Dr. Mueller and Denis Lynch work in the anechoic wind tunnel.

Experimental Aeroacoustics

For almost 10 years, Roth-Gibson Professor Thomas J. Mueller has been conducting research in Hessert's anechoic wind tunnel. Why? Because propellers, engines, and fans do not operate in steady or uniform flow environments. Unsteady or non-uniform flows enter the rotor, and they produce varying thrust components and pressure waves which radiate from the rotor, becoming noise in the far field.

As the population increases, background noise from air conditioning fans, computers, and other electronic devices also increases. There are already stringent restrictions on the permissible sound levels of aircraft. Much of this noise is a nuisance, but carries few harsh penalties. However, sound generated by the propeller on a submarine, for instance, significantly increases its detection range. This places the crew and its mission in jeopardy, making it crucial for the Office of Naval Research to understand the sources of propeller noise.

Noise is very difficult to measure, because the pressure variations are small. If there's any interference, the measurements will not be accurate. By running experiments in the anechoic chamber, the sound does not reflect off the walls of the chamber and cannot interfere with measurements. This is similar to what would happen in a real-world environment, where there are no walls present.

In addition to the unique capabilities the anechoic chamber offers, Mueller and graduate students have developed their own specialized equipment. For example, they built a low-turbulence, free-jet wind tunnel with an open test section in the chamber. With it, they can create a low-turbulence stream to study the flow over airfoils and propellers.

Mueller and his students also created a low-cost unsteady pressure sensor with which they can capture aeroacoustic phenomena previously unmeasurable. This Notre Dame innovation took two years to develop, with much of that time focused on determining ways to calibrate the sensor so it could provide accurate measurements. Using these sensors, which are embedded in a model mounted in the anechoic wind tunnel, Mueller can correlate unsteady pressure with the far field noise, giving the Navy experimental verification of theoretical response models.

Dr. Mueller and students with a micro air vehicle.

Low Reynolds Number Aerodynamics

While he focusers on aeroacoustics, Mueller is also studying the aerodynamics of micro-air vehicles. There is a serious effort to design aircraft that are as small as possible for special, limited-duration military and civil missions. These aircraft, called micro-air vehicles (MAVs), are of interest because electronic surveillance and detection sensor equipment can now be miniaturized so that the entire payload mass is about 18 grams. The advantages of a MAV include compact system transport by a single operator, rapid deployment, real-time data, low radar cross-section; they are difficult to see, and very quiet. The potential for low production cost is also an advantage. The primary missions of interest for fixed wing MAVs include surveillance, detection, communications, and the placement of unattended sensors. Surveillance missions include video (day and night) and infrared images of battlefields (referred to as the "over the hill" problem) and urban areas (referred to as "around the corner"). These real-time images can give the number and location of opposing forces. This type of information can also be useful in hostage rescue and counter-drug operations. Because of the availability of very small sensors, detection missions include the sensing of biological agents, chemical compounds, and nuclear materials (i.e., radioactivity). MAVs may also be used to improve communications in urban or other environments where full-time line of sight operations are important. The placement of acoustic sensors on the outside of a building during a hostage rescue or counter-drug operation is another possible mission.

The requirements for fixed wing MAVs cover a wide range of possible operational environments including urban, jungle, desert, maritime, mountains and arctic environments. Furthermore, MAVs must be able to perform their missions in all weather conditions (i.e., precipitation, wind shear, and gusts). Because these vehicles fly at relatively low altitudes (i.e., less than 100 m) where buildings, trees, hills, etc. may be present, a collision avoidance system is also required.

The recent interest in the development of small unmanned air vehicles (UAVs) and MAVs has highlighted the need for a more thorough understanding of the aerodynamics of small, slow-flying aircraft. Current research is directed toward a comprehensive study of lift, drag and pitching moment characteristics of low aspect ratio wings at low Reynolds numbers. Wind tunnel studies on wings with aspect ratios from 0.5 to 2.0, with four different planforms, at chord Reynolds numbers between 20,000 and 200,000 are all in progress. The goal of this research is to provide the necessary aerodynamic data so that designers can develop efficient small aircraft.

