QuasiActive Control of Axial Fan Blade Tones

1

Engineered Adaptive Structures V Conference
Maiori, Amalfi, Italy June 18-23
Quasi-Active Control of Axial Fan Blade
Tones Using Optimally Tuned Quarter Wavelength
Resonators Gary H. Koopmann, Dean E. Capone,
Lee J. Gorny Center for Acoustics and Vibration,
The Pennsylvania State University United States
of America Olaf Lemke Technische Universität
Berlin, Sonderforschungsbereich 557 Berlin,
Germany Wolfgang Neise Institut für
Antriebstechnik Abteilung Triebwerksakustik Deutsc
hes Zentrum für Luft-und Raumfahrt (DLR) Berlin,
Germany
2

• Axial Flow Fans Are Common Noise Sources in
  Turbomachines (Ventilation Units, Compressors,
  Jet Engines, etc.)
• - Sound Radiated Has Both Broadband and
  Narrowband Components
• - Tonal Noise Occurs at the Blade Passing
  Frequency (BPF) and its Harmonics
• Use of Flow-Excited Resonators Serves as a
  Non-Intrusive, Quasi-Active Means of Attenuating
  Blade Tones of Axial Flow Fans

3
High By-Pass Ratio Jet Engine
Blade Tips Near Shroud
Jet Exhaust
Airflow into turbofan
Turbofan Blades
Airflow Exiting at High Speeds Generating Thrust
4
Fan/Resonator Design
Stators Offset and Small in Diameter Single
Resonator Incorporated In Initial Design
Resonator Locations Useful for Mounting
Sensors
Resonators Driven Directly By Passing Blades To
Generate Sound in Anti-phase To Blade Passing
Tones
5
Concept for Quasi-Active Noise Control
Resonators as Acoustic Sources
Unsteady Surface Pressure
6
Resonator Design
Resonator mouths perforated openings
interchangeable
Variable Length Allows For Tuning
Conventional Quarter Wave Tube Behavior
N1 1/4?
N2 3/4?
N3 5/4?
Nn (2n-1)/4?
Pressure inside of quarter inch resonator
Velocity inside of quarter inch resonator
7
Optimization of Resonator Mouth Perforations
Perforated Surface Provides a Porous Boundary
Through Which Sound Can Propagate
Advantages of Perforated Mouth Geometry
8
Penn State Experimental Test Facility
Anechoic Terminations
Flow Control Door
Transition Ducting
Inlet Duct
Upstream BK In-stream Microphone
Downstream BK In-stream Microphone
Locations for Resonators
Removable Fan Assembly
9
Acoustic Cancellation Mechanism

• Blade Tone Noise Radiates as a Dipole Source

• Flow-excited Resonator Acts
• as a Secondary Monopole Source

• Resonator is Tuned To Provide Anti-phase Sound
• Near Blade Tips Resulting in Acoustic
  Cancellation

Resonator Source 180 deg out of Phase
with Downstream Noise
Flow Direction
Downstream Fan Noise (Negative Dipole Lobe)
Reduced Noise Level
Upstream Fan Noise (Positive Dipole Lobe)
Center Plane of Fan Blade
10
Characterization of Resonators Response with
Varying Mouth Perforations
Resonator mounted in baffle excited with sound
from loudspeaker Sound pressure incident on
solid baffle used as resonator driving pressure
Internal pressure measured at closed and open
ends of resonator
11
Characterization of Resonators Response with
Varying Mouth Perforations
Each of 6 Opening Configurations areTested as
well as Fully Open Resonator
Measured Resonance Frequencies for Each
Configuration
Resonance Frequency and Quality Factor Decrease
as Open Area is Reduced
12
Flow-Excited Resonator Model
Resonator Can be Modeled Damped, Mass/Spring
System
Uniform Incident Flow
Perforated Resonator Mouth
Mass mc Stiffness kc Damping
cc
m r (density of air)A (A open area of
perforate) k rAcot(wL/A) (L Tube
Length) c p n a
(nnumber of holes)
(ahole diameter)
Oscillating Surface Pressure
Radiated Tonal Acoustic Pressure
13
Transmission Line Theory Resonator Model
p0
w1
w2
l
w3
pl Measured Blocked Impedance Sound Pressure at
Each Frequency p0 Calculated Sound Pressure at
Closed Resonator End c Speed of Sound wm
Frequency of Excitation l Resonator Length a
Attenuation Constant
Incident Sound Pressure ( plane wave)
pl
Radiated Tonal Acoustic Pressure
Loudspeaker Generating White Noise
14
Comparison of TL Theory Model to Measured
Resonator Behavior
Calculated Closed End Resonator Pressure Levels
(Magnitude)

