Assessment of high-resolution methods for numerical simulations of compressible turbulence with shock waves

Flows in which shock waves and turbulence are present and interact dynamically occur in a wide range of applications, including inertial confinement fusion, supernovae explosion, and scramjet propulsion. Accurate simulations of such problems are challenging because of the contradictory requirements of numerical methods used to simulate turbulence, which must minimize any numerical dissipation that would otherwise overwhelm the small scales, and shock-capturing schemes, which introduce numerical dissipation to stabilize the solution. The objective of the present work is to evaluate the performance of several numerical methods capable of simultaneously handling turbulence and shock waves. A comprehensive range of high-resolution methods (WENO, hybrid WENO/central difference, artificial diffusivity, adaptive characteristic-based filter, and shock fitting) and suite of test cases (Taylor–Green vortex, Shu–Osher problem, shock-vorticity/entropy wave interaction, Noh problem, compressible isotropic turbulence) relevant to problems with shocks and turbulence are considered. The results indicate that the WENO methods provide sharp shock profiles, but overwhelm the physical dissipation. The hybrid method is minimally dissipative and leads to sharp shocks and well-resolved broadband turbulence, but relies on an appropriate shock sensor. Artificial diffusivity methods in which the artificial bulk viscosity is based on the magnitude of the strain-rate tensor resolve vortical structures well but damp dilatational modes in compressible turbulence; dilatation-based artificial bulk viscosity methods significantly improve this behavior. For well-defined shocks, the shock fitting approach yields good results.     

Acyl Group Migration in Pyranosides as Studied by Experimental and Computational Methods

Acyl group migration affects the synthesis, isolation, manipulation and purification of all acylated organic compounds containing free hydroxyl groups, in particular carbohydrates. While several isolated studies on the migration phenomenon in different buffers have been reported, comprehensive insights into the overall migration process in different monosaccharides under similar conditions have been lacking. Here, we have studied the acyl migration in different monosaccharides using five different acyl groups by a combination of experimental, kinetic and theoretical tools. The results show that the anomeric configuration in the monosaccharide has a major influence on the migration rate, together with the relative configurations of the other hydroxyl groups and the nature of the migrating acyl group. Full mechanistic model, based on computations, demonstrates that the acyl migration proceeds through an anionic stepwise mechanism with linear dependence on the [OH⁻] and the pK_a of the hydroxyl group toward which the acyl group is migrating.     

Water Formation in Non-Hydrolytic Sol–Gel Routes: Selective Synthesis of Tetragonal and Monoclinic Mesoporous Zirconia as a Case Study

Several non-hydrolytic sol–gel syntheses involving different precursors, oxygen donors, and conditions have been screened aiming to selectively produce mesoporous t-ZrO₂ or m-ZrO₂ with significant specific surface areas. The in situ water formation was systematically investigated by Karl Fisher titration of the syneresis liquids. XRD and nitrogen physisorption were employed to characterize the structure and texture of the ZrO₂ samples. Significant amounts of water were found in several cases, notably in the reactions of Zr(OnPr)₄ with ketones (acetone, 2-pentanone, acetophenone), and of ZrCl₄ with alcohols (benzyl alcohol, ethanol) or acetone. Conversely, the reactions of Zr(OnPr)₄ with acetic anhydride or benzyl alcohol at moderate temperature (200 °C) and of ZrCl₄ with diisopropyl ether appear strictly non-hydrolytic. Although reaction time and reaction temperature were also important parameters, the presence of water played a crucial role on the structure of the final zirconia: t-ZrO₂ is favored in strictly non-hydrolytic routes, while m-ZrO₂ is favored in the presence of significant amounts of water. ¹H and ¹³C NMR analysis of the syneresis liquids allowed us to identify the main reactions responsible for the formation of water and of the oxide network. The morphology of the most interesting ZrO₂ samples was further investigated by electron microscopy (SEM, TEM).     

