ME 322: Instrumentation Lecture 32

1. ME 322: InstrumentationLecture 32 April 11, 2014 Professor Miles Greiner

2. Announcements/Reminders • Next week: Lab 10 Vibrating Beam • 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 • Did you know? • HW solutions are posted on WebCampus • Exam solution posted outside PE 213 (my office) • Help wanted (see me greiner@unr.edu) • Summer: Help construct a convection heat transfer augmentation experiment • Spring 2015: ME 322r Lab Assistant

3. Cylinder in Cross Flow V∞ Velocity Probe • Speed is reduced in the wake region • Instability of steady flow causes periodically-shed vortices • Karman Vortex Street • Figure shows unsteady speed measured by a probe in wake • Fairly regular oscillations, period P ~ 0.01/6 = 0.0017 sec • Peak oscillatory frequency of f = 1/P ~ 600 Hz • Broad spectrum of frequencies • Can a Pitot probe measure oscillations at these high frequencies? • How to measure rapidly changing speeds?

4. Strouhal Number V∞ f Q • What does the vortex shedding frequency depend on? • Increases with • Decreases with • Dimensionless Strouhal Number • ; • For , 0.20 < < 0.21 (~constant) • Frequency increases linearly with speed and flow rate • This phenomena used to measure pipe volume flow rate Q D Q

5. Example • A car in Reno is moving at 60 miles/hour and has a ¼-inch diameter antenna. At what frequency will vortices be shed from it? The air temperature is 27°C and the atmospheric pressure is 86 kPa. • 0.20 < < 0.21 • For

6. How to measure Rapidly Varying Speed? • Pressure Method • Pitot probes transmit pressure to transducers using tubes • Ok for slowly varying speeds • At high frequencies, pressure response at transducer is attenuated and delayed compared to probe (2nd order system) • Heat Transfer Method • Hot Wire or Hot Film probe • Very small wire or metal plated quartz on a support fork • Electrically heat surface • Heat transfer to the surrounding fluid increases with fluid speed • Two modes: • Constant Current • Constant Temperature

7. Circuit V∞T∞ R2 I I VO VE TS RS • Probe electrical resistance heating • Q = IVO (can be measured) • Heat is mostly removed by convection • Q = IVO= hA(TS-T∞) • Neglecting radiation and conduction • Convection Coefficient for small cylinders in cross flow • ; M and N are constants • If we can find TS then we can find • and

8. How to find TS? • Wire resistance depends on TS • Temperature Coefficient of Resistance (material property) • RS0 = RS at T = T0 • , • We can find • So, theoretically we can find TS and so • and • Two modes of operation

9. Constant Current Mode V∞T∞ R2 I I VO VE TS RS V0 • VE= constant, and R2 >> RS • = constant • Probe temperature TSand resistance RSgo downs as V∞ go up • Measure V0 = IRS • V0 will decrease as V∞ increases • Calibrate • Problem: TS must reach equilibrium with surroundings • Takes time, ~ 0.01 sec, or frequency 100 Hz • Too slow! V∞

10. Constant Temperature Anemometer (CTA) • Incorporates hot sensor into a Wheatstone bridge • If V∞ increases, TS and RS“start” to go down • This decreases VBridge, but Feedback amplifier (op-amp) very quickly increases VO to increase current to sensor and restore its temperature and resistance (RS = RR) • The current and power to sensor adjusts to make its temperature constant • Output is VCTA (voltage across sensor) VCTA V∞ RR TS RS VBridge

11. CTA Transfer Function • Convection Heat Transfer from probe to fluid • ) • So • ) • ) • Or find constants a and b by calibration • Feedback amplifiers respond very quickly • Accurate for up to f = 400,000 Hz • Requires feedback control (Lab 12) • To use CTA, measure VCTA. • Calculate , Constants

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

13. Lab 11 Unsteady Speed in a Karman Vortex Street • Use the same wind tunnels as Lab 6 • Sign up for 1.5 hour periods with your partner in lab next week • Two steps • Statically calibrate hot film CTA using a Pitot probe • Measure unsteady speed downstream from a cylinder • Measure “steady” speed without cylinder V • Perform spectral analysis and find frequency with peak amplitude, fP • Calculate StD= DfP/V and compare to expectations

14. Setup myDAQ Variable Speed Blower VCTA Barometer PATM TATM Plexiglas Tube CTA • Add CTA and cylinder in cross flow • Do not use Venturi tube or Gage Pressure Transducer • Assume Pstat = PATM (Pgage= 0) • Tunnel Air Density DTube Cylinder Static Pitot-Static Probe VC Total PP - + 3 in WC IP

15. 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.

16. Fig. 2 VI Block Diagram

17. Fig. 1 VI Front Panel

18. Calibrate CTA using Pitot Probe • Remove Cylinder • Align hot film and Pitot probes (carefully) • 4 probes cost $600 • Measure VCTA,AVG and IPitot for different blower speeds

19. Calibration Measurements and Calculations • Average Velocity

20. Table 2 Calibration Data • The initial and final no-wind hot film voltages and Pitot transmitter currents are the same.

21. Standard Error of the Estimate x x • Find best fit line • Find Standard Error of the Estimate • Now measure VCTA to determine x x VCTA2 x x x x

22. Measure VCTA to determine • Invert • Uncertainty • But we want

24. Cylinder in cross flow Wake: region of reduced speed Frequency Strughold #: Constant Page 360 to 361 Measure flow rate in a pipe For

25. Example A car antenna D = 0.25 in and car s=60 mph What will the frequency of the shed vortices be?

26. Before we used: Pressure Method Pito-probe/pressure transmitter (too slow) Heat transfer method: -hot film or hot wire probe -small electrically heated surface Probe: Acid etched wire (hot wire) -small but brittle Metal plated quartz cylinder (hot film).

27. Probe electrical resistance heating → Can measure I, V0 Q [watts] Heat is mostly dissipated by convection For small cylinders in cross flow

28. How to find TS: Wire resistance changes with its temperature TS: α ≡ material property So theoretically by measuring: A, I, V0, & known α.

29. Tow modes of operation: • 1) Constant current VE ≡ constant & R2 >> RS As U↑, h↑, TS↓, RS↓ Problem: TS must reach equilibrium with surroundings. Takes time Max frequency Response

30. 2) Constant Temp Anemometer (CTA) • Uses electronic feedback (op-amp) to very VE so TS (and RS) • stay constant. Wheat stone bridge circuit