1,846,372 research outputs found
Comparison of Subgrid-scale Viscosity Models and Selective Filtering Strategy for Large-eddy Simulations
Full text linkExplicitly filtered large-eddy simulations (LES), combining high-accuracy schemes with the use of a selective filtering without adding an explicit subgrid-scales (SGS) model, are carried out for the Taylor-Green-vortex and the supersonic-boundary-layer cases. First, the present approach is validated against direct numerical simulation (DNS) results. Subsequently, several SGS models are implemented in order to investigate if they can improve the initial filter-based methodology. It is shown that the most accurate results are obtained when the filtering is used alone as an implicit model, and for a minimal cost. Moreover, the tests for the Taylor-Green vortex indicate that the discretization error from the numerical methods, notably the dissipation error from the high-order filtering, can have a greater influence than the SGS models
Telegrama de Eddy Torres, New York (Abril 11 de 1959)
No full textTelegrama de Eddy Torres, New York en abril 11 de 195
Demo 20-24 Eddy Current Pendulum
No full textA pendulum with a large bob that is constructed from a metal disc swings between the pole tips of a strong permanent magnet. Its motion damps quickly due to the large eddy currents produced in the pendulum bob. Non-conducting bobs or bobs formed from long, thin conducting elements that cannot support large eddy currents damp more slowly. Cooling a circular copper bob to liquid nitrogen temperature allows very large eddy currents that cause very strong damping, as shown in the video
When Does Eddy Viscosity Damp Subfilter Scales Sufficiently?
Full text linkLarge eddy simulation (LES) seeks to predict the dynamics of spatially filtered turbulent flows. The very essence is that the LES-solution contains only scales of size ≥Δ, where Δ denotes some user-chosen length scale. This property enables us to perform a LES when it is not feasible to compute the full, turbulent solution of the Navier-Stokes equations. Therefore, in case the large eddy simulation is based on an eddy viscosity model we determine the eddy viscosity such that any scales of size <Δ are dynamically insignificant. In this paper, we address the following two questions: how much eddy diffusion is needed to (a) balance the production of scales of size smaller than Δ; and (b) damp any disturbances having a scale of size smaller than Δ initially. From this we deduce that the eddy viscosity νe has to depend on the invariants q = ½tr(S^2) and r =−⅓tr(S^3) of the (filtered) strain rate tensor S. The simplest model is then given by νe = 3/2(Δ/π)^2|r|/q. This model is successfully tested for a turbulent channel flow (Reτ = 590).
Large eddy simulation of buoyant plumes
Full text linkA 3d parallel CFD code is written to investigate the characteristics of and differences
between Large Eddy Simulation (LES) models in the context of simulating a thermal
buoyant plume. An efficient multigrid scheme is incorporated to solve the Poisson
equation, resulting from the fractional step, projection method used to solve the Low
Mach Number (LMN) Navier-Stokes equations.
A wide range of LES models are implemented, including a variety of eddy models,
structure models, mixed models and dynamic models, for both the momentum stresses
and the temperature fluxes. Generalised gradient flux models are adapted from their
RANS counterparts, and also tested.
A number of characteristics are observed in the LES models relating to the thermal
plume simulation in particular and turbulence in general. Effects on transition,
dissipation, backscatter, equation balances, intermittency and energy spectra are all
considered, as are the impact of the governing equations, the discretisation scheme,
and the effect of grid coarsening. Also characteristics to particular models are
considered, including the subgrid kinetic energy for the one-equation models, and
constant histories for dynamic models.
The argument that choice of LES model is unimportant is shown to be incorrect as a
general statement, and a recommendation for when the models are best used is given
A Neutrally Stratified Boundary-Layer: A Comparison of Four Large-Eddy-Simulation Computer Codes
No full textA neutrally stratified Ekman layer limited in depth by a stress-free rigid lid has been simulated using four different large-eddy simulation computer codes. This is the second large-eddy simulation intercomparison study
in a series of simulations of different flows aimed at a systematic analysis of large-eddy simulation sensitivity to subgrid scale model assumptions and numerical methods. The resolution was chosen to be coarse so that
the results might be sensitive to the details of the various subgrid scale model formulations. Stochastic backscatter of subgrid kinetic energy and scalar variance was included in one subgrid scale model.
Profiles of first and second moments, budgets of resolved-scale second moments, and spectra have been analyzed. It is found that significant differences in the lowest third of the boundary layer result from moderate
changes in subgrid scale eddy diffusivities or from the inclusion of backscatter.
To isolate effects of the various numerical methods, a set of tests with a fixed subgrid scale K-profile were made. When run with a common subgrid scale model the results from the four large-eddy simulation codes converge. This test indicates that the large-eddy simulation results are more sensitive to the subgrid scale model formulations than to the various numerics
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
Eddy boys
No full textEddy boys off to school with helmets. Men's quarters of A.E. Jolly and Co. behind. Jolly's full name: Alfred Edward Jolly.Unknown.Date:194
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