1,739,606 research outputs found
Statistics of the subgrid scales after the shock-turbulence interaction
No full textThe interaction of a normal shock with isotropic turbulence (IT) represents a basic problem for studying some of the phenomena associated with high speed flows, such as hypersonic flight, supersonic combustion and Inertial Confinement Fusion (ICF). In general, in practical applications, the shock width is much smaller than the turbulence scales and the upstream turbulent Mach number is modest. In this case, recent high resolution shock-resolved Direct Numerical Simulations (DNS) (Ryu and Livescu, J. Fluid Mech., 756, R1, 2014) show that the interaction can be described by the Linear Interaction Approximation (LIA). By using LIA to alleviate the need to solve the shock, DNS post-shock data can be generated at much higher Reynolds numbers than previously possible. Here, such results with Taylor Reynolds number around are used to investigate the properties of the subgrid scales (SGS). In particular, it is shown that the shock interaction decreases the asymmetry of the SGS dissipation PDF as the shock Mach number increases, with a significant enhancement in size of the regions and magnitude of backscatter
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the use of effective thermophysical properties to predict the melting process of composite phase change materials with coarse structures
Full text linkComposite PCMs combining metallic foam and paraffin are widely used as phase change materials (PCMs) to tailor the properties of pure PCMs and enhance the thermal energy storage/release. For the complex composites structures, the transient thermal response prediction by direct simulation (DS) is not easy in term of geometry generation and computation. The volume-averaged model (1T model) considering the composite PCMs as homogeneous media is sometimes used to deal with thermal transport in Composite PCMs, not always with a sufficiently good local description of non-steady conditions. The paper carries out a set of cases where a composite PCM modelled as an open-pore body-centred cell made of Aluminium (Al) filled with paraffin (i) to investigate the combined effects of the geometry of the unit cell (side length, porosity), the composite sample (sample height) and boundary conditions (heat input) on the heat response; (ii) to identify the local/overall errors in temperature and volume fraction of liquid PCM (and thus of stored heat) induced by the use of 1T model for various geometry/heat flux combinations. Analytical equations are proposed to predict the maximum temperature difference between Al and PCM as well as the maximum temperature difference calculated by applying the 1T or DS model as a function of the open cell structure geometry and heat flux. The main novelty introduced in the paper is the analytical model used to quantify the maximum local error on molten PCM volume fraction for the 1T model, and thus on heat stored/released. The model supplies good local thermal response predictions for fine structures and lower heat flux input. Nevertheless, errors in the volume fraction of molten PCMs predicted for the whole sample are far lower and the 1T model can be easily applied in a wider range of geometry/conditions
Analysis of the applicability of effective thermophysical properties to composite phase change materials
No full textCoarse form-stable phase change materials (FS-PCMs) can tailor the properties of pure PCMs. This is often attained by the presence of high-melting, high-thermal conductivity metallic phase which enhances the thermal energy storage/release. The evaluation of the thermal response of these composite materials in unsteady conditions, is not an easy task, and simplifications introduced to deal with them must be carefully considered. A set of FS-PCMs of prismatic geometry with polymeric wax as PCM and an Al foam with various pore sizes, modelled as BCC lattice has been considered in this paper. The thermal response under a set of boundary conditions with constant heat flux at the bottom surface, all other being adiabatic, was investigated both by direct simulations approach modelling the two phases and the '1-temperature model’, which considers the material as homogeneous and characterized by a proper set of effective properties. The '1-temperature model’ is able to closely reproduce the whole the local thermal history only within certain validity ranges, even if it can well reproduce the ‘average’ energy storage due to the transformation of the PCM phase
Modelling the conditions for natural convection onset in open-cell porous Al/paraffin composite phase change materials: Effects of temperature, paraffin type and metallic structure geometry
Full text linkComposite Phase Change Materials (PCMs) can be made combining a PCM, i.e., a material that is able store/release heat by its melting/solidification, and a low-amount of well distributed high-melting and high-thermal conductivity phase with the aim of improving the overall conductivity of the material and keeping its high heat storage capability. The composite made by a paraffin and a porous structure of aluminium (Al) has been considered as the representative of this material class. The design of these materials should not only take into account the melting temperature (Tm) and the volume fraction of the paraffin, but also the geometrical distribution and coarseness of the Al phase, which relate to the effective thermal conductivity of the composite as well as the occurrence of natural convection once the PCM is in the molten state. In the present paper, the inverse Body Centred Cubic (BCC) structure has been confirmed to be the most suitable to model high porosity Al foams. For their BCC modelled structure, an analytical equation is proposed for the evaluation of the overall thermal conductivity of the composite PCMs. Also, new best fit equations for predicting permeability of BBC structure are proposed. Analytical description is also given for the Rayleigh-Darcy number obtained as a product of material-dependent term (related to Tm and volume fraction of PCM) and the geometry dependent term (related to volume fraction of PCM, permeability as well as to material coarseness alternatively given in terms of pores per inch, pore size or unit cell length). The model has been validated by means of literature available experimental data. The proposed simplified model can further be adjusted to correlate the onset of natural convection through the local temperature gradient for the composite PCMs
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