References

1) Mueller, T.J., “On the Historical Development of Apparatus and Techniques for Smoke Visualization of Subsonic and Supersonic Flows,” AIAA Paper No. 80-0420-CP, presented at the AIAA 11th Aerodynamic Testing Conference, March 1980.

2) Batill, S.M, and Mueller, T.J., “Visualization of the Laminar Turbulent Transition in the Flow Over an Airfoil Using the “Smoke-Wire” Technique,” AIAA Paper No. 80-0421-CP, presented at the AIAA 11th Aerodynamic Testing Conference, March 1980.

3) Arena, A.V., and Mueller, T.J., “Laminar Separation, Transition and Turbulent Reattachment Near the Leading Edge of Airfoils,” AIAA Journal, Vol. 18, No. 7, pp. 747-753, July 1980.

4) Mueller, T.J, and Burns, T.F., “Experimental Studies of the Eppler 61 Airfoil at Low Reynolds Numbers,” AIAA Paper No. 82-0345, presented at the AIAA 20th Aerospace Sciences Meeting, Orlando, Florida, January 11-14, 1982.

5) Mueller, T.J. and Jansen, Jr., B.J., “Aerodynamic Measurements At Low Reynolds Numbers,” AIAA Paper No. 82-0598, presented at the AIAA 12th Aerodynamic Testing Conference, March 1982

6) Mueller, T.J., and Batill, S.M., “Experimental Studies of Separation on a Two-Dimensional Airfoil at Low Reynolds Numbers,” AIAA Journal, Vol. 20, No. 4 pp. 456-463, April 1982.

7) Mueller, T.J., Pohlen, L.J., Conigliaro, P.E., and Jansen, B.J., Jr., “The Influence of Free Stream Disturbances on Low Reynolds Number Airfoil Experiments,” Experiments in Fluids, Vol. 1, pp. 3-14, 1983.

8) Jansen, Jr., B.J. and Mueller, T.J., “Experimental Studies of the Boundary Layer on an airfoil at Low Reynolds Numbers,” AIAA Paper No. 83-1671, presented at the AIAA 16th Fluid and Plasma Dynamics Conference, July 1983.

9) Pohlen, L.J. and Mueller, T.J., “Boundary Layer Characteristics of Miley Airfoil at Low Reynolds Number,” AIAA Journal of Aircraft, Vol. 21, No. 9, pp. 658-664, September 1984.

10) Bastedo, W.G., Jr., and Mueller, T.J., “The Spanwise Variation of Laminar Separation Bubbles on Finite Wings at Low Reynolds Numbers,” AIAA Paper No. 85-1590, presented at the AIAA 18th Fluid Dynamics, Plasma Dynamics, and Laser Conference, July 1985.

11) Mueller, T.J., “The Influence of Laminar Separation and Transition on Low Reynolds Number Airfoil Hysteresis,” AIAA Journal of Aircraft, Vol. 22, No. 9, pp. 763-770, September 1985.

12) Perry, M.L. and Mueller, T.J., “The Influence of leading and Trailing Edge Flaps on the Peformance of a Low Reynolds Number Airfoil,” AIAA Paper No. 86-1787-CP, presented at the AIAA 4th Applied Aerodynamics Conference, San Diego, California, June 9-11, pp. 141-151.

13) Bloch, D.R., and Mueller, T.J., “The Effects of Distributed Grit Roughness on Separation and Transition on an Airfoil at Low Reynolds Numbers,” AIAA Paper No. 86-1788-CP Proceedings of the AIAA 4th Applied Aerodynamics Conference, San Diego, California, June 9-11, 1986, pp. 152-161.

14) Bastedo, W.G., Jr., and Mueller, T.J., “The Spanwise Variation of Laminar Separation Bubbles on Finite Wings at Low Reynolds Numbers,” AIAA Journal of Aircraft, Vol. 23, No. 9, pp. 687-694, September 1986.

