34,036 research outputs found
Numerical studies towards practical large-eddy simulation
Large-eddy simulation developments and validations are presented for an
improved simulation of turbulent internal flows. Numerical methods are proposed
according to two competing criteria: numerical qualities (precision and
spectral characteristics), and adaptability to complex configurations. First,
methods are tested on academic test-cases, in order to abridge with fundamental
studies. Consistent results are obtained using adaptable finite volume method,
with higher order advection fluxes, implicit grid filtering and "low-cost"
shear-improved Smagorinsky model. This analysis particularly focuses on mean
flow, fluctuations, two-point correlations and spectra. Moreover, it is shown
that exponential averaging is a promising tool for LES implementation in
complex geometry with deterministic unsteadiness. Finally, adaptability of the
method is demonstrated by application to a configuration representative of
blade-tip clearance flow in a turbomachine
An equivalent-effect phenomenon in eddy current non-destructive testing of thin structures
The inductance/impedance due to thin metallic structures in non-destructive
testing (NDT) is difficult to evaluate. In particular, in Finite Element Method
(FEM) eddy current simulation, an extremely fine mesh is required to accurately
simulate skin effects especially at high frequencies, and this could cause an
extremely large total mesh for the whole problem, i.e. including, for example,
other surrounding structures and excitation sources like coils. Consequently,
intensive computation requirements are needed. In this paper, an
equivalent-effect phenomenon is found, which has revealed that alternative
structures can produce the same effect on the sensor response, i.e. mutual
impedance/inductance of coupled coils if a relationship (reciprocal
relationship) between the electrical conductivity and the thickness of the
structure is observed. By using this relationship, the mutual
inductance/impedance can be calculated from the equivalent structures with much
fewer mesh elements, which can significantly save the computation time. In eddy
current NDT, coils inductance/impedance is normally used as a critical
parameter for various industrial applications, such as flaw detection, coating
and microstructure sensing. Theoretical derivation, measurements and
simulations have been presented to verify the feasibility of the proposed
phenomenon
Large-Eddy Simulation of combustion instabilities in a variable-length combustor.
This article presents a simulation of a model rocket combustor with continuously variable acoustic properties thanks to a variable-length injector tube. Fully compressible Large-Eddy Simulations are conducted using the AVBP code. An original flame stabilization mechanism is uncovered where the recirculation of hot gases in the corner recirculation zone creates a triple flame structure. An unstable operating point is then chosen to investigate the mech- anism of the instability. The simulations are compared to experimental results in terms of frequency and mode structure. Two-dimensional axi-symmetric computations are com- pared to full 3D simulations in order to assess the validity of the axi-symmetry assumption for the prediction of mean and unsteady features of this flow. Despite the inaccuracies in- herent to the 2D description of a turbulent flow, for this configuration and the particular operating point investigated, the axi-symmetric simulation qualitatively reproduces some features of the instability
Comparison of Lattice Boltzmann Method and Finite Volume Method with Large Eddy Simulation in Isothermal Room Flow
Lattice Boltzmann method (LBM), as a new computational fluid simulation method, has aroused widespread attention in recent decades within engineering practice. LBM with large eddy simulation (LBM-LES) model is commonly used in predicting high Reynolds flow, and is considered to have a prediction accuracy similar to traditional finite volume method (FVMLES). Nonetheless, a systematic discussion on the accuracy of LBM-LES, and its consistency with FVM-LES, in indoor turbulent flow situations, is still insufficient. In this study, simulations of an indoor isothermal forced convection benchmark case (from IEA Annex 20) are implemented by using both LBM-LES and FVM-LES, with the aim of comparing the accuracies of LBM-LES and FVM-LES, in indoor turbulent flow situations. A comparison of their relative computation speeds, and parallel computation performances, is also implemented. The results show that LBM-LES can achieve the same level of accuracy as FVM-LES, in indoor turbulent flow situations; however, more refined meshes are required. Compared with FVMLES, half size grids are required for LBM-LES to approach similar levels of accuracy, meaning that the meshes of LBM-LES are approximately eight times as large as FVM-LES. The computation speeds of both LBM-LES and FVM-LES scale well, with the increase in the number of computation cores in one node. Their computation speeds (with the same accuracy) approach a similar level; however, the parallel computation speed of the LBM-LES speed can be larger than FVM, owing to its superior parallel speedup performance
Proper orthogonal decomposition closure models for fluid flows: Burgers equation
This paper puts forth several closure models for the proper orthogonal
decomposition (POD) reduced order modeling of fluid flows. These new closure
models, together with other standard closure models, are investigated in the
numerical simulation of the Burgers equation. This simplified setting
represents just the first step in the investigation of the new closure models.
It allows a thorough assessment of the performance of the new models, including
a parameter sensitivity study. Two challenging test problems displaying moving
shock waves are chosen in the numerical investigation. The closure models and a
standard Galerkin POD reduced order model are benchmarked against the fine
resolution numerical simulation. Both numerical accuracy and computational
efficiency are used to assess the performance of the models
Optimising UCNS3D, a High-Order finite-Volume WENO Scheme Code for arbitrary unstructured Meshes
UCNS3D is a computational-fluid-dynamics (CFD) code for the simulation of viscous flows on arbitrary unstructured meshes. It employs very high-order numerical schemes which inherently are easier to scale than lower-order numerical schemes due to the higher ratio of computation versus communication. In this white paper, we report on optimisations of the UCNS3D code implemented in the course of the PRACE Preparatory Access Type C project “HOVE” in the time frame of February to August 2016. Through the optimisation of dense linear algebra operations, in particular matrix-vector products, by formula rewriting, pre-computation and the usage of BLAS, significant speedups of the code by factors of 2 to 6 have been achieved for representative benchmark cases. Moreover, very good scalability up to the order of 10,000 CPU cores has been demonstrated
Modeling of the subgrid-scale term of the filtered magnetic field transport equation
Accurate subgrid-scale turbulence models are needed to perform realistic
numerical magnetohydrodynamic (MHD) simulations of the subsurface flows of the
Sun. To perform large-eddy simulations (LES) of turbulent MHD flows, three
unknown terms have to be modeled. As a first step, this work proposes to use a
priori tests to measure the accuracy of various models proposed to predict the
SGS term appearing in the transport equation of the filtered magnetic field. It
is proposed to evaluate the SGS model accuracy in term of "structural" and
"functional" performance, i.e. the model capacity to locally approximate the
unknown term and to reproduce its energetic action, respectively. From our
tests, it appears that a mixed model based on the scale-similarity model has
better performance.Comment: 10 pages, 5 figures; Center for Turbulence Research, Proceedings of
the Summer Program 2010, Stanford Universit
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