Fluids

Boundary Condition of Ffowcs Williams Hawkings model

• Gorilla Koala
Subscriber

Hello

I'm calculating sound generated by water flow in pipe by using LES and Ffowks Williams Hawkings model (FW-H model).

My question is what boundary condition does FW-H model use?

The theory guide mentions that the model is applicable to the far-field acoustic wave and acoustic propagation toward free space. Does it means each source grid is a point source? The pipe wall isn't the boudary condition such as reflection?

Also I'm a little bit confusing. Can we change the boundary condition of FH-W model by using the Boudnary Acoustic Wave Models?  https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v212/en/flu_ug/flu_ug_sec_nrbc.html

Thank you.

• Federico Alzamora Previtali
Subscriber

In general, the FW-H model is not applicable to internal flow acoustics such as noise propagation in ducts, but you could have a case where the internal flow is your source and you will be using FW-H to model sound propagation downstream in an external domain.
FW-H solution assumes that the source region radiates sound through a free field environment with no obstacles between the source and receivers. Hence, FW-H will be less sensitive to BCs outside of your source region.

• Gorilla Koala
Subscriber

Thank you so much for your responce. I still cannot get completely. Even it calculates acoustically far-field, doesn't it need boundary condition? Does free field become the boudary condition?

FH-W model assumes free field, does it mean the wave is radiated uniformly to all the direction like point source?

Thanks!

• Federico Alzamora Previtali
Subscriber

The boundary conditions will depend on the model and application that you are solving for. The FW-H model solution is developed as integral terms of the acoustic pressure from the integration surfaces surrounding the source region. Outside of this source region, yes the sound is radiated through a uniform medium. Because of this, you can even set up receiver outside of your simulation domain.

• Gorilla Koala
Subscriber

I see… There’re something that I still don’t understand. In case, what does decide the wave number? It means if you place the several receivers in same r with different direcections in the figure, they show same wave?