15) Mueller, T.J., et al., “Low Reynolds Number Wind Tunnel Measurements: The Importance of Being Earnest,” Proceedings of the International Conference on Aerodynamics at Low Reynolds Numbers 104 <Re <106, Vol. II, London, England, October 15-18, 1986, pp.14.1-14.18.

16) Huber II, A.F., and Mueller, T.J., “The Effect of Trip Wire Roughness on the Performance of the Wortmann FX63-137 Airfoil at Low Reynolds Numbers,” Experiments in Fluids, Vol. 5, No. 4, pp. 263-272, 1987.

17) O’Meara, M.M., and Mueller, T.J., “Laminar Separation Bubble Characteristics on an Airfoil at Low Reynolds Numbers, AIAA Journal, Vol. 25, No. 8, pp. 1033-1041, August 1987.

18) Michelsen, W.D. and Mueller, T.J., “Low Reynolds Number Airfoil Performance Subjected to Wake Interference From an Upstream Airfoil,” AIAA Paper No. 87-2351/CP, presented at the AIAA Applied Aerodynamics Conference, Monterey, California, August 17-19, 1987.

19) Mueller, T.J., “The Visualization of Low Speed Separated and Wake Flows,” AIAA Paper No. 87-2422, presented at the AIAA Atmospheric Flight Mechanics Conference, Monterey, California, August 17-19, 1987.

20) Perry, M.L., and Mueller, T.J., “Leading and Trailing-Edge Flaps on a Low Reynolds Number Airfoil,” AIAA Journal of Aircraft, Vol. 24, No. 9, pp. 653-672, September 1987.

21) Brendel, M., and Mueller, T.J., “Boundary Layer Measurements on an Airfoil at a Low Reynolds Numbers in an Oscillating Freestream,” AIAA Journal, Vol. 26, No. 3, pp. 257-263, March 1988.

22) Brendel, M., and Mueller, T.J., “Boundary Layer Measurements on an Airfoil at Low Reynolds Numbers,” AIAA Journal of Aircraft, Vol. 25, No. 7, pp. 612-617, July 1988.

23) Ellsworth, R. and Mueller, T.J., “Boundary Layer Measurements on an Airfoil at Low Reynolds Numbers in an Accelerating Flow from a Nonzero Base Velocity,” AIAA Paper No. 89-0569, presented at the 27th Aerospace Sciences Meeting, Reno, Nevada, January 9-12, 1989.

24) Schmidt, G.S and Mueller, T.J, “Analysis of Low Reynolds Number Separation Bubbles Using Semiempirical Methods,” AIAA Journal, Vol. 27, No. 8, pp. 993-1001, August 1989.

25) Brendel, M. and Mueller, T.J., “Transition Phenomenon on Airfoil’s Operating at Low Chord Reynolds Numbers in Steady and Unsteady Flows”, Part 3, Number 20 in Numerical and Physical Aspects of Aerodynamic Flows IV, edited by T. Cebeci, pp. 333-344, Springer-Verlag, Berlin, 1990.

26) Fitzgerald, E.J. and Mueller, T.J., “Measurements in a Separation Bubble on an Airfoil Using Laser Velocimetry,” AIAA Journal, Vol. 28, No. 4, pp. 584-592, April 1990.

27) Scharpf, D.F. and Mueller, T.J., “An Experimental Study of Closely Coupled Tandem Wing Configuration at Low Reynolds Numbers,” AIAA-90-3094-CP, presented at the AIAA 8th Applied Aerodynamics Conference, Portland, Oregon, August 20-22, 1990.

28) Khan, F.A. and Mueller, T.J., “Tip Vortex/Airfoil Interaction for a Low Reynolds Number Canard/Wing Configuration,” AIAA Journal of Aircraft, Vol. 28, No. 3, pp. 181-186, March 1991.

