Shock wave instability and the carbuncle phenomenon: same intrinsic origin ?

The theoretical linear stability of a shock wave moving in an unlimited homogeneous environment has been widely studied during the last fifty years. Important results have been obtained by Dyakov (1954), Landau & Lifchitz (1959) and then by Swan & Fowles (1975) where the fluctuating quantities are written as normal modes. More recently, numerical studies on upwind finite difference schemes have shown some instabilities in the case of the motion of an inviscid perfect gas in a rectangular channel. The purpose of this paper is first to specify a mathematical formulation for the eigenmodes and to exhibit a new mode which was not found by the previous stability analysis of shock waves. Then, this mode is confirmed by numerical simulations which may lead to a new understanding of the so-called carbuncle phenomenon

Mirror-Coupled Nanoantennas with Hexagonal-Boron Nitride Spacers

Transparent dielectric materials are employed in a wide range of optical resonators. Combining plasmonic nanoantennas with such resonators enables the formation of nanoscale optical resonances. The novel nano-optics observed by combining these heterostructures with optically active media is laying the foundations for devices such as ultra-high efficiency optical switches. In this thesis, mirror-coupled nanoantennas consisting of the layered dielectric hexagonal-Boron Nitride encapsulated between gold nanoparticles and a gold substrate, are probed using high-angle optical scattering measurements. During illumination of individual nanoparticles, light is trapped at the nanoscale inside the hBN, between particle and gold beneath. As the thickness of hBN is decreased, the coupling between nanoparticle and substrate changes dramatically. Here, the inert properties of hBN are used to remove extraneous influences on scattering spectra, thereby revealing new nanoscale light confinement mechanisms and sub-nanometre structural changes. In the first experiment presented, submicron-thick hBN crystals embedded in gold form planar Fabry-Perot half-microcavities. Gold nanoparticles on top of these microcavities form previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes. In the second experiment presented, the thickness of hBN is reduced down to a singleatom- thick layer leading to greatly enhanced field intensities and confinement when compared to thicker layers, via plasmonic coupling. By comparing results to an analytic model, resultant ultra-sensitive scattering signals from nanoparticles atop these thinner layers reveal field interactions sub-nanometre structural changes. Finally, photoluminescence measurements on defects in as-grown monolayer WS2 on gold lead to clear emission despite significant substrate quenching. Measurements using nanoplatelets of the metal halide perovskite methyl-ammonium lead-iodide, lead to suppression of resonant modes and a blue-shifting behaviour correlated with light emission

Investigating pseudohomophone interference effects in young second-language learners

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A New Implementation of the Extended Helmholtz Resonator Acoustic Liner Impedance Model in Time Domain CAA

The application of wall acoustic lining is a major factor in the reduction of aircraft engine noise. The Extended Helmholtz Resonator impedance model is widely used since it is representative of the behavior of realistic liners over a wide range of frequencies. Its application in time domain CAA methods by means of z-transform has been the subject of several papers. In contrast to standard liner modeling in time domain CAA, which consists in imposing a boundary condition modelling both the cavities and the perforated sheet of the liner, an alternative approach involves adding the cavities to the computational domain and imposing a condition between these cavities and the duct domain to model the resistive sheet. However, the original method may not be used for broadband acoustics since it implements an impedance condition with frequency independent resistance. This paper describes an extension of this method to implement the extended Helmholtz Resonator impedance model in a time domain CAA method

Global linear stability analysis of flow in a lined duct

Eigenmodes of the linearised Euler equations are computed in order to study lined flow duct global stability. A simplified configuration is considered and the governing equations are discretised by means of the discontinuous Galerkin method. A biorthogonal technique is used to decompose the global eigenfunctions into local eigenmodes. The system dynamics switches from noise amplifier to resonator as the length of the liner is increased. The global mode in the liner region is shown to be mainly composed of a hydrodynamic unstable wave and a left-running acoustic mode

Nano-immuno assay development for the detection of cancer biomarkers

The quantitative analysis of protein markers is a promising strategy in diagnosis, prognosis and therapy monitoring of malignant tumors. The optimization of the clinical detection of these markers is directed towards the use of a small volume system. We developed a multiplexing nano-immuno array for proteomic analysis in low sample volumes with potential capability of pathological screening of cancer biomarkers. Nanografting, a tip assisted Atomic Force Microscopy (AFM) nanolithography technique is used to fabricate DNA nanoarrays. DNA nanospots created by nanografting are exploited in order to immobilize DNA-antibody conjugates that recognize specific proteins of interest. The determination of the antigen content of a biological sample was obtained from the analysis of AFM topographic profiles of the nanopatches before and after the incubation. As a proof of principle, we focused on the biomarker Human Epidermal Growth Factor Receptor 2 (Her2), relevant antigen found in some human cancers such as breast, lung and gastric ones. By measuring spot height variation we performed the detection of low concentrations of the biomarker and we were able to optimize the device sensitivity by correlating the density of the DNA-antibody conjugates on the surface and their capability to bind the Extra Cellular Domain (ECD) of Her2. As future perspectives we are also exploring the use of new binders, as aptamers and nanobodies (VHH), as new tool for the recognition of specific biomarkers with higher affinity

How language affects children’s use of derivational morphology in visual word and pseudoword processing:evidence from a cross-language study

