Les techniques de mesure optique ont dévoilé le potentiel de la conception d'aile en s’inspirant de la nature
Friday, 4. March 2016
Les solutions de LaVision pour les interactions Fluide-Structure (FSI)
La dynamique de vol des insectes et des oiseaux fascine la communauté du génie aérospatial depuis longtemps : la complexité et la variété des mécanismes de vol des insectes et des oiseaux prouve bien qu’il y a de nombreuses façons, très optimisées, de voler, que l’ingénierie est toujours incapable de reproduire. Cependant, notre compréhension de ces problèmes progresse au fur et à mesure que la recherche en aérospatial continue de se développer vers des territoires encore inexplorés. Un de ces groupes de recherche est le laboratoire de recherche en mécanique des fluides expérimentale de l’université de Southampton, dirigé par le professeur Ganapathisubramani. Dans ce groupe, Robert Bleischwitz, doctorant, a porté son attention sur la conception d’une aile inspirée par les chauves-souris et il décrit son travail de la façon suivante:
"The increasing interest in small scale Unmanned Air Vehicles (UAV) and Micro Air Vehicles (MAV) turned the attention of our research community to the development of flexible, biologically inspired membrane wings known from bats. In order to understand the dynamics of bat flight, which is a complex Fluid-Structure Interaction (FSI) phenomenon, an experimental wind tunnel study was conducted at the University of Southampton. This study needed to simultaneously capture loads, wing deformations as well the flow dynamics in high speed. While the loads where measured using load cells instrumentation, the wing displacements and flow characteristics needed to be investigated using non-intrusive, full-field measurement techniques. 3D Digital Image Correlation (3D-DIC) was used to capture the deformation of the wing with 0.1 mm resolution and Time-Resolved Particle Image Velocimetry (TR-PIV) was used to measure the flow evolution at a high sampling rate of 800 Hz. The optical measurement system from LaVision (consisting of both FlowMaster and StrainMaster modules) allowed seamless integration of the two DIC cameras and LED lamps along with the two PIV cameras and a high power laser. Moreover, the added benefits of using this LaVison equipment, such as blue LED lamps and green laser, allowed for simultaneous measurements to be performed because it was possible to separate the respective wavelengths for the two systems. The full concerto was managed from a highly accurate high-speed controller which was triggered externally, thus making it possible to achieve synchronized measurements.
The knowledge of all three measurement techniques allowed to find clear answers on the highly dynamic FSI coupling effects and the source of the physical problem. The benefit of capturing the Fluid-Structure Interactions became also useful in combining similar high-speed experiments of the membrane with different PIV based flow-planes. So we were running the same membrane wing case with two different PIV-setups (one planar streamwise plane, one stereo-spanwise) independently on different days. Later, we were able to extract the DIC based membrane information as guiding master, allowing to phase-average the different PIV-planes into one cycle, thus reconstructing a full image of the phenomenon under investigation. Due to the high dynamic resolution (high signal to noise ratio) of cameras, we were able to identify membrane induced flow oscillation even in the wake one chord behind the wings trailing edge. As a result, we are capable to tell what kind of wing has what kind of frequency footprint from the wake only. And, we know how the structures with this frequency content look like. POD analysis on membrane and flow were critical to identify the main frequency components of the wing deformation and their relation to the flow evolution."