Interaction of large scale strucutres in high Re round jets

International audienceThe dynamical significance of vortical structures in the near field of round jets, viz. the vortex rings and the stream-wise vortices, has been known for several decades (Ho & Huerre, 1984). A fundamental understanding of their interactions , especially in turbulent jets at high Reynolds number (Re), is expected to provide an impetus to devising control strategies; for instance in improving combustion efficiency and reducing jet noise. One such possible interaction was recently observed in Davoust et al. (2012) that involved production of additional streamwise vorticity, as a result of an influence of the fluctuations of streamwise vorticity on those of azimuthal vorticity in high Re jets. This additional vorticity showed up in streamwise vorticity correlations in the form of radial organization as opposed to its azimuthal organization classically found. In further exploring this possibility, we have observed in our experiments, that strengthening the vortex rings relative to the streamwise vortices, renders them less susceptible to an influence from the streamwise vortices. This paper presents the results for a Re = 1.5 × 10 5 and low Mach number round jet in which the most energetic vortex rings are acoustically excited at different excitation amplitudes. A comparison of the relative strengths of the two structures with increasing excitation levels, corroborates the interaction proposed in Davoust et al. (2012)

Uniformly rotating flow in a straight duct followed by a contraction: experimental and theoretical study of the mean flow

We study the mean dynamics of a radially confined, uniformly rotating flow of high Reynolds number. The duct geometry consists of a portion of constant cross-section followed by a final contraction, downstream of which the flow exhausts as a swirling jet. The duct flow is investigated by Stereoscopic PIV. To understand the specific effect of the contraction, we also consider the case where it is replaced by a straight duct portion. These experimental results are compared with a simulation of axisymmetric steady flow at a moderate Reynolds number. Good agreement is obtained between the two methods. Both show that the flow is strongly influenced by the contraction when it is subcritical with respect to axisymmetric Kelvin waves. In that regime, no vortex breakdown is found inside the duct. Instead, large-amplitude standing waves of positive azimuthal vorticity are observed

Optimum control of cavity flow

For high Reynolds numbers Re = UL/nu, with an order of magnitude of a few thousands, a flow over a square cavity becomes unsteady with the growth of two-dimensional instabilities. This phenomenon is studied by computing : 1/ the branch of steady solutions with respects to the Reynolds number, using a branch tracking method ; 2/ the eigenvalues and eigenvectors of the global linearized operator with respects to Re. We thus show that the cavity is subject to a Hopf bifurcation at a critical Reynolds number denoted by Rec. After setting the computations in a supercritical case for which Re > Rec, we use an optimum control algorithm to minimize the energy of the perturbations at various terminal times T. The control will consist in unsteady blowing and succion on the cavity wall. We will analyze the phenomenology of the control law with a description of the influence of the target time T and the cost of the control which will be denoted by m

Counter-Rotating Open Rotor (CROR) : flow physics and simulation

La propulsion par hélices contrarotatives est à l'étude dans le contexte des recherches menées sur la réduction de la consommation et du bruit aéronautiques. Dans un Open Rotor, on cherche notamment à maîtriser la nappe tourbillonnaire issue de l'hélice amont car elle vient impacter l'hélice aval, contribuant majoritairement au bruit rayonné. Des simulations avancées de ce problème seront présentées et discutées sur le plan de la mécanique des fluides et de l'acoustique

Prandial states modify the reactivity of the gustatory cortex using gustatory evoked potentials in humans

Previous functional Magnetic Resonance Imaging studies evaluated the role of satiety on cortical taste area activity and highlighted decreased activation in the orbito-frontal cortex when food was eaten until satiation. The modulation of orbito-frontal neurons (secondary taste area) by ad libitum food intake has been associated with the pleasantness of the food's flavor. The insula and frontal operculum (primary taste area) are also involved in reward processing. The aim was to compare human gustatory evoked potentials (GEP) recorded in the primary and secondary gustatory cortices in a fasted state with those after food intake. Fifteen healthy volunteers were enrolled in this observational study. In each of two sessions, two GEP recordings were performed (at 11:00 am and 1:30 pm) in response to sucrose gustatory stimulation, and a sucrose-gustatory threshold was determined. During one session, a standard lunch was provided between the two GEP recordings. During the other session, subjects had nothing to eat. Hunger sensation, wanting, liking, and the perception of the solution's intensity were evaluated with visual analog scales. GEP latencies measured in the Pz (p < 0.001), Cz (p < 0.01), Fz (p < 0.001) recordings (primary taste area) were longer after lunch than in the pre-prandial condition. Fp1 and Fp2 latencies (secondary taste area) tended to be longer after lunch, but the difference was not significant. No difference was observed for the sucrose-gustatory threshold regardless of the session and time. Modifications in the primary taste area activity during the post-prandial period occurred regardless of the nature of the food eaten and could represent the activity of the frontal operculum and insula, which was recently shown to be modulated by gut signals (GLP-1, CCK, ghrelin, or insulin) through vagal afferent neurons or metabolic changes of the internal milieu after nutrient absorption. This trial was registered at clinicalstrials.gov as NCT02472444

On the linear receptivity of trailing vortices

The present work investigates the excitation process by which free-stream disturbances are transformed into vortex-core perturbations. This problem of receptivity is modelled in terms of the resolvent in frequency space as the linear response to forcing. This formulation of receptivity suggests that non-normality of the resolvent is necessary to allow free-stream disturbances to excite the vortex core. Considering a local (in frequency) measure of non-normality, we show that vortices are frequency-selectively non-normal in a narrow frequency band of retrograde perturbations while the rest of the range is governed by an effectively normal operator, thus not contributing to receptivity. Canonical decomposition of the resolvent reveals that vortices are most susceptible to coiled filaments localised about the critical layer that induce bending waves on the core. Considering Lamb–Oseen, Batchelor and Moore–Saffman vortices as reference-flow models, we find free-stream receptivity to be essentially generic and independent of the axial wavelength on the considered range. A stochastic interpretation of the results could be a model for trailing-vortex meandering.This work has been supported by the French Ministry of Civil Aviation (DGAC) under PHYWAKE (PHYsics of WAKE vortices) research program. We are grateful to the anonymous referees for their exceptional investment and insightful suggestions