How Wind tunnels Work - Measurement techniques, PIV, pressure taps, force balance, pressure probe.

For more videos on how wind tunnels work:
Part 1: • How Wind tunnels Work ...
Part 2: • How Wind tunnels Work ...
In this third video in our series "How Wind Tunnels Work" we look at wind tunnel measurement techniques - both some of the most widely used as well as some innovative ones.
Forces & moments
One of the most important output parameters of a wind tunnel test are the forces & moments on the object. To obtain these, the object is typically connected to a 3D force balance, which is a device that can measure the forces and moments around three perpendicular axes.
It's important to make sure that the influence of the connection part does not disturb the airflow too much, which is why they're often given an aerodynamic profile.
Surface pressure measurement
To measure the static pressure on the surface, you can use pressure taps. These are small holes drilled perpendicularly to the surface with a tube fitted to each hole in the model. These tubes are then sent to a pressure transducer which is typically located outside the wind tunnel, where the pressures are measured & logged.
If you want to measure the pressure on many locations on the surface of your object, you'll quickly end up with a huge set of cables that needs to be led outside. This is something you need to take into account during the design of your prototype.
Alternatively, you can work with miniaturized pressure transducers that fit within your model and then convert the measurements into digital data which is then sent to the outside world via a single cable to save space.
3D flow measurement
Smoke: one of the oldest & most common techniques is to release smoke upstream of the object to trace the path of the air. This can lead to very insightful information in terms of where the separation point is located, what the size is of certain vortex structures and so on. But it's quite difficult to determine local flow velocities, so over the years, a number of alternatives have been developed.
PIV - particle image velocimetry: tracer particles are released upstream of the area of interest. The size, density, ... of these particles are chosen in such a way that they follow the real flow nicely, without dropping to the ground or disturbing the flow pattern. They are then illuminated, typically in a 2D plane, using a laser. Then, they're filmed perpendicularly to that plane by high-speed cameras. By tracking & tracing the position of the individual particles between consecutive shots, one can reconstruct their path and calculate the velocity vector and flow field.
3D measurement probes: by measuring the differential pressures at the head of these multi-hole probes, you can obtain the static pressure and total pressure, the angle of attack and the velocity. If you then trace the position of the probe itself in 3D, you can reconstruct a 3D flow field.
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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).
We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!
For more information, visit www.airshaper.com

[@erockromulan9329]

We just got a PIV system at work. Can't wait to get it up and running!

[@AirShaper]

Ah great! Feel free to share your experiences here!

[@jurepecar9092]

I wanted to build my own wind tunnel in high school .. I got the air flowing through it quite nicely but then I started to think how to actually do some quantifiable observations and this is where my project fell apart. Looking back, I should have limited myself to some simple smoke and a high speed camera ...

[@AirShaper]

Hi Jure, in a previous job we did the same and indeed, the measuring equipment cost almost as much as the tunnel itself! Force balances aren't cheap 🙂 perhaps the cost ratio changes for larger tunnels

[@Tom-ir6or]

Brilliant video I will be starting my role as wind tunnel engineer next year and I cannot wait. This has given me a little insite on how we measure different aero dynamics. Thank you.

[@AirShaper]

Thanks a lot Tom, I'm glad you liked it! Feel free to connect on Linkedin as well (Wouter Remmerie) to stay in touch once you move to the wind tunnel.
Best of luck!

[@TheOnlyRaceEngineer]

Thank you Wouter for identifying the different measurement techniques.

[@AirShaper]

You're welcome!

[@yossriwidad7579]

These series in this channel are amazing! Thank you so much for making this information simplified and accessible !

[@AirShaper]

You're welcome!

[@jorbedo]

Excellent, learning a lot!

[@AirShaper]

Thanks a lot!!

[@MeirbhinOConaill]

Fantastic series Wouter. Quantifing pressure is a hot topic amongst competitive speed skydivers & wingsuiters. Thanks for putting out this high quality content

[@AirShaper]

Thanks a lot Meirbhin! We've been discussing simulations a lot with other speed sky diving professionals. Wouldn't feel good to drill holes in the body, but there should be alternatives to measure the surface pressure on the body 🙂 we do sims on 3D scans of athletes to analyze and optimize their equipment and position. See this video:
th-cam.com/video/DqlDpUFcScc/w-d-xo.html

[@MeirbhinOConaill]

@@AirShaper I've been following your work with cyclists and skiers. Very interesting indeed. Unfortunately I sustained an injury which impacted my plans on a mesh scan this year, but I did commit to drilling holes in the body haha

[@AirShaper]

@@MeirbhinOConaill LOL :D I hope they didn't have to put too much metal inside your body to recover from the injury. For athletes, our customers sometimes work with "parametric" 3D models which can then be fitted to measurements on the real person. Less accurate but perhaps more accessible.

[@MeirbhinOConaill]

@@AirShaper luckily I got away with no permanent metal this time. thank you for this insight. I'll look into parametric 3D models, thanks again

[@The_Japanese_Fox]

that is so fucking cool wtf

[@adiantosetiawan-aan2548]

Good video! I have a suggestion, next time could you make a video that discuss about the aeorodynemics of aptera 3? (Just like when you discuss about the tesla semi truck aeorodynemics)

[@AirShaper]

We've contacted them for an interview - should it come through then we'll make a video on it!

[@gyshalom]

is there air between the stream lines indicated via the smoke? what is the distance between the actual particles hitting an object in a wind tunnel?

[@AirShaper]

The streamlines indicate a specific path for the particles released at the smoke outlet. In between, the air likely follows a trajectory that is an average of the streamlines around it, although local differences may exist.
The distance between individual "air molecules" is very, very small - but I wouldn't know exactly how small (and it will likely vary a lot, due to kinetic energy / the mix of various types of molecules / ...)