# Velocity field with phase
**URL:** https://bempp.discourse.group/t/velocity-field-with-phase/245
**Category:** Applications
**Created:** [12 March 2024 19:15 UTC](https://bempp.discourse.group/t/velocity-field-with-phase/245 "2024-03-12T19:15:09Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@soundintubes](https://bempp.discourse.group/u/soundintubes)
#### Post date: [12 March 2024 19:15 UTC](https://bempp.discourse.group/t/velocity-field-with-phase/245/1 "2024-03-12T19:15:09Z")
Hello, apologies in advance but I am an engineer not a mathematician(!)
I’m working on an acoustics problem of a cube with one face radiating.
I based it on this tutorial: [https://nbviewer.org/github/mscroggs/bempp-acoustic-tutorials/blob/main/tutorials/5\_loudspeaker.ipynb](https://nbviewer.org/github/mscroggs/bempp-acoustic-tutorials/blob/main/tutorials/5_loudspeaker.ipynb)
It works quite nicely, but now I would like to try making the opposite face radiate with an amplitude and phase difference to look at the directivity patterns and generally play around with it.
I have my tagged mesh set up with different IDs for the different faces.
How do I set up the two velocity fields in Bempp with different amplitude and phase?
How do I set up the solver with the multiple spaces?
Thanks!
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### Author:  [@soundintubes](https://bempp.discourse.group/u/soundintubes)
#### Post date: [14 March 2024 15:10 UTC](https://bempp.discourse.group/t/velocity-field-with-phase/245/2 "2024-03-14T15:10:39Z")
I worked this out.
The velocity field is defined in this function:
@bempp.api.complex\_callable
def u\_total\_callable(x, n, domain\_index, result):
if domain\_index == 2:
result[0] = n[2];
else:
result[0] = 0
To add another field to another domain, I included an elif for that domain index/tag.
The complex values for each field can be entered as required.
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### Author:  [@Kevin](https://bempp.discourse.group/u/Kevin)
#### Post date: [7 October 2024 05:23 UTC](https://bempp.discourse.group/t/velocity-field-with-phase/245/3 "2024-10-07T05:23:25Z")
I am an engineering student working on a similar problem. I have a few questions and It would be really helpful if we can connect. Could you please suggest a way for me to contact you?
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### Author:  [@soundintubes](https://bempp.discourse.group/u/soundintubes)
#### Post date: [7 October 2024 10:24 UTC](https://bempp.discourse.group/t/velocity-field-with-phase/245/4 "2024-10-07T10:24:40Z")
Hi Kevin, post your questions here.
I’m not an expert in the field, but there are experts on this forum.