Chlovalicin B,a Chlorinated Sesquiterpene Isolated from the Marine Mushroom Digitatispora marina

As part of our search for bioactive metabolites from understudied marine microorganisms, the new chlorinated metabolite chlovalicin B (1) was isolated from liquid cultures of the marine basidiomycete Digitatispora marina, which was collected and isolated from driftwood found at Vannøya, Norway. The structure of the novel compound was elucidated by spectroscopic methods including 1D and 2D NMR and analysis of HRMS data, revealing that 1 shares its molecular scaffold with a previously isolated compound, chlovalicin. This represents the first compound isolated from the Digitatispora genus, and the first reported fumagillin/ovalicin-like compound isolated from Basidiomycota. Compound 1 was evaluated for antibacterial activities against a panel of five bacteria, its ability to inhibit bacterial biofilm formation, for antifungal activity against Candida albicans, and for cytotoxic activities against malignant and non-malignant human cell lines. Compound 1 displayed weak cytotoxic activity against the human melanoma cell line A2058 (~50% survival at 50 µM). No activity was detected against biofilm formation or C. albicans at 50 µM, or against bacterial growth at 100 µM nor against the production of cytokines by the human acute monocytic leukemia cell line THP-1 at 50 µM.     

Oscillator strengths for resonance transitions in neutral selenium and tellurium derived from time-resolved laser spectroscopy

We have measured radiative lifetimes for selenium and tellurium resonance levels by using short pulse laser excitation in differentially heated resonance cells. We obtained τ(³ S ₁ Se I)=2.9(5) ns and τ(³ S ₁ Te I)=3.1(5) ns, which together with branching ratio values given by Ubelis and Berzinsh yield the absorption oscillator strengthf _abs(Se I, 196.0 nm)=0.074(16) andf _abs(Te I, 214.3 nm)=0.098(17).     

Synthesis of 2,6-bridged piperazine-3-ones by N-acyliminium ion chemistry

Several 2-substituted and 2,5-disubstituted piperazine-3,6-diones were synthesized starting from readily available alpha-amino acids. After activation of a lactam carbonyl via introduction of a methoxycarbonyl group onto nitrogen, this carbonyl was selectively reduced. Treatment of the resulting urethane with protic acid generated the corresponding N-acyliminium ion, which was trapped by a nucleophilic C2-side chain to provide 2,6-bridged piperazine-3-ones. Several aromatic, heteroaromatic, and nonaromatic side chains were used as pi-nucleophiles. In addition, the effect of the presence of a C5-methyl group on the stereochemical outcome of the cyclization was examined.     

Ready-made matrix-assisted laser desorption/ionization target plates coated with thin matrix layer for automated sample deposition in high-density array format.

The methodology for ready-made matrix-assisted laser desorption/ionization (MALDI) target plates covered with an optimized thin layer consisting of matrix and nitrocellulose has been developed. Piezoelectric microdispensing enabled sample depositions in a high-density array format of 2000 sample depositions on a conventionally sized target plate (45 x 47 mm). The sample depositions were made reproducibly in a fully automated mode by using an in-house developed computer-controlled piezoelectric flow-through microdispenser. Additionally, the piezoelectric technique facilitated significant analyte enrichment that increased the detection sensitivity. The MS signal was obtained rapidly, generally within ten laser pulses. An airbrush device was used to generate a fine spray of matrix and nitrocellulose dissolved in acetone. The acetone evaporated instantly when reaching the target plate leaving the entire surface with a thin and uniform matrix/nitrocellulose coating consisting of very small crystals of matrix embedded in the nitrocellulose. These crystals acted as a seed-layer on subsequent analyte depositions, rendering homogeneous sample spots when using alpha-cyano-4-hydroxycinnamic acid (CHCA) as matrix. The relative standard deviation of the signal intensity between spots was (20-30)% (n = 30). The detection sensitivity was improved by restricting the sample spot diameter to 300 microm. The spot size was affected by the deposition rate and the evaporation rate of the dispensed sample volume. Mass spectra of a 25-amol peptide mixture deposition were successfully recorded.