• The Model is Used to Characterize a Fully Open
  Quarter Wave Tube

• The Analytical Results (Magnitude and Phase)
  Demonstrate Effectiveness of TL Theory Model

Calculated Closed End Resonator Pressure Levels
(Phase)
15
Model Adapted to Account for Various Mouth
Perforations
A Simple Weighting Factor (C) is Incorporated to
Account for Impedance of Perforated Resonator
Mouth

• Resonator Pressure p0 is Calculated for all
  Opening Configurations
• Once Values for C, a, and w1,2,n are Determined,
  the Resonator Behavior is Characterized

16
Measurement of Unsteady Pressure at Blade Tip
That Excites Resonator

• Probe Microphone Measures Wall Pressure Along
• Blade Passing Region (Magnitude and Phase)
• Phase Relative to Stationary Tachometer
  Reference Source

17
Measurement of Unsteady Pressure at Blade Tip
That Excites Resonator
Probe Microphone
Position 1
Position 11
Downstream Direction
The maximum pressure amplitude along the blade
occurs at the leading edge There is a phase
shift of approximately 180 degrees across the
passing blade
18
Modeling the Driving Pressure of Perforated Flow
Driven Resonator

• Pressure Measurements Across the Passing Blades
  are Used as Driving Pressure (Averaged over
  Perforate)
• A First Model Is Used for Quick Resonator
  Characterization (Does Not Account for the Phase
  Variation of the Moving Blade)
• As Frequency Increases Phase Variation from
  Moving Blades Across Adjacent Holes must be
  Included for Accurate Modeling

• A Second Model Includes Phase Corrections Dphase
  for Holes Located Away From Mouth Centerline
  (Reference Phase Location)

19
Transmission Line Theory Model Driven Resonator
Behavior

• Measured Shroud Pressure Used as Driving Pressure
  for Analytical Model

Mic
Comparison of Modeled Closed Resonator End
Pressure With Measured Values

• The First Model Allows for Calculation of
  Resonator Response at the Fundamental
• The Second Model is Effective for Calculating
  Higher Order Mode Response of the Resonator

20
Measured and Predicted Noise Cancellation Results

• Flow Driven Resonator Response can be Determined
  Through Measurement Closed End Sound Pressure.
• This Measurement with Transmission Line Theory
  Can Be Used to Track Resonator Response
  (Potential for Adaptive Control)
• Downstream Sound Pressure is Measured to
  Determine the Minimum Noise Output

Closed End SPL Measurement
Resonator Sound Field
Resonator Microphone
Downstream Microphone
21
Measured and Predicted Noise Cancellation
Results

• Phase Diagram
• Downstream Noise With and Without Resonator,
• (phase Corrected to SRC Location)
• 2. Resonator Response (measured and Calculated)

22
Blade Tone SPL Reductions With Optimized
Resonator Mouth Perforations
23
Reduction of Downstream Blade Tone SPL Using
Flow-excited Resonators
Data Acquired with 2 Hz Bandwidth over 1.6 kHz
Range
24
Incorporation of Upstream Obstructions in the
Flow Field

• 1/2 Inch Diameter Rod 2 inches from Fan
• Blade Tone Noise Substantially Increased
• (10-12 dB)
• Phasing Across Blade is Unchanged Due to This
• Addition
• Resonators Are Still Effective
• Reducing Blade Tone Noise (23 dB Reduction)
• Configuration of the Resonator Mouth Must Be
• Adjusted to Reduce Noise

25
Incorporation of Multiple Resonators and Testing
of a Conformable Resonator

• Double Resonator
• Attempt To Create an Acoustically Driven Dipole
• Draw Off Of both Sides of the Blade Separately
• Allows For Independent Control of Each Harmonic

