This Letter aims at understanding the dynamical process that leads to the onset of chaos in the flow past a blunt-based axisymmetric bluff body. On the basis of direct numerical simulations, conducted for Reynolds numbers ranging from 100 to 900, we show that the flow undergoes multiple transitions, successively giving rise to the SS, RSPa, RSPb, RSPc and RSB wake states. In particular, the RSPc state, revealed in this work via long-term computations, is characterized by intermittent vortex stretching denoting the onset of chaos before the symmetry breaking and the occurrence of the RSB state. 相似文献
Recently, two ionization sources, electrospray (ESI) and matrix-assisted laser desorption (MALDI) have been used in parallel to exploit their complementary nature and to increase proteome coverage. In this study, a method using bidimensional (2D) nanoLC coupled online with ESI quadrupole time-of-flight (Q-TOF) with the simultaneous collection of fractions for analyses by LC–MALDI Q-TOF–MS/MS was developed. A total of 39 bovine proteins were identified to a high degree of confidence. To help in differentiating peptide detection following ESI and MALDI with the same mass spectrometer, we compared physico–chemical characteristics of the peptides (molecular mass, charge and size) by principal component analysis (PCA) and analysis of variance on the results of PCA. More hydrophobic peptides with a wider mass coverage were identified when ESI was used, whereas more basic and smaller peptides were identified when MALDI was used. However, the generally accepted differentiation between ESI and MALDI according to the presence of basic amino acids residues Lys and Arg and the ratio Lys/Arg was not shown as significant in this study. Moreover, we pointed out the importance of the type of mass spectrometer used in complement to both ionization sources for achieving a global increase of proteome coverage. 相似文献
Deviations from Miner's linear law of cumulative damage have been modelled and observed many times for the fatigue of metals, but almost no analogous studies have been performed for elastomers. Such a study is reported here. 相似文献
Plasmon-loss spectra of clean polycrystalline molybdenum surfaces have been determined in the primary energy range Ep = 50–3000 eV. Spectra a distributions (nonderivative mode). A simplified model is described for evaluating plasmon-loss spectra using elastic-peak electron spectroscopy, as de of elastically reflected electrons is determined by integrating the N(E) spectrum of secondary and backscattered electrons. The ratio of the ar (23–24 eV) to that of the elastic peak is Pλ, the product of the probability for creating a volume plasmon loss and the inelastic mean free pat second plasmon-loss peak is (Pλ)2. Evaluation of our experimental plasmon-loss spectra gives Pλ = 0.4–0.5 for Ep > 500 eV. Th constitutes ~50% of all losses determining the IMFP, interband loss processes being important in the remainder. For the low energy range, Ep found. For Ep < 100 eV, no volume plasmon-loss peak could be detected in our N(E) spectra. The simplified model proves to be valid fo plasmon-loss peak (11–12 eV), i.e., such that Npls/Ne ? 10?2. Some results are presented concerning surface plasmon losses as molybdenum surface. 相似文献
Numerical simulations of the four-field extended magnetohydrodynamics (MHD) equations with hyper-resistivity terms present a difficult challenge because of demanding spatial resolution requirements. A time-dependent sequence of r-refinement adaptive grids obtained from solving a single Monge–Ampère (MA) equation addresses the high-resolution requirements near the x-point for numerical simulation of the magnetic reconnection problem. The MHD equations are transformed from Cartesian coordinates to solution-defined curvilinear coordinates. After the application of an implicit scheme to the time-dependent problem, the parallel Newton–Krylov–Schwarz (NKS) algorithm is used to solve the system at each time step. Convergence and accuracy studies show that the curvilinear solution requires less computational effort than a pure Cartesian treatment. This is due both to the more optimal placement of the grid points and to the improved convergence of the implicit solver, nonlinearly and linearly. The latter effect, which is significant (more than an order of magnitude in number of inner linear iterations for equivalent accuracy), does not yet seem to be widely appreciated. 相似文献
The three-dimensional flow that develops around a finite flapping wing is investigated using a tomographic scanning PIV technique.
