MECH 391 Instrumentation Lab 11 Unsteady Velocity in a Karman Vortex Street Performed: 04/07/05 Sinan Ozcan: I believe I performed 50% of this lab Participation grade __ / 50 Soma: I believe I performed 50% of this lab Participation grade __ / 50

ABSTRACT The objective of this lab was to calibrate a hot film anemometer using pitot probe and use it to measure the unsteady speed and natural frequency in a Karman Vortex Street. The dominant frequency in the unsteady wake was measured for a range of wind speeds The Strouhal number (dimensionless frequency) was consistently one half the predicted value for the full Reynolds number range investigated.

Table 1 Air Properties and Cylinder Diameter Air viscosity from A.J. Wheeler and A.R. Ganji, Introduction to Engineering Experimentation, 2 nd Edition, Pearson Prentice Hall, 2004, p. 430.

Table 2 Calibration Data The initial and final no-wind hot film voltage readings are close but not the same

Figure 1 Hot Film System Calibration The fit equation V HF 2 = aU 0.5 +b appears to be appropriate for these data The dimensional parameters are a= volts 2 s 1/2 /m 1/2 and b = volts 2

Table 3 Karmon Frequency and Incoming Wind Speed

Fig. 2 Spectral Content in Wake for Highest Wind Speed Largest spectral content is at 834 hz. Secondary peak are also observed.

Fig. 3 Measured Speed vs. Time for Highest Wind Speed 5 large peaks in sec corresponds to frequency 833 hz, in agreement with the most energetic frequency in Fig. 2.

Figure 4 Stroughal versus Reynolds Measured Stroughal number is roughly 15% below the expected value. The reference value is from A.J. Wheeler and A.R. Ganji, Introduction to Engineering Experimentation, 2 nd Edition, Pearson Prentice Hall, 2004, p. 337 Noise in the hot film signal may cause this difference.

Fig. 5 VI Front Panel

Fig. 6 VI Block Diagram