Dr. Mueller
conducts his research at the Hessert
Laboratory for Aerospace Research and employs the Anechoic
Free-Jet Wind Tunnel, the Low-Speed, Low- Turbulence Closed Test Section Tunnels,
and the Water Tunnel.
Course AE240; AE240 Design Projects
11th International Symposium on Flow Visualization
Current Research
The performance of airfoils operating in low Reynolds number flows has been of increasing interest. This interest has been a result of the desire to improve the low speed performance of general aviation aircraft and high aspect ratio sailplane wings, as well as to improve the design of remotely piloted vehicles, jet engine fan blades, and propellers at high altitudes. Prediction of performance requires that the entire airfoil flow field, including the leading edge separation bubble and all other separated regions as well as the location of transition, be calculable. Techniques for predicting the location of transition on smooth airfoils are still not adequate, let alone methods which include the important influences of free stream turbulence and/or distributed surface roughness. An extensive experimental program using flow visualization in conjunction with quantitative measurements of pressure, and velocity, has been performed. The continuing goal is to improve analytical methods of predicting these complex flows.
Aerodynamics of Micro-Air VehiclesRecently, there has been a serious effort to design aircraft that are as small as possible for special, limited-duration missions. These vehicles might carry visual, acoustic, chemical or biological sensors. These aircraft, called micro-air vehicles, are of interest because electronic detection and surveillance sensor equipment can now be miniaturized so that the entire payload weighs about one ounce. The long term goal of this project is to develop aircraft systems that weigh less then one ounce, have about a three inch wing span and can fly for 20 to 50 minutes at between 25 and 35 m.p.h.. Although it is not possible to meet all of the design requirements for a micro-air vehicle with current technology, research is proceeding on all of the system components at various government laboratories, companies and universities. One of the areas of interest is the aerodynamic efficiency of various fixed wing, flapping wing, or rotary wing concepts since these vehicles are very small and must fly at very low speeds. The corresponding chord Reynolds number range is from 20,000 to about 150,000. Very little if any, information on the performance of various airfoil/wing shapes exists, however, there has been a long history of natural flight studies (e.g. insects and small birds) which may be helpful. Experiments to determine the flow physics and aerodynamic performance of various airfoil/wing shapes at chord Reynolds number below 150,000 have begun at the Hessert Center. Both fixed and flapping wing configurations are of interest.
Experimental Investigation of Unsteady Aerodynamics and the Sources of Propeller NoiseThe primary objective of this research is to identify noise sources and obtain a better understanding of how inflow nonuniformities interact with propeller blades, thereby producing vibrations and the radiation of unwanted noise. In this research, carefully controlled experiments are being conducted in a low speed anechoic wind tunnel facility utilizing an advanced propeller-dynamometer system. The fluid mechanical characteristics of the upstream and downstream flows are documented and related to blade sound sources. The blade response, as characterized by its thrust, torque, surface pressure distribution and radiated sound, are measured for the cases of both steady and unsteady inflows of variable turbulence intensity and scale. Cyclic inflows are also being studied.
Experimental Inversion and Simulation Techniques to Study Unsteady Aerodynamics and Propeller Blade Response to Inflow Distortion
Marine propellers usually operate either in the nonuniform wake of a submarine or skewed flow under the stern of a surface ship. Therefore, the flow upstream of marine propeller blades is characterized by irregular patterns which are caused by various mean flow distortions due to the boundary layer growth on the hulls of the ship, turbulent viscous wakes of upstream bodies or obstacles as well as propeller/shroud or propeller/hull interactions. Whether the inflow to the propeller is steady or unsteady, uniform or non-uniform, the flow over and downstream of the blades is unsteady. This unsteadiness can be due to a separation bubble near the leading edge of the blade, separated flow from the blade before the trailing edge as swell as tip and hub vortices. The unsteady flow over the blades produces unsteady hydrodynamic forces (i.e., excitations) which result in unwanted vibrations and noise. Inflow nonuniformities give rise to tonal and narrowband noise centered around the blade passing harmonics. Additional broadband signatures are superimposed due to turbulence-trailing edge interaction. Highly nonuniform and asymmetric inflows further exaggerate these effects. In order to provide guidelines for water tunnel and full scale ship experiments and the development of theoretical models to predict these phenomena, wind tunnel experiments appear to be the most cost effective approach.
