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number up to the transitional range. It is shown how flow can undergo separation and reattachment also at very low-Re. The influence of flow separation mechanisms on local and global heat transfer rates is discussed, also considering the effect of different thermal boundary conditions imposed at the channel walls
- …
