Effects of Membrane Vibration on the Flow Field Surrounding Flat-Plate Membrane Airfoils

This investigation examines the fluid-structure interaction between flexible membrane airfoils and their surrounding flow field for the purpose of physically interpreting aerodynamic performance benefits attributed to certain membrane airfoil configurations in low Reynolds number (< 100,000) flight regimes. Specifically, this research effort identifies airfoil membrane vibration onset through quantifiable characteristics and presents simultaneous time-resolved hotwire anemometry and laser vibrometry to measure membrane vibration and flow field oscillation velocities, respectively. Evaluations were executed with a series of test articles consisting of rigid, flat plate airfoils containing a single, variably-sized membrane cell located at the test article’s spanwise centerline. Results demonstrate that membrane vibration onset criteria involving direct assessment of membrane oscillation velocity via laser vibrometry more closely and consistently match visual assessment of vibration onset than do onset criteria involving flow field oscillation velocity measured via hotwire anemometry. At low freestream velocities, membrane vibration does not significantly affect the surrounding flow and vibration onset cannot be consistently identified via flow field assessment. Comparison of total membrane vibration energy at varying test conditions proves to be the most consistently accurate and quantifiable indicator of vibration onset.