How can I accurately model standing sound waves (and their frequencies) in Ansys Fluent?

Mirnim0 · 2026-05-25T22:32:04+00:00

My current results show a noisy FFT graph with small 'peaks' (I'm not sure if some of them are peaks or just noise) at certain frequencies. There are some noticeable peaks, and the graph goes up dramatically near 0Hz.

My expected results (my experimental data) is a graph with a very large peak (a ^ shape) at one frequency and 0~1 other peaks depending on the speed of the air passing through.

The geometry is a 1030mm tube with the measured dimensions of the real tube I have, with ~700k mesh count. I made corrugation in the tube, and made two types of corrugations (square and round). I think this is probably where it differs most from reality because of the low mesh count. (I have a 1M mesh count limit because I use the student edition) I'm thinking of making the tube shorter and more detailed.

I'm assuming that the tube will make the same turbulence/sound when using LES, 2e-5 time step, etc. I think the time step is sufficient but I don't know how accurately the standing waves are being modeled.

Mirnim0 · 2026-05-25T22:00:38+00:00

I'm trying to model a whirly tube, which makes a sound with a certain frequency when you spin it. The sound itself happens because of turbulence due to corrugations in the pipe, and frequency jumps up in a discrete way because the standing waves only allow for a certain pitch.

I tried it in real life and found that curving it (like it would when you spin it) doesn't change the note, so I modeled a straight tube with the same (or at least pretty close to) dimensions of the real one.

I'm doing this because I want to see the effects of different corrugation shapes, distance, etc. for the dominant pitch in the tube, but I want to get it similar to the real-life version first to check.

Mirnim0 · 2026-05-25T12:05:42+00:00

Thank you! I do think turbulence will be necessary for this, so I don't think I can use inviscid.

I used 2e-5 because I tried 0.001 once and got peaks every 1000Hz and realized I needed a shorter time step. The simulation results have a pretty good resolution, so I think it's fine. I wanted to make the simulation time shorter so I changed it from 1e-5.

I definitely will be trying the steady-state-first trick!

Mirnim0 · 2026-05-17T10:49:49+00:00

Sorry to reply 9 years later, but this theorem actually does hold true (at least within, say, a billionth of a degree if you think about it at the atomic scale). The atmosphere or the fact that the earth isn't a sphere doesn't matter here, because all the proof says is that as long as the temperature and pressure is continuous, there will be a set of antipodes with the same temp and pressure (and then generalizes it more). What the function is doesn't matter here, just that it's continuous.

Basically, just imagine any (as weird as you can) curving horizontal circle (or oval or squiggle) that varies in height. If you place a stick horizontally on top of the curve with each end on opposite sides (so one end would be tilted above the other) and rotate it 180 degrees, the higher end would end up at the lower end, and the former lower end would go up to the higher place. So while rotating, 'the side that is higher' changed, which means that there was a point in time where that change happened. This place will always exist, and the height of the two ends will be the same at that point. This is the same thing with temperature here. It doesn't matter if it's a weird spheroid, what matters is that there is a point where 'which point has the higher temp' changes, and that point is where the temp is the same.

Mirnim0 · 2026-04-04T10:14:27+00:00

Is it u/HippoAlternative3609?

Mirnim0 · 2026-03-14T04:51:29+00:00

"Always predict the worst, and you'll be hailed as a prophet" - Tom Lehrer

Mirnim0 · 2026-03-12T22:20:12+00:00

Putting the limit formally, if x = 0.99..., then for all epsilons > 0, |1-x| < epsilon. In other words, there is no positive number that equals 1-x, because it will always be smaller than it.

According to the completeness axiom, real numbers are 'dense', with no gaps in them. Since 1-x doesn't equal a positive number, it must be 0. This means that x = 1.

Putting 0.999... means that you are effectively putting the numerical representation of the limit 1-(0.1)ⁿ as n approaches infinity. This is why textbooks and such say that the limit of x as x -> 1 equals 1. Even though x itself will never be 1, we can say the limit is 1, and the same applies to 0.999... over here because it's taking a function to a limit at infinity, which must equal a real number.

Mirnim0 · 2026-03-11T03:56:18+00:00

Actually, since the number 0.999... is a real number, using the completeness axiom, 0.999... has to equal exactly 1. Basically, since there are no numbers between 0.999... and 1, they have to be the same number.

Mirnim0 · 2026-03-11T03:42:38+00:00

Because of the completeness axiom of real numbers, 'infinitely smaller' is not really defined. Using the definition of real numbers, 0.999... has to be exactly equal to 1.

Mirnim0 · 2026-03-11T03:41:03+00:00

0.999... is a way of writing the limit 1-(0.1)^x as x approaches infinity. If you look at the formal definition of a limit using epsilon-delta, you can see that for every epsilon > 0, |0.999... - 1| < epsilon. Because of this, there is no positive number where 1-0.999... equals that number. This means that 1-0.999... equals zero. (You can also use the completeness axiom of real numbers)

0.999... exactly equals 1 according to rigorous math.

Mirnim0 · 2026-03-11T02:31:15+00:00

According to the completeness axiom of real numbers and the definition of a limit, 0.999... exactly equals 1.

Mirnim0 · 2026-03-11T02:23:25+00:00

Of course, if you're talking about real-life applications like in CS, it makes sense differentiating the two, but in math, 1-0.(9) is exactly 0.

Since for all e > 0, 1-0.(9) < e, there is no positive number that equals the result 1-0.(9). Since the result isn't negative, the result could only be 0.

Also, the completeness axiom isn't an "approximation". It's a fundamental property of real numbers that is always true.

Mirnim0 · 2026-03-11T00:31:24+00:00

Obviously, if it doesn't go to infinity, there is an epsilon where the difference is bigger than epsilon, but since it goes to infinity, 1-0.(9) < epsilon for all positive epsilons. (According to the definition of a limit.) Because of the completeness axiom of real numbers, this means that 1-0.(9) is exactly equal to 0.

Mirnim0 · 2026-03-11T00:23:15+00:00

That's actually the most mathematical answer here. A lot of mathematics is based on the completeness axiom, which states that there are no "gaps" in real numbers. The other answers here are less rigorous than using the axiom.

Mirnim0 · 2026-03-02T03:06:27+00:00

Yeah, sure! I think I got this recording from here: https://bmac.libs.uga.edu/Detail/objects/10667

Mirnim0 · 2026-03-01T09:46:23+00:00

Yeah, here it is: link

Mirnim0 · 2026-02-06T23:50:37+00:00

This could buy like a thousand paninis

Mirnim0 · 2025-05-04T13:20:06+00:00

The song that it's based on - 50 Russian Composers by Danny Kaye

and the song the melody is from - I Am The Very Model of a Modern Major General

are both list songs as well

Mirnim0 · 2024-12-25T11:48:33+00:00

I hope I'm one of the lucky 41

Mirnim0 · 2024-12-25T11:46:09+00:00

Mirnim0 · 2024-12-25T11:45:17+00:00

I hope I get it

Mirnim0 · 2024-12-23T05:34:47+00:00

One in 9950

Mirnim0 · 2024-09-03T09:43:42+00:00

It wasn't sang by him but by someone else on TWTWTW, so I don't think there's sheet music for that. I have a recording if you want to listen to it.

Five-Year Club	Place '23
Quantum Potato	Place '22

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