Force Generation of Bio-Inspired Hover Kinematics: An experimental campaign

Force Generation of Bio-Inspired Hover Kinematics: An experimental campaign

Vandenheede, R.

Van Oudheusden, B.W. (mentor)
Bernal, L.P. (mentor)

Aerospace Engineering

Aerodynamics, Wind Energy & Propulsion

2012-06-18

The field of remote sensing and information gathering is being revolutionized by the recent developments in Micro Air Vehicles, MAVs. The need for maneuverability and flight in confined spaces has directed the focus of research towards flapping flight. Biological flapping flyers exhibit all the characteristics that are desired by MAVs. Biological flyers are able to hover, make rapid changes in their attitude, and navigate through very narrow spaces. For the purpose of this Master thesis the hawkmoth was used as a starting point and source of inspiration. First and foremost the hawkmoth is a fairly large insect, with a wingspan of roughly 10 cm. The larger scale of the insect will translate to an easier design in terms of the scalability of structural and electronic components. Furthermore, the hawkmoth displays consistent and simple kinematics. The complex aerodynamics are not fully understood, it is still unclear how we can design flapping kinematics that will lead to an optimum performance with respect to maneuverability, speed, and energy efficiency and are feasible to manufacture. The problem is tackled through an experimental campaign in a water channel. The kinematics investigated are two pitch-plunge motions based on the hawkmoth and a third purely sinusoidal motion. The Reynolds number is 4, 800 and the reduced frequency is 0.38, similar to the hawkmoth. The kinematics are subjected to force data acquisition, flow visualization, and particle image velocimetry. The results are compared to flapping experiments and calculations using computational fluid dynamics and an unsteady vortex model. The results show that the effect of the elevation angle is very important when looking at the details of the force development. A strong correlation between the strength of the leading and trailing edge vortex and the forces was found. The force data acquired during the experiments compares well to the to the CFD calculations. The calculated force coefficients are between 82 and 87% of the magnitude measured during the experiments. The bias towards a lower value can be explained partially by the assumptions of the CFD model and the presence of blockage effects during the experiments. Comparison with the unsteady aerodynamic vortex model suggests that the dominant force generating mechanisms are the leading and trailing edge vortices. The impact of spanwise flow and tip vortices on the overall magnitude of the forces is not as significant. The comparison of pitch-plunge experiments with flapping experiments showed a good agreement as well. The force coefficients measured during flapping are about 50% smaller compared to the pitch-plunging case. The difference is attributed due to the fundamental difference in kinematics and the definition of the reference velocity. The agreement of the shape of the time history of the forces suggests that the underlying flow topology is analogues in both cases. It remains to be evaluated if the spanwise flow component is also present and whether the evolution of the tip vortices is comparable. The application of advanced and delayed rotation proved to have dramatic effects on the force generation. Delayed rotation is detrimental to the force production, thrust produced was 60% less compared to the baseline cases. Advanced rotation yields an increase in thrust of 34 to 47% with a reduction of up to 20% in efficiency. The flow topology was remarkably similar to the baseline kinematics with a slight shift in phase. Both the application of advanced and delayed rotation provides a large potential in maneuverability. An increase in reduced frequency to a value of 0.7 lead to an increase in the thrust produced. A qualitative study of the flow topology showed that the vortical structures were similar to the baseline cases. A small decrease in efficiency was measured for the bio-inspired cases and a small increase for the harmonic motion. It might thus be favorable for roboflyers to flap at higher reduced frequencies.

http://resolver.tudelft.nl/uuid:e51a4d39-dc63-4e1b-b5ed-681af812d887

Student theses

master thesis

(c) 2012 Vandenheede, R.