Alain Farcy

Unsteady Aerodynamics of Flapping Airfoil in Hovering Flight at Low Reynolds Numbers

The major aim of flapping motion research is based on the understanding of the relation between the temporal and the spatial changes of the wake structure and the resulting instantaneous aerodynamic forces over the flapping wings. The essential physics of nonsteady airfoil problems can be observed from simplified two-dimensional experiments, and the interpretations of the behavior can be supported by theoretical or numerical models. The aim of this study is to find optimum parameters to generate maximum lift during this motion, by using numerical methods and analytical models. A great number of cases are investigated involving the changes in the parameters such as angle of attack, location of start of change of incidence, location of start of change of velocity, axis of rotation, and Re number. In addition to the instantaneous aerodynamic forces, pressure distributions and vorticity contours, the average lift and drag coefficient values are also calculated. Positive lift values along the motion are obtained for angle of attack greater than 30°. The vortices shed during the flapping motion generate the lift. A modelization program is developed by use of Duhamel integral in order to compare with DNS results.

On the non-intrusive evaluation of fluid forces with the momentum equation approach

The aim of this paper is to discuss the advantages and difficulties linked with the experimental application of the momentum equation approach as a non-intrusive way to predict the unsteady loads experienced by an airfoil in motion. First, in order to evaluate the influence of the varying parameters relative to the calculation of the corresponding drag and lift coefficients, numerical flow fields obtained by means of DNS are used. The comprehension of the impact of the spatial and temporal resolutions, velocity accuracy or third velocity component on the estimation of forces allows us to quantify the accuracy of the approach and helps in specifying the parameters setting which could lead to a consistent experimental application. In a second step, the approach is applied to experimental flow fields measured through the use of time resolved particle image velocimetry (TR-PIV). A low Reynolds number flow around an impulsively started airfoil is considered. The loads and vorticity flow fields are correlated and compared with those obtained by DNS.

Laser Sheet Visualization for Flapping Motion in Hover

§The aim of the present study is to understand the aerodynamics phenomena and the vortex topology of the highly unsteady flapping motion. Instead of the use of real insect/bird wing geometries and motions which are highly complex and difficult to imitate by an exact modeling, a simplified model is used to understand the unsteady aerodynamics and vortex formation during the different phase of the flapping motion.

A Parametrical Study with Laser Sheet Visualization for an Unsteady Flapping Motion

§In order to better understand the vortex dynamics and evaluation of the flow structures during the unsteady flapping motion, experimental visualizations are considered. The visualizations are represented for the phenomenological analysis of the flow. The flow is assumed to be laminar with the Reynolds number of 1000. The evaluation of the complex flow topology is investigated instantaneously for the effect of the different parameters. The vortex identification is performed for the change of incidence position xa, the change of velocity position xv and the starting angle of attack α0.