Module 9 - Hot Wire Anemometry Flashcards
What are some advantages of the seven-hole probe over the hot wire probe?
- reduced sensitivity to misalignment with the flow
- seven hole probe can measure 3 components of velocity
- seven hole probe only has to be calibrated once
- seven hole probe more robust
What are hot wires typically made of?
Tungsten alloys (good mech strength, inexpensive but oxidizes)
Platinum alloys (good thermal props but low mech strength and expensive, alloy with iridium or rhodium)
What is the coding cooling law?
Simple approach to analyzing HWA data - assumes wire responds to normal component of velocity only, pitch angle does not influence measurement, wire is perfect (symmetric). This assumption allows us to replace U in kings law with Ueff which can be related to the yaw angle. This is only reasonable for low turbulence and low angles (~10)
What are the reasons for a small sensor size in HWA?
- Better spatial resolution
- Wire reacts to velocity changes quicker (better temporal resolution)
What is the general form of Kings law for HWA?
E2 = A+BV^n
What are the benefits of direct interpolation over a theoretical HWA approach?
- Removes cosine cooling law assumptions
- we don’t need to know wire angle
- can measure flows at greater yaw angles than 10 degrees
What is static calibration?
Mean velocities and voltages are used to determine the response equation for a probe, which is assumed to be able to predict the instantaneous voltage-velocity relationship
What are two sources of error in HWA?
- Temperature drifts
- Directional sensitivity
- Flow effects of prongs/solder
What are some advantages of a cup anemometer? Disadvantages?
Advantages: simple construction, calibration, setup, omnidirectional, durable
Disadvantages: limited to one component of velocity, directional ambiguity, doesn’t respond well to low flow rates
