Self-sustained oscillations of a confined impinging jet

The present paper investigates the dynamics of a laminar plane jet impinging on a flat plate in a channel. An experimental parametric study is carried out to determine the flow regimes at different levels of confinement and Reynolds numbers. For very confined jets, the flow is steady whatever the Reynolds number. The overall structure of the flow is symmetric with respect to the jet axis and is characterized by the presence of recirculation zones at the channel walls. The dynamics is radically different for less confined jets. Above a critical Reynolds number, the flow bifurcates in the form of an oscillating flapping mode of the impinging jet. Analyses of the experimental results provide with a quantitative characterization of this regime in terms of amplitude, wavelength and frequency. This self-oscillating bifurcated flow induces strong sweepings of the target plate by the jet and intense vortex dipole ejections from the impacted wall. Such a regime is expected to be particularly useful in the enhancement of the local heat transfer at relatively low cost in terms of flow rate

Interaction of unequal anti-parallel vortex tubes

A computational study is reported of the close interaction of nominally anti-parallel vortex tubes with unequal strengths. The computations are performed using a spectral method, with periodic boundary conditions and vortex Reynolds number Re=1500. The vortices are perturbed by a wavelength for which the pair is unstable because of their mutual interaction. The numerical method is tested for the case of equal-strength vortices, which exhibits the classic vortex reconnection phenomenon typifed by bridging between the vortex cores and formation of thin vorticity threads as the bridged sections advect away under their self-induced velocity. Computations for vortices of unequal strengths are reported for cases with small, moderate and large strength differences. The bridges between the vortex structures form loops that twist owing to the unequal vortex strengths. In the thread region, the vortex interaction is controlled by competition between the effects of stretching of the weak vortex as it wraps around the stronger vortex and the core distortion induced on each vortex owing to the straining imposed by the opposing vortex. For cases with large vortex strength difference, the strong vortex remains nearly straight as the weak vortex wraps around it, inducing an interlaced pattern of positive and negative vorticity spirals within the core of the strong vortex. Over long time, the bridge regions form loops that propagate away from the thread region for cases with small strength difference and wrap around the nearly columnar strong vortex for cases with large strength difference

Une méthode conservative de couplage instationnaire de codes en aérothermique

Une méthode conservative de couplage de solveurs de mécanique des fluides et de diffusion thermique est présentée. L'utilisation optimale des codes et la perspective de paralléliser l'intégration incitent à effectuer un couplage pour lequel les échanges de données entre domaines intégrés sont effectués selon des pas de temps indépendants des conditions classiques de stabilité numérique. La conservativité est alors maintenue par des corrections qui sont une nouvelle source de déstabilisation. L'étude détermine des critères permettant de réaliser des calculs stables et conservatifs

A conservative coupling numerical method for transient conjugate heat transfer

A conservative fluid mechanics-heat diffusion solver coupling method is presented. Optimal use of solvers can be achieved by coupling according to a cycle time step independent of classical numerical stability conditions. Solvers integrate their domains independently during a cycle. Between cycles, data are exchanged to compute a coupling boundary condition, which is imposed at the interface between the coupled domains. Conservativity is one of the main purposes of this coupling method. Consequently, Finite Volume method is used for the solvers. But during independent integrations by solvers, thermal flux losses happen at the interfaces between coupled domains. Conservative corrections are defined and used in order to maintain conservativity. But they can destabilize time integration. Stability criteria are established in order to achieve a robust conservative coupling, that eventually also improves integration accuracy

Implementation and validation of a slender vortex filament code: its application to the study of a four-vortex wake model

A computational code EZ-vortex is developed for the motion of slender vortex filaments of closed or open shape. The integro-differential equations governing the motion of the vortex centrelines are either the Callegari and Ting equations, which are the leading order solution of a matched asymptotic analysis, or equivalent forms of these equations. They include large axial velocity and nonsimilar profiles in the vortical cores. The fluid may be viscous or inviscid. This code is validated both against known solutions of these equations and results from linear stability analyses. The linear and non-linear stages of a perturbed two-vortex wake and of a four-vortex wake model are then computed

Active control of transitional channel flows with pulsed and synthetic jets using vortex methods

International audienceIn this work a vortex method is used to simulate and control an incompressible two-dimensional transitional flow over a backward-facing step. Two different control strategies are implemented to modify the shedding, the recirculation zone behind the step and the transport in the channel. The first technique consists in using a pulsing inlet velocity and the second one is based on local oscillating jets implemented on the step vertical wall. The influence of these controls on the flow behavior is carefuly investigated. Both, open-loop and closed-loop active control approaches are performed

Réponse d'un jet plan à des excitations sinusoïdales

Le forçage fréquentiel au mode naturel est utilisé pour stabiliser les structures tourbillonnaires d'un écoulement cisaillé. Dans une étude précédente, nous avons soumis un jet plan à des excitations sinusoïdales monochromatiques à la fréquence naturelle en utilisant la simulation numérique. Dans cette étude, nous présentons pour la même configuration de jet, les résultats obtenus en faisant varier la fréquence d'excitation. L'objectif est déterminer la réponse du jet et de la comparer à celle de couches de mélange et de jets en mettant en évidence les phénomènes d'appariement. On étudie les jets à profil d'entrée uniforme et à profil d'entrée parabolique. Les simulations numériques montrent que la fréquence de réponse est identique à la fréquence d'excitation dans un intervalle assez large comprenant la fréquence naturelle pour les deux cas. Au delà d'une certaine fréquence d'excitation, les fréquences de réponses sont plus faibles que la fréquence d'excitation et sont liées à la fréquence naturelle. On observe des appariements des structures tourbillonnaires traduits sur le spectre par l'apparition de fréquences sub-harmoniques

Smart Atlas for Supporting the Interpretation of probe-based Confocal Laser Endomicroscopy (pCLE) of Biliary Strictures: First Classification Results of a Computer-Aided Diagnosis Software based on Image Recognition

International audiencepCLE enables microscopic imaging of biliary strictures, in vivo and in real time, during an ERCP procedure. Results of a multicentric study (Meining et al., GIE 2011) have shown that pCLE allows endoscopists to diferentiate benign from malignant strictures in real time with high sensitivity and NPV. A computer-aided diagnosis software called Smart Atlas has been developed to assist endoscopists with the interpretation of pCLE sequences. This study aims at evaluating the performance of this software for the diferentiation of benign and malignant strictures

Smart Atlas for Supporting the Interpretation of needle-based Confocal Laser Endomicroscopy (nCLE) of Pancreatic Cysts: First Classification Results of a Computer-Aided Diagnosis Software based on Image Recognition

International audiencenCLE enables microscopic imaging of pancreatic cysts, in vivo and in real time, during an EUS-FNA procedure. Diferentiating branch duct-type Intraductal Papillary Mucinous Neoplasm (IPMN) and Serous Cystadenoma (SCA) of the pancreas can be difcult, especially in case of a solitary lesion without clear communication with the pancreatic duct. Recent studies (Konda et al., Endoscopy 2013; Napoléon et al., DDW 2013) have identifed reliable nCLE descriptive features (superfcial vascular network in SCA; fngerlike projections in IPMN), allowing endoscopists to discriminate between SCA and IPMN. In parallel, a computer-aided diagnosis software called Smart Atlas has been developed to assist endoscopists with the interpretation of nCLE video sequences. This study aims at evaluating the performance of this software for the diferentiation of SCA and IPMN cases