Antoine Guitton

A study in the near pressure field of co-axial subsonic jets

An experimental investigation of the near pressure field of unbounded subsonic jets has been performed. The near-field pressure was sampled, using linear and azimuthal arrays, on conical surfaces surrounding free jets generated by (1) a single axisymmetric nozzle, (2) a co-axial short-cowl nozzle, and (3) a co-axial short-cowl nozzle with serrations (the co-axial experiments were performed as part of the EU program, CoJeN (AST3-CT-2003-502790), where velocity and temperature-ratios were varied). The objective of the study is to better understand differences in the structure of the flows in terms of their sound production mechanisms. A model representation of the source mechanism associated with coherentstructures in the flow is considered, using both the pressure fluctuations themselves and the pressure-derivative source term from Curle’s acoustic analogy. A filtering operation is then applied in order to identify the structure of the radiating source field.

Dislocation analysis of Ti2AlN deformed at room temperature under confining pressure

Compression experiments of the brittle MAX phase Ti2AlN were performed under confining gas pressure at room temperature. Subsequently, a complete dislocation analysis was performed by transmission electron microscopy. In particular, the Burgers vectors and the dislocation lines were studied via the weak beam technique: dislocation reactions are reported for the first time in a MAX phase, as well as dipole interactions. Footprints of a high lattice friction were also observed. All these features point towards classical dislocation activity, eventually leading to hardening.

Measurements in a co-axial subsonic jet.

A series of experimental measurements are performed on a subsonic flow exiting from a co-axial nozzle geometry. The measurements are acquired at the University of Poitiers, France, as part of the EU program CoJEN, where the turbulence characteristics of unbounded subsonic co-axial jet flows as they pertain to aeroacoustic sources of noise are to be investigated. This paper presents for the first time, measurements obtained via LDA, PIV, CTA, near-field line and azimuthal pressure arrays and far-field arc and azimuthal pressure arrays from this new facility.

Pressure-enforced plasticity in MAX phases: from single grain to polycrystal investigation

Ti4AlN3, Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases were plastically deformed at room temperature (RT) under gaseous confining pressure. Microstructures of as-grown and deformed samples are carefully analysed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is demonstrated that high level of plastic deformation can be reached under confining gas pressure; the later suppresses the brittle failure at RT to the profit of plasticity. Multiscale characterization techniques are shown to provide a unique insight into all the scales of the plastic deformation; in particular, the effect of the mesoscale. Indeed, grain shape and orientation relative to the compression axis are shown to play a key role in the deformation process, intergranular stresses leading to a complex stress field in the polycrystalline samples. The TEM results show that dislocation activity highly depends on the grain orientation. The observation of dislocation entanglements unambiguously demonstrates that dislocations may be organized in such a configuration so that their glide in the basal plane can be hindered when deep plastic regime is reached.

Evidence of dislocation cross-slip in MAX phase deformed at high temperature

Ti2AlN nanolayered ternary alloy has been plastically deformed under confining pressure at 900°C. The dislocation configurations of the deformed material have been analyzed by transmission electron microscopy. The results show a drastic evolution compared to the dislocation configurations observed in the Ti2AlN samples deformed at room temperature. In particular, they evidence out-of-basal-plane dislocations and interactions. Moreover numerous cross-slip events from basal plane to prismatic or pyramidal planes are observed. These original results are discussed in the context of the Brittle-to-Ductile Transition of the nanolayered ternary alloys.

Identifying the dynamics underlying the large-scale and fine-scale jetnoise similarity spectra