• Bent Resonator
• Tunable Resonators Can Be Conformed to the
• Perimeter of the Outer Fan Assembly

• These Systems Behave Nearly Identically to
  Straightened Quarter Wave Tube
• Allows For Application of Resonator Technology in
  a Confined Space (e.g., Jet Engine Nacelles)

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• Conclusions From PSU Study
• Flow Excited Resonators Can Substantially Reduce
  Blade Tone SPLs of Axial Fans By Approximately
  20 dB
• Minimal Loss in Fan Performance (1)
• Adaptable Tuning (mouth/length) Potential for
  Adaptive Noise Control
• Resonators Can be Designed Compactly to allow Low
  Profile Integration Into Turbomachinery Geometry

27
Present Research Goals at the DLR Facility in
Berlin
Extend the Blade Tone Attenuation Technology
Developed for the PSU Low Speed/Low Pressure Fan
to DLRs High Speed/High Pressure Axial Flow Fan

• Technical Challenges
• -Increased Rotor-Stator Interaction
• -Onset of Higher Order Duct Modes
• -Design a More Robust Resonator with
• Motorized Control
• - Increase Number of Resonators to 16
• - Control Multiple Resonators
  Simultaneously to
• Achieve Optimal Reductions in
  Blade Tone SPLs

28
General Electrics GE90-115B Jet Engine

• Guinness Book of Records certifies GE90-115B as
  the most powerful commercial jet engine ever
  built.
• GE90-115B generated 123,000 pounds of
  steady-state thrust during its initial ground
  testing in 2001, a record it would later break.
• To put that achievement in historic perspective,
  the HE S-1, the hydrogen-powered turbojet
  prototype developed by German engineer Dr. Hans
  von Ohain and built by the Ernst Heinkel in 1937,
  cranked out a modest 250 pounds of thrust.

29

Challenges for Extending Quasi-Active Method to
Control Blade Tones on Ducted Axial Propulsors
-Increased Rotor-Stator Interaction -Onset
of Higher Order Duct Modes - Increased Number of
Resonators to 16 - Control
Multiple Resonators Simultaneously to
Achieve Optimal Reductions
in Blade Tone SPLs
30
Challenges for Extending Quasi-Active Method to
Control Blade Tones on Ducted Axial Propulsors
-Design a More Robust Resonator
with Motorized Control - Increase
Number of Resonators to 16 -
Control Multiple Resonators to
Simultaneously Achieve Optimal
Reductions in Blade Tone SPLs.



31
New Resonator Design with Piezomotor Controlled
Tuning

• Length Modification
• Variable Length Allows For Conventional Resonator
  Tuning

32
New Resonator Design with Piezomotor Controlled
Tuning

• Conformable Mouth Tuning
• Resonator Mouth Impedance Changes by
• Increasing or Decreasing Open Area
• Frequency Response of Resonator Effected
• as Mouth Area is Modified
• Optimal Tuning Achieved by Tailoring
• Both Resonator Parameters

33
New Resonator Design with Piezomotor Controlled
Tuning
Piezo Electric Motor
Quarter wave resonator body
Motor-Driven Resonator Mouth Opening
34
Fabricating 16 Resonators Work in Progress
35
DLR Axial Fan Testing Facility
Quarter-Wave Resonator
Anechoic Termination
36
DLR Axial Fan Testing Facility
16-Bladed Turbofan
Resonator
37
DLR Axial Fan Testing Facility
Fan Blades of 16-Bladed, High Speed, High
Pressue DRL Turbofan.
38
DLR Axial Flow Fan Testing Facility
Array of surface-mounted microphones spaced in
four planes evenly around periphery of duct to
measure content of higher order modes of
acoustic propagation in the duct
39
Calibration of Quarter Wave Resonator Response
with Piezomotor Driving Resonator Mouth Open
Area
40
Results From Initial Resonator Calibration Study
Resonance Frequency and Quality Factor Shift with
Mouth Porosity Percentage
41
A Vision for Using Quarter Wavelength Resonators
to Control Turbofan Noise from Jet Engines
Resonators integrated into nacelle