The Center for Energy, Environmental and Technological Research (CIEMAT), till 1986 Junta de Energía Nuclear (JEN), is a Spanish public research institution.
The dynamics of a planar premixed flame is analyzed using the Zel'dovich-Li & ntilde;& aacute;n-Dold two-step chemical-kinetic mechanism. The chemistry model involves an initial autocatalytic reaction with large activation energy converts the premixed fuel into intermediate radical species without heat release, followed by a temperature-independent recombination reaction that releases heat and produces the final combustion products. Focusing on low-diffusivity conditions, we revisit the modal stability of planar deflagrations and compare the results with asymptotic predictions obtained in the high-activation-energy limit. To characterize transient amplification mechanisms at finite activation energies, the stability analysis is further extended through pseudospectral and endogeneity analyses, which quantify the intrinsic sensitivity of the system to perturbations and identify the regions that most strongly influence its dynamical behavior. The adjoint-based analysis reveals transient-growth dynamics closely consistent with the optimal-response predictions. In particular, significant transient growth is observed even when the flame is linearly stable for modal disturbances, suggesting the possibility of triggering nonlinear dynamics that may ultimately lead to flame extinction. These findings are further supported by nonlinear time-dependent simulations, which reveal distinct regimes of transient flame evolution, including stable propagation, global instability, and extinction events associated with transient growth phenomena.
The sound horizon scale rs is a key source of information for measurements of H0 from early-time data, and is therefore a common target of new physics proposed to solve the Hubble tension. We present a sub-2% measurement of the Hubble constant that is independent of this scale, using data from the first data release of the Dark Energy Spectroscopic Instrument (DESI DR1). Building on previous work, we remove dependency on the sound horizon size using a heuristic rescaling procedure at the power spectrum level. A key innovation is the inclusion of uncalibrated (agnostic to rs) post-reconstruction BAO measurements from DESI DR1, as well as using the CMB acoustic scale theta & lowast; as a high-redshift anchor. Uncalibrated type-Ia supernovae are often included as an independent source of ohm m information; here we demonstrate the robustness of our results by additionally considering two supernova-independent alternative datasets. We find somewhat higher values of H0 relative to our previous work: 69.2+1.3-1.4, 70.3+1.4-1.2, and 69.6+1.3-1.8 km s-1 Mpc-1 respectively when including measurements from i) Planck/ACT CMB lensing & times; unWISE galaxies, ii) the DES Year 3 6 & times;2pt analysis, and iii) Planck/ACT CMB lensing + the DES Year 5 supernova analysis. These remarkably consistent constraints achieve better than 2% precision; they are among the most stringent sound horizon-independent measurements from LSS to date, and provide a powerful avenue for probing the origin of the Hubble tension.
In this study, the friction surfacing process was investigated to assess its influence on the microstructure evolution and mechanical properties of two novel high-strength aluminum alloys, HighZn + Zr and LowZn + Y, from the feedstock to the deposited and heat-treated states. The results indicate that LowZn + Y exhibited superior deposition performance, achieving higher deposition efficiency and rate while consuming less energy compared to HighZn + Zr. Both alloys experienced significant microstructural refinement during friction surfacing, characterized by fine, equiaxed grains and an inhomogeneous intermetallic particle distribution due to intense shear forces, precipitate dissolution, and dynamic recrystallization. However, subsequent heat treatment led to abnormal grain growth, influenced by the inhomogeneous particle distribution and the extended duration of the thermal exposure. In terms of mechanical properties, both alloys demonstrated increased yield strength, ultimate tensile strength, and maximum elongation in the as-deposited state relative to the feedstock, attributed to grain refinement and improved particle dispersion. After heat treatment, both yield strength and ultimate tensile strength exhibited a slight increase, attributed to the formation of nanometric precipitates. However, maximum elongation decreased as a result of grain coarsening, which promoted plastic strain localization and increased the susceptibility to crack initiation and propagation under mechanical loading.
The tens of millions of spectra being captured by the Dark Energy Spectroscopic Instrument (DESI) provide tremendous discovery potential. In this work we show how Machine Learning, in particular Variational Autoencoders (VAE), can detect anomalies in a sample of approximately 200,000 DESI spectra comprising galaxies, quasars and stars. We demonstrate that the VAE can compress the dimensionality of a spectrum by a factor of 100, while still retaining enough information to accurately reconstruct spectral features. We then detect anomalous spectra as those with high reconstruction error and those which are isolated in the VAE latent representation. The anomalies identified fall into two categories: spectra with artefacts and spectra with unique physical features. Awareness of the former can help to improve the DESI spectroscopic pipeline; whilst the latter can lead to the identification of new and unusual objects. To further curate the list of outliers, we use the Astronomaly package which employs Active Learning to provide personalised outlier recommendations for visual inspection. In this work we also explore the VAE latent space, finding that different object classes and subclasses are separated despite being unlabelled. We demonstrate the interpretability of this latent space by identifying tracks within it that correspond to various spectral characteristics. For example, we find tracks that correspond to increasing star formation and increase in broad emission lines along the Balmer series. In upcoming work we hope to apply the methods presented here to search for both systematics and astrophysically interesting objects in much larger datasets of DESI spectra.
A novel spectroscopic gas puff imaging (SGPI) diagnostic is presented for the purpose of simultaneous two-dimensional measurements of the electron density (ne) and temperature (Te). The based on the He line-ratio technique, this diagnostic achieves a spatiotemporal resolution in the edge plasma of the TJ-II stellarator of a few mm and down to 5 & micro;s. A high-speed intensified camera records a scene that combines images of the plasma volume where helium is injected using three lenses. Each lens is fitted with a band-pass filter for one of the emission lines of He-I (667 nm, 706 nm and 728 nm). The electron density and temperature are deduced from the ratios of intensities using a collisional radiative model. The location of the helium injector was optimised to align the line(s) of sight of the diagnostic with the magnetic field within the volume of the injected thermal neutral cloud of helium. This specific alignment serves to minimise any artefacts caused by line integration, thereby enhancing spatial resolution. A series of experiments were conducted under varying plasma heating conditions-electron cyclotron resonance and neutral beam injection-to cross-validate the electron density and temperature profiles obtained with the novel SGPI diagnostic against Thomson scattering and the supersonic helium beam. Furthermore, the use of the diagnostic for the purpose of turbulent fluctuation analysis is explored, and preliminary results are presented.
