How Wind tunnels Work - Blockage factor, wall effects, scaled model, similarity number, moving floor

[@TheOnlyRaceEngineer]

I cannot thank you more for sharing such high level knowledge on aerodynamics. GOD bless Wouter.

[@justinchang6763]

Nice job!!!!!!!! This will be so so helpful for some people

[@AirShaper]

Thanks a lot Justin!!

[@benykla]

very clear information, thank you.

[@AirShaper]

Thank you very much!

[@Guess2know]

Hi Wouter, I'm building a big 2m x 2m x 10m high speed tunnel to test different RC wing designs, aiming to fly the RC inside the tunnel to see the reactions in different air speeds, Will this work? Before i invest too much 💰

[@gibran653]

This is a good video, thank you. I have a question. What could be an alternative when the Reynolds number of a scale model cannot be matched in a low speed wind tunnel testing? (I mean, when you are limited by the wind tunnel facility, you do not have a water tunnel available, the model cannot be scaled to a larger size and the speed to match the Reynolds number is very high so the tunnel I cannot match that required speed) Could a solution be to carry out numerical simulations with the available wind tunnel conditions? (model length and wind speed) and validate the CFD results. Once you have found the correct CFD parameters (also check the Y +), the next step could be to keep the same parameters and simulate the model in real size, with real wind conditions Or matching the Mach number in the wind tunnel test instead of the Reynolds number might be acceptable? (Considering that the speed of the real model is 100 km/hr. So, the Mach number is not as relevant as the Reynolds Number in this case) Or is there some other method to approximate of the results in this type of situations?

[@AirShaper]

Hi Gibran, I don't know of any method that would provide equally similar results compared to the similarity numbers. You could indeed consider running a simulation on the scaled model to see how close it gets to a simulation on the full-size model. But first of all, it might be luck when the numbers match (so do check flow patterns as well) and second of all, the relative improvements you get on the scaled model might not be correct or even make things worse on the full-size model, as you're operating in a different Reynolds regime... In some cases, if you're looking at an isolated part (like the landing gear of a small plane), you could consider testing just that one at full scale/speed. But then of course you lose the proper surroundings. It's always a compromise :)

[@gibran653]

@@AirShaper Thanks for your answer it was really useful!

[@nikoskorobos100]

In case it is a wing or smthing similar, there are some techniques to induce turbulence earlier than it supposed to occur. In that way you get "closer" to flow structures corresponding to the Re you can't actually achieve.... I don't know how accurate it is and as a fluid dynamicist I wouldn't recommend that. In any case, there is some literature over it. It is called boundary layer tripping. Hope it helps 😎

[@Brian20351]

Does this same scaling apply when using CFD? For example, if I had a 20 meter wing that flies at Mach 0.85, but it is 1 meter in my CFD, would I have to change my Mach number and reynolds number input to get the correct coefficients? Also, are there any other changes I need to make to get the correct results?

[@AirShaper]

Hi Brian, the benefit of CFD is that you don't have to scale your model. But indeed, if you would scale it virtually, you also need to respect these changes in velocity and so on. And indeed, this goes beyond the Reynolds number. If you want to achieve similitude between flows, I suggest this page: en.wikipedia.org/wiki/Similitude_(model)

[@haji_amzar]

thanks

[@themeantuber]

I need to talk to you. I have an idea that can only be confirmed or disproved by someone with your particular knowledge. If it's viable we could both profit from it, along with the entire world. If not, you could still make a video of it. Any way I could contact you?

[@AirShaper]

You can reach us at [e-mail]

[@oplkfdhgk]

can you talk about vehicle called aptera. they say their vehicle is really aerodynamic and that's why it can go 1 mile with 100w. i just want to make sure that it sounds possible to you :D

[@AirShaper]

Yes, it's a fantastic vehicle!! We're looking to interview them, so if you know anyone on the team, just let me know. It has a claimed Cd value of 0.13, which easily cuts aerodynamic losses at highway speeds in half compared to an already aerodynamic sedan. Given those are well over half of the total losses, that alone makes a huge difference in range. And it looks pretty light too, so I'm guessing rolling losses are also cut in half (or more). So yes, I do believe they have made it drastically more efficient!

[@oplkfdhgk]

@@AirShaper i don't know anyone in their team. I am just normal dude who is thinking about buying it and want to make sure it's real :)

[@AirShaper]

@@oplkfdhgk It definitely looks & sounds very credible - and really makes sense compared to conventional EVs (which still put a heavy load on the environment). Drop us a line once you have one!!

[@oplkfdhgk]

@@AirShaper well I don't have the money right now :D maybe in couple years :p

[@oplkfdhgk]

I guess i could have worded my question little bit better :D

[@Jkauppa]

spin a barrel instead, in open air

[@AirShaper]

Would result in curved flow 🙂

[@Jkauppa]

@@AirShaper well thats easy way of doing that tho, circular duct around it

[@Jkauppa]

@@AirShaper its like earth, locally approximately non-circular, ie area and not so curved, spin a large cylinder to make it approximately linear, and yep it will have outwards negative gravitation acceleration

[@aniketsarpate5279]

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