127,865 research outputs found
Comparison of Subgrid-scale Viscosity Models and Selective Filtering Strategy for Large-eddy Simulations
Get PDFExplicitly 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?
Get PDFLarge 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
Get PDFA 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
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
Large-eddy simulation of low-frequency unsteadiness in a turbulent shock-induced separation bubble
No full textThe need for better understanding of the low-frequency unsteadiness observed in shock wave/turbulent boundary layer interactions has been driving research in this area for several decades. We present here a large-eddy simulation investigation of the interaction between an impinging oblique shock and a Mach 2.3 turbulent boundary layer. Contrary to past large-eddy simulation investigations on shock/turbulent boundary layer interactions, we have used an inflow technique which does not introduce any energetically significant low frequencies into the domain, hence avoiding possible interference with the shock/boundary layer interaction system. The large-eddy simulation has been run for much longer times than previous computational studies making a Fourier analysis of the low frequency possible. The broadband and energetic low-frequency component found in the interaction is in excellent agreement with the experimental findings. Furthermore, a linear stability analysis of the mean flow was performed and a stationary unstable global mode was found. The long-run large-eddy simulation data were analyzed and a phase change in the wall pressure fluctuations was related to the global-mode structure, leading to a possible driving mechanism for the observed low-frequency motions
Large-eddy simulation of turbulent flow over a rough surface
Get PDFA family of wall models is proposed that exhibits more satisfactory performance than previous models for the large-eddy simulation (LES) of the turbulent boundary layer over a rough surface.The time and horizontally averaged statistics such as mean vertical profiles of wind velocity, Reynolds stress, turbulent intensities, turbulent kinetic energy and also spectra are compared with wind-tunnel experimental data. The purpose of the present study is to obtain simulated turbulent flows that are comparable with wind-tunnel measurements for use as the wind environment for the numerical prediction by LES of source dispersion in theneutral atmospheric boundary layer
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