This work concerns the development of a diagnostic tool for the analysis of jet noise source mechanisms. The technique comprises three steps: (1) synchronous measurement of pressure and velocity fluctuations (pressure measured in the irrotational nearand farfield regions; velocity measured in the rotational region of the flow); (2) filtering of the near pressure field into radiating and non-radiating components; (3) reconstruction of the velocity fluctuations which are linearly associated with the radiating component of the near pressure field. The technique is here applied to an isothermal, co-axial jet (primary Mach=0.5; secondary Mach=0.25; Re=1,000,000), and we perform a further filtering of the near pressure field so as to extract the space-time structure of the field which radiates to 30 degrees and to 90 degrees (finally, the flow dynamics associated with each of these components is to be estimated). Preliminary results show that: (1) the radiating ‘source’ activity does not occur in the regions of peak turbulence (in agreement with previous numerical results of Freund); and, (2) the shape of the radiating sound spectrum looks to be made up of three main components: a coherent low-frequency component associated with the dynamics at the end of the potential core; a coherent high-frequency component associated with the near-nozzle dynamics; and a more broadband high frequency component which may be associated with ’fine-grained’ turbulence. The ’fine-grained’ component appears to be an order of magnitude less energetic than the two coherent components, and the flatness of the 90◦ spectrum appears to be a result of a superposition of the highfrequency near-nozzle and the low-frequency potential-core dynamics.

Dislocation modelling in Ti2AlN MAX phase based on the Peierls–Nabarro model

In this study, we determined the core structure and the Peierls stress of dislocations in Ti2AlN MAX phase. We use a generalized Peierls–Nabarro model, called Peierls–Nabarro–Galerkin (PNG), coupled with first principles calculations of generalized stacking fault (GSF). The GSF calculations show that dislocation glide in the basal plane will occur preferentially between M (here Ti) and A (here Al) planes. Additionally, the results of PNG calculations demonstrate that whatever the dislocation character, dislocations are dissociated in the basal plane, with a dissociation distance below the experimental resolution of transmission electron microscopy observations. Finally, the Peierls stress calculations show that the edge and screw characters are the easiest characters to glide in the basal plane.

Effect of microstructure anisotropy on the deformation of MAX polycrystals studied by in-situ compression combined with neutron diffraction

In situ compression tests combined with neutron diffraction were performed on Ti2AlN MAX polycrystals with lamellar anisotropic microstructure: the diffraction peak evolution (position and profile) with applied stress reveals that lamellar grains parallel to compression axis remain elastic while lamellar grains perpendicular to compression plastify, both families being subjected to strong variations of heterogeneous strains (types II and III). We demonstrate that this behavior originates from the complex response of the very anisotropic lamellar microstructure and explains the observation of reversible hysteretic loops when cycling MAX polycrystals even in the elastic regime.

Experimental Investigation of a Cylindrical Cavity

A cylindrical cavity (H/D=1.357) has been investigated experimentally through wall pressure measurements in the test section of a low speed closed-circuit wind tunnel. The upstream boundary layer is fully turbulent and the free stream velocity was varied in the range (M=0.015-0.15). Wall pressure measurements on the cavity side wall exhibit a symmetric pattern with respect to the streamwise central plane. The first three shear layer modes were identified by a spectral analysis of the velocity above the cavity and of the wall pressure measurements. The intensity of those modes are modulated by the depth acoustic resonance mode of the cavity. A comparison with a numerical study of a cylindrical cavity of similar aspect ratio and Reynolds number at M=0.235 is presented. The aim is to validate the physical insight proposed in this paper. It is found that non-dimensional spectra agreed with the experimental one although the intensity was over-predicted by the numerical model.

The sound production mechanism associated with coherent structures in subsonic jets

In this work we address the question regarding the mechanism by which coherent structures in high Reynolds number jets generate sound. Previous work by Coiffet et al. and Guitton et al. has provided evidence that the mechanism may correspond to a linear, wavy-wall scenario—coherence nodes observed in nearfield measurements, and thought to correspond to destructive interference between convecting hydrodynamic perturbations and propagating sound waves, were interpreted as the signature of such a mechanism. We investigate this hypothesis by means of a simple model problem comprising an axisymmetric, subsonically-convecting wavepacket structure, with Gaussian axial and radial envelopes. The model reproduces the interference nodes observed experimentally. We then compare the results with experimental data acquired in the nearfield of a subsonic jet (M = 0.6, Re = 1.10). This data is filtered to separate hydrodynamic and acoustic nearfield signatures, such that quantitative comparison is possible between the sound field generated by the model and that observed experimentally. Results show...