29) Ellsworth, R.H. and Mueller, T.J., “Airfoil Boundary Layer Measurements at Low Re in an Accelerating Flow From a Nonzero Velocity”, Experiments in Fluids, Vol. 11, No. 6, pp. 368-374, 1991.

30) Scharpf, D.F. and Mueller, T.J., “Experimental Study of a Low Reynolds Number Tandem Airfoil Configuration,” AIAA Journal of Aircraft, Vol. 29, No. 2, pp. 231-236, March-April 1992.

31) Khan, F.A. and Mueller, T.J., “Visualization of Tip Vortex/Airfoil Interaction,” Journal of Flow Visualization and Image Processing, Vol. 1, No. 1, pp. 35-41, January-March 1993.

32) Prazak, M.W. and Mueller, T.J., “Hydrogen Bubble Visualization of the Flow Over a Thin Wing at Chord Reynolds Numbers From 12,000 to 21,000,” 8th International Symposium on Flow Visualization, 1998.

33) Mueller, T.J., “Aerodynamic Measurements at Low Reynolds Numbers for Fixed Wing Micro-Air Vehicles,” presented at the RTO AVT Course on “Development and Operation of UAVs for Military and Civil Applications, Rhode-Saint-Genese, Belgium, September 13-17, 1999.

34) Pelletier, A. and Mueller, T. J., “Aerodyanmic Force/Moment Measurement at Very Low Reynolds Numbers,” Proceedings of the 46th Annual Conference of the Canadian Aeronautics and Space Institute, Montreal, Quebec, Canada, pp. 59-68, May 2-5, 1999.

35) Pelletier, A. and Mueller, T. J., “Low Reynolds Number Aerodynamics of Low Aspect-Ratio Wings,” AIAA-99-3182, AIAA 17th Applied Aerodynamics Conference, Norfolk, Virginia, June 28-July 1, 1999.

36) Mueller, T.J., “Aerodynamic Measurements at Low Reynolds Numbers for Fixed Wing Micro-Air Vehicles,” Proceedings of the Development and Operation of UAVs for Military and Civil Applications Short Course, Belgium, September 13-17, 1999, published in Rto-EN-9 by NATO, April 2000.

37) Torres, G. E. and Mueller, T. J., “Aerodynamic Characteristics of Low Aspect Ratio Wings at Low Reynolds Numbers,” Proceedings of the Conference on Fixed, Flapping and Rotary Wing Vehicles at Very Low Reynolds Numbers, University of Notre Dame, Notre Dame, Indiana, pp. 278-305, June 5-7, 2000.

38) Torres, G. E. and Mueller, T. J., “Effects of Propeller Slipstream on the Aerodynamic Characteristics of Low Aspect Ratio Wings at Very Low Reynolds Numbers,” Proceedings of the Conference on Fixed, Flapping and Rotary Wing Vehicles at Very Low Reynolds Numbers, University of Notre Dame, Notre Dame, Indiana, pp. 278-305, June 5-7, 2000.

39) Torres, G. E. and Mueller, T. J., “Micro Aerial Vehicle Development: Design, Components, Fabrication, and Flight-Testing,” Proceedings of “Unmanned Systems 2000,” available on a CD, Orlando, Florida, July 11-13, 2000. (www.auvsi.org).

40) Pelletier, A. and Mueller, T.J., “Low Reynolds Number Aerodynamics of Low-Aspect-Ratio, Thin/Flat/Cambered-Plate Wings,” Journal of Aircraft, Vol. 37, No. 5, pp. 825-832, September-October 2000.

41) Pelletier, A. and Mueller, T.J., “Effect of Endplates on Two-Dimensional Airfoil Testing at Low Reynolds Number,” Journal of Aircraft, Vol. 38, No. 6, pp. 825-832, November-December 2001.

42) Torres, G.E. and Mueller, T.J., “Low Aspect Ratio Wing Aerodynamics at Low Reynolds Numbers,” AIAA Journal, Vol. 42, No. 4, May 2004 (forthcoming).