GouldJoseph
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Wind Tunnel Issues

Postby GouldJoseph » Mon Feb 17, 2020 8:31 am

Hello,

So I am taking a senior design engineering class and the engineering topic I chose at the beginning of the year deals with improving aircraft fuel efficiency by increasing aerodynamic efficiency. I have taken the shark fin vortex generator design and modified it. Theoretically, through my calculations, my design should be able to improve the effectiveness of the trailing edge devices on aircraft wings. But for me to actually test it "in the real world" I have to test it in my school's wind tunnel but my wind tunnel can only go up to 45 mph and is open ended. My scaled down device is one on a 1:3 scale and it must be able to sustain ~180 mph winds. How do I do this in the wind tunnel that my school has? We are not allowed to take it apart and I have already thought of putting a funnel in the chamber that would decrease the cross sectional area of the air flow, ergo increasing the air's velocity but that has proven to be difficult to incorporate into the wind tunnel without having issues. What should I do?

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Re: Wind Tunnel Issues

Postby bfinio » Mon Feb 17, 2020 2:13 pm

Hello,

Are you a college student or a senior in high school? These forums are intended for K-12 students doing school science projects. I wanted to check because this sounds like a college-level project; but if you are in high school we might be able to help.

Thanks,

Ben

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Re: Wind Tunnel Issues

Postby GouldJoseph » Mon Feb 17, 2020 2:48 pm

Hello Ben,

I am a senior in high school and the reason why I chose this project is that I love aerospace engineering and aerodynamics.

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Re: Wind Tunnel Issues

Postby bfinio » Mon Feb 17, 2020 3:35 pm

Hi GouldJoseph,

In that case, great! Happy to help. I am a mechanical engineer - admittedly it has been quite a while since I took fluid dynamics, but I'll do my best. If you haven't already, I would recommend starting research on something called the Reynolds number (see the equation in the Definition section)

https://en.wikipedia.org/wiki/Reynolds_number

In general, when doing aerodynamic experiments at different scales, you try to keep the Reynolds number the same. Reynolds number depends on fluid density and viscosity, as well as velocity and the length of the physical object in question (in the case of an aircraft wing, it's the chord length). So if you change the length, you have to change something else to keep the Reynolds number constant. The first answer in this post gives a good explanation:

https://physics.stackexchange.com/quest ... ng-problem

Unfortunately I realize that doesn't give a simple answer to your question, but the point for now is that it's more complicated than trying to make the wind tunnel go at the "real world" speed. If you scale the length down by a factor of 3, then (assuming you are still using room-temperature air at atmospheric pressure), you actually need to increase the speed by a factor of 3 in order to keep the Reynolds number the same - and I assume we aren't getting your wind tunnel up to 540mph.

If you haven't seen it yet, we have a (somewhat dated) guide on building a wind tunnel that may be useful (not that you should build your own from scratch, but some of the references on the last tab might be helpful: https://www.sciencebuddies.org/science- ... troduction).

In general I would recommend looking around for more resources on scaling experiments for wind tunnels. I have a few contacts who know fluid dynamics better than I do, I can ask them and get back to you.

-Ben

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Re: Wind Tunnel Issues

Postby GouldJoseph » Mon Feb 17, 2020 6:01 pm

Hello Ben,

Thank you so much for the information! I was focusing on keeping the velocities at 40k feet and sea level constant that I forgot about the Reynolds Number. So you're saying that if instead of keeping the velocity constant, I instead keep the Reynolds Number constant, I will be able to calculate the velocity required in the wind tunnel? If so, I just did the math and using the data below, I've calculated the Reynolds Number to be 44,505,918 (in metric out of habit)

Re = 44,505,918
Altitude = 12192 m
Velocity = 245.85 m/s
Average Commercial Chord Length = 8.230 m
Density = 0.30286 kg/m^3
Dynamic Viscosity = 1.3789 x 10^-5 N-s/m^2

Now, if I use the same conditions but instead of being at 40k feet, I am at sea level and I have the same Reynolds Number and I use the following data to solve for the velocity:

Re = 44,505,918
Altitude = 0 m
Velocity =?
Density = 1.22528 kg/m^3
Average Commercial Chord Length = 8.230 m
Dynamic Viscosity = 1.7332e-5 N-s/m^2

Once plugged into the Reynolds Number equation, we get a resulting velocity of 76.442 m/s or 170.99 mph.

