The Experts below are selected from a list of 2046 Experts worldwide ranked by ideXlab platform
Carlos Vicente - One of the best experts on this subject based on the ideXlab platform.
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High-Power Low-Pass Harmonic Filters With Higher-Order ${\rm TE}_{{\rm n}0}$ and Non-${\rm TE}_{{\rm n}0}$ Mode Suppression: Design Method and Multipactor Characterization
IEEE Transactions on Microwave Theory and Techniques, 2013Co-Authors: Ivan Arregui, Fernando Teberio, Israel Arnedo, Aintzane Lujambio, Magdalena Chudzik, David Benito, Txema Lopetegi, Rolf Jost, Franz-josef Görtz, Carlos VicenteAbstract:In this paper, a method to design high-power low-pass harmonic Filters in rectangular waveguide technology is proposed. The new Filters consist of a collection of smooth E-plane bandstop elements along the propagation direction and a smooth variation of the Filter Width. This yields to a broad rejected band for the fundamental TE10 mode, together with higher-order ( TEn0 and non- TEn0) mode suppression. Two different examples with stringent requirements of the space industry are provided to demonstrate the capabilities of the new methodology. By means of high-power simulations and an extensive measurement campaign, it will be shown that the smoothness of the Filter profile guarantees high-power operation even with small minimum mechanical gaps. Moreover, unlike classical techniques, our method is not restricted to Filters with small gaps. Hence, Filters with larger gaps (always fulfilling the demanding frequency specifications) are fabricated for even higher power-handling performance.
Bernardus J. Geurts - One of the best experts on this subject based on the ideXlab platform.
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A physical length-scale for LES of turbulent flow
ERCOFTAC Series, 2020Co-Authors: Ugo Piomelli, Bernardus J. GeurtsAbstract:The fundamental assumption underlying large-eddy simulations (LES) is that the large, energy-carrying, eddies are resolved, while only the smaller eddies are modeled. An implication of this assumption is that the Filter-Width Δ, the length scale that separates the resolved from the unresolved eddies, should be a fraction of the integral scale, which is characteristic of the large eddies. In practice, however, the Filter Width is taken to be proportional to the grid size, h. This approach is generally legitimate, since the grid is usually refined where the important turbulence scales are smaller; it presents, however, two problems. First, rapid variations of the mesh (especially in methods that use local mesh refinement) may cause commutation and aliasing errors, and unphysical results (Vanella et al., 2008). Second, it requires knowledge, on the part of the user, on the characteristics of turbulence; in complex flows it may not be possible to predict the turbulence behavior a priori.
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How Can We Make LES to Fulfill Its Promise
Fluid Mechanics and Its Applications, 2020Co-Authors: Bernardus J. GeurtsAbstract:We review three important issues in LES which need to be addressed in order to systematically advance LES towards flows of realistic complexity. First, we consider rigorous modeling consequences arising from the analytic – and algebraic structure of the LES modeling problem. In this context, we develop a new generalized similarity model and apply this to a spatially developing mixing layer. Second, we estimate the commutation error arising in LES for flows in complex geometries which involvenon-uniform Filter-Widths, and propose direct similarity modeling of these contributions. Third, we consider the numerical contamination of a ‘Smagorinsky fluid’ in order to illustrate the role of spatial discretization errors in LES. Suitable ratios between Filter-Width Δ and grid-spacing h are identified. Some guidelines for developing LES are proposed.
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Regularization Modeling of Commutator-Errors in Large-Eddy Simulation of Wall-Bounded Turbulence
2019Co-Authors: Bernardus J. GeurtsAbstract:Commutator errors arise in large-eddy simulation of incompressible turbulent flow from the application of non-uniform Filters to the continuity and Navier-Stokes equations. Control over the commutator errors compared to the turbulent stress fluxes can only be obtained by appropriately restricting the spatial variations of the Filter-Width and Filter-skewness. For situations in which the dynamical consequences of the commutator errors are significant, e.g., near solid boundaries, explicit similarity modeling for the commutator errors is proposed, including Leray regularization. The performance of this commutator error parameterization is illustrated for the one-dimensional Burgers equation. The Leray approach is found to capture the Filtered flow with higher accuracy than conventional similarity modeling, which is particularly relevant for large Filter-Width variations.
