Sound and vorticity interactions: transmission and scattering

We review several aspects of the propagation of sound in vortical flows. We restrict ourselves to isothermal, humidity-free flows at low Mach number M. Since vorticity plays a major role in vortex-flow interactions we focus on vortical flows. We consider two main canonical situations. The first concerns the transmission of sound. We analyze the evolution of acoustic wavefronts as they propagate across a single vortex. The second situation addresses the scattering of sound waves by nonstationary vortices. We study the evolution of the acoustic pressure emitted in the far field, at an angle with the initial direction of propagation. In this geometry one performs direct spectroscopy of the flow vorticity field. In each case, we review theoretical results and compare with experimental measurements and numerical simulations when available. We also briefly report how the following new acoustic techniques have recently been used to study complex or turbulent flows: time-resolved acoustic spectroscopy, speckle interferometry and Lagrangian particle tracking. PACS 43.25, 43.28, 47.32, 67.40     

Effects of damping on brake squeal coalescence patterns – application on a finite element model

Brake squeal is referred to, in most publications, as a flutter instability triggered by a mode coupling phenomenon. A lot of clues tend to prove that damping would be a key parameter in brake squeal modelling. This study aims at investigating the effects of damping on coalescence patterns, that is to say on the way the modes couple. A finite element model of the whole brake corner has been used to compute the brake modal behaviour. Then a complex eigenvalue analysis has been undertaken to assess the brake stability as a function of the friction coefficient. Different kinds of damping spreading over the modes have been studied. Two main effects have been noticed: a shifting effect and a smoothing effect. The first one always stabilize the brake, whereas this is not the case of the second one. The combination of the two effects may make the brake more unstable depending on the spreading of the additionnal damping.     

Rethinking Borole Cycloaddition Reactivity

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes.     

Capillary electrophoresis for fast detection of heterogeneous population in colistin‐resistant Gram‐negative bacteria

It has been shown that diverse strains of bacteria can be separated according to their characteristic surface properties by means of CE. We employed here this analytical technique to the study of colistin‐resistance in Gram‐negative bacteria, which involves the selection of mutants with modified outer membrane composition resulting in changes of surface cell properties. In the same way as with molecular entities, we performed firstly the validation of an ITP‐based CE method for three common pathogenic Gram‐negative bacteria namely Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Secondly, we compared the electrophoretic profiles of bacterial samples from a colistin‐susceptible clinical isolate of K. pneumoniae and from the corresponding colistin‐resistant derivative. By a simple CE run taking a few minutes, the coexistence of several bacterial subpopulations in the colistin‐resistant derivative was clearly evidenced. This work encourages further research that would allow applications of CE in clinical laboratory for a daily monitoring of bacterial population in cared patients when “last‐chance” colistin treatment is initiated against multidrug‐resistant bacteria.     

A convenient one-pot synthesis of boroxoles from diboronic acid

The preparation of the boroxole motif traditionally relies on a 3-step process and the use of n-butyl lithium that can limit substrate scope. Herein during our exploration toward novel RORγ inhibitors, we identified a convenient one-pot preparation of the motif in good yields with good substrate scope.     

The Structure of the Aluminum Fumarate Metal–Organic Framework A520

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid‐state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL‐53(Al)‐BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by in situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.     

Fine‐Tuning the Nucleophilic Reactivities of Boron Ate Complexes Derived from Aryl and Heteroaryl Boronic Esters

Boron ate complexes derived from thienyl and furyl boronic esters and aryllithium compounds have been isolated and characterized by X‐ray crystallography. Products and mechanisms of their reactions with carbenium and iminium ions have been analyzed. Kinetics of these reactions were monitored by UV/Vis spectroscopy, and the influence of the aryl substituents, the diol ligands (pinacol, ethylene glycol, neopentyl glycol, catechol), and the counterions on the nucleophilic reactivity of the boron ate complexes were examined. A Hammett correlation confirmed the polar nature of their reactions with benzhydrylium ions, and the correlation lg k(20 °C)=s_N(E+N) was employed to determine the nucleophilicities of the boron ate complexes and to compare them with those of other borates and boronates. The neopentyl and ethylene glycol derivatives were found to be 10⁴ times more reactive than the pinacol and catechol derivatives.     

Long-cavity Nd:YAG laser used in single-shot spontaneous Raman scattering measurements

The use of a 100 ns pulse Nd:YAG laser is proposed to perform single-shot measurements by spontaneous Raman scattering. The long pulse duration is obtained thanks to a White cell placed inside the laser oscillator. This lengthens the pulse duration within the laser cavity, keeping almost the same beam quality as standard Nd:YAG lasers. The laser system is described, the advantages are compared with extra-cavity stretched lasers, and single-shot measurement of concentration in gas is demonstrated.