Sound Sources in a Ducted Rotor David B. Stephens and Scott C. Morris University of Notre Dame Aerospace and Mechanical Engineering Department

Research Objective Develop dipole source models to predict sound generated by a subsonic rotor in a rigid duct Applications: -Jet Engines -HVAC Blowers -Submarine Propulsion

Motivation Open Rotor Aircraft Propeller Ship Propeller Cooling Fans Ducted Rotor Jet Engines Automobile Cooling Fans HVAC Equipment Axial Flow Compressors Duct effects include reflections, cut-on and organ pipe modes

Research Questions Relate fluid motions to unsteady forcing in a simplified turbomachine model Need to relate far field sound measurements to unsteady forcing on rotor blades

Approach Develop quantifiable experimental model –Experimental setup complete ’04-’05 –Future work: PIV measurements of rotor wake Identify source terms separate from transfer function –Complete ‘05-‘06 Develop dipole models to account for source terms –Current work

Experimental Model Far Field Acoustic Measurements

Blocked Modeling Far Field Sound Unsteady Forces Green’s Function Separable Green’s Function Derivation of Blocked Model

Rotor Loading

Results for T (He) All four loading conditions agree to 0.26 dB Levine, H., Schwinger, J., On the Radiation of Sound from an Unflanged Circular Pipe, Physical Review 73(4), 1948.

Unsteady Loading Spectra, 5000 RPM Tonal and broadband sound Quietest operating point Different sources

Sound Spectra Decomposition

Mechanisms of Rotor Noise Blake, W., Mechanics of Flow-induced Sound and Vibration (two volumes), Academic Press Inc., Different Source Contributions at Different Operating Points

Approach Turbulence to Rotor Hot Wire Anemometry Axial Mean VelocityAxial RMS Velocity

Unsteady Forces due to Approach Flow Turbulence Ingestion Noise, Part 2: Rotor Aeroacoustic Response to Grid-Generated Turbulence J. P. Wojno, T. J. Mueller and W. K. Blake AIAA Journal (2002) v40 n1

Correlated Forces Between Blades Nature of Inlet Turbulence and Strut Flow Disturbances and their Effect on Turbomachinery Rotor Noise R. Trunzo and B. Lakshminarayana and D. E. Thompson Journal of Sound and Vibration(1981)v76 n2p Asymptotic Theory of Broadband Rotor Thrust, Part 1: Manipulation of Flow Probabilities for a High Number of Blades R. Martinez Transactions of the ASME (1996) v63

Experimental model provides data to study source modeling Algorithm effective at identifying source separate from transfer function Unsteady force spectra can be compared with analytical models A range of different source conditions can be studied and modeled –Blade loading –Ingested turbulence –Tip gap Conclusions

Experimental Model Emphasis on quantifiable boundary conditions By the numbers: –D = 0.2 m, L/D = 4.6 and 6.8, B = 10 –C = 2.54 cm, Tip gap 5% of chord –M tip ≤ 0.15 Plane of Rotor For Hot Wire Survey

Velocity Spectra of Approach Flow