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ME 322: Instrumentation Lecture 33 April 14, 2014 Professor Miles Greiner

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Announcements/Reminders This week: Lab 10 Vibrating Beam Sign up for 1.5-hour Lab 11 periods with your partner in lab Extra-Credit LabVIEW Workshop – Friday, April 18, 2014, 2-4 PM, Jot Travis Room 125D – Sign-up on WebCampus – If enough interest then we may offer a second session Noon-2 Help wanted (see me greiner@unr.edu)greiner@unr.edu – Summer: Help construct a convection heat transfer augmentation experiment – Spring 2015: ME 322r Lab Assistant

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Lab 11 Unsteady Speed in a Karman Vortex Street Nomenclature – U = air speed (instead of V) – V CTA = Constant temperature anemometer voltage Two steps – Statically calibrate hot film CTA using a Pitot probe – Find frequency, f P with largest U RMS downstream from a cylinder of diameter D for a range of air speeds U Compare to expectations (St D = Df P /U = 0.2-0.21)

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Setup D Tube P Static Total + - IPIP Variable Speed Blower Plexiglas Tube Pitot-Static Probe V C 3 in WC Barometer P ATM T ATM CTA myDAQ Cylinder V CTA

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Before Experiment Construct VI (formula block) Measure P ATM, T ATM, and cylinder D Find and for air Air Viscosity from A.J. Wheeler and A. R. Ganji, Introduction to Engineering Experimentation, 2 nd Edition, Pearson Prentice Hall, 2004, p. 430.

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Fig. 2 VI Block Diagram

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Fig. 1 VI Front Panel

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Calibrate CTA using Pitot Probe Cylinder Hot Film Probe Pitot Probe

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Calibration Calculations

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Hot Film System Calibration The fit equation V CTA 2 = aU 1/2 +b appears to be appropriate for these data. Using least squares the best values for the dimensional parameters are – a = 2.643 volts 2 s 1/2 /m 1/2 – b = 4.5742 volts 2

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Standard Error of the Estimate x x x x x x x x V CTA 2

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Unsteady Speed Downstream from a Cylinder Enter values of a and b in VI For each measurement use f S ~ 48,000 Hz, sampling time t T ~ 1 sec For each blower speed – Remove cylinder to measure average speed approaching cylinder U A – Return cylinder and measure unsteady speed Determine frequency f P with highest U RMS – Eyeball – Uncertainty in f P is larger of » Frequency resolution: ½(1/t T ) ~ 1/2 Hz, or » Eyeball range Repeat for ~5 different blower speeds UAUA Hot Film

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Fig. 2 VI Block Diagram

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

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Dimensionless Frequency and Uncertainty

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Comparison with Expectations Are the values you get for St within the expected range?

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Demo Construct VI – Formula Block – Convert to Dynamic Data Perform calculations

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