Bubble column Substracting the drag on the liquid phase

sarrazs · October 2, 2019, 10:47

Hi,

I'm setting up a multiphase simulation using CFX for a bubble column. I set the water as the continuous Fluid and the bubbles as the dispersed fluid. I want to set the interfacial forces acting on the liquid phase to zero. I know it is not accurate but it is for a verification purpose. I put the drag = 0 and I added a drag force by a user expression as a Momentum source for the bubbles, the simulation didn't work. Than I kept the drag and I substract it by a user function Momentum source on the liquid. The simulation worked but it the liquid didn't stay stagnant. I may be doing it wrong.
Is there any other way to do it??

Opaque · October 2, 2019, 12:06

How did you add the momentum source?

Can you show your setup for the momentum source only?

sarrazs · October 2, 2019, 12:57

I added a subdomain, I put the location as the whole domain, then in the fluid Sources, I selected Water - Momentum Source - General Momentum Source and in x,y,z component I put the drag force components as for the x component -(3/4)*air.vf*water.density*sqrt(UrU^2+UrV^2+UrW^2)*CD/air.mean particle diameter *(air.Velocity u-water.Velocity u)

UrU = (air.Velocity u-water.Velocity u)…
and CD is equal to 0.44 because in the fluid pair model I put the drag coefficient = 0.44 for simplification .

Opaque · October 2, 2019, 13:07

How much source/linearization coefficient are you using?

sarrazs · October 2, 2019, 14:03

Quote:

Originally Posted by Opaque

How much source/linearization coefficient are you using?

I did not use a momentum linearization coefficient. Is there any default value used by CFX for the drag?? and what is the best value? or how can I choose the right value?

Thank you very much

Opaque · October 3, 2019, 08:25

You need to linearize such a source term; otherwise, it will only converge for a very small timestep (if it ever converges).

The linearization coefficient is the derivative of the source term respect to the variable being solved for, say velocity u/v/w, in the equation.

In the case of the momentum equation, it should be a vector, but since a single linearization entry is provided (isotropic linearization), you must pick the most conservative value, that is, the maximum between any of the 3 possible derivatives.

October 2, 2019, 10:47	Bubble column Substracting the drag on the liquid phase	#1
sarrazs Member cfxtwophaseflow Join Date: Aug 2017 Posts: 46 Rep Power: 8	Hi, I'm setting up a multiphase simulation using CFX for a bubble column. I set the water as the continuous Fluid and the bubbles as the dispersed fluid. I want to set the interfacial forces acting on the liquid phase to zero. I know it is not accurate but it is for a verification purpose. I put the drag = 0 and I added a drag force by a user expression as a Momentum source for the bubbles, the simulation didn't work. Than I kept the drag and I substract it by a user function Momentum source on the liquid. The simulation worked but it the liquid didn't stay stagnant. I may be doing it wrong. Is there any other way to do it??

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October 2, 2019, 12:06		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	How did you add the momentum source? Can you show your setup for the momentum source only?

October 2, 2019, 12:57		#3
sarrazs Member cfxtwophaseflow Join Date: Aug 2017 Posts: 46 Rep Power: 8	I added a subdomain, I put the location as the whole domain, then in the fluid Sources, I selected Water - Momentum Source - General Momentum Source and in x,y,z component I put the drag force components as for the x component -(3/4)air.vfwater.densitysqrt(UrU^2+UrV^2+UrW^2)CD/air.mean particle diameter *(air.Velocity u-water.Velocity u) UrU = (air.Velocity u-water.Velocity u)… and CD is equal to 0.44 because in the fluid pair model I put the drag coefficient = 0.44 for simplification .

October 2, 2019, 13:07		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	How much source/linearization coefficient are you using?

October 3, 2019, 08:25		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	You need to linearize such a source term; otherwise, it will only converge for a very small timestep (if it ever converges). The linearization coefficient is the derivative of the source term respect to the variable being solved for, say velocity u/v/w, in the equation. In the case of the momentum equation, it should be a vector, but since a single linearization entry is provided (isotropic linearization), you must pick the most conservative value, that is, the maximum between any of the 3 possible derivatives.