To cite this version:Christophe Maqua, Guillaume Castanet, Fabrice Lemoine, Nicolas Doué, Gérard Lavergne. Temperature measurements of binary droplets using three-color laser-induced fluorescence. Experiments in Fluids, Springer Verlag (Germany), 2006, 40 (5), pp.786 -797. 10.1007/s00348-006-0116-y. hal-01570422Temperature measurements of binary droplets using three-color laser-induced fluorescence Abstract Evaporation of multicomponent droplets is a critical problem in many engineering applications, for example spray combustion. Knowledge of droplet temperature is a key issue in understanding the highly complex heat and mass-transfer phenomena related to multicomponent droplet evaporation and combustion. In this work, optical diagnosis based on three colorlaser-induced fluorescence was developed: the objective was to measure the temperature of binary droplets (ethanol and acetone mixtures), even when the composition varies with time. Demonstration on an overheated droplet stream of acetone-ethanol mixtures is described and the experimental data are compared with results from a numerical simulation based on the discretecomponents model.

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We numerically investigated the unsteady dynamics of a two-dimensional airfoil undergoing a continuous, prescribed pitch-up motion and freely translating as a response to aerodynamic forces and the gravity field. The pitch-up motion was applied about an axis located $1/6$ chord away from the leading edge and was parameterized using the shape change number, with a Reynolds number set to 2000. It was shown that the minimum kinetic energy reached by the airfoil depends stochastically and asymptotically on shape change numbers for values below and above 1, respectively. Very low kinetic energy levels (close to zero) can be reached in both stochastic and asymptotic regions but high shape change numbers are accompanied by significant gain in altitude which may be undesirable from a practical perspective. Rather, shape change numbers in the range [0.1–0.3] allow us to reach relatively low levels of kinetic energy for close perching locations. We showed that highly nonlinear fluid–structure interactions induced by massive flow separations and strong vortices are conducive to low kinetic energy, but responsible for the stochastic dependence of kinetic energy to shape change number, which can make perching manoeuvres hardly controllable for flying vehicles.

The influence of periodic blade pitching on rotor aerodynamics is numerically investigated at a Reynolds number typical of micro-air vehicles. Blade pitching motion is parameterized using three variables, giving rise to a large parameter space that is explored through 74 test cases. Results show that a relevant tuning of pitching variables can lead to an increase in rotational efficiency and thrust, which is found to be primarily related to the occurrence of reversed von Karman street, leading edge vortex (LEV) formation and dynamic stall phenomenon. In addition, for cases where reversed von Karman street occurs, the flow is found to be quasi-two-dimensional, suggesting that quasi-twodimensional approaches can provide relevant approximations of the global aerodynamics. Overall, the analysis demonstrates that blade pitching can be beneficial to the aerodynamic performance of micro-air vehicles and helps draw guidelines for further improvements of flapping-rotor concepts.

International audienceThis paper extends Froude’s momentum theory for free propellers to the analysis of shrouded rotors. A one-dimensional analytical approach is provided, and a homokinetic normal inlet surface model is proposed. Formulations of thrusts and power for each system component are derived, leading to the definition of optimum design criteria and providing insight into the global aerodynamics of shrouded rotors. In the context of micro-air vehicles applications, assessment of the model is conducted with respect to numerical data. Overall, comparison between numerical and analytical results shows good agreement and highlights the sensitivity of the model to viscous effects

The work presented in this paper is part of a project called ARChEaN (Aerodynamic of Rotors in Confined ENvironment) whose objective is to study the interactions of a micro drone rotor with its surroundings in the case of flight in enclosed environments such as those encountered, for example, in archeological exploration of caves. To do so the influence of the environment (walls, ground, ceiling, etc) on the rotor’s aerodynamic performance as well as on the flow field between the rotor and the surroundings is studied. This paper focuses on two different configurations, flight near the ground and flight near a corner (wall and ground), and the results are analyzed and compared to a general free flight case (i.e. far away from any obstacle). In order to carry out this analysis both numerical and experimental approaches are conducted. The objective is to validate the numerical model with the results obtained experimentally and to benefit from the advantages of both approaches in terms of flow analysis. This research work will provide knowledge on how to operate these systems as to minimize the possible negative environment disturbances, reduce power consumption and predict the micro drone’s behaviour during enclosed flights.

In the framework of an optimization study based on a multi-objective optimization formulation, the consideration of optimization parameters such as the actuator location and the outlet design implies a re-meshing procedure that add complexity (even if the actuator is modeled with simple boundary conditions at the jet orifice exit since, locally, the re-mesh is still required). This strongly impacts the global computational cost, in particular if the considered geometry is complex. In this paper we propose an alternative method to model ZNMF synthetic jet actuators through the implementation of volumetric reduced-order models of ZNMF synthetic jet actuators as an additional source terms, in the form of body forces on given local control volumes corresponding to the actual locations of the actuators. This approach is promising as it allows to skip re-meshing procedures (velocity inlet boundaries or full synthetic jet actuator modeling) by directly plugging the volumetric source terms at appropriate locations and, consequently, save a considerable computational time during the optimization process.

This paper describes the setting of two measuring techniques on a rectilinear monosized droplets stream allowing the validation of droplet vaporizing models for internal spatial temperature distribution. Indeed, the vaporizing rate depends strongly on droplet surface temperature and consequently on physical phenomena inside droplets. However, the measurement of temperature in small droplets whose diameter is about 100μm is not easy. To determine the thermal gradient, two experimental techniques have to be coupled: the first must be sensitive to the thermal gradient and the second to the mean temperature. The two chosen experimental techniques, the rainbow refractometry and the Laser Induced Fluorescence (LIF) in liquid phase, agree with these criteria. Some experiments have been done on ethyl alcohol and acetone droplet streams with different injection temperatures and in vaporizing or burning conditions. Then, the results have been compared to numerical predictions.