3,386 research outputs found
Adaptively refined large eddy simulations of clusters
We present a numerical scheme for modelling unresolved turbulence in
cosmological adaptive mesh refinement codes. As a first application, we study
the evolution of turbulence in the intra-cluster medium and in the core of a
galaxy cluster. Simulations with and without subgrid scale model are compared
in detail. Since the flow in the ICM is subsonic, the global turbulent energy
contribution at the unresolved length scales is smaller than 1% of the internal
energy. We find that the production of turbulence is closely correlated with
merger events occurring in the cluster environment, and its dissipation locally
affects the cluster energy budget. Because of this additional source of
dissipation, the core temperature is larger and the density is smaller in the
presence of subgrid scale turbulence than in the standard adiabatic run,
resulting in a higher entropy core value.Comment: Submitted to ApJ, 14 pages, 14 figures, 3 table
Scanner Invariant Representations for Diffusion MRI Harmonization
Purpose: In the present work we describe the correction of diffusion-weighted
MRI for site and scanner biases using a novel method based on invariant
representation.
Theory and Methods: Pooled imaging data from multiple sources are subject to
variation between the sources. Correcting for these biases has become very
important as imaging studies increase in size and multi-site cases become more
common. We propose learning an intermediate representation invariant to
site/protocol variables, a technique adapted from information theory-based
algorithmic fairness; by leveraging the data processing inequality, such a
representation can then be used to create an image reconstruction that is
uninformative of its original source, yet still faithful to underlying
structures. To implement this, we use a deep learning method based on
variational auto-encoders (VAE) to construct scanner invariant encodings of the
imaging data.
Results: To evaluate our method, we use training data from the 2018 MICCAI
Computational Diffusion MRI (CDMRI) Challenge Harmonization dataset. Our
proposed method shows improvements on independent test data relative to a
recently published baseline method on each subtask, mapping data from three
different scanning contexts to and from one separate target scanning context.
Conclusion: As imaging studies continue to grow, the use of pooled multi-site
imaging will similarly increase. Invariant representation presents a strong
candidate for the harmonization of these data
Holographic Turbulence in a Large Number of Dimensions
We consider relativistic hydrodynamics in the limit where the number of
spatial dimensions is very large. We show that under certain restrictions, the
resulting equations of motion simplify significantly. Holographic theories in a
large number of dimensions satisfy the aforementioned restrictions and their
dynamics are captured by hydrodynamics with a naturally truncated derivative
expansion. Using analytic and numerical techniques we analyze two and
three-dimensional turbulent flow of such fluids in various regimes and its
relation to geometric data.Comment: 36 pages, 22 figure
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