Overview
Recent interest in Micro Aerial Vehicles (MAVs) and Unmanned Aerial Vehicles (UAVs) have revived the research on the performance of airfoils at relatively low Reynolds numbers. A common problem with low Reynolds number flow is that separation is almost inevitable without the application of some means of flow control. Applying an apt flow control mechanism involves studying the nature of flow in these regimes. This research is focused on studying the separated flow regime and applying a particular flow control approach that involves oscillating a portion of the upper surface of airfoil, thereby changing shape of the airfoil. In order to decide the optimal frequencies to be employed for the oscillation of the upper surface, flow behavior for the non oscillating cases or non morphing cases needs to be known. This lays the groundwork for the oscillating cases or morphing cases. The overall goal of this research is to perform numerical simulations for various cases involved in the study and comparing those results with the experimental results for the purpose of validation.
A series of experiments for the non morphing cases have been performed in the Fluid Mechanics lab (at UKY) for Reynolds numbers between 2.5 × 104 and 2 ×105 and over a range of angles of attack. Most of these cases have been reproduced by numerical simulations through CFD, and effective comparison with the experimental results has been made. Preliminary comparisons were made for the morphing cases for some selected Reynolds numbers and angles of attack.
Wing Morphing
The present research is mainly motivated by the fact that the UAV’s and MAV’s fly in the same Reynolds number region as insects and most birds . By closely looking at the insects and bird flight we can deduce that they fly more comfortably and with less effort in the low Reynolds number region which is not the case with man-made flight. Clearly, new techniques have to be developed to improve the performance of man-made flight at these Reynolds numbers.
Taking a closer look at the bird flight, the wings of the birds adapt themselves to the flight conditions so that less energy is consumed, which also decreases the flow separation and increases the lift to drag ratio. Another reason for easy adaptation of birds is the fluttering of the wings. Several researchers have explored the idea of wing flutter, but the wing adaptation technique has not been explored to a great extent. Hence the present research throws some light in this area of flow control where the wing adapts itself to the flow conditions. The technique of wing adaptation can also be called as shape changing or morphing. The shape changing phenomenon can be accomplished by using sensors and actuators with the advent of ‘smart’ materials. The other details about the wing construction can be found in various publications at this location.
Sample Results
The variation of the lift curves with angle of attack for non oscillating cases in the following figures. At Re=25,000, the trend is one of increasing lift coefficient, greater lift variation, and decreasing frequency of oscillation with increasing angle of attack. The lift curves also become increasingly irregular at higher angles. The drag coefficient also shows significant increases with increasing angle of attack. At Re=50,000, the lift curves are more irregular throughout, but the trend of decreasing frequency and increasing lift coefficient with higher angle appear to hold. A trend in lift variation, however, is not readily discernable. The increase in drag coefficient with angle of attack is notably reduced, a tendency that continues with the Re=100,000 case. The periodic nature of the lift curves at Re=100,000 was result of the increase in the flow velocity and in comparison to the lower Reynolds number values the variation in lift is significantly less. Also, the nature of the periodic curves has changed, with broad peaks and narrow valleys at Re=100,000 versus broad valleys and narrow peaks at Re=25,000. With the implementation of morphing, l/d ratio has doubled for Re = 25,000 and AoA = 90.
No Morphing at Re = 25,000 and AoA = 90 (l/d = 4.65)
Morphing at Re = 25,000 and AoA = 90 at f = 45 Hz (l/d = 8.07)
Relevant Publications and Presentations
Katam, V., R.P. LeBeau, and J.D. Jacob, “Simulation of Separation Control on a Morphing Wing with Conformal Camber,” Accepted to the 35th AIAA Fluid Dynamics Conference and Exhibit, Toronto, Canada, July 6-9, 2005. (abstract)
Katam, V., R.P. LeBeau, and J.D. Jacob, “Separation Control Using a Wind with Oscillatory Camber in Low-Re Flows," Accepted for the 30th Annual Dayton-Cincinnati Aerospace Science Symposium, Dayton, OH, March 8, 2005. (abstract)
Katam, V., R.P. LeBeau , and J.D. Jacob, "Experimental and Computational Investigation of a Modified NACA 4415 in Low-Re Flows," 22nd Applied Aerodynamics Conference and Exhibit, AIAA-2004-4972, Providence, RI, August 16-19, 2004. (abstract)
Katam, V.K., R.P. LeBeau , and J.D. Jacob, "Investigation of Low Reynolds Number Flows over a Modified NACA 4415 Airfoil," 29th Annual Dayton-Cincinnati Aerospace Science Symposium, Dayton, OH, March 9, 2004. (abstract)