Real Time Demodulation of the A.C. Plasma Anemometer

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Real Time Demodulation of the A.C. Plasma
Anemometer

• T. Corke and E. Matlis
• University of Notre Dame
• Center for Flow Physics and Control
• Aerospace and Mechanical Engineering Department

Sivaram P. Gogineni Innovative Scientific
Solutions, Inc. 2766 Indian Ripple Rd. Dayton,
OH 45440
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Objectives - Develop robust, high-bandwidth
sensor based on a.c. glow-discharge. -
Develop calibration procedure for high enthalpy
flows. - Acquire and process signal in
real-time. - Demonstrate Wireless
Transmission.
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Probe Fixture
H.V. 350 Vrms
Electrode Gap 0.002
Nylon Locking Screw
Ground
Platinum-Iridium Tips
Plastic Probe Holder
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Mechanism for Flow Sensitivity (Mettler, 1949)
For D.C. Glow

• Flow convects charge-carrying ions downstream.
• Loss of current increases internal resistance
  increases voltage output.
• Mechanism has no dependence on temperature.
• Native frequency response.
• Robust, small spatial volume.

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A.C. Carrier A.M. Approach
Fc 3 MHz
fm
Velocity Fluctuations
U
A.C. Carrier
Modulated Carrier
Spectral Domain
fc
Carrier rms Gives Mean
Side-Band rms Give Fluctuations
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Closed-Loop Feedback

• Current used to maintain plasma independent of
  flow velocity.
• Real time demodulation

GnuRadio
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Wireless Acquisition and Real-Time Processing
Raw FFT 3 MHz Carrier, 1 kHz modulation
Demodulated FFT Spectrum converted to baseband
Received wirelessly!
Demodulated Time Series
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1 - 2 kHz blade passing frequency
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Plasma Boundary Layer Probe in Mach 1.4 Tunnel
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Frequency Response Cylinder Wake200 kHz
Response at Mach 0.9
254 m/s
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Plasma Sensor Mean Calibration
Mach 1.4 Boundary Layer Profile
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Calibration Hot-wire vs Plasma Sensor
- Hotwire rms fluctuations depend non-linearly on
mean flow calibration.
Ec
U ao a1V a2V2 a3V3 a4V4
Plasma Sensor
Em
m modulation index Em/Ec
- Fluctuations are linear ratio to mean.
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Wireless Transmission For Turbo-Machinery
Applications
(Surface-mount)
Inductive Pick-Up (Magneto)
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ND Transonic Compressor Facility

• 400 hp.
• Tip Mach 1.1
• Inlet Mach 0.45

Stream Measurements
Plasma Probe Locations
Supported by AFOSR DURIP
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MEMS Plasma Sensor

• Advantages
• Smaller size
• Greater spatial resolution
• Reduced power required
• Improved dimensional tolerances
• Single piece design

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Summary

• Plasma probe shows excellent potential for high
  speed time-
• resolved velocity measurements.
• - Offers extremely high frequency response
  without
• compensation.
• - Robust capable of measurements from Mach
  1 5 .
• - Low noise.
• - Calibration model possible.
• - Temperature independent.
• - Real time processing and wireless
  transmission.
• - Miniaturization through MEMS process.

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Plasma Fluidic Response

• Flow causes convection of ions in gap.
• Voltage output increases with velocity.
• Mechanism independent of temperature.

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Proposed for Phase II

• Develop laboratory plasma anemometer for
  distribution.
• Develop surface mount sensors for velocity,
  shear stress,
• and acoustic measurements.
• Develop wireless transmission.
• Integrated sensor and plasma actuators.
• Micro-sensor packaging.
• Applications Turbo-jet Engines, Pulsed
  Detonation,
• High Enthalpy and
  Ionized Gas Tunnels.

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Laboratory Plasma Anemometer
mmodulation coef.
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Modulation Coefficient Metric
Sample m1
Example from plasma sensor Too large gap (m1)
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Cavity Flow Mach 5 WPAFB Rarified Air Tunnel
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A.C. Carrier A.M. Approach
Fc 3 MHz
fm
Velocity Fluctuations
U
NL
A.C. Carrier
Modulated Carrier
Spectral Domain
Carrier rms Gives Mean
Side-Band rms Give Fluctuations
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Comparison to H.W.
6.6 m/s
Plasma Probe Fm3242 Hz.
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Comparison to H.W.
20.7 m/s
Plasma Probe Fm12.4kHz.
H.W. Probe F12.4kHz.
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Comparison to H.W.
30 m/s
Plasma Probe Fm18kHz.
H.W. Probe F18kHz.
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Plasma Probe Mean Flow Response
Hysteresis Test
Decreasing Velocity
Increasing Velocity
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Plasma Probe Electrodes Tips
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Cylinder Wake Mach 5 WPAFB Rarified Air Tunnel
35kHz
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Voltage-Current Flow Dependence
Current
Voltage
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Closed-Loop Feedback

• - Current used to maintain plasma independent of
  flow velocity.

Wireless Transmission
5 Watt 3 MHz
R
Sensor Current
error
power
reference
Amplifier

-
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• A.C. Glow Discharge Plasma Sensor
• - Frequency response 1MHz.
• - Low power 5 Watt.
• - Use in high-enthalpy flows.
• - Independent of temperature.
• - Robust, no moving parts.
• - NATIVE high frequency response up to
  carrier
• frequency!
• - Capable of wireless transmission.

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Current-Voltage Power Dependence
Voltage
Saturation
Current
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Plasma Sensor Starting Hysteresis
Power Consumed Voltage (rms)
Begin start sequence
Operating Point
Input Power (Gain)
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Frequency Response Cylinder Wake
Fm27 54 kHz.
190 m/s
Fm24 48 kHz.
Fm21 42 kHz.
U
Fm18 36 kHz.
Fm17 34 kHz.
120 m/s
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Mach 1.4 Turbulent B.L.Mean Profile
Mean Response
fc
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Mach 1.4 Turbulent B.L.Fluctuation Profile
Fluctuation Response
fc