Wind Tunnels: Exploring Operation Principles and Test Rhombuses

[alchemist]

Guys,

i need some help on wind tunnels here. Basically i need to find out more on the operation principles behind wind tunnels.

Basically, wind tunnels provide simulation environment of high speed (transonic) flows by mimicking the reynolds number on the actual flow, because of dynamic similarity (i hope i got this right).

1. My question is, how exactly does wind tunnel achieves this reynolds number?

by looking at the equation: Re = (density*velocity*dimension)/dynamic viscosity.

so to achieve a desired reynolds number, or to attain a maximum possible reynolds number, we would only have to adjust the components that make up Re? eg: changing the medium of fluid flow? presurrizing the air to gain density?

2. The next question is about 'test rhombus'. Its been mention in many articles that a test rhombus is placed in the testing section of a wind tunnel to be tested for aerodynamic performance. Is this 'test rhombus' a generic term for arifoil sections that are to be tested? or its literally a double wedge (rhombus) airfoil?

also, is there a different 'test rhombus' for different wind tunnels?(low speed, transonic or supersonic,hypersonic)

thank you!

regards

 

[Cyrus]

Hi Alchemist, I'll answer your questions for you.

Guys,

i need some help on wind tunnels here. Basically i need to find out more on the operation principles behind wind tunnels.

Basically, wind tunnels provide simulation environment of high speed (transonic) flows by mimicking the reynolds number on the actual flow, because of dynamic similarity (i hope i got this right).

Sort of.

1. My question is, how exactly does wind tunnel achieves this reynolds number?

by looking at the equation: Re = (density*velocity*dimension)/dynamic viscosity.

so to achieve a desired reynolds number, or to attain a maximum possible reynolds number, we would only have to adjust the components that make up Re? eg: changing the medium of fluid flow? presurrizing the air to gain density?

While some tunnels do just as you said: pressurizing the air or changing the working fluid, the majority do not. They are what is known as 'distorted' in Reynolds number. The underlying assumption is that the plot of drag coefficient vs Reynolds number on a log scale is more or less flat. This isn't true, and makes scaling from a model to a prototype tricky. Usually the drag and pitching moment is the most sensitive to Re. The Re number will always be off by the inverse of the model scale ratio if it is unpressurized and has air as the working fluid. We usually assess the degree of variation (for an entire aircraft configuration) based on the characteristics of the airfoil section data. This is obviously an assumption, but fairly accurate.

2. The next question is about 'test rhombus'. Its been mention in many articles that a test rhombus is placed in the testing section of a wind tunnel to be tested for aerodynamic performance. Is this 'test rhombus' a generic term for arifoil sections that are to be tested? or its literally a double wedge (rhombus) airfoil?

also, is there a different 'test rhombus' for different wind tunnels?(low speed, transonic or supersonic,hypersonic)

thank you!
regards

I never used one when I did work in our Low Speed tunnel (7'x10') test section

www.windvane.umd.edu

This might be something for transonic tunnels. Keep asking questions, because I have lots of answers, its an interesting subject.

 

[alchemist]

Thank you Cyrus!

Here's more questions from me,

1. you mentioned that

"They are what is known as 'distorted' in Reynolds number. The underlying assumption is that the plot of drag coefficient vs Reynolds number on a log scale is more or less flat. This isn't true, and makes scaling from a model to a prototype tricky. Usually the drag and pitching moment is the most sensitive to Re. The Re number will always be off by the inverse of the model scale ratio if it is unpressurized and has air as the working fluid. We usually assess the degree of variation (for an entire aircraft configuration) based on the characteristics of the airfoil section data. This is obviously an assumption, but fairly accurate."

I'm not sure i get what you mean here. Are you saying that apart from altering reynolds component of the flow, we could 'distort' the number? How would we do that?

How about given a situation where we would have to maximise the Reynolds number in a transonic wind tunnel, how could we go about doing that other than changing the flow medium (to cryogenic tunnel) or pressurizing the gas?

2. I've tried looking through several textbooks and the internet on test rhombus.

As a matter of fact, i have absolutely no idea what a test rhombus section is, what it is for or even how it looks like. My initial thought was that its just an experimental airfoil which is to be tested. (ignorant, really)

But unfortunately all I've manage to get is that its part of a shock-expansion cancellation technique used in the diverging part of a supersonic nozzle where the is brought back to free stream direction.

I'm thinking that this rhombus section is something (a physical object or a segment of the tunnel?)that will cause/create shocks in the wind tunnel ahead of the testing section in order 'force' the airflow to converge into an uniform axial flow before reaching the airfoil section to ensure accurate results? that could be why the section is not used in subsonic speed tunnels, is this correct?

regards,

 

[Cyrus]

1.
I'm not sure i get what you mean here. Are you saying that apart from altering reynolds component of the flow, we could 'distort' the number? How would we do that?

No, what I'm saying is that it is impossible to match the Reynolds number in an air driven unpressurized wind tunnel; therefore, it is distorted in Reynolds number (you cannot have dynamic similitude in Re). Note that it does match in Mach number and Geometric scaling, but for Ma < 0.3 (incompressible flow) you want to match Re first and foremost. So while its nice that Ma matches, it really doesn't help.

How about given a situation where we would have to maximise the Reynolds number in a transonic wind tunnel, how could we go about doing that other than changing the flow medium (to cryogenic tunnel) or pressurizing the gas?

In transonic tunnels, they are special built for high speed flow. Sometimes you can change the working fluid, sometimes not. Pressurizing the gas is very, very hard to do because the facility required to withstand the pressures are extremely expensive.

2. I've tried looking through several textbooks and the internet on test rhombus.

As a matter of fact, i have absolutely no idea what a test rhombus section is, what it is for or even how it looks like. My initial thought was that its just an experimental airfoil which is to be tested. (ignorant, really)

I'll ask the director of our wind tunnel about this, he will know.

So what you need to get out of Reynolds distortion is this: Reynold number is all about the boundary layer. The distortion means the transition and separation points between laminar and turbulent flow will not exactly match the full size prototype. So to account for this stall strips are sometimes placed on the wing at locations the engineers expect stall to occur on the full size airplane. This ensures the transition location is more to scale, and extrapolation to a full size airplane is justifiable. You can see if strips are needed by varying the Reynolds number (velocity) of the wind tunnel. If the drag coefficients are flat over a wide range, it means the transition has probably developed pretty early on and is consistent. If, on the other hand, the drag starts varying wildly, you have a big problem on your model.

Wind tunnel testing is really still an art form as much as it is a science. There is a tremendous amount of subtitles that you need to know when you work in the tunnel. Even things like how the data is measured on the force balance is not as direct as you would expect. I.e. you don't just measure whatever force it reads out and write that down: it uses statistical convergence criteria. It's a really really interesting subject.

 

[anvesh111]

Hi alchemist
the rhombus test sections are used for supersonic speed,the problem with supersonic speed are the bow wave(type of shock wave) which form at the leading edge of the wing causes an high pressure there resulting in pressure drag,the following link is about the supersonic design,just have a look you might have an idea

http://home.scarlet.be/comicstrip/Supersonic%20Wing%20Design_files/Supersonic%20Wing%20Design.htm .

For the uniform fluid flow there is a principle called kutta condition