Now, because my school's wind tunnel has a wing that is only 6 inches (0.1524 m) long, and can't fit an 8.230 m wing, I used most of the data from the above equation, with an exception to the chord length:

Re = 44,505,918
Altitude = 0 m
Velocity =?
Density = 1.22528 kg/m^3
Wind Tunnel Wing Chord Length = 0.1524 m
Dynamic Viscosity = 1.7332e-5 N-s/m^2

Once plugged into the equation, I got 4128.054 m/s (9234.19 mph) as the velocity needed to maintain the Reynolds Number.

My device, full-scaled, has the dimensions of a regular vortex generator (1 in. in height, 3 in. in length and 0.14 in. in thickness) and is supposed to fit on a full-scale aircraft wing. Now, to fit on the wind tunnel's wing, and to maintain its effectiveness, I had to scale it down to 1/3 of its size.

Now, once scaled down to fit on the wind tunnel wing and after doing these Reynolds Number calculations, would there be any viable ways of testing my design? Would I have to go to an outside institution or business to help me test it?

To delay the onset of flow separation, my device will be placed on the external surface of the airfoil. They will be placed at sixteen percent of the leading edge of the airfoil to maintain steady airflow over the surface of the airfoil and the control surfaces at the trailing edge. Regular micro vortex generators are usually triangular or rectangular but my device will be a shark fin shape, will be as tall as the boundary layer, and run in spanwise lines near the thickest part of the wing.
The device will be positioned obliquely so that it will have an angle of attack, AoA, to the local airflow around them. This will create a tip vortex that will draw energetic, turbulent airflow into the relatively slow-moving boundary layer in contact with the surface. A turbulent boundary layer is less likely to separate than a laminar airflow and is, therefore, better in ensuring the effectiveness of trailing edge control surfaces. Wouldn't my device be decreasing the Reynolds Number?

I apologize if this is a lot of information I am throwing at you. If you want, I can send you my preliminary design review to get a better feel for my project.

Thank you so much for taking the time out of your day to help me with my dilemma!

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Re: Wind Tunnel Issues

Postby bfinio » Mon Feb 17, 2020 7:57 pm

Hi GouldJoseph,

I don't have time to check your calculations or give you a detailed reply tonight, but wanted to let you know that I saw this. Honestly this is outside of my area of expertise and I will have to ask around a bit (I have a friend who teaches fluid dynamics I can check with). In the meantime I would suggest that you try to do more background research on airfoil testing in wind tunnels and appropriate scaling. You might need to get your hands on a college level fluid dynamics or aerodynamics textbook, or look around at academic literature (try Google Scholar). Clearly this is something other people have done before - we just have to figure out how to do it properly.

-Ben

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Re: Wind Tunnel Issues

Postby bfinio » Wed Feb 19, 2020 6:48 am

Hi GouldJoseph,

A few updates after checking with a fluid dynamics prof. Here's her initial reply (edited slightly):

"Yes, you're right, you need to jack up the air speed 10x. So yes, doing model tests of things like planes and cars in normal wind tunnels is dicy. Either you can have a huge wind tunnel so your model can be bigger, or you mess with the air in other ways to make it more inertial. so since p = rho R T, you can either lower the temp of the air which would make it denser and more viscous (so there are cryogenic test facilities), or pressurize the tunnel (to increase rho). On the other hand, if he's doing something like, measuring drag / calculating drag coefficient, there's a reasonable chance that something like that is relatively insensitive to reynolds number, like C_d at Re=10^5 is probably reasonably close to C_d at Re=10^6"

So that leads to a few follow-up questions from me:

1) How exactly are you measuring/calculating aerodynamic efficiency? If you're doing this by measuring lift and drag and then calculating the lift/drag ratio, based on her response about drag coefficients then it might be OK even if the Reynolds numbers aren't the same. You would still be able to calculate a relative change in efficiency.

