3,526 research outputs found
A global adaptive velocity space for general discrete velocity framework in predictions of rarefied and multi-scale flows
The rarefied flow and multi-scale flow are crucial for the aerodynamic design
of spacecraft, ultra-low orbital vehicles and plumes. By introducing a discrete
velocity space, the Boltzmann method, such as the discrete velocity method and
unified methods, can capture complex and non-equilibrium velocity distribution
functions (VDFs) and describe flow behaviors exactly. However, the extremely
steep slope and high concentration of the gas VDFs in a local particle velocity
space make it very difficult for the Boltzmann method with structured velocity
space to describe high speed flow. Therefore, the adaptive velocity space (AVS)
is required for the Boltzmann solvers to be practical in complex rarefied flow
and multi-scale flow. This paper makes two improvements to the AVS approach,
which is then incorporated into a general discrete velocity framework, such as
the unified gas-kinetic scheme. Firstly, a global velocity mesh is used to
prevent the interpolation of the VDFs at the physical interface during the
calculation of the microscopic fluxes, maintaining the program's high level of
parallelism. Secondly, rather than utilizing costly interpolation, the VDFs on
a new velocity space were reconstruction using the ``consanguinity"
relationship. In other words, a split child node's VDF is the same as its
parent's VDF, and a merged parent's VDF is the average of its children's VDFs.
Additionally, the discrete deviation of the equilibrium distribution functions
is employed to maintain the proposed method's conservation. Moreover, an
appropriate set of adaptive parameters is established to enhance the automation
of the proposed method. Finally, a number of numerical tests are carried out to
validate the proposed method
A gas-surface interaction algorithm for discrete velocity methods in predicting rarefied and multi-scale flows
The rarefied flow and multi-scale flow are crucial for the aerodynamic design
of spacecraft, ultra-low orbital vehicles and plumes. By introducing a discrete
velocity space, the discrete velocity method (DVM) and unified methods can
capture complex and non-equilibrium distribution functions and describe flow
behaviors exactly. The unified methods predict flows from continuum to rarefied
regimes by adopting unified modeling, and they can be further applied to other
multi-scale physics such as radiation heat transfer, phonon heat transfer and
plasma. In the flow field, the concrete dynamic process needs to describe the
gas-gas interaction and gas-surface interaction (GSI). However, in both DVM and
unified methods, only a simple but not accurate GSI is used, which can be
regarded as a Maxwell GSI with a fixed accommodation coefficient of 1 (full
accommodation) at the present stage. To overcome the bottleneck in extending
DVM and unified methods to the numerical experiment and investigate real
multi-scale flow physics, this paper realizes precise GSI in the DVM framework
by constructing the boundary conditions of a concrete Maxwell GSI with an
adjustable accommodation coefficient. In the constructing process, the problems
of macro-conservation and micro-consistency in the DVS at the boundary are well
solved by reflected macroscopic flux and interpolation distribution function
and interpolation error correction, respectively. Meanwhile, considering that
the multi-scale flows in the background of aeronautics and aerospace are often
at supersonic and hypersonic speeds, the unstructured velocity space (UVS) is
essential. From the perspective of generality, the GSI is forced on UVS.
Besides, by combined with the unified method (the unified gas-kinetic scheme in
the paper), the effectiveness and validity of the present GSI on the DVM
framework are verified by a series of simulations
An efficient numerical method for charged particle transport based on hybrid collision model and machine learning
Charged particle transport is an important energy transport mode in the
combustion process of inertial confinement fusion plasma. On the one hand,
charged particles inside the hot spot have a strong non-equilibrium effect, so
it is necessary to solve the Boltzmann transport equation to simulate the
energy transport process of charged particles accurately. On the other hand,
charged particle transport has the characteristics of high collision frequency
and complex blocking power, so the calculation amount of the traditional Monte
Carlo algorithm is difficult to bear under the existing calculation conditions.
Aiming at the computational bottleneck caused by the large Coulomb potential
collision cross-section, we developed a hybrid collision model which greatly
reduced the computational cost while maintaining the second-order accuracy of
the collision process. In order to solve the computational bottleneck caused by
the complex blocking power model, we developed a neural network model based on
machine learning to achieve formal unity and efficient calculation of different
blocking power. Based on the calculation method, we developed the charged
particle transport MC function modules of the RDMG program and LARED-S program
and applied them to the study of critical target performance of inertial
confinement fusion, which showed good computational efficiency and accuracy
Towards Neutron Transformation Searches
To probe the origins of the baryon asymmetry, baryon number violation, the last unconfirmed Sakharov condition, must be definitively observed experimentally. Similarly, the nature of dark matter is currently unknown, and calls out for new candidates to be investigated. Each of these issues can be considered through the study of neutron transformations.
Some rare baryon number violating processes, such as neutron-antineutron transformations, are expected to probe baryogenesis. Here, I show progress on this discovery target through construction of more accurate Monte Carlo models, the design of future detectors, creation of more complete atmospheric neutrino background simulations, and use of automated analysis techniques within the the NNBAR/HIBEAM experimental program at the European Spallation Source (ESS) and the Deep Underground Neutrino Experiment (DUNE). First simulation-based sensitivities for these experiments will be discussed. Modeling of rare neutron-antineutron transformation and subsequent annihilation will be discussed at length for multiple nuclei useful to these and other collaborations. To go along with this work, more comprehensive lepton-scattering nuclear models must be integrated into neutrino event generators for proper atmospheric neutrino background simulations. I discuss the first furnishing of these backgrounds for DUNE, and I highlight a potential path forward for the community in this vein using precision electron scattering modeling as a facsimile.
Aspects of other potentially related neutron--mirror-neutron oscillations pertinent to dark matter and the neutron lifetime anomaly will also be considered for the ESS HIBEAM experiment. Here, I will present the first experimental sensitivity calculations for a broad range of modular experimental setups which will serve as research and design stepping stones toward NNBAR while producing a multitude of physics results over short time scales
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