Hot wire anemometers

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Hot wire anemometers
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Agenda

• Review of lab 6 and what is coming.
• What is a hot wire?
• How does it work?
• How do we control things?

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Our circuit
Vin
Vresistor
Vlamp
V0
How does current through the resistor relate to
current through the lamp?
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Our circuit
Vin
Vresistor
What is the voltage on this wire?
Vin
Vlamp
V0
Now we can build a system that can measure the
temperature of the filament and control it by
regulating the voltage input to the op-amp
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Hot wire/hot film anemometer
Hot film for liquids
Hot wire for gases
Why do we want the probe so small?
6
Why do we want the probe so small?

• Resolution in space
• Resolution in time (high frequency)
• Dont want to disturb what we are measuring.

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Uses are found anywhere fluids flow

• Aerodynamics lift, drag
• Combustion IC, gas turbine engines
• Meteorology
• Fires and fire safety
• Ocean currents
• How bugs fly and how fish swim
• Turbulence (Richard Feynman, "the most important
  unsolved problem of classical physics." )
• Ordinary measurement tools, i.e. HVAC probes

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Use of hot wire in turbulence
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Hot-wire measurements connect large scales to
small scales in turbulence
50,000 km 12 orders of magnitude of structure
in this flow
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So now, the theory.
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Last time.the stove
Heat flow out
Power in
hA
T (K)
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Results from changes..
T (K)
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Constant power device
P
A
Response time dictated by the response of the
system.
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Constant T device
Constant power mode needs to wait for steady
state to correlate T to h
Constant T mode always at steady state -
instantly correlate P to h
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So how can we control the temperature?
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Proportional Control
kp
Power input
dT/dt
T
hA(T-Tair)
h
T-Tair
Physical system
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Proportional control
Kp1
Kp0.1
Kp0.01
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kp
Why cant p-gain be set to be very large?
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Time delay can cause instability
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What is happening?

• Cant make P-gain too high, time delay can cause
  instability.
• If P-gain is too low there is high offset.

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Integral Control
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Result with Integral Control
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Problems with the model

• Power cannot be removed by the controller easy
  to fix.
• Filament is not precisely a single temperature
  small error for small filament.
• Small time delays sample rate of computer,
  thermal response, response of op-amp, etc these
  are small.

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The End