1,907 research outputs found
Critical Scaling Properties at the Superfluid Transition of He in Aerogel
We study the superfluid transition of He in aerogel by Monte Carlo
simulations and finite size scaling analysis. Aerogel is a highly porous silica
glass, which we model by a diffusion limited cluster aggregation model. The
superfluid is modeled by a three dimensional XY model, with excluded bonds to
sites on the aerogel cluster. We obtain the correlation length exponent
, in reasonable agreement with experiments and with previous
simulations. For the heat capacity exponent , both experiments and
previous simulations suggest deviations from the Josephson hyperscaling
relation . In contrast, our Monte Carlo results support
hyperscaling with . We suggest a reinterpretation of
previous experiments, which avoids scaling violations and is consistent with
our simulation results.Comment: 4 pages, 3 figure
Don't Pay Attention to the Noise: Learning Self-supervised Representations of Light Curves with a Denoising Time Series Transformer
Astrophysical light curves are particularly challenging data objects due to the intensity and variety of noise contaminating them. Yet, despite
the astronomical volumes of light curves available, the majority of algorithms used to process them are still operating on a per-sample
basis. To remedy this, we propose a simple
Transformer model –called Denoising Time Series Transformer (DTST)– and show that it excels
at removing the noise and outliers in datasets of
time series when trained with a masked objective, even when no clean targets are available.
Moreover, the use of self-attention enables rich
and illustrative queries into the learned representations. We present experiments on real stellar light
curves from the Transiting Exoplanet Space Satellite (TESS), showing advantages of our approach
compared to traditional denoising techniques1
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Study of multi-component fuel premixed combustion using direct numerical simulation
Fossil fuel reserves are projected to be decreasing, and emission regulations are becoming more stringent due to increasing atmospheric pollution. Alternative fuels for power generation in industrial gas turbines are thus required able to meet the above demands. Examples of such fuels are synthetic gas, blast furnace gas and coke oven gas. A common characteristic of these fuels is that they are multi-component fuels, whose composition varies greatly depending on their production process. This implies that their combustion characteristics will also vary significantly. Thus, accurate and yet flexible enough combustion sub-models are required for such fuels, which are used during the design stage, to ensure optimum performance during
practical operating conditions.
Most combustion sub-model development and validation is based on Direct Numerical Simulation (DNS) studies. DNS however is computationally expensive. This, has so far limited DNS to single-component fuels such as methane and hydrogen. Furthermore, the majority of DNS conducted to date used one-step chemistry in 3D, and skeletal chemistry in 2D only. The need for 3D DNS using skeletal chemistry is thus apparent. In this study, an accurate reduced chemical mechanism suitable for multi-component fuel-air combustion is developed from a skeletal mechanism. Three-dimensional DNS of a freely propagating turbulent premixed flame is then conducted using both mechanisms to shed some light into the flame structure and
turbulence-scalar interaction of such multi-component fuel flames.
It is found that for the multi-component fuel flame heat is released over a wider
temperature range contrary to a methane flame. This, results from the presence of individual species reactions zones which do not all overlap. The performance of the reduced mechanism is also validated using the DNS data. Results suggest it to be a good substitute of the skeletal mechanism, resulting in significant time and memory savings. The flame markers commonly used to visualize heat release rate in laser diagnostics are found to be inadequate for the multi-component fuel flame, and alternative markers are proposed. Finally, some popular mean reaction rate closures are tested for the multi-component fuel flame. Significant differences are observed between the models’ performance at the highest turbulence level considered in this study. These arise from the chemical complexity of the fuel, and further parametric studies using skeletal chemistry DNS would be useful for the refinement of the
models
The habitability of a stagnant-lid Earth
Plate tectonics is a fundamental component for the habitability of the Earth.
Yet whether it is a recurrent feature of terrestrial bodies orbiting other
stars or unique to the Earth is unknown. The stagnant lid may rather be the
most common tectonic expression on such bodies. To understand whether a
stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we
model the thermal evolution of the mantle, volcanic outgassing of HO and
CO, and resulting climate of an Earth-like planet lacking plate tectonics.
We used a 1D model of parameterized convection to simulate the evolution of
melt generation and the build-up of an atmosphere of HO and CO over 4.5
Gyr. We then employed a 1D radiative-convective atmosphere model to calculate
the global mean atmospheric temperature and the boundaries of the habitable
zone (HZ). The evolution of the interior is characterized by the initial
production of a large amount of partial melt accompanied by a rapid outgassing
of HO and CO. At 1 au, the obtained temperatures generally allow for
liquid water on the surface nearly over the entire evolution. While the outer
edge of the HZ is mostly influenced by the amount of outgassed CO, the
inner edge presents a more complex behaviour that is dependent on the partial
pressures of both gases. At 1 au, the stagnant-lid planet considered would be
regarded as habitable. The width of the HZ at the end of the evolution, albeit
influenced by the amount of outgassed CO, can vary in a non-monotonic way
depending on the extent of the outgassed HO reservoir. Our results suggest
that stagnant-lid planets can be habitable over geological timescales and that
joint modelling of interior evolution, volcanic outgassing, and accompanying
climate is necessary to robustly characterize planetary habitability
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