23,188 research outputs found
Intergranular stress distributions in polycrystalline aggregates of irradiated stainless steel
In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of
post-irradiated austenitic stainless steel in Light Water Reactor (LWR)
environment, reliable predictions of intergranular stresses are required.
Finite elements simulations have been performed on realistic polycrystalline
aggregate with a recently proposed physically-based crystal plasticity
constitutive equations validated for neutron-irradiated austenitic stainless
steel. Intergranular normal stress probability density functions are found with
respect to plastic strain and irradiation level, for uniaxial loading
conditions. In addition, plastic slip activity jumps at grain boundaries are
also presented. Intergranular normal stress distributions describe, from a
statistical point of view, the potential increase of intergranular stress with
respect to the macroscopic stress due to grain-grain interactions. The
distributions are shown to be well described by a master curve once rescaled by
the macroscopic stress, in the range of irradiation level and strain considered
in this study. The upper tail of this master curve is shown to be insensitive
to free surface effect, which is relevant for IGSC
Parallel Anisotropic Unstructured Grid Adaptation
Computational Fluid Dynamics (CFD) has become critical to the design and analysis of aerospace vehicles. Parallel grid adaptation that resolves multiple scales with anisotropy is identified as one of the challenges in the CFD Vision 2030 Study to increase the capacity and capability of CFD simulation. The Study also cautions that computer architectures are undergoing a radical change and dramatic increases in algorithm concurrency will be required to exploit full performance. This paper reviews four different methods to parallel anisotropic grid generation. They cover both ends of the spectrum: (i) using existing state-of-the-art software optimized for a single core and modifying it for parallel platforms and (ii) designing and implementing scalable software with incomplete, but rapidly maturating functionality. A brief overview for each grid adaptation system is presented in the context of a telescopic approach for multilevel concurrency. These methods employ different approaches to enable parallel execution, which provides a unique opportunity to illustrate the relative behavior of each approach. Qualitative and quantitative metric evaluations are used to draw lessons for future developments in this critical area for parallel CFD simulation
Airborne and Terrestrial Laser Scanning Data for the Assessment of Standing and Lying Deadwood: Current Situation and New Perspectives
LiDAR technology is finding uses in the forest sector, not only for surveys in producing forests but also as a tool to gain a deeper understanding of the importance of the three-dimensional component of forest environments. Developments of platforms and sensors in the last decades have highlighted the capacity of this technology to catch relevant details, even at finer scales. This drives its usage towards more ecological topics and applications for forest management. In recent years, nature protection policies have been focusing on deadwood as a key element for the health of forest ecosystems and wide-scale assessments are necessary for the planning process on a landscape scale. Initial studies showed promising results in the identification of bigger deadwood components (e.g., snags, logs, stumps), employing data not specifically collected for the purpose. Nevertheless, many efforts should still be made to transfer the available methodologies to an operational level. Newly available platforms (e.g., Mobile Laser Scanner) and sensors (e.g., Multispectral Laser Scanner) might provide new opportunities for this field of study in the near future
Statistical mechanics of Beltrami flows in axisymmetric geometry: Theory reexamined
A simplified thermodynamic approach of the incompressible axisymmetric Euler
equations is considered based on the conservation of helicity, angular momentum
and microscopic energy. Statistical equilibrium states are obtained by
maximizing the Boltzmann entropy under these sole constraints. We assume that
these constraints are selected by the properties of forcing and dissipation.
The fluctuations are found to be Gaussian while the mean flow is in a Beltrami
state. Furthermore, we show that the maximization of entropy at fixed helicity,
angular momentum and microscopic energy is equivalent to the minimization of
macroscopic energy at fixed helicity and angular momentum. This provides a
justification of this selective decay principle from statistical mechanics.
These theoretical predictions are in good agreement with experiments of a von
Karman turbulent flow and provide a way to measure the temperature of
turbulence and check Fluctuation-Dissipation Relations (FDR). Relaxation
equations are derived that could provide an effective description of the
dynamics towards the Beltrami state and the progressive emergence of a Gaussian
distribution. They can also provide a numerical algorithm to determine maximum
entropy states or minimum energy states.Comment: 25 pages, 2 figure
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