Developing readers have been shown to rely on morphemes in visual word recognition across several naming, lexical decision and priming experiments. However, the impact of morphology in reading is not consistent across studies with differing results emerging not only between but also within writing systems. Here, we report a cross-language experiment involving the English and French languages, which aims to compare directly the impact of morphology in word recognition in the two languages. Monolingual French-speaking and English-speaking children matched for grade level (Part 1) and for age (Part 2) participated in the study. Two lexical decision tasks (one in French, one in English) featured words and pseudowords with exactly the same structure in each language. The presence of a root (R+) and a suffix ending (S+) was manipulated orthogonally, leading to four possible combinations in words (R+S+: e.g. postal; R+S-: e.g. turnip; R-S+: e.g. rascal; and R-S-: e.g. bishop) and in pseudowords (R+S+: e.g. pondal; R+S-: e.g. curlip; R-S+: e.g. vosnal; and R-S-: e.g. hethop). Results indicate that the presence of morphemes facilitates children’s recognition of words and impedes their ability to reject pseudowords in both languages. Nevertheless, effects extend across accuracy and latencies in French but are restricted to accuracy in English, suggesting a higher degree of morphological processing efficiency in French. We argue that the inconsistencies found between languages emphasise the need for developmental models of word recognition to integrate a morpheme level whose elaboration is tuned by the productivity and transparency of the derivational system

An experimental study on the effects of two-dimensional positive surface defects on the laminar–turbulent transition of a sucked boundary layer

International audienceLaminar–turbulent transition can be effectively delayed using Laminar Flow Control (LFC) by boundary layer suction. However, major obstacles to the industrial implementation of this technique are related to practical limitations, such as proper integration of the suction system or unreliability of current design tools. The influence of surface discontinuities that can arise from installing an LFC system (and then potentially cancel or deteriorate its stabilizing effect on the boundary layer) is scarcely documented in the open literature, adding to the complexity of improving numerical models. The present investigation, therefore, focuses on experimentally characterizing the effects of surface defects on the laminar–turbulent transition of a sucked boundary layer in a two-dimensional flow, in an effort to address some of the issues mentioned above. The experimental facility and protocol for conducting this transition study are first presented, followed by a baseline characterization of the effects of wall suction alone on transition. Surface defects, in the form of cylindrical roughness elements (wires), are then introduced on the flat plate and their effects, coupled to those of wall suction, on boundary layer stability are discussed. The critical relative height (where the onset of transition coincides with the position of the surface defect) was found to be the same for cases both with and without wall suction. For the critical cases, spectral analysis of the flow immediately downstream of the defect for all suction configurations revealed a range of amplified high frequencies in addition to or in place of the natural Tollmien–Schlichting instabilities. Wall suction was, therefore, ineffective in delaying the critical relative wire height, since, in the critical cases, the transition mechanism seemed to be governed by inflection-type instabilities, rather than viscous instabilities

A non-linear analysis of fiscal multipliers and consumption drivers

This thesis explores the non-linear features of fiscal multipliers in the US economy and of consumption drivers in the euro area. In Chapter 1 we examine a Smooth Transition implementation of a VAR model by Auerbach and Gorodnichenko (2012). We show how the difference in the fiscal multiplier disappears when the design matrix is not augmented with business cycle lags and the model is estimated in first differences. Furthermore, we build on this original approach by using generalized impulse analysis to produce authentic non-linear impulse responses. Our results highlight the Great Recession as a significant turning point, the inclusion of which in the sample enables us to reverse the sign of the effect on GDP of a fiscal shock and endorse the expansionary budget cuts narrative. Chapter 2 presents the use of the same STVAR and generalized impulse response analysis to examine the non-linear effects of unanticipated government expenditure shocks on US GDP, controlling for private credit and public debt. We also perform a scenario analysis exercise to investigate shock responses during recessions and expansions, while making explicit the effect of the shock on public debt. We find that (i) the results support the inclusion of a measure of fiscal burden in the model; (ii) the GDP response to fiscal shocks is asymmetric in sign and magnitude; (iii) there exists a phenomenon of diminishing returns to expansionary shocks, which limits counter-cyclical fiscal policy; (iv) scenario analysis shows stronger multipliers on average in typical recessions. We investigate the combined effect of business and financial cycles on a non-linearly fluctuating economy in Chapter 3, designing and estimating a joint economic cycle. A STVAR model and generalized impulse response analysis enable us to examine the non-linear effect on GDP of unanticipated government expenditure shocks, which we complement by performing scenario analysis. The main findings are that (i) every specification shows concordance between signs of shock and GDP response; (ii) the inclusion of an indicator of fiscal capacity in the model leaves the baseline key findings unchanged; (iii) the main results show diminishing returns to increasing expansionary stimuli; (iv) public debt and private credit generally behave pro-cyclically; (v) scenario analysis suggests higher yield to shocks during recessions. Chapter 4 studies the cyclical dynamics of consumption in the euro area (EA) and the large EA countries by distinguishing between durable and nondurable expenditures. We adopt a theoretical partial equilibrium framework to justify the identification strategy of our empirical model, a time-varying parameter structural vector autoregression (TVP-SVAR). Following the main insight from the theoretical model -- that liquidity constraints induce important interactions between durables and nondurables -- we distinguish durable-specific demand and supply shocks, while taking into account monetary and credit conditions. Our main findings are: (i) durables react faster and more strongly than nondurables after monetary shocks in the euro area and in the largest EA countries; (ii) there is large degree of cross-country heterogeneity in the factors that drive consumption; (iii) strength of spillovers from durables to nondurables is empirically correlated with the likelihood of being liquidity-constrained across countries