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Commutator errors in large-eddy simulation
Journal of Physics A, 2006Co-Authors: Bernardus J. Geurts, Darryl D. HolmAbstract:Commutator errors arise in large-eddy simulation of incompressible turbulent flow from the application of non-uniform Filters to the continuity -- and Navier-Stokes equations. For non-uniform, high order Filters with bounded moments the magnitude of the commutator errors is shown to be of the same order as that of the turbulent stress fluxes. Consequently, one cannot reduce the size of the commutator errors independently of the turbulent stress terms by any judicious construction of such Filter operators. Independent control over the commutator errors compared to the turbulent stress fluxes can, instead, be obtained by appropriately restricting the spatial variations of the Filter-Width and Filter-skewness. For situations in which the dynamical consequences of the commutator errors are significant, e.g., near solid boundaries, explicit similarity modelling for the commutator errors is proposed, including the application of Leray regularization. The performance of this commutator error parametrization is illustrated for the one-dimensional Burgers equation. The Leray approach is found to capture the Filtered flow with higher accuracy than conventional similarity modelling, which is particularly relevant for large Filter-Width variations.
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Lagrangian dynamics of commutator errors in large-eddy simulation
Physics of Fluids, 2005Co-Authors: Bernardus J. GeurtsAbstract:In large-eddy simulations of turbulent flow only the flow structures with length scales larger than the local Filter Width Δ are explicitly resolved. We analyze the dynamic effect associated with spatial variations in the Filter Width. With the introduction of such a nonuniform Filter Width a number of additional closure terms emerges, generally referred to as commutator errors. The dynamic effect of the commutator errors is shown to correspond to the apparent local creation or destruction of turbulent flow scales, depending on, respectively, a decrease or an increase in Δ along the flow path. This Lagrangian context suggests significant correlation between the material derivative of the Filter Width and the production or dissipation of kinetic energy due to the commutator error. This is confirmed by novel a priori analysis of turbulent mixing. An explicit Lagrangian model for the commutator error in the momentum equations is proposed. Additionally, the dynamic effect of a skewed Filter on the commutator ...
Kenneth E Jansen - One of the best experts on this subject based on the ideXlab platform.
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A parameter-free dynamic subgrid-scale model for large-eddy simulation
Computer Methods in Applied Mechanics and Engineering, 2006Co-Authors: Andrés E. Tejada-martínez, Kenneth E JansenAbstract:Abstract We present a new dynamic Smagorinsky subgrid-scale model in which the sole model parameter, the Filter Width ratio, is computed dynamically. The resulting expression for the Filter Width ratio is parameter-free, as difficult-to-compute Filter Widths required in earlier models are no longer necessary. Traditionally, the Filter Width ratio is taken as a constant based on assumptions which do not hold in general for all numerical discretizations. Previous work has shown that simulation results may strongly depend on the Filter Width ratio parameter, thus motivating its accurate determination. The new dynamic model is tested on large-eddy simulation of decaying isotropic turbulence on hexahedral, tetrahedral, and wedge topologies using the stabilized finite element method developed by Whiting and Jansen [C.H. Whiting, K.E. Jansen, A stabilized finite element method for the incompressible Navier–Stokes equations using a hierarchical basis, Int. J. Numer. Methods Fluids 35 (2001) 93–116].
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a dynamic smagorinsky model with dynamic determination of the Filter Width ratio
Physics of Fluids, 2004Co-Authors: Andres E Tejadamartinez, Kenneth E JansenAbstract:Various low-pass, spatial test Filters specific to dynamic model large-eddy simulation (LES) on finite element topologies are proposed and analyzed. A number of simulations of decaying isotropic turbulence are performed using the stabilized finite element method of Whiting and Jansen with the purpose of understanding the dependence of the dynamic model on the test Filter of choice and the elusive Filter Width ratio. From these numerical experiments, a key assumption is extracted leading to the derivation of a new dynamic model in which the sole model parameter, the Filter Width ratio, is computed dynamically. Traditionally, the dynamic model parameter has taken the Filter Width ratio as a constant. The new dynamic model is tested on LES of decaying isotropic turbulence on hexahedral, tetrahedral, and wedge topologies.
Ivan Arregui - One of the best experts on this subject based on the ideXlab platform.
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High-Power Low-Pass Harmonic Filters With Higher-Order ${\rm TE}_{{\rm n}0}$ and Non-${\rm TE}_{{\rm n}0}$ Mode Suppression: Design Method and Multipactor Characterization
IEEE Transactions on Microwave Theory and Techniques, 2013Co-Authors: Ivan Arregui, Fernando Teberio, Israel Arnedo, Aintzane Lujambio, Magdalena Chudzik, David Benito, Txema Lopetegi, Rolf Jost, Franz-josef Görtz, Carlos VicenteAbstract:In this paper, a method to design high-power low-pass harmonic Filters in rectangular waveguide technology is proposed. The new Filters consist of a collection of smooth E-plane bandstop elements along the propagation direction and a smooth variation of the Filter Width. This yields to a broad rejected band for the fundamental TE10 mode, together with higher-order ( TEn0 and non- TEn0) mode suppression. Two different examples with stringent requirements of the space industry are provided to demonstrate the capabilities of the new methodology. By means of high-power simulations and an extensive measurement campaign, it will be shown that the smoothness of the Filter profile guarantees high-power operation even with small minimum mechanical gaps. Moreover, unlike classical techniques, our method is not restricted to Filters with small gaps. Hence, Filters with larger gaps (always fulfilling the demanding frequency specifications) are fabricated for even higher power-handling performance.