The acquisition and correlation processes employed to achieve such measurements have been carefully validated. Issues regarding
the relevant timescales of the flow and the spanwise space-resolution are addressed. Results obtained on a hovering flapping
wing whose plunging phase is described by a rectilinear motion highlight the influence of the free end condition and the formation
of the tip vortex on the leading edge vortices behavior, wing/wake interactions, and wake stabilization. 相似文献
The vast majority of research works on low aspect ratio rotating wings report that, at high angle of attack, the leading edge vortex that forms on the upper surface of the wing is stable. This ‘trick’ is used by insects and auto-rotating seeds, for example, to achieve the desirable amount of lift. Yet, a few experimental studies suggest that leading edge vortices might be unstable under similar, low Rossby number, conditions. While it is unclear what causes vortex shedding in these studies, the present communication explores the sensitivity of leading edge vortex attachment to perturbations of the rotating speed and demonstrates that shedding can be triggered even for very small perturbations, corresponding to wing tip displacements lower than 1% of the wing chord.
Flight agility, resistance to gusts, capability to hover coupled with a low noise generation might have been some of the reasons
why insects are among the oldest species observed in nature. Biologists and aerodynamicists focused on analyzing such flight
performances for diverse purposes: understanding the essence of flapping wings aerodynamics and applying this wing concept
to the development of micro-air vehicles (MAVs). In order to put into evidence the fundamentally non-linear unsteady mechanisms
responsible for the amount of lift generated by a flapping wing (Dickinson et al. in Science 284:1954–1960, 1999), experimental and numerical studies were carried out on typical insect model wings and kinematics. On the other hand, in
the recent context of MAVs development, it is of particular interest to study simplified non-biological flapping configurations
which could lead to lift and/or efficiency enhancement. In this paper, we propose a parametrical study of a NACA0012 profile
undergoing asymmetric hovering flapping motions at Reynolds 1000. On the contrary to normal hovering, which has been widely
studied as being the most common configuration observed in the world of insects, asymmetric hovering is characterized by an
inclined stroke plane. Besides the fact that the vertical force is hence a combination of both lift and drag (Wang in J Exp
Biol 207:1137–1150, 2004), the specificity of such motions resides in the vortex dynamics which present distinct behaviours, whether the upstroke
angle of attack leads to a partially attached or a strong separated flow, giving more or less importance to the wake capture
phenomenon. A direct consequence of the previous remarks relies on the enhancement of aerodynamic efficiency with asymmetry.
If several studies reported results based on the asymmetric flapping motion of dragonfly, only few works concentrated on parametrizing
asymmetric motions (e.g. Wang in Phys Rev Lett 85:2216–2219, 2000). The present study relies on TR-PIV measurements which allow determination of the vorticity fields and provide a basis to
evaluate the resulting unsteady forces through the momemtum equation approach. 相似文献
Actinides (thorium, uranium, neptunium, plutonium, and americium) were infiltrated into a porous Nd1.8Zr2O6.7 matrix, prepared by gel-supported precipitation. (Nd1.8An0.2)Zr2O7+x pyrochlores were formed after sintering in Ar/H2 and the pyrochlore structure remains during oxidation at 800 °C in air. X-ray diffraction reveals a linear relationship between the pyrochlore lattice parameter and the ionic radii of the actinides. EXAFS measurements on actinide L3-edge show a split shell of nearest neighbour oxygen atoms similar to that surrounding of Nd. The actinide-oxygen bond distances decrease with the actinide ionic radii, which verifies that these actinides adopt the Nd site in the (Nd1.8An0.2)Zr2O7+x pyrochlore. The oxidation susceptibility of Np is related to the availability of oxygen vacancies and in contrast to stabilised zirconia Np(V) can be obtained in zirconia based pyrochlore. 相似文献