I am searching for an anemometer to measure turbulence intensity of a wind tunnel. Most of the hot wire aneomemeters on market are designed for HVAC applications so their accuracy is around 5%. Since moderate turbulence intensity is defined to be between 1% and 5%, using a anemometer with 5% accuracy does not sounds like appropriate. What should be the accuray of the device for this measurement?
Additional Question: What should be the sampling rate of the device?
Hi Harun,
As you probably know, the HWA uncertainty is influenced by many factors. See, for example, Jörgensen, where the basic aspects are nicely described. You can reduce uncertainty through careful calibration procedures. Additionally, using a good A/D converter can lower the uncertainty. Another issue with HWA is temperature effects, which also impact measurements.
There is a possibility to develop a fast response pressure probe with, for example, a Kulite sensor placed at the probe tip. This sensor has some advantages over HWA, such as temperature compensation. However, there are also some issues. As always, for the best results, this sensor needs to be calibrated prior to measurements, and furthermore, the eigenfrequency of such a probe should be determined.
You can consider using fast PIV as well, although I haven't personally used this method, so I can't offer specific advice about it.
I recommend trying multiple methods if you have the resources and time, and then combining the results.
Regarding your additional questions, I usually start with the highest sampling rate to check the signals. After some preliminary evaluations, I adjust it to capture all of the interesting phenomena, similar to adjusting the signal filter.
I hope that my comment helps you a little bit.
Best regard,
Erik.
An anemometer is not an appropriate instrument with which to measure turbulence intensity. There are many papers published on the subject which include details of the various instruments used.
Hi Harun,
In a wind tunnel, Hot-Wire Anemometry (HWA) should do the job. Although you will have to calibrate the probes. Normally they can have an acquisition frequency of about 30kHz, which should be more than enough.
Other methods are Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV). For these techniques you will need a particle generator as the velocity of such particles will be used as a proxy of the flow velocity. Normally in wind tunnels a smoke generator is used to generate particles.
Laser Doppler Anemometry is, like HWA, a point wise technique. That means that you measure point-to-point. However, it's probably the most precise technique to measure instantaneous fluid velocities.
Particle Image Velocimetry can also be used, but in this case the requirements are higher: fast camera synchronized with a laser to light a section of the wind tunnel, etc.
Acquistion frequency will depend on the time scales of your phenomenon. Always consider the Nyquist-Shannon criterion (acquisition frequency should be at least the double of the signal's maximum frequency) when performing measurements, so you will be sure you'll capture all the important features.
Let me suggest you an event that may be of your interest: the W.A.T.E.R. Summer School, an event dedicated to experimental techniques in fluid mechanics: https://water.sciencesconf.org/
And some bibliographic references that can be useful for you:
The Handbook of Experimental Fluid Mechanics: Book Springer Handbook of Experimental Fluid Mechanics
The experimental hydraulics methods.
Book Experimental Hydraulics: Methods, Instrumentation, Data Proc...
Feel free to send me a message with more questions.
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