Optical measurement techniques have uncovered the potential of nature-inspired wing design

Friday, 4. March 2016

LaVision’s Fluid-Structure Interaction (FSI) solutions
The flight dynamics of insects and birds has long fascinated the aerospace engineering community and for good reason. The complexity and high variety of insect and bird flight mechanisms means that there are numerous, highly optimized ways of flight that engineering is still unable to reproduce. However, as aerospace research continues to advance into uncharted territory, so does our understanding of these problems. One such group is the Experimental Fluid Mechanics Research Laboratory within the University of Southampton, led by Professor Ganapathisubramani. Within this group, attention to bat-inspired wing design has been pursued by Robert Bleischwitz, a PhD researcher, who has been conducting ground-breaking research in this field and who had this to say about his work:

"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."

The work conducted by Robert Bleischwitz and his colleagues, led to their development of a bio-inspired UAV prototype that they have successfully tested in a wind tunnel as well as over water, as described in the article entitled ‘Is it a bird? Is it a bat? No, it's the future of drone technology’1

1 (c) Guardian News & Media Ltd