Angle of the contraction cone in an open circuit wind tunnel 1

[Alen94]

Hello everybody

My name is Alen and I am currently building a wind tunnel as my complementary work in school. For most of my plannings and designs I considered the book "Low-Speed Wind Tunnel Testing".
During the plannings of the contraction cone, I encountered a blur: It is written that the area ratio ought to be in the range of 7-12.
But what about the lenght (and its resulting angle) of the contraction cone? Is there an effect on wind-speed or flow similarity or can I just neglect the lenght/angle of the contraction cone? And how can this effect be reasoned on aerodynamical basis? I've been searching for a long time, to find some more information on that, but there was no chance: I checked various books and my mentor has no idea as well.

Just to be sure you understand what I mean: Here is an extract of the 3D-model of my tunnel. What about this angle?



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Thank you very much for your help!


Best wishes

Alen

 

[berkshire]

Alen 94
For the most part Students are expected to do their own homework, however since this looks like more than homework, I would recomend that the angle be no more than 15 degrees, or you may have flow separation problems where the cone joins the straight portion of your duct.
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[rb1957]

i doubt i'd go with a design as shown ... i think the test section is Way to close to kink.

a straight sided inlet is easy to build but will have issues in getting a uniform test section flow. i'd use something like a parabola, gently leading into the test section, or at least a radius blend on the kink.

 

[Alen94]

Good Day and thank you very much for answering so quickly !

@berkshire: Is there a way to reason the angle of 15 degrees? I've seen a lot of tunnels with angles in this area, but why? Is there a formula to explain that? I would really like to understand the relation between the angle and its aerodynamic effects. I know that the flow may seperate, but how can i predict at which angle? Thank you very much!

@rb1957: I've seen some tunnels constructed like this, and people also confirmed its flow similarity. I will try it like shown i think and if i experience any problems i will bring the kink a bit down to a round figure. Thank you anyway!


Best wishes

Alen

 

[IRstuff]

Have you looked at:

https://moodle.polymtl.ca/file.php/1047/JEROME/Contractions/Bell_1988.pdf

? There appears to be some sort of "sweet spot" relative to the cone length.

TTFN
faq731-376

 

[rb1957]

do you hve access to a CFD code ? if so, i'd baseline the code by analyzing some known designs and comparing analysis with experiment.

in IR's ref (gotta be woth an LPS), fig 13 shows the profiles considered; note the smooth transition into the test section. The effectiveness of your simple design will depend on the test spection velcity, ie the Reynolds number of the flow over the kink.

 

[Compositepro]

The science behind a vena contracta nozzle is what you want to study. The purpose of the cone is to convert pressure difference to fluid velocity while lessening energy losses due to turbulence and sudden changes in flow direction. Too long, or too short, a cone introduces drag. Your best bet is to simply find pictures of wind tunnels and copy the design. All designs involve numerous compromises.

 

[berkshire]

Alen94,
In a ducting system which is essentialy what you are designing, an abrupt change in direction of more than 15 degrees will make the flow detach itself from the wall of the duct creating a Vena Contracta at that point. Think stall on an aircraft wing,wery few airfoils will maintain attached flow at angles greater than 15 degrees. If you can make the entrance cone curved like the mouth of a trumpet, you can do that with 4 flat sides but curve the pieces. then you will have better results.
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----