Theoretical and Computational Fluid Dynamics | Year: 2012
An efficient generalized Zonal Detached Eddy Simulation method (ZDES) is presented, which aims at performing hybrid Reynolds Averaged Numerical Simulation (RANS)/Large Eddy Simulation (LES) calculations for both internal and external aerodynamics problems. It is based on a zonal formulation of the hybrid length scale that allows to combine the zonal approach with the best features of Delayed Detached Eddy Simulation (DDES) (Spalart et al. Theor Comput Fluid Dyn 20:181-195, 2006). In other words, the presumed weak point of a zonal approach, namely that the location of separation has to be known in advance, is now overcome. What is more, the problem of slow LES content development in mixing layers when they are treated neither in RANS nor in LES mode is investigated. It is argued that the subgrid length scale δmax = max (δx,δy,δz) entering DDES is physically justified to shield the boundary layer but is definitely not a good subgrid length scale in LES mode. Remedies are proposed based on new zonal subgrid length scales that depend not only on the grid spacing but also on the flow solution and especially on the local vorticity vector. The method is validated on a spatially developing mixing layer as well as in a backward facing step flow and then applied to a three-element airfoil. It is argued in this latter case that a precise control of the RANS mode thanks to a zonal approach is essential. More generally, in all simulated cases in this study, ZDES has proven to be very efficient as regards the behavior in LES mode while retaining the strongest asset of DDES, namely the treatment of the attached boundary layer in RANS mode. The issue of zonal or non-zonal treatment of turbulent flows is also briefly discussed. © Springer-Verlag 2011.
Journal of Fluid Mechanics | Year: 2012
This article deals with open-loop control of open-cavity flows with harmonic forcings. Two-dimensional laminar open-cavity flows usually undergo a supercritical Hopf bifurcation at some critical Reynolds number: a global mode becomes unstable and its amplitude converges towards a limit cycle. Such behaviour may be accurately captured by a Stuart-Landau equation, which governs the amplitude of the global mode. In the present article, we study the effect on such a flow of a forcing characterized by its frequency ω f, its amplitude E and its spatial structure f E. The system reacts like a forced Van der Pol oscillator. In the general case, such a forcing modifies the linear dynamics of the global mode. It is then possible to predict preferred forcing frequencies ω f, at which the global mode may be stabilized with the smallest possible forcing amplitude E. In the case of a forcing frequency close to the frequency of the global mode, a locking phenomenon may be observed if the forcing amplitude E is sufficiently high: the frequency of the flow on the limit cycle may be modified with a very small forcing amplitude E′ In each case, we compute all harmonics of the flow field and all coefficients that enter the amplitude equations. In particular, it is possible to find preferred forcing structures f that achieve strongest impact on the flow field. In the general case, these are the optimal forcings, which are defined as the forcings that trigger the strongest energy response. In the case of a forcing frequency close to the frequency of the global mode, a forcing structure equal to the adjoint global mode ensures the lowest forcing amplitude E′. All predictions given by the amplitude equations are checked against direct numerical simulations conducted at a supercritical Reynolds number. We show that a global mode may effectively be stabilized by a high-frequency harmonic forcing, which achieves suppression of the perturbation frequencies that are lower than the forcing frequency, and that a near-resonant forcing achieves locking of the flow onto the forcing frequency, as predicted by the amplitude equations. © 2012 Cambridge University Press.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013
The resonant behavior of vacancy states in graphene is well known but some ambiguities remain concerning, in particular, the nature of the so-called zero energy modes. Other points are not completely elucidated in the case of low but finite vacancy concentration. Here we concentrate on the case of vacancies described within the usual tight-binding approximation. More precisely, we discuss the case of a single vacancy or of a finite number of vacancies in a finite or infinite system. © 2013 American Physical Society.
Agency: Cordis | Branch: H2020 | Program: MSCA-ITN-ETN | Phase: MSCA-ITN-2015-ETN | Award Amount: 3.90M | Year: 2016
The increasing environmental awareness of the European society has been always present in the aeronautical community, industry and research centres, having a definite influence on the way the aircraft of the future should be. In this line, the ACARE Vision for 2020, a Group of Renowned Personalities in the aeronautical field, has formulated a clear set of requirements for civil transport aircraft operations in order to reach the following specific environmental goals: halving perceived aircraft noise, 50% cut in CO2 emissions per passenger-km and 80% cut in NOx emissions. Many of these goals have a direct connection with the aerodynamic performance of the aircraft; mainly with aerodynamic technologies. Most of the elements of the aerodynamics of conventional aircraft are modelled and understood to some degree but reliable solutions are not available due to new challenges appearing as the technology matures. One of the most common problems is related to stability analysis for configurations in the limits of the flight envelope or when unsteady effects are dominant. This challenge is the object of the research of SSeMID, and is the focus of the international training plan for young engineers employed within the network. The project will provide valuable information for such aerodynamic structures paving the way to its complete industrialization while increasing the academic knowledge regarding instability mechanisms and covering the necessary skills and knowledge to train experts in this area
Arenal R.,ONERA |
Blase X.,CNRS Neel Institute |
Advances in Physics | Year: 2010
We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.