ME 322: Instrumentation Lecture 33

ME 322: InstrumentationLecture 33 April 14, 2014 Professor Miles Greiner

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) • Summer: Help construct a convection heat transfer augmentation experiment • Spring 2015: ME 322r Lab Assistant

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

Setup myDAQ Variable Speed Blower VCTA Barometer PATM TATM Plexiglas Tube CTA • Same as Lab 6 but add CTA and cylinder, and do not use Venturi tube or Gage Pressure Transducer • Tunnel Air Density DTube Cylinder Static Pitot-Static Probe VC Total PP - + 3 in WC IP

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

Fig. 2 VI Block Diagram

Fig. 1 VI Front Panel

Pitot Probe Calibrate CTA using Pitot Probe Hot Film Probe • Remove Cylinder • So air speed is relatively steady • Align hot film and Pitot probes • So both are exposed to same air speed • Careful, hot film probes cost $150 each • Based on physical analysis (last lecture) we expect • For different blower speeds (and outlet covering) measure • VCTA (use myDAQ, average using fS ~ 48,000 Hz, tS ~ 1 sec) • IP (Pitot probe, DMM, “eyeball” average) • In Lab 11 use 8-12 wind speed • including blower off • In Final used fewer if time is an issue Cylinder

Calibration Calculations

Hot Film System Calibration • The fit equation VCTA2 = aU1/2 +b appears to be appropriate for these data. • Using least squares the best values for the dimensional parameters are • a = 2.643 volts2s1/2/m1/2 • b = 4.5742 volts2

Standard Error of the Estimate x x x x VCTA2 x x • Find coefficients of best fit line, a and b • Find Standard Error of the Estimate x x

Measure VCTA to determine and • Invert transfer function • (use this function in VI) • Uncertainty • (68%) • uncertainty in is independent of U • But we want the uncertainty in U, (not uncertainty in ) • Power Product? • But (68%) • (68%) • (68%) • uncertainty in U increases with U

Unsteady Speed Downstream from a Cylinder UA • Enter values of a and b in VI • For each measurement use fS ~ 48,000 Hz, sampling time tT ~ 1 sec • For each blower speed • Remove cylinder to measure average speed approaching cylinder UA • Return cylinder and measure unsteady speed • Determine frequency fP with highest URMS • Eyeball • Uncertainty in fP is larger of • Frequency resolution: ½(1/tT) ~ 1/2 Hz, or • Eyeball range • Repeat for ~5 different blower speeds Hot Film

Fig. 2 VI Block Diagram

Fig. 4 Spectral Content in Wake for Highest and Lowest Wind Speed (a) Lowest Speed fp = 751 Hz fp = 2600 Hz (b) Highest Speed • 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. • The frequency resolution is ½ (1/tT) = ½ hz • However, the shape of the peaks are somewhat broad, leading to

Dimensionless Frequency and Uncertainty • UA from LabVIEW VI • fP from LabVIEW VI plot • ½(1/tT) or eyeball uncertainty • Re = UADr/m (power product) • StD = DfP/UA(power product)

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

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

ME 322: Instrumentation Lecture 33