Josette Bellan - One of the best experts on this subject based on the ideXlab platform.
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Explicit Filtering to obtain grid-spacing-independent and discretization-order-independent large-eddy simulation of compressible single-phase flow
Journal of Fluid Mechanics, 2012Co-Authors: Senthilkumaran Radhakrishnan, Josette BellanAbstract:In large-eddy simulation (LES), it is often assumed that the Filter Width is equal to the grid spacing. Predictions from such LES are grid-spacing dependent since any subgrid-scale (SGS) model used in the LES equations is dependent on the resolved flow field which itself varies with grid spacing. Moreover, numerical errors affect the flow field, especially the smallest resolved scales. Thus, predictions using this approach are affected by both modelling and numerical choices. However, grid-spacing-independent LES predictions unaffected by numerical choices are necessary to validate LES models through comparison with a trusted template. First, such a template is created here through direct numerical simulation (DNS). Then, simulations are conducted using the conventional LES equations and also LES equations which are here reformulated so that the small-scale-producing nonlinear terms in these equations are explicitly Filtered (EF) to remove scales smaller than a fixed Filter Width; this formulation is called EFLES. First, LES is conducted with four SGS models, then EFLES is performed with two of the SGS models used in LES; the results from all these simulations are compared to those from DNS and from the Filtered DNS (FDNS). The conventional LES solution is both grid-spacing and spatial discretization-order dependent, thus showing that both of these numerical aspects affect the flow prediction. The solution from the EFLES equations is grid independent for a high-order spatial discretization on all meshes tested. However, low-order discretizations require a finer mesh to reach grid independence. With an eighth-order discretization, a Filter-Width to grid-spacing ratio of two is sufficient to reach grid independence, while a Filter-Width to grid-spacing ratio of four is needed to reach grid independence when a fourth- or a sixth-order discretization is employed. On a grid fine enough to be utilized in a DNS, the EFLES solution exhibits grid independence and does not converge to the DNS solution. The velocity-fluctuation spectra of EFLES follow those of FDNS independent of the grid spacing used, in concert with the original concept of LES. The reasons for the different predictions of conventional LES or EFLES according to the SGS model used, and the different characteristics of the EFLES predictions compared to those from conventional LES are analysed.
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Grid-spacing-independent large-eddy simulations
Journal of Fluid Mechanics, 2012Co-Authors: Sunita Radhakrishnan, Josette BellanAbstract:AbstractIn large-eddy simulation (LES), it is often assumed that the Filter Width is equal to the grid spacing. Predictions from such LES are grid-spacing dependent since any subgrid-scale (SGS) model used in the LES equations is dependent on the resolved flow field which itself varies with grid spacing. Moreover, numerical errors affect the flow field, especially the smallest resolved scales. Thus, predictions using this approach are affected by both modelling and numerical choices. However, grid-spacing-independent LES predictions unaffected by numerical choices are necessary to validate LES models through comparison with a trusted template. First, such a template is created here through direct numerical simulation (DNS). Then, simulations are conducted using the conventional LES equations and also LES equations which are here reformulated so that the small-scale-producing nonlinear terms in these equations are explicitly Filtered (EF) to remove scales smaller than a fixed Filter Width; this formulation is called EFLES. First, LES is conducted with four SGS models, then EFLES is performed with two of the SGS models used in LES; the results from all these simulations are compared to those from DNS and from the Filtered DNS (FDNS). The conventional LES solution is both grid-spacing and spatial discretization-order dependent, thus showing that both of these numerical aspects affect the flow prediction. The solution from the EFLES equations is grid independent for a high-order spatial discretization on all meshes tested. However, low-order discretizations require a finer mesh to reach grid independence. With an eighth-order discretization, a Filter-Width to grid-spacing ratio of two is sufficient to reach grid independence, while a Filter-Width to grid-spacing ratio of four is needed to reach grid independence when a fourth- or a sixth-order discretization is employed. On a grid fine enough to be utilized in a DNS, the EFLES solution exhibits grid independence and does not converge to the DNS solution. The velocity-fluctuation spectra of EFLES follow those of FDNS independent of the grid spacing used, in concert with the original concept of LES. The reasons for the different predictions of conventional LES or EFLES according to the SGS model used, and the different characteristics of the EFLES predictions compared to those from conventional LES are analysed.