Introduction

Micro air vehicles (MAVs), with a maximal size of 15 cm and flight speed from 0 to approximately 10 m/s, have gained growing interest from both military as well as civilian community. Equipped with a video camera or a sensor, these vehicles can perform surveillance and reconnaissance, targeting, and bio-chemical sensing at hazardous location. With the rapid progress made in structural and material technologies, miniaturization of power plants, communication, visualization, and control devices, numerous groups have developed successful MAVs, see Figure 1.

MAVs provide rich research topics, including low Reynolds number aerodynamics, fluid-structure interaction, and unsteady aerodynamics, for both engineering and science societies. Based on the wing type, MAVs can be generally categorized in three groups: fixed wing, rotary wing, and flapping wing. Our group has actively involved in fixed wing and flapping wing study in the past decade. In the following we briefly present the challenges involved based on wing type and the current state-of-art results.

Fig. 1 Examples of MAVs. (a) Flexible fixed wing; (b) Rotary wing; (c) Hybrid flapping-fixed wing, using fixed wing for thrust generation; and (d) Flapping wing.

Objectives

• Gain insight into the fundamental mechanisms governing the unsteady low Reynolds number aerodynamics (i.e. similar flight regimes as those encountered by biological flyers) during flapping wing flight.

• Elucidate the role laminar to turbulent transition and the role turbulence plays in flight conditions which may exhibit laminar, transitioning, and turbulent flow simultaneously in different regions of the flow domain.

• Identify and evaluate the relationships when the fluid and (anisotropic) structure responses are coupled. This is currently being examined in the context of adding flexibility to the wing and simultaneously solving the fluid and structure interactions.

• Study the role shape and wing morphology have on the resulting aerodynamics.

• Assessment the effect of variance of the relevant non-dimensional parameters such as Reynolds number, Strouhal number, reduced frequency, and aspect ratio are examples currently under investigation.

• Evaluate environment sensitivity (e.g. wind gusts) and the relationship between wing kinematics, structural flexibility, etc.

References

Shyy, W., Lian, Y., Tang, J., Viieru, D., and Liu, H. (authors), “Aerodynamics of Low Reynolds Number Flyers”, Cambridge University Press, New York, 2008.

Shyy, W., Trizila, P., Kang, C., Aono, H., “Can Tip Vortices Enhance Lift of a Flapping Wing?”, AIAA Journal, Vol. 47, (2009), pp. 289-293.

Shyy, W., Lian, Y., Tang, J., Liu, H.,Trizila, P., Stanford, B., Bernal, L., Cesnik, C., Friedmann, P., and Ifju, P., “Computational aerodynamics of low Reynolds number plunging, pitching and flexible wings for MAV applications”, Acta Mechanica Sinica, Vol. 24, (2008), pp. 351-371.

Shyy, W. and Liu, H., “Flapping Wings and Aerodynamic Lift: The Role of Leading-Edge Vortices,” AIAA Journal, Vol. 45, (2007), pp. 2817-2819.

Lian, Y., Shyy, W., Viieru, D. and Zhang, B., “Membrane Wing Aerodynamics for Micro Air Vehicles,” Progress in Aerospace Sciences, Vol. 39, (2003), pp. 425-465.

Shyy, W., Berg, M. and Ljungqvist, D., “Flapping and Flexible Wings for Biological and Micro Air Vehicles,” Progress in Aerospace Sciences, Vol. 35, (1999), pp. 155-205.

Hovering Kinematics and Unsteady Fluid Physics

Bird, bat, and insect flights are highly unsteady and thus some of the traditional views carried over from steady state aerodynamics are sometimes incomplete or even not applicable. For instance given the same airfoil at the same angle of attack and velocities, the lift and drag can change appreciably depending on the time history of its movement. Furthermore, the role of wing tip vortices plays. In classic steady state aerodynamics these are strictly a performance penalty whereas we have seen that they can be used to enhance lift generation while not appreciably affecting the drag. Unsteady mechanisms such as delayed stall of the leading edge vortex (LEV), Weis-Fogh’s clap-and-fling, the persistent jet, wake capture, and wing tip vortices can all play a significant role in the resultant aerodynamic loadings experienced by natural flapping flyers, see Figure 2.

Fig.2 Highlights of unsteady fluid physics in flapping wing flight. (Left: Leading edge vortex generation and delayed stall; Center: Jet interaction and wake capture mechanism: Right:  vorticity contours around hovering flat plate)

References

Shyy, W., Trizila, P., Kang, C., Aono, H., “Can Tip Vortices Enhance Lift of a Flapping Wing?”, AIAA Journal, Vol. 47, (2009), pp. 289-293.

Shyy, W., Lian, Y., Tang, J., Liu, H.,Trizila, P., Stanford, B., Bernal, L., Cesnik, C., Friedmann, P., and Ifju, P., “Computational aerodynamics of low Reynolds number plunging, pitching and flexible wings for MAV applications”, Acta Mechanica Sinica, Vol. 24, (2008), pp. 351-371.

Shyy, W., Lian, Y., Tang , J., Liu, H., Trizila, P., Stanford, B., Bernal, L., Cesnik, C., Friedmann, P., and Ifju, P., “Computational Aerodynamics of Low Reynolds Number Plunging, Pitching and Flexible Wings for MAV Applications”, AIAA paper 2008-253, 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan 7-10, 2008.

