Hi all! I creat a fluid sub-domain in CFX Build. When I press "Bulk Sources" and enable "Momentum Source", momentum along three direction can be found. I'm confused by the unit of the momentum, [kg m^-2 s^-2]. It also can't be changed into another one. Is anyone kind enough to let me know what it means?

Miko

Is it just the momentum per unit volume, as you are applying the source term to a subdomain volume ? Bob

Hi Bob!

But if it means the momentum per unit volume, it should be [kg m^-2 s^-1}, I think.

Miko

Hi Miko It means Force per unit volume.

Thank you all! I think Zahid is right! [kg m^-2 s^-2] means force per uint volume, such as: (density)*(gravitational acceleration), (electric current density)*(magnetic field). Their dimensions are both equal to [kg m^-2 s^-2].

Miko

Hi Miko, yes you are right (or Zahid is), I was missing the seconds out. So would it be the same as saying Momentum per unit volume per second ? Bob

Hi Bob Yes you can say it rate of change of momentum per unit volume, and according to Newton's second law of motion it is equal to force per unit volume. Zahid

Yes, I agree with Zahid. In fact, momentum=force*time

most probably, it can be presure per unit length, cause, if you simplify the unit of pressure [F/A] it becomes kg/(m.s^2). check it out if my suggestion gives sound to what you are doing

bye

it means temporary change of momentun per unit of volume, analyze it and you'll see.

[ama294]

In fluid, it is the unit of the specific weight (gamma = rho . g)