2) Something I didn't catch in your previous post - you said "my school's wind tunnel has a wing that is only 6 inches (0.1524 m) long, and can't fit an 8.230 m wing" - so the model wing is less than 2% the size of a real wing. However, you said you only scaled your vortex generator down to 1/3 its actual size, or about 33%. That means it's over 15x bigger relative to the model wing than the full-scale model would be relative to a real airplane wing. I assume for this to work properly, you need to scale everything together and keep their sizes proportional - so either your model wing needs to be much bigger, or your model vortex generator needs to be much smaller.

I know that didn't exactly answer all the questions in your last post, but I'm trying to make sure we're heading in the right direction, and wanted to provide an update while I'm still waiting to her back from my fluid dynamics professor friend about a few things.

*edit to include an additional comment from her: "My best explanation is that the Reynolds number, for his type of experiment, is usually something that's calculated a priori. It's a number that's somewhat predictive of turbulent vs laminar, but the resulting aerodynamics can have a wide range. It's like saying, make a chocolate chip cookie -- we have a good sense of what to expect, but there's a lot of latitude in between. "

Thanks,

Ben

GouldJoseph
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Re: Wind Tunnel Issues

Postby GouldJoseph » Wed Feb 19, 2020 6:06 pm

Hello Ben,

First off, thank you so much for the information about different ways of modifying our wind tunnel. Because my wind tunnel is open ended, I can't really modify the pressure so I'll probably focus on trying to decrease the temperature (without making my teacher mad at me).

1.) With regards to measuring aerodynamic efficiency, I was planning on recording how the airflow acts around the airfoil at different angles of attack and compare the results to how the airflow reacts to the airfoil with my prototype attached to it at the same velocity in the wind tunnel. Previously, I have attempted to calculate lift, drag, and their coefficients to find L/D ratio and each time, I calculated a value of ~4. Now, when I search up the L/D ratio of the average commercial aircraft, the internet it should be around 18. I don't know where I've gone in the calculations so I've determined to stick to comparing the airfoil at different AOA with and without the prototype.

2.) Yes, so the wind tunnel's wing chord length is 6 inches. If I were to scale down my prototype to the proper scale, our 3D printers filament would be too thick to print the device, the other prototype made from an aluminium alloy would be too thick, and I wouldn't be able to cut out the aluminium at that scale. I can try to scale down the prototype to the right scale but I doubt that it will print successfully. The aluminium alloy that I am using for non 3D printed prototype is 1/4" inch aluminium alloy 7075-T7351.

I cannot change the dimensions of the wind tunnel testing chamber so I guess the best option is to find a company or institution where I can change the pressure, temperature, and test the prototype at the proper scale to get the best result.

I have already done testing of the 3D printed prototype on the wing and I have seen promising results, especially at high AOA. When the prototype is not on the wing, you can clearly see the airflow go over the wing and not stick to the wing's surface BUT when the prototype is on the wing, you can clearly see how it makes the airflow conform to the length of the wing, which would theoretically improve the effectiveness of the trailing edge control surfaces such as the flaps and ailerons.

Where do you suggest I should go from here?

Thanks!

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Re: Wind Tunnel Issues

Postby bfinio » Wed Feb 19, 2020 6:28 pm

Hi GouldJoseph,

I don't know what resources are available to you, so unfortunately it's kind of hard for me to suggest where to go from here. I'm not sure where you're located (please don't post it though - students aren't supposed to post contact information in the forums), but it might be worth trying to reach out to a local university or aircraft/aerodynamics company to see if you can get some direct advice from someone with more of an aerodynamics background. I think it's pretty rare for high schools to even have wind tunnels (mine certainly didn't), and I don't know what university/company policies would be about letting students use their wind tunnels for experiments. In general, I'd say it certainly doesn't hurt to email people and ask, or try and get a teacher at your school to reach out and get you in touch with someone in case they already have contacts at a local university. Don't take it personally if you don't hear back though - a lot of research labs get a lot of unsolicited requests for information/collaboration etc, and it's simply more than they can reply to.

Some further clarification about point #1 in your latest reply though - does your school's wind tunnel have a force sensor that allows you to experimentally measure lift and drag? How are you visualizing the airflow (e.g. by introducing smoke)? If you are calculating lift and drag, what measurements are you using as a basis for the calculations?

