CFX Momentum Inputs Question

gsmith · September 27, 2013, 13:50

Hi all!

I am new to using CFX and a novice with CFD analysis. I'm working on a problem with a moving tank. To simulate the movement of the tank in the fluid, I am trying to input momentum source information. The momentum info I am using was calculated from Reynolds Transport. However, for a general Reynolds Transport in Momentum, the units come out to be kg*m/s. The CFX Momentum input asks for a momentum source coefficient with units kg/(m^3 s) and a vector with units kg/(m^2 s^2). How do I go from a general momentum case to the needed values/equations for the CFX momentum input?

Thank you,

G. Smith

evcelica · September 27, 2013, 17:09

Don't use the momentum source coefficient. That is for making it a function of velocity.
Just use the Component option, which has units of kg/(m^2 s^2) = momentum per unit volume per second = (kg*m/s) / (m^3 s)

gsmith · October 21, 2013, 10:46

Thank you for the reply!
The per unit volume makes sense if I divide the general momentum case by the total fluid volume. But I still don't understand how to go from general momentum to the per time. Is the general momentum divided by the timestep? or by the overall time of the run?

September 27, 2013, 13:50	CFX Momentum Inputs Question	#1
gsmith New Member G. Smith Join Date: Sep 2013 Posts: 3 Rep Power: 12	Hi all! I am new to using CFX and a novice with CFD analysis. I'm working on a problem with a moving tank. To simulate the movement of the tank in the fluid, I am trying to input momentum source information. The momentum info I am using was calculated from Reynolds Transport. However, for a general Reynolds Transport in Momentum, the units come out to be kg*m/s. The CFX Momentum input asks for a momentum source coefficient with units kg/(m^3 s) and a vector with units kg/(m^2 s^2). How do I go from a general momentum case to the needed values/equations for the CFX momentum input? Thank you, G. Smith

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September 27, 2013, 17:09		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,167 Rep Power: 23	Don't use the momentum source coefficient. That is for making it a function of velocity. Just use the Component option, which has units of kg/(m^2 s^2) = momentum per unit volume per second = (kg*m/s) / (m^3 s)

October 21, 2013, 10:46		#3
gsmith New Member G. Smith Join Date: Sep 2013 Posts: 3 Rep Power: 12	Thank you for the reply! The per unit volume makes sense if I divide the general momentum case by the total fluid volume. But I still don't understand how to go from general momentum to the per time. Is the general momentum divided by the timestep? or by the overall time of the run?