35 research outputs found
Direct Numerical Simulation of High Prandtl Number Fluid Flow in the Downcomer of an Advanced Reactor
The design of advanced nuclear reactors (Gen IV) involves an array of
challenging fluid-flow issues that affect its safety and performance. The
calculated DNS database will be instrumental in understanding the flow behavior
in the downcomer. Ultimately, we aim to evaluate existing heat transfer
correlations and some modifications are proposed.Comment: arXiv admin note: substantial text overlap with arXiv:2203.1415
Biglobal linear stability analysis for the flow in eccentric annular channels and a related geometry
Recently, it has been observed that simple geometry characterized by a low level of symmetry present interesting peculiarities in the process of transition from laminar Poiseuille flow to turbulent flow. Examples of this type of geometry are eccentric channels and, more generally, parallel channels containing a narrow gap. In the present work, a global linear stability analysis for the flow in this class of geometry has been performed. The problem is discretized through spectral collocation and the eigenvalue problem has been solved with the Arnoldi-method based algorithms and the QZ algorithm. Since no numerical studies of this type have yet been performed to address the issue of transition in this geometry, the codes have been validated toward results obtained in simplified geometries _e.g., concentric annular channel and square channel_. The eigenvalue spectra of the Poiseuille flow in eccentric channels and a U-shaped channel have then been computed and analyzed for a wide range of geometric parameters. After comparison with spectra typical of channel flow and pipe flow it is shown that an additional linear mechanism of instability is present, related to the spanwise variation of the laminar velocity profile
An Energy-Based Lengthscale for Reduced Order Models of Turbulent Flows
In this paper, we propose a novel reduced order model (ROM) lengthscale that
is constructed by using energy distribution arguments. The new energy-based ROM
lengthscale is fundamentally different from the current ROM lengthscales, which
are built by using dimensional arguments. To assess the novel, energy-based ROM
lengthscale, we compare it with a standard, dimensionality-based ROM
lengthscale in two fundamentally different types of models: (i) the
mixing-length ROM (ML-ROM), which is a ROM closure model; and (ii) the
evolve-filter-relax ROM (EFR-ROM), which is a regularized ROM. We test the four
combinations (i.e., ML-ROM and EFR-ROM equipped with the energy-based and
dimensionality-based lengthscales) in the numerical simulation of the turbulent
channel flow at . The numerical investigation yields the
following conclusions: (i) The new energy-based ROM lengthscale is
significantly (almost two orders of magnitude) larger than the standard
dimensionality-based ROM lengthscale. As a result, the energy-based lengthscale
yields more stable ML-ROMs and EFR-ROMs than the dimensionality-based
lengthscale. (ii) The energy-based lengthscale displays the correct asymptotic
behavior with respect to the ROM dimension, whereas the dimensionality-based
lengthscale does not. (iii) The energy-based lengthscale yields ML-ROMs and
(when significant filtering is effected) EFR-ROMs whose parameters are less
sensitive (i.e., more robust) than the parameters of the ML-ROMs and EFR-ROMs
based on the dimensionality-based lengthscale. The novel energy-based
lengthscale could enable the development of better scale-aware ROM strategies
for flow-specific applications and is expected to have long term applications
in nuclear reactor thermal-hydraulics.Comment: arXiv admin note: substantial text overlap with arXiv:2108.0225