The goal of this project is to correlate the fluid dynamics to the aeroacoustics and develop a better understanding of the influence of inflow distortions on the noise generating mechanisms. Further, the measurement techniques and viscous flow results obtained will be used to determine how much of the pressure field is viscous dominated. These studies will assist future water tunnel and full scale ship experiments and help in the development of theoretical models.
Trailing Edge Noise ResearchTrailing edge
sound is an aeroacoustic phenomenon that contributes to the noise from lifting
bodies such as airfoils, hydrofoils, surface flaps, and rotor and stator blades
in turbomachines. Aerodynamic trailing edge noise can be regarded as a superposition
of source contributions. The first two consist of broadband sound from the
turbulent sources on each side of the foil which are developed upstream of
and near the edge on each side of the lifting surface. Simplistically, they
are regarded as incoherent, from each of the wall flows of the airfoil. The
third-source is a quasi-periodic vortex street source that is due to the large
scale, nearly periodic, flow structures in the near wake of the trailing edge.
In practice, these disturbances are not tones, but have some bandwidth that
is due to the weak, yet finite, space-time growth of the vortices in the near
wakes of un-symmetrically beveled edges. In cases of blunt trailing edges,
these disturbances are the familiar vortex shedding “singing” tones.
Currently no generally applicable approach exists for the engineering of quiet-design
lifting surfaces and turbomachinery blades that give the simultaneous reduction
of all three of these sources.
A great deal of work has been done in the theoretical and experimental study
of trailing edge noise. Still, the relationship between the turbulent flowfield,
unsteady surface pressure, and farfield acoustics is not fully understood.
Advances in technology now allow for the development of complex numerical
methods to predict the acoustic radiation from an airfoil trailing edge in
turbulent flow. These methods lack an experimental database, which would serve
both as a source of realistic boundary conditions and as a measure of merit
for computational results. While computations are performed at thousands of
points in the flowfield at every instant in time, experimental measurements
can only be made at discrete points over a period of time. Therefore, the
number and location of experimental points used for numerical code validation
and verification must be chosen carefully. The objective of this research
is to provide the aeroacoustic data needed for the development of numerical
codes and to improve the understanding of the complex flow features responsible
for noise generation.
A phased microphone
array system has been designed, fabricated and calibrated for use in the anechoic
wind tunnel (AWT) in the Hessert Laboratory. This system adds a powerful measurement
capability for aeroacoustic research. One of two 40-microphone acoustic arrays
is currently in use. This acoustic array effectively localizes noise sources
in the anechoic wind tunnel environment. Preliminary measurements of trailing
edge noise from a hydrofoil model have shown that the array correctly depicts
the power spectrum of the trailing edge noise. Extraneous flow noise sources
not correlated among the microphones and not related to trailing edge noise
are suppressed using the phase averaging techniques among the microphones.
These methods demonstrate the power of the acoustic array to increase the
signal to noise ratio over that of a single microphone measurement..
Mueller, T. J., "Aeroacoustic Measurements," Springer-Verlag, Berlin, ISBN: 3-540-41757-5, 321 pages, 2002.
Wojno, J.P., Mueller, T. J., and Blake, W.K., "Turbulence Ingestion Noise Part 1: Experimental Characterization of Grid-Generated Turbulence," AIAA Journal, Vol. 40, No. 1, pp. 16-25, January 2002.
Wojno, J.P., Mueller, T. J., and Blake, W.K., "Turbulence Ingestion Noise Part II: Rotor Aeroacoustic Response to Grid-Generated Turbulence," AIAA Journal, Vol. 40, No. 1, pp. 26-32, January 2002.
Torres, G.E., and Mueller, T.J., "Aerodynamics Characteristics of Low Aspect Ratio Wings at Low Reynolds Numbers," AIAA Progess in Astronautics and Aeronautics, Chapter 7, pp. 115-141, Vol. 195, 2001.
Mueller, T.J. (Editor), "Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications," AIAA Progess in Astronautics and Aeronautics, Vol. 195, Published by the American Institute of Aeronautics an Astronautics, Inc., Reston, VA, 2001.
Direct comments, questions, and corrections to amedept@nd.edu