1,721,163 research outputs found
Semi-implicit-linearized multiple-relaxation-time formulation of lattice Boltzmann schemes for mixture modeling
No full textA lattice Boltzmann model for mixture modeling is developed by applying the multiple-relaxation-time MRT approach to the Hamel model, which allows one to derive from a general framework different model equations independently proposed, like the Gross-Krook model and the Sirovich model. By imposing some physical constraints, the MRT lattice-Boltzmann Hamel model reduces to the generalized MRT lattice-Boltzmann Gross-Krook model involving the local Maxwellian centered on the barycentric velocity, which allows one to tune independently the species diffusivity, the mixture kinematic viscosity, and the mixture bulk viscosity. Reducing the number of moving particles over the total is possible to deal effectively with mass particle ratios far from unity and, for this reason, to model the pressure-driven diffusion. A convenient numerical approach is proposed for solving the developed model, which essentially widens the stability range of conventional schemes in terms of dimensionless relaxation frequencies, by solving explicitly the advection operator together with the nonlinear terms of the collisional operator and solving implicitly the residual linear terms. In this way, the calculations are drastically reduced and the operative matrices can be computed once for all, at the beginning of the calculation implying moderate additional computational demand. Following this approach, a semi-implicit-linearized backward Euler scheme, ideal for parallel implementations, is proposed. In order to achieve the previous results, the asymptotic analysis, recently suggested for analyzing the macroscopic equations corresponding to lattice-Boltzmann schemes in the low-Mach-number limit, proves to be an effective tool. Some numerical tests are reported for proving the consistency of the proposed method with both the Fick model and Maxwell-Stefan model in the macroscopic limi
Multiple-relaxation-time lattice Boltzmann scheme for homogeneous mixture flows with external force
Full text linkNumerical Prediction of Turbulent Convective Heat Transfer in Mini/Micro Channels for Carbon Dioxide at Supercritical Pressure
Full text linkFinite-volume and Finite-element Hybrid Technique for the Calculation of Complex Heat Exchangers by Semiexplicit Method for Wall Temperature Linked Equations (SEWTLE)
Full text linkComparison among Phenomenological Correlations for Convective Heat Transfer of Supercritical Carbon Dioxide Flowing in Mini/Micro Channels under Cooling Conditions
No full textAsymptotic analysis of multiple-relaxation-time lattice Boltzmann schemes for mixture modeling
Full text linkAbstractA new lattice Boltzmann model for simulating ideal mixtures has been developed by means of the multiple-relaxation-time (MRT) approach. When compared with the previous single-relaxation-time (SRT) formulation of the same model, based on the continuous kinetic theory, the new model offers the possibility to independently tune the mutual diffusivity and the effects of cross collisions on the effective stress tensor. The additional degrees of freedom, due to the increased set of relaxation time constants used for modeling the cross collisions, allow us to match the experimental data on macroscopic transport coefficients. Two different integration rules, i.e. the forward Euler and the modified mid-point integration rule, were used in order to numerically integrate the developed model. Unfortunately the simpler forward Euler integration rule violates the mass conservation and there is no way to fix the problem by changing the definition of the macroscopic velocity. On the other hand, a small correction has been purposely designed for compensating this error by means of the mid-point integration rule. Some numerical simulations are reported for proving the effectiveness of the proposed corrective factor. For the considered application, the asymptotic analysis, recently suggested as an effective tool for analyzing the macroscopic equations corresponding to the lattice Boltzmann schemes, offers a remarkable advantage in comparison with the classical Chapman–Enskog technique, because it easily deals with leading terms in the distribution functions, which are no more Maxwellian
Lattice Boltzmann scheme for mixture modeling: analysis of the continuum diffusion regimes recovering Maxwell-Stefan model and incompressible Navier-Stokes equations
Full text linkViscous coupling based lattice Boltzmann model for binary mixtures
Full text linkA new lattice Boltzmann model for binary mixtures, which can naturally include both the two-fluid approach and the single-fluid approach, is developed. The model is derived from the continuous
kinetic model proposed by Hamel, which independently takes into account self-collisions and cross collisions. The original kinetic model is discussed in order to appreciate that cross collisions realize an internal coupling force, proportional to the diffusion velocity, and an additional coupling effect in the effective stress tensor, proportional to the deformation of the barycentric velocity field. For this reason, Hamel’s model is the natural forerunner of all linearized models based on the two-fluid approach and allows us to describe binary mixtures at different limiting regimes consistently. A discrete lattice Boltzmann model, which recovers the original Hamel’s model with second-order accuracy in both time and space, is proposed. This discrete model can analyze ordinary diffusion, pressure diffusion, and forced diffusion
Metodo e sistema per la generazione di vapore in struttura planare mediante radiazione solare od altra radiazione
No full textL’invenzione riguarda un metodo per la generazione di vapore (V) che prevede, in particolare, di dirigere una radiazione solare od altra radiazione (R) su una struttura planare (100), e specificamente su un supporto (2) di detta struttura planare (100) comprendente un materiale (4) assorbente di detta radiazione (R), in modo che l’energia termica rilasciata da detta radiazione (R) induca l’evaporazione di un fluido (F) che fluisce sotto forma di film sottile in un’intercapedine (3) di detta struttura planare (100), con conseguente produzione di vapore (V); l’invenzione riguarda anche il relativo sistema per la generazione di vapore (V) che utilizza tale metodo e che comprende almeno una struttura planare (100) comprendente, a sua volta, un elemento superiore (1), un supporto (2) comprendente un materiale (4) assorbente di radiazione solare od altra radiazione (R), un’intercapedine (3) tra detto elemento superiore (1) e detto supporto (2) in cui fluisce un fluido (F) sotto forma di film sottile, su detta struttura planare (100) essendo diretta detta radiazione (R) riflessa da almeno un dispositivo riflettente (16) rivolto verso detta struttura planare (100), detto sistema comprendendo inoltre almeno un condotto (13) attraverso cui viene fatto fuoriuscire il vapore (V) prodotto dall’evaporazione di detto fluido (F) per effetto dell’energia termica rilasciata da detta radiazione (R).
Il metodo ed il sistema secondo la presente invenzione sono preferibilmente e vantaggiosamente impiegati per la generazione di vapore e la sua successiva condensazione in impianti di purificazione e/o dissalazione di acqua di mare, fiume, palude o piovana; la presente invenzione risulta particolarmente adatta all’uso in regioni remote e per applicazioni cosiddette “off-grid”
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