For point #2 - is it possible for you to make a larger wing and use that, at least to get closer to the scale of your prototype? Or is there only one fixed wing that's available for use with the tunnel?

GouldJoseph
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Re: Wind Tunnel Issues

Postby GouldJoseph » Wed Feb 19, 2020 6:39 pm

Hello Ben,

1.) I apologize, I forgot to mention that I had been using a fog machine to visualize the airflow. It has been very effective at showing the airflow's interaction with the wing. Unfortunately, my school's wind tunnel does not have a force sensor, I will relying primarily on my phone recordings of the tests.

2.) The wind tunnel only has a fixed wing attached to a point where it can pivot to achieve the angles of attack.

Thank you for the all the information and insight that you have provided me. You have helped me tremendously with my project.
I'll start emailing professors at local universities to see if they have a facility that I can test at. It'll be interesting to see who replies and what they say.

Thanks!

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Re: Wind Tunnel Issues

Postby bfinio » Wed Feb 19, 2020 7:11 pm

Hi GouldJoseph,

Glad I could help (somewhat)! We would love to hear how this project goes, so please keep us posted. You can always reply to this thread with more questions, or email us at scibuddy@sciencebuddies.org with updates.

-Ben

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Re: Wind Tunnel Issues

Postby GouldJoseph » Thu Mar 26, 2020 10:13 am

Hello Ben,

Again, thank you so much for helping me out! It's been a crazy last few weeks with schools closing for the remainder of the school year and the governor closing all non-essential businesses. With that being said, "the show must go on" and our research project has to get completed. When the governor initially announced the closure of all schools, I assumed that our projects had come to an end because we weren't allowed access to our school anymore but in the days since, our teacher assigned the next steps for our projects, so we can finish them by the end of the school year.

For my research project, the next step is to get my project evaluated by qualified stakeholders and field experts. Would you be interested in evaluating my prototype testing and project? If not, I completely understand. If so, let me know so I can send you the files of the data we collected. I do not think I'll be able to get a hold of our actual prototype because it is at my high school but I am currently in talks with my teacher to see if he'll allow me to go in one day and grab both our prototype and our huge sheet of aircraft-grade aluminum alloy. Have a great day!

Thank you so much for everything you've done!
Joey Gould

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Re: Wind Tunnel Issues

Postby bfinio » Thu Mar 26, 2020 11:37 am

Hi Joey,

It depends on a few things:

1. What exactly the "evaluation" consists of
2. When you need it by
3. Your teacher's definition of "qualified expert"

For 1 & 2 - obviously life has been a little nuts lately. So if this is something that requires me to write a lot of detailed feedback and you need it very soon, I probably can't do it. If it's just answering a few questions and you don't need it for a few weeks, that would probably be OK. For #3 - remember that I have an undergraduate degree in mechanical engineering, but my PhD is in robotics. I'm not really an "aerospace person" and I don't have any real experience working with wind tunnels. So I'm less qualified to provide feedback than someone with more direct experience in that area. If that's OK, then I'm happy to try and help.

Thanks,

Ben

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Re: Wind Tunnel Issues

Postby bfinio » Mon Mar 30, 2020 2:32 pm

Hi Joey,

Sorry for the delay in responding. Unfortunately, even under normal circumstances without the COVID-19 situation, there's no way I'd be able to adequately review a 196 page document in sufficient detail. We are checking to see if any other Science Buddies volunteers would be able to do it, but it will be difficult to find someone with enough spare time to read a document of that length. We will get back to you if we're able to find someone, but I think it would be best to start pursuing other options for now.

For the record, I just checked and my PhD thesis was exactly 196 pages! That is quite a coincidence!

Please keep in touch, if you do have other shorter questions, I may still be able to help out.

Thanks,

Ben

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Re: Wind Tunnel Issues

Postby GouldJoseph » Mon Mar 30, 2020 2:43 pm

Ben,

I completely understand! Thank you for helping me throughout my entire project! If it weren't for your guidance, I don't think I would have gotten this far. What topic was your PhD thesis on? I am curious because you said that you got your PhD in Robotics so I am intrigued about what specifically in robotics you focused on. I will keep you in the loop and will let you know if I have any more (shorter) questions. Have an amazing rest of your week and try to stay safe!

Thank you again for all the help!
Joey


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