The present work reports a simple, inexpensive method for synthesis of calcium hydroxide [Ca(OH) 2 ] nanoparticles (CHNPs). The method involves chemical precipitation (CP) in aqueous medium at room temperature. Calcium nitrate dihydrate [Ca (NO 3 ) 2 .2H 2 O] and sodium hydroxide were used as precursors. The CHNPs were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Rietveld analysis, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), BET surface area evaluation as well as particle size distribution analysis techniques. The results confirmed the synthesis of CHNPs as the major phase. The CHNPs exhibited an average size of about 350 nm. In addition, some calcite phase formed due to the inevitable carbonation process. A very minor amount of aragonite phase was also present. A schematically developed new qualitative model is proposed to explain the genesis and subsequent evolution of the various phases at the nanoscale. The model helps to identify the rate-controlling step. It also highlights the implication of reaction kinetics control in synthesis of predesigned nanophase assembly.

Computational models are developed to predict the natural convection heat transfer and buoyancy for a Montgolfiere under conditions relevant to the Titan atmosphere. Idealized single-and double-walled balloon geometries are simulated using algorithms suitable for both laminar and (averaged) turbulent convection. Steadystate performance results are compared with existing heat transfer coefficient correlations. The laminar results, in particular, are used to test the validity of the correlations in the absence of uncertainties associated with turbulence modeling. Some discrepancies are observed, which appear to be primarily associated with temperature nonuniformity on the balloon surface. The predicted buoyancy for both the single-and double-walled balloons in the turbulent convection regime, predicted with standard two-equation turbulence models, showed trends similar to those with the empirical correlations. There was also good agreement with recently conducted experiments in a cryogenic facility designed to simulate the Titan atmosphere.

We have demonstrated a novel solution chemistry route for the synthesis of nanoscale hierarchical flower-like MnO 2 through a light-assisted decomposition of a manganese precursor over the surface of a clay nanotube. By tuning the reaction conditions, we have successfully synthesized δ-MnO 2 flowery nanostructures comprising assemblies of intersected nanosheets and subsequently studied their photocatalytic activity for the degradation of organic dyes under natural sunlight irradiations. The crystallographic phase dependent photocatalytic activity of MnO 2 nanocomposites has also been carried out toward the photodegradation of dyes, indicating δ-MnO 2 nanostructures possess higher catalytic efficiency compared to α-MnO 2 . The underlying mechanism demonstrates the formation of reactive oxygen species, which in turn facilitate the degradation of dyes and also substantiates that there is no need of any supplementary oxygen sources during photodegradation. The outstanding performance of the hierarchical δ-MnO 2 nanocomposites, together with the convenient fabrication method, represents an alternative and environmentally benign route to develop heterogeneous photocatalyst for the degradation of refractory pollutants. Thus, these new insights will shed light in the practical applications of heterogeneous catalysts for environmental remediation through wastewater treatment in a greener approach.

As the vortical disturbances of a shrouded jet pass the sharp edge of the shroud exit some of the energy is scattered into acoustic waves. Scattering into upstream-propagating acoustic modes is a potential mechanism for closing the resonance loop in the ‘howling’ resonances that have been observed in various shrouded jet configurations over the years. A model is developed for this interaction at the shroud exit. The jet is represented as a uniform flow separated by a cylindrical vortex sheet from a concentric co-flow within the cylindrical shroud. A second vortex sheet separates the co-flow from an ambient flow outside the shroud, downstream of its exit. The Wiener–Hopf technique is used to compute reflectivities at the shroud exit. For some conditions it appears that the reflection of finite-wavelength hydrodynamic vorticity modes on the vortex sheet defining the jet could be sufficient to reinforce the shroud acoustic modes to facilitate resonance. The analysis also gives the reflectivities for the shroud acoustic modes, which would also be important in establishing resonance conditions. Interestingly, it is also predicted that the shroud exit can be ‘transparent’ for ranges of Mach numbers, with no reflection into any upstream-propagating acoustic mode. This is phenomenologically consistent with observations that indicate a peculiar sensitivity of resonances of this kind to, say, jet Mach number.

Developing highly efficient and less-expensive electrocatalysts toward the oxygen evolution reaction (OER) is an ongoing effort for sustainable energy generation and faces great challenges in achieving more effective and renewable energy conversion systems. Herein, we report the synthesis of Ni-doped Fe2O3 (denoted as Ni–Fe2O3) nanoclews as a highly efficient electrocatalyst via a facile light-assisted solution chemistry route without employing any template. Ni–Fe2O3 nanoclews demonstrate superior electrocatalytic activity toward OER with a low overpotential of 277 mV at current density of 10 mA cm–2 and small Tafel slope of 68 mV dec–1, accompanied by their excellent durability. Outstanding OER activity with long-standing structural and morphological stability of the nanoclews has been achieved by virtue of their clew-like morphology, comprising tiny nanorods with uniform distribution of constituent elements. Their unique structural features provide more exposed catalytically active sites, and the atomic-scale synergistic effect aroused from Fe and Ni contributes to improved intrinsic catalytic activity of Ni–Fe2O3 nanoclews.

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Acoustic analogies for the prediction of flow noise are exact rearrangements of the flow equations N q 0 into a nominal sound source Sq and sound propagation operator L such that Lq Sq. In practice, the sound source is typically modeled and the propagation operator inverted to make predictions. Because the rearrangement is exact, any sufficiently accurate model of the source will yield the correct sound, and so other factors must determine the merits of any particular formulation. Using data from a two-dimensional mixing-layer direct numerical simulation, we evaluate the robustness of several formulations to different errors intentionally introduced into the source. The motivation is that because S cannot be perfectly modeled, analogies that are less sensitive to errors in S are preferable. Our assessment is made within the framework of Goldstein's generalized acoustic analogy. A uniform base flow yields a Lighthill-like analogy, which we evaluate against a formulation in which the base flow is the actual mean flow of the direct numerical simulation and also against a globally parallel base flow that gives a Lilley-like analogy. The more complex mean-flow formulations are found to be significantly more robust to errors in the energetic turbulent fluctuations, but the advantage is less clear when errors are introduced at smaller scales. Nomenclature a i = eigenfunction time coefficients a 1 = ambient speed of sound e ij , j = source terms defined in (5) h = enthalpy L = propagation operator M 1 , M 2 = Mach numbers of the mixing layer N = compressible flow operator N t = number of eigenmodes p = pressure p e = modified pressure defined in (4) q = flow field S = nominal noise source t = time T ij , H ij , H 0 = source terms defined in Sec. II u i = modified flow velocity defined in (4) v i = flow velocity V 1;2 = mixing-layer velocity corresponding to M 1;2 y p = perturbation at mixing-layer centerline = directivity (downstream is 0) = specific heat ratio = vorticity thickness at inflow boundary = wavelength = fluid density ij = stress tensor defined in (3) i = empirical eigenfunctions (POD modes) ! = angular frequency Superscripts 0 = perturbed quantity = time-averaged quantity = Favre averaged quantitŷ = filtered quantity

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