1. Objectives Velocity and flow measurement
Lab tour and data acquisition use

2. From the last class: Wheatstone bridge+
Known resistor that we select based on R4 R1 Vo + R2 - -
VEX
Calculate R4
Our sensor

3. Converting Analog signal to Digital signal
Analog-to-digital converter (ADC) - electronic device that converts analog signals to an equivalent digital form
- heart of most data acquisition systems
Loss of information in conversion,
but no loss in transport and processing

4. Velocity and flow measurement
How to measure velocity?
Hot wire anemometer – rate of heat transfer
Propeller – rate of rotation, correlated with flow or velocity
Pitot tube – magnitude of velocity pressure
Laser – measure velocity of aerosol movement
Ultrasonic anemometer
Thermistor based –measure temperature
Other methods?
How to measure flow?
Calibrated fan – magnitude of fan pressure
Flow hood – Capture flow in known area/measure velocity
Orifice – magnitude of pressure drop
Vortex flowmeter
Rotameters
Masflowmeters
In all cases:
Flow conditions are important
Flow disturbance is an issue

5. Propeller Rotational speed is calibrated to flow rate
Does this disturb flow?
What flows are hard to measure?
Example: Multifunction meter

6. Pitot Tube From Bernoulli Equation
ρ = 1.2 kg/m3 = lb/m3 at std. conditions

7. Ultrasonic Anemometer
- No moving parts
- Use ultrasonic sound waves to measure wind
speed and direction
- Good precision
Relatively high frequency (up to 60Hz)
Several principle of operation
- Transmission (contrapropagating transit time) flowmeters
- Reflection (Doppler) flowmeters – for liquids
Transmission
Send sound pulses and measure transit time between
an ultrasonic pulse sent in the flow direction and an
ultrasound pulse sent opposite the flow direction.

8. RTD Temperature Based Velocity Sensor
Differential between two RTDs mounted on the sensor tube. The upper sensor measures the ambient temperature of the gas and continuously maintains the second RTD (near the tip of the probe) at 60°F above ambient.
The higher the gas velocity, the more current is required to maintain the temperature differential.
Good for high rangeability measurements of very low flows.

9. Hot Wire Anemometer (HWA)
Issues
Measures velocity at a single point
Omnidirectional
Directional (1D, 2D & 3D)
Minimal disturbance to flow
High frequency
Very Expensive
Fragile for field measurements
Require frequent calibration

10. Hot Wire Anemometer Constant Temperature 3-D Constant Power
Temperature control based on measured velocity
- Prevents overheating

11. Laser LDV or LDA Laser Doppler Velocimetry
Non-intrusive 1D, 2D and 3D point measurement of velocity and turbulence distribution
Requires particles seeded or from flow
Ultra high precession
High spatial and temporal resolution
Very expensive

12. LDA (LDV)
As particles pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector. Since, the fringe spacing d is known (from calibration), the velocity can be calculated to be
u = f \times d
where f is the frequency of the signal received at the detector.

13. Laser Particle Image Velocimetry (PIV)
Provide two- or three-dimensional velocity maps in flows using whole field
techniques based on imaging the light scattered by small particles in the flow
illuminated by a laser light sheet.
Is this CFD?

14. PIV
Properties similar to LDV

15. Schlieren flow visualization

16. Flow Measurements
Flow hood
Orifice and Venturies tube
Rotameter

17. Orifice Pressure drop through a known (circular, sharp edged) hole
Flow is smoothed before entry (usually need ~10 diameters upstream)
Q = C √ΔP
C provided by manufacturer (sometimes √ too)
Concerns/issues
Example: Trueflow Plate

18. Thermistor Based Velocity Sensor

19. Vortex flowmeter For given geometry V~f You measure sped of
pressure oscillations (frequency)

20. Flow with Pitot tube
Flow measurement Multiple measurements with pitot tube

21. Positioning of flow station / measuring point

22. Gas Mass Flowmeter The measuring gas is split.
Most goes through a bypass tube,
while a fraction goes through a sensor tube containing two
temperature coils. Heat flux is introduced at two sections
of the sensor tube by means of two wound coils. As gas flows
through the device, it carries heat from the upstream, to the downstream,
coils. The temperature differential, generates a proportional change in the resistance
of the sensor windings.

23. Bubble flow meter