MECH 322 Instrumentation

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MECH 322 Instrumentation

• Lab 11 Unsteady Velocity in a Karman Vortex
  Street

Performed 04/19/07
Group 0 Miles Greiner Lab Instructors Mithun
Gudipati, Venkata Venigalla
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ABSTRACT

• The objective of this lab was to measure the
  natural frequency of a Karman vortex street.
• A hot film anemometer was calibrated at steady
  speeds using a Pitot probe in a wind tunnel. It
  was then used to measure unsteady speeds in the
  wake behind a cylinder. The dominant frequency
  was determined for a range of wind speeds using a
  LabVIEW function.
• The Strouhal number was nearly independent of
  Reynolds number. Two of the six Strouhal numbers
  were above the predicted range.

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Fig. 1 VI Front Panel
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Fig. 2 VI Block Diagram
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Table 1 Air Properties and Cylinder Diameter

• Air Viscosity from A.J. Wheeler and A. R. Ganji,
  Introduction to Engineering Experimentation, 2nd
  Edition, Pearson Prentice Hall, 2004, p. 430.

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Table 2 Calibration Data

• The initial and final no-wind hot film voltages
  and Pitot transmitter currents are the same.

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Figure 3 Hot Film/CTA System Calibration

• The fit equation VCTA2 aSA0.5b appears to be
  the appropriate form.
• The standard deviation of the fit input is SI
  0.10 m1/2/s1/2.

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Fig. 4 Spectral Content in Wake for Highest and
Lowest Wind Speed
(a) Lowest Speed
fp 751 Hz
(b) Highest Speed
fp 2600 Hz

• The sampling frequency and period are fS 48,000
  Hz and TT 1 sec.
• The minimum and maximum detectable finite
  frequencies are 1 and 24,000 Hz.
• It is straightforward to distinguish fP from
  this data. Its uncertainty is Wfp 0.5 Hz.

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Table 3 Karman Frequency and Incoming Wind Speed

• The dominant frequency f and centerline speed SA
  were determined as several wind speeds.
• The Reynolds number is ReD SADr/m
• The Strouhal number is StD fD/SA

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Fig. 5 Strouhal versus Reynolds

• The reference value is from A.J. Wheeler and A.R.
  Ganji, Introduction to Engineering
  Experimentation, 2nd Edition, Pearson Prentice
  Hall, 2004, p. 337.
• Four of the six Strouhal numbers are within the
  expected range.

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Extra (not part of report)2006 Summary
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Histogram
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Measured Speed vs. Time for Lowest Wind Speed

• The sampling rate appears to be sufficiently fast
  to capture the shape of the unsteady trace.
• Turbulence or instabilities at the tunnel
  entrance may cause unsteadiness in the speed even
  when there is no cylinder in the flow
• With the cylinder, there are roughly 6 large
  peaks in 0.01 seconds, corresponds to a frequency
  600 Hz.

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Hot Film System Calibration

• The fit equation VCTA2 aSA0.5b appears to be
  appropriate for these data.
• The dimensional parameters are a 1.366
  volts2s1/2/m1/2 and b 2.2057 volts2