Analysis and treatment of errors due to high velocity gradients in particle image velocimetry

International audienceThis paper deals with errors occurring in 2D cross-correlation PIV algorithms (with window shifting), when high velocity gradients are present. A first bias error is due to the difference between the Lagrangian displacement of a particle and the real velocity. This error is calculated theoretically as a function of the velocity gradients, and is shown to reach values up to 1 pixel if only one window is translated. However, it becomes negligible when both windows are shifted in a symmetric way. A second error source is linked to the image pattern deformation, which decreases the height of the correlation peaks. In order to reduce this effect, the windows are deformed according to the velocity gradients in an iterative process. The problem of finding a sufficiently reliable starting point for the iteration is solved by applying a Gaussian filter to the images for the first correlation. Tests of a PIV algorithm based on these techniques are performed, showing their efficiency, and allowing the determination of an optimum time separation between images for a given velocity field

Physics of vortex merging

20 p.This article deals with the interaction of co-rotating vortices, in configurations similar to those found in the extended near-wake of typical transport aircraft. The fundamental process of vortex merging is analyzed and modeled in detail in a two-dimensional context, giving insight into the conditions for merging and its physical origin, and yielding predictions for the resulting flow. Three-dimensional effects, in the form an elliptic short-wave instability arising in the initial co-rotating vortex flow, are described and analyzed theoretically. They are found to cause significant changes in the merging process, like earlier merging and larger final vortex cores. Illustrations from recent experimental, numerical and theoretical studies are given, and the relevance of the results for applications to real aircraft wakes is discussed

The evolution of a subharmonic mode in a vortex street

The development of a subharmonic three-dimensional instability mode in a vortex street is investigated both numerically and experimentally. The flow past a ring is considered as a test case, as a previous stability analysis has predicted that for a range of aspect ratios, the first-occurring instability of the vortex street is subharmonic. For the flow past a circular cylinder, the development of three-dimensional flow in the vortex street is known to lead to turbulent flow through the development of spatio-temporal chaos, whereas subharmonic instabilities have been shown to cause a route to chaos through the development of a period-doubling cascade. The three-dimensional vortex street in the flow past a ring is analysed to determine if a subharmonic instability can alter the route to turbulence for a vortex street. A linear stability analysis and non-axisymmetric computations are employed to compute the flow past a ring with an aspect ratio AR=5, and comparisons with experimental dye visualizations are included to verify the existence of a subharmonic mode in the wake. Computations at higher Reynolds numbers confirm that the subharmonic instability does not initiate a period-doubling cascade in the wake

Numerical investigation of the vortex roll-up from a helicopter blade-tip using a novel fixed-wing adaptation method

This contribution relates to the simulation of the flow around the tip of a helicopter rotor blade in hovering flight conditions. We here propose a new methodology of framework adaptation, using a comprehensive rotor code and high-fidelity numerical simulations. We construct an equivalent fixed-wing configuration from a rotating blade, in which centrifugal and Coriolis forces are neglected. The effect of this approximation on the solution is analysed. The method is validated by a detailed comparison with wind tunnel data from the literature, concerning aerodynamic properties and tip vortex roll-up. This validation also includes variations of the pitch angle and rotational speed, up to transonic tip velocities. Compared to previously published methods of framework adaptation, the new hybrid method is found to reproduce more accurately the flow around a rotating blade tip

A merging criterion for two-dimensional co-rotating vortices

10 p.We propose a quantitative criterion for the merging of a pair of equal two-dimensional co-rotating vortices. A cross-validation between experimental and theoretical analyses is performed. Experimental vortices are generated by the roll-up of a vortex sheet originating from the identical and impulsive rotation of two plates. The phenomenon is then followed up in time until a rapid pairing transition occurs for which critical parameters are measured. In the theoretical approach, the nonlinear Euler solution representing a pair of equal vortices is computed for various nonuniform vorticity distributions. The stability analysis of such a configuration then provides critical values for the onset of merging. From this data set, a criterion depending on global impulse quantities is extracted for different shapes of the vorticity distribution. This theoretical statement agrees well with our experimentally based criterion

PAJARITA Y BIGOTILLO [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201

Vortex dynamics and breakdown in the wake of a wind turbine rotor

International audienceA century ago, Joukowsky proposed an aerodynamic rotor model utilizing an actuator disc with constant circulation. Since then, this model has been the subject of much controversy, as it predicts a power performance that for all tip speed ratios exceeds the Betz limit and that goes to infinity when the tip speed ratio approaches zero. Recently, it was demonstrated that the Joukowsky model is fully compatible with the inviscid Euler equations and that the apparent inconsistency can partly be explained by the fact that the model neglects lateral forces due to pressure or friction [1]. Furthermore, CFD computations using an actuator line model have shown that the excessive swirl of the hub vortex at small tip speed ratios generates vortex breakdown on the wake axis, causing a recirculating zone that limits the power yield [2]. In the past years we have studied in detail the flow behavior of Joukowsky rotors at small tip speed ratios (TSR). It is shown that the excessive swirl appearing towards the rotor center at small tip speed ratios generates vortex breakdown, causing a recirculating zone in the wake that limits the power yield of the rotor. The appearance of vortex breakdown has a similar effect on the flow behavior as the vortex ring state that usually appears at higher tip speed ratios. Limits to where vortex breakdown might occur with tip speed ratio and rotor loading as parameter are investigated. The limits found correspond to well-known criterion for vortex breakdown onset for swirling flows in general. By applying a criterion for vortex breakdown in combination with the general momentum theory, the power performance always stays below the Betz limit

Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry

International audienceResults from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes

Etude numérique de l'instabilité elliptique dans une paire de vortex co-rotatifs avec écoulement axial

L'instabilité elliptique est un phénomène tridimensionnel apparaissant dans un vortex dont les lignes de courant ont une forme elliptique. Elle est notament suceptible d'accélérer la destruction des sillages avions. Au cours de notre éude, nous avons étudié numériquement l'influence d'un écoulement axial sur la stabilité d'une paire de vortex co-rotatifs. Les taux de croissance des modes les plus instables sont calculés en fonction de l'amplitude W0 de l'écoulement axial et du nombre d'onde k des perturbations, pour un nombre de Reynolds (basé sur la circulation d'un vortex) de 14000, et pour deux tailles du coeur des tourbillons. Il a été mis en évidence l'existence de nombreuses zones instables dans l'espace W0-k. En particulier, pour une taille du coeur élevée, les paires de vortex sont instables quasiment dans l'ensemble de l'espace des paramètres, et on observe une prédominance de modes avec couche critique

REPORTAJE FOTOGRÁFICO DEL TABOR DE AGÜIMES [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201