Is there a tutorial on acoustic in CFX! If CFX claims it can simulate acoustic flows then they should provide some sort of example on it!

Usman

Hi,

Talk to your support office. They will have some examples.

Also, an acoustic flow is just one with pressure waves. This is such a wide class of flows that almost any flow would fall into it - for instance my IC engine modelling work can be considered acoustics in a broad sense.

Glenn Horrocks

Glenn i have been looking through your work but i didnt find the chapter on acoustics. What chapter is it in your theses?

Usman

I have recently started my PhD and I am still using academic version of CFX. CFX does not help you unless you are registered user of CFX (i.e you have research license for cfx). Are you aware of any acoustic tutorial which i can use to learn how to set up an acoustic problem in CFX. I am particularly intertesed in prediction of far field noise.

Also how do we set up the boundary conditions for acoustic problem once we have obtained LES solution. I suppose solver has to solve light-hill equtions to get the pressure fluctuations. How do we set up that.

Thanks Usman

CFX does not solve Lighthill equation. There is no tutorial on how to setup acoustic problem. You can use either LES, DES, or SAS to predict near-field noise directly. If you want to predict far-field noise, you probably have to export CFX calculation result done in the near-field into 3rd party software, e.g., SYSNOISE, to do acoustic calculation.

Hi longbow

I am really confused about whole issue right now. I thought CFX was able to predict noise (pressure fluctuations) because they have documentation on it in help section (its not much of help though). Let me get this straight then. CFX can not solve Lighthill equations and thus is not able to predict far field noise. However, we can predict near field noise (i.e pressure fluctuations) which involves solving compressible Navier-Stokes equations and then we can use SPL equation to calculate pressure level. Are you sure that is all we can do in CFX?

Hi,

You are correct. For typical acoustics work CFX is used to predict the pressure waves in the near field using LES or DES and linked to a package such as SYSNOISE to do the far field acoustics.

As for my thesis - I was not studying acoustics so there is no discussion of acoustics in there, but it is an example of a prediction of pressure waves, and this could be used to predict near field noise.

Glenn Horrocks

How exactly did you predict pressure waves. I have following questions with regards to this.

1. I understand that compressibility of fluid can cause acoustic waves in the flow. However, what is the cause of these acoustic waves? Is it fluctuations in velocity field or density field.

2. More importantly, how do we set up near-field noise prediction problem in CFX-Pre. Do we make the denisty function of pressure or we set up user points to take pressure readings at each time step. In either case how do we do it in CFX-Pre.

3. How small my time step should be or how would you calculate it.

Thank you very much. I really apprecaite your help. Usman

Hi,

It looks like you need to do some basic research into acoustics - these are pretty basic questions. Visit your library and grab any decent acoustics textbook.

To answer your questions: 1) The pressure, velocity and density waves are coupled together. They all move as the waves pass. 2) Talk to CFX support about setting up an acoustics simulation. You will probably want to link to SYSNOISE for the far field and you will probably need their help to get the link working. 3) I would start with around 0.1 of the period of the highest frequency wave you wish to resolve and do a timestep sensitivity study from there.

Glenn Horrocks

Thank you very much for your help Glenn. I was in fact doing back ground reading on acoustics. I am planning to do some reading before i make an attempt to set up an acoustic problem. Thanks

Regards Usman

[flyingd]

everyone!I am doing the problem on acoustic in CFX.
Who has the tutorials to help me. I want to understand it.
Thinks!
Regards best!

[flyingd]

Hello:longbow
I want to know how to calculate near-field noise using LES, DES, or SAS in CFX.
Please help me!!!

[flyingd]

Hi,Glenn Horrocks
I want to ask for near field noise problem. Do you know how to simulate it in cfx? Expericially, how to deal with the *.cgns file? Using CFX post ?
Please help me!

[ghorrocks]

In concept it is very easy - set up the flow and model the feature which is causing the noise generation. It will generate pressure waves and this is the noise. You then can do acoustic analysis on the pressure wave signal to determine dBa, frequency, or couple to acoustic packages like sysnoise to work out far field conditions. Simpel in concept, but actually doing this simulation so it does not require a super computer and is accurate will not be simple.

Glenn

[flyingd]

Hello:Glenn[/QUOTE]
Thanks your help!
But I don't know how to do acoustic analysis if I have the *.cgns file.
Please tell me detailly.
At the same time, I don't know weahter acoustic analysis can be completed in cfx post.

[ghorrocks]

I am not going to give you a lesson on acoustic modelling. That is why they have universities and libraries full of books on the subject. I suggest you ask you local acoustics professor or get an acoustics modelling textbook.

Glenn Horrocks