2,465 research outputs found
A three-dimensional inverse finite-element method applied to experimental eddy-current imaging data.
Full text linkEddy-current techniques can be used to create electrical conductivity mapping of an object. The eddy-current imaging system in this paper is a magnetic induction tomography (MIT) system. MIT images the electrical conductivity of the target based on impedance measurements from pairs of excitation and detection coils. The inverse problem here is ill-posed and nonlinear. Current state-of-the-art image reconstruction methods in MIT are generally based on linear algorithms. In this paper, a regularized Gauss-Newton scheme has been implemented based on an edge finite-element forward solver and an efficient formula for the Jacobian matrix. Applications of Tikhonov and total variation regularization have been studied. Results are presented from experimental data collected from a newly developed MIT system. The paper also presents further progress in using an MIT system for molten metal flow visualization in continuous casting by applying the proposed algorithm in a real experiment in a continuous casting pilot plant of Corus RD&T, Teesside Technology Centre
Going beyond eddy viscosity: Finding a minimal representation of subgrid-scale stresses in large-eddy simulation
No full textIn the current study we aim to go beyond the dissipative description of turbulent flows that is provided by eddy viscosity models for large-eddy simulation. As a starting point, we consider a general subgrid-scale model that is nonlinear in the velocity gradient. To reduce the number of degrees of freedom of the model, we propose a first-principles-based procedure to find a minimal representation of subgrid-scale stresses. Then, several criteria to determine the dependence of model coefficients on flow properties are detailed. Ultimately, this would lead to a better understanding of the role of different nonlinear model terms in the description of turbulent flows
Circulation characteristics in three eddy-permitting models of the North Atlantic
Full text linkA systematic intercomparison of three realistic eddy-permitting models of the North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma) coordinates, respectively. Although these models were integrated under nearly identical conditions, the resulting large-scale model circulations show substantial differences. The results demonstrate that the large-scale thermohaline circulation is very sensitive to the model representation of certain localised processes, in particular to the amount and water
mass properties of the overflow across the Greenland-Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at small or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different
model representation of these processes
Large eddy simulation for turbulent non-premixed fuel-rich combustion in a cylindrical combustor
Full text linkNo abstract available
Comparative assessment of grid-spacing-based filter width formulations for Very Large Eddy Simulation
No full textWhen applying hybrid LES/RANS turbulence models the relevant filter width plays a crucial role concerning switching between different operating modes. Presently, the influence of the filter width and the choice of the corresponding criterion within the so-called VLES (Very Large Eddy Simulation) computational framework is investigated. Results obtained by using three eddy-viscosity-based background RANS models in conjunction with different filter-width formulations and two representative wall-bounded flow configurations are presented
Building proper invariants for subgrid-scale eddy-viscosity models
No full textSince direct simulations of the incompressible Navier-Stokes equations are limited to relatively low-Reynolds numbers, dynamically less complex mathematical formulations are necessary for coarse-grain simulations. Eddy-viscosity models for Large-Eddy Simulation is probably the most popular example thereof: they rely on differential operators that should be able to capture well different flow configurations (laminar and 2D flows, near-wall behavior, transitional regime...). Most of them are based on the combination of invariants of a symmetric second-order tensor that is derived from the gradient of the resolved velocity field. In the present work, they are presented in a framework where all the models are represented as a combination of elements of a 5D phase space of invariants. In this way, new models can be constructed by imposing appropriate restrictions in this space. The performance of the proposed models is successfully tested for a turbulent channel flow
TIME-CONSERVATIVE FINITE-VOLUME METHOD WITH LARGE-EDDY SIMULATION FOR COMPUTATIONAL AEROACOUSTICS
Full text linkThis thesis presents a time-conservative finite-volume method based on a modern flow simulation technique developed by the author. Its applicability to technically relevant aeroacoustic applications is demonstrated. The time-conservative finite-volume method has unique features and advantages in comparison to traditional methods.
The main objectives of this study are to develop an advanced, high-resolution, low dissipation second-order
scheme and to simulate the near acoustic field with similar accuracy as higher-order (e.g., 4th-order, 6th-order, etc.) numerical schemes. Other aims are to use a large-eddy simulation (LES) technique to directly predict the near-field aerodynamic noise and to simulate the turbulent flow field with high-fidelity.
A three-dimensional parallel LES solver is developed in order to investigate the near acoustic field. Several cases
with wide ranges of flow regimes have been computed to validate and verify the accuracy of the method as well as to demonstrate its effectiveness. The time-conservative finite-volume method is efficient and yields high-resolution results with low dissipation similar
to higher-order conventional schemes. The time-conservative finite-volume approach offers an accurate way to compute the most relevant frequencies and acoustic modes for aeroacoustic calculations. Its accuracy was checked by solving demonstrative test cases including the prediction of narrowband and broadband cavity acoustics as well as the screech tones and the broadband shock-associated noise of a planar supersonic jet.
The second-order time-conservative finite-volume method can solve practically relevant aeroacoustic problems with high-fidelity which is an exception to the conventional second-order schemes commonly regarded as inadequate for
computational aeroacoustic (CAA) applications
Large eddy simulations of weakly heated stratocumulus top boudary layer
No full textPerforming Large Eddy Simulations (LES) of marine stratocumulus in the weakly heated boundary layer is an opportunity to evaluate the relative importance of radiative cooling and of a wind shear in cloud top region on cloud structure. It is shown that cooling due to longwave radiation influences the convective circulation in the atmospheric boundary layer and counteracts dilution caused by the wind shear
A Dynamic Subfilter-scale Stress Model for Large Eddy Simulations Based on Physical Flow Scales
No full textWe propose a new definition of the length scale in an eddy-viscosity model for large-eddy simulations (LES). This formulation extends and generalizes a previous proposal [Piomelli, Rouhi and Geurts, Proc. ETMM10, 2014], in which the LES length scale was expressed in terms of the integral length-scale of turbulence determined by the flow characteristics and explicitly decoupled from the simulation grid; this approach was named Integral Length-Scale Approximation (ILSA). As in the original ILSA, the model coefficient was determined by the user, and required to maintain a desired contribution of the unresolved, subfilter scales (SFS) to the global transport. We propose a local formulation (local ILSA) in which the model coefficient is local in space, allowing a precise control over SFS activity as a function of location. This new formulation preserves the properties of the global model; application to channel flow and backward-facing step verifies its features and accuracy
Large-eddy simulation of turbulent channel flow using the explicit algebraic subgrid-scale model
No full textLarge-eddy simulation (LES) of turbulent channel flow are performed with a new subgrid-scale (SGS) stress model. The simulations show that with this model we can well predict turbulent wall flows at coarse resolutions and moderately high Reynolds numbers. The commonly used dynamic Smagorinsky model fails at coarser resolutions
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