Trizila, P., Kang, C., Visbal, M., and Shyy, W., “Unsteady Fluid Physics and Surrogate Modeling of Low Reynolds Number, Flapping Airfoils”, AIAA-2008-3821, 38th Fluid Dynamics Conference and Exhibit, Seattle, Washington, June 2008.

Trizila, P., Kang, C., Visbal, M., and Shyy, W., “A Surrogate Model Approach in 2-D Versus 3-D Flapping Wing Aerodynamic Analysis”, AIAA-2008-5914, 12th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, Victoria, British Columbia, September, 2008.

Computational, Experimental and Theoretical Examinations of Flow at Re=O (10⁴)

So far most of the studies on flapping wing aerodynamics at low Reynolds number have focused on the qualitative aspects at the Reynolds number regimes of order of magnitudes of 10²-10³. Current research is in the framework of the RTO (NATO’s Research and Technology Organization) and aims at

i)  Elucidating fluid dynamics at higher Reynolds numbers O(10⁴) to understand the interplay between the pitching and plunging  kinematics, response of the flow structures, and aerodynamic loadings, see Figure 3.

ii) Quantitatively (link to velocity line plots) assessing predictive capabilities: numerical (CFD), experimental (PIV), analytical (Theodorsen’s theory), see Figure 3.

Another challenging question at these Reynolds numbers is how much a role the turbulence, including the transition from laminar to turbulence, plays on the flow structures and the resulting aerodynamic loading.

Fig. 3 Highlights of the cross comparisons among computational, experimental, and theoretical approaches for a pitching and plunging SD7003 airfoil at Re of 6.0×10⁴ as a RTO case. (a) Flow field by stream velocity contours; (b) Prescribed kinematics and the resulting aerodynamic loading; (c) Qualitative comparison between experimental and computational results on streamwise velocity profiles.

References

Kang, C., Baik, Y., Bernal, L., Ol, M.V., and Shyy, W., “Fluid Dynamics of Pitching and Plunging Airfoils of Reynolds Number between 1.0×10⁴ and 6.0×10⁴“, AIAA-2009-536, 47th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Orlando, Florida, Jan. 5-8, 2009.

Impact of Wing Flexibility on Flapping Wing Aerodynamics

Animal and insect wings are not perfectly rigid. Examples of this include that a bat which has a membrane structure spread between its fingers and the feathers of a bird which will deform as they adjust to the incoming flow. Most of insect wing has also substantial variation in its stiffness between the spanwise and chordwise directions. Currently we have been examining the impact of varying the chordwise or spanwise stiffness on flapping wing aerodynamics and comparing the results to experimental findings, see Figure 4. Current our efforts showed that the performance gains can be realized by introducing flexibility, though both aerodynamic and structural responses are not monotonic meaning too much flexibility will lead to a decrease in performance characteristics.

Fig.4 Highlights of flow structures and pressure distributions around flexible wings undergoing a pure plunging at Re of 3.0×10⁴. (Left: Spanwise flexibility is beneficial for aerodynamic performance; Right: Spanwise flexibility is detrimental for aerodynamic performance)

References

Aono, H., Chimakurthi, S.K., Cesnik, C.E.S, Liu, H., and Shyy, W., “Computational Modeling of Spanwise Flexibility Effects on Flapping Wing Aerodynamics”, AIAA-2009-1270, 47th AIAA Aerospace Sciences Meeting including The New Horizons Forum and Aerospace Exposition, Orlando, Florida, Jan. 5-8, 2009.

Aono, H., Chimakurthi, S.K., Liu, H., Cesnik, C.E.S., and Shyy, W., “Effect of Spanwise Flexibility on Aerodynamics of a Plunging Wing”, The DFD08 Meeting of The American Physical Society, San Antonio, Texas, Nov. 23-25, 2008.

Corrugated dragonfly airfoil aerodynamics

In conventional aerodynamics only smooth airfoils are considered, as sharp corners are known to cause flow separation and degrade performance.  However, experimental studies at smaller scales have shown some corrugated (or zigzag) wing profiles to perform better than their smooth counterparts due to the scaling laws governing fluid flow. Of particular interest to us is the wing of the Southern Hawker dragonfly (aeshna cyanea) due to the insect’s large size (such that it flies at a Reynolds number relatively close to intended MAV designs) and its strong degree of non-smoothness.  As is common in natural flyers, the airfoil profile varies along the wing span; analyzing the full 3D wing would be extremely computationally expensive, so we focus our studies on the mid-span profile, where the greatest degree of corrugation occurs. Using computational approaches, we are able to examine the flow field around the airfoil.  At a chord-based Reynolds number of 3.4×10⁴, trapped unsteady vortices are observed in the valleys, which pump high-speed flow into the boundary layer, giving it more kinetic energy to overcome the adverse pressure gradient and thus suppress separation. Our current research efforts aim to better understand aerodynamics around corrugated airfoil profiles through numerical simulations, in order to apply them to MAV designs.

Fig.5 Pressure contour and streamline around the corrugated dragonfly wing in forward flight at Re of 3.4×10⁴.

Researchers

Faculty advisor: Professor Wei Shyy

Postdoctoral Researchers: Dr. Jian Tang, Dr. Hikaru Aono

Graduate Students: Pat Trizila, Chang-kwon Kang, Wenbo Du, Andrea Mazzola