1. Flow Measurement

2. Flow Measurement Positive Displacement Flow Obstruction Drag Effects
Hot Wire Anemometers
Magnetic Effects
Doppler Effects
Interference Effects
Pressure Probes

3. Positive Displacement
Bucket & Stopwatch Methods
Buckets for low flows
Tanks of known dimensions
Can measure either volume or weight
Accuracy limited by time and volume or weight measurements

4. Positive Displacement
Fluid is captured in fixed “pockets”
Fluid flow moves pockets from inlet to outlet
Count pocket movements per unit time
Compute flow from known pocket volume
Best for incompressible fluids (i.e. liquids)
Can be used for gases if know pressure and composition

5. Positive Displacement

6. Obstruction Methods Based on Bernoulli’s equation:1 2
Energy Considerations:
Mass Flow Considerations:

7. Obstruction Methods
If the flow is incompressible and very little temperature change, then:

8. Obstruction Methods
Unfortunately, can’t meet the rigorous conditions for the theory to be absolutely true, so modify with an empirical coefficient:
Note: if always running same fluid at about the same conditions, then write as

9. Obstruction Methods

10. Drag Effects
Fluid flowing past a body exerts a drag on the object. The body resists this force through gravitational, friction, and other forces.
Rotameter: Discussed in experiment 2
Turbine Flow Meter
Vortex Shedding Flow Meter

11. Drag Effects Turbine Flow Meter Like a fan running in reverse
Flow turns blades
Count revolutions
Calibrate flow vs rotational speed
Accurate, with modest pressure drop

12. Drag Effects Vortex Shedding Meter Turbulent flow conditions
Vortices (circular flow patterns) are shed from downstream edge of body in the flow stream.
Pressure sensor in body can sense departure of vortex.
Frequency of vortex departure related to flow velocity
Require special installation and care

13. Hot Wire Anemometers
As fluid flows past a hot object, extracts heat according to:

14. Hot Wire Anemometers
But the amount of heat delivered to the wire is related to the resistance of the wire, and the applied current:

15. Accurately measure i and Rw Compute q and Tw Compute u
Hot Wire Anemometers
Accurately measure i and Rw
Compute q and Tw
Compute u
Can be used in liquids and gases
Very fast response times, so can measure fluctuating flows

16. Magnetic Flow Meters
When a conductor moves through a magnetic field, an electric potential is induced in the conductor:
Use magnet to supply field
Measure induced potential
Calibrate to flow

17. Magnetic Flow Meters Non-intrusive, so don’t disrupt flow
Minimal pressure drop
Fluid must be conductive
Relatively expensive

18. Doppler Effects
Wave reflected off moving particle undergoes a frequency shift (Doppler effect)
Ultrasonic probes use ultrasound
Laser Doppler Anemometer uses light
(Same principle as police radar)

19. Doppler Effects Ultrasonic meters Meters fairly simple
Need good contact with pipe
Need fluid with particles
(Actually measure particle velocity)
Can be made in “clamp on” form, so portable

20. Doppler Effects Laser Doppler Anemometers Requires particles in fluid
Velocity determination requires sophisticated signal processing
Can detect velocity in very small volume, so good for turbulent flow measures

21. Interference Effects
Changing flow fields result in pressure changes which give rise to density changes.
The index of refraction is a function of density.
Can use this change in index of refraction to build a picture of the density, and, hence, an estimate of the velocity.

22. Interference Effects The Shadowgraph
Light shining through the flow field will be bent in regions of differing density.
On leaving the flow field, some areas will have more light than others and get light and dark areas.
This is the phenomenon that is observed when you see the wavy patterns rising from a hot surface such as a road. You are seeing the density differences.

23. Interference Effects Schlieren effect & Interferometers
Use lenses to send a beam through the sample.
Use sharp edge (shlieren) or split beam (interferometer) to generate interference pattern.
Produces a visual representation of the flow, but no numbers.

24. Interference Effects

25. Pressure Probes Measure static pressure Measure Stagnation Pressure
Compute flow from Bernoulli’s equation

26. Pressure Probes