An a priori assessment of the Partially Stirred Reactor (PaSR) model for MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion has drawn increasing attention as it allows to avoid the thermo-chemical conditions prone to the formation of pollutant species while ensuring high energy efficiency and fuel flexibility. MILD combustion is characterized by a strong competition between turbulent mixing and chemical kinetics so that turbulence-chemistry interactions are naturally strengthened and unsteady phenomena such as local extinction and re-ignition may occur. The underlying physical mechanisms are not fully understood yet and the validation of combustion models featuring enhanced predictive capabilities is required. Within this context, high-fidelity data from Direct Numerical Simulation (DNS) represent a great opportunity for the assessment and the validation of combustion closure formulations. In this study, the performance of the Partially Stirred Reactor (PaSR) combustion model in MILD conditions is a priori assessed on Direct Numerical Simulations (DNS) of turbulent combustion of MILD mixtures in a cubical domain. Modeled quantities of interest, such as heat release rate and reaction rates of major and minor species, are compared to the corresponding filtered quantities extracted from the DNS. Different submodels for the key model parameters, i.e., the chemical time scale τ_c and the mixing time scale τ_mix, are considered and their influence on the results is evaluated. The results show that the mixing time scale is the leading scale in the investigated cases. The best agreement with the DNS data regarding the prediction of heat release rate and chemical source terms is achieved by the PaSR model that employs a local dynamic approach for the estimation of the mixing time scale. An overestimation of the OH species source terms occurs in limited zones of the computational domain, characterized by low heat release rates.     

Plateau's Problem as a Singular Limit of Capillarity Problems

Soap films at equilibrium are modeled, rather than as surfaces, as regions of small total volume through the introduction of a capillarity problem with a homotopic spanning condition. This point of view introduces a length scale in the classical Plateau's problem, which is in turn recovered in the vanishing volume limit. This approximation of area minimizing hypersurfaces leads to an energy based selection principle for Plateau's problem, points at physical features of soap films that are unaccessible by simply looking at minimal surfaces, and opens several challenging questions. © 2020 Wiley Periodicals LLC.     

Theoretical and experimental determination of the aerodynamic characteristics of a helicopter rotor

Summary A fairly simple theory which can determine the aerodynamic characteristics of a freely hinged rotor in any condition of forward flight is discussed in the present work.Theoretical results are compared with NASA experimental data and with data from tests on a helicopter model in the wind tunnel of Istituto di Meccanica Applicata alle Macchine, Aerodinamica e Gasdinamica (Politecnico of Torino).The agreement between theoretical and experimental results is well suited for practical purpose.

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Sommario Nella nota viene presentata una teoria relativamente semplice per la determinazione delle caratteristiche aerodinamiche di un rotore di elicottero completamente incernierato o basculante nelle condizioni di volo in avanti a regime. I risultati numerici vengono confrontati con dati sperimentali ottenuti dalla NASA e con misure sperimentali effettuate su di un modello di elicottero nella galleria del vento dell'Istituto di Meccanica Applicata alle Macchine, Aerodinamica e Gasdinamica (Politecnico di Torino).L'accordo tra i risultati teorici e i risultati sperimentali è giudicato valido nelle applicazioni pratiche.

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Research supported by the National Research Council (C.N.R.) of Italy.     

A CNDO/2 STUDY OF ROTATIONAL BARRIERS IN THIOAMIDOPYRIDINES

Abstract

Barriers to rotation about C[sbnd]C and C[sbnd]N bonds in 2-, 3- and 4-thioamidopyridine have been calculated by the CNDO/2 method. Comparisons between computed and experimental data are used to assess reliability of the calculations.     

Structural evidence of differential forms of nanosponges of beta-cyclodextrin and its effect on solubilization of a model drug

Nanosponges (NS) are a recently developed class of hyper-branched polymers, nano-structured to form three dimensional meshwork; obtained by reacting cyclodextrins with a cross linker like diphenyl carbonate. Herein, we report an anomalous behavior of NS with regards to physical and morphological characteristics and drug encapsulation behavior by minor synthetic modification. Two distinct forms viz. crystalline and para-crystalline of NS were identified and extensively characterized by use of high resolution transmission electron microscopy (HR-TEM), X-ray powder diffraction (XRPD), scanning electron microscope, atomic force microscope, optical microscope and Fourier transform infra-red attenuated total reflectance spectroscopy (FTIR-ATR). Dimension of the crystal lattice was found to be equal to 0.61 nm. Higher magnifications clearly showed a zone axis with a hexagonal symmetry as that of beta-cyclodextrin. XRPD patterns were in concurrence with the HR-TEM results. Solubility studies with a model drug dexamethasone (DEX) showed more than three folds increase in the solubility of the drug in the crystalline NS as compared to the para-crystalline ones. Percent drug association and drug loading for DEX was found to be higher in the crystalline type of NS. An In vitro drug kinetic study evidenced a faster release of DEX from the crystalline type NS. The particle sizes of the formulations were as follows: crystalline NS: 688.6 ± 38.0 nm, para-crystalline NS: 702.2 ± 21.2 nm with polydispersity indices of 0.155 and 0.132; zeta-potential of ?26.55 ± 1.7 and ?23.42 ± 2.1 respectively. Differential scanning calorimetry and thermogravimetric analysis revealed that both forms encapsulated the drug satisfactorily. FTIR-ATR and Raman spectroscopy showed weak interactions. Crystallinity of NS was thus found to be an important factor in solubilization, in vitro kinetics and encapsulation behavior and can be tuned to give a tailored drug release profile or formulation characteristics.     

Orthogonal geometry of wave interaction in a photorefractive crystal for linear phase demodulation

We present a novel configuration of an interferometer which is implemented by using vectorial wave mixing in the orthogonal geometry of wave interaction in a cubic photorefractive crystal, for the detection of small phase transients. The orthogonal geometry of wave interaction allows the linear regime of the phase-to-intensity transformation even when phase transients are encrypted in a depolarized wave without using any polarizing element. The use of a depolarized beam without a polarizing element strongly diminishes the noise level, thus increasing the sensitivity to phase transients.     

Was Poisson's spot an intentional discovery?

This short historical note deals with the phenomenological aspect of diffraction patterns in obstacles having central or axial symmetry. In the special case of the circular symmetry the light spot at the centre of the shadow is called “Poisson's spot” and it is often quoted as a crucial moment in the development of the theories about light. In this paper the phenomenon is seen only as a typical case of poor knowledge of the scientific Literature and of an unattentive observation of the men involved.