Learning Forum

[Emine Goktepe]

Hello,

 

I have been running particle tracking simulatons in both |CFX and Fluent. I am applying almost same conditions for both cases (there are few different settings between two software but I am setting them similarly with workaround methods). However, the particle tracks I get from CFX is always different from Fluent. I want to give example of two basic cases. The first one is a rectangular pipe and simulation details are:

	CFX	Fluent
One-way/two-way particle coupling?	Fully Coupled	Fully Coupled
#of mesh	40000	40000
Mesh type	CFX	Fluent
Particle shape	Spherical	Spherical
Particle material	DOS (liquid)	DOS (liquid)
Particle Diameter (Cunningham slip corr:/ 1)	3um	3um
Geometry	Same	Same
Boundary Conditions	Same	Same
Injection	Inlet/ surface/ uniform	Inlet/ surface /uniform
Flow model	Laminar/no turbulence model added	Laminar/no turbulence model added
# of particles tracked	100	100

I am using here Cunningham slip correction in ansys and dividing the drag coefficient with slip correction factor in CFX. Actually I used all possible combinations(selecting sphericalin ansys and schiller neuman in cfx... all of them). But Ansys shows no trapped particles while CFX shows trapped ones. (second pic)

To be sure about I applied another basic case elbow. Again here I used the same parameters. You can see the resulting behavior for both fluent and CFX. While ansys resulting no particles are trapped on boundaries, CFX results shows too many particles are trapped. I know the cfx case is correct. The only difference between these two set up is boundary absorption coefficient in CFX. I am assuming all of the particles which touch to the boundary should be absorbed immediately (assigned this value 1). For fluent, I couldnt find how to assign this value. In the boundary conditions in fluent, there is only a selection for reflect,trap and escape. Even though I select trap, I can not have the same result.  I feel somehow I am missing a step. 

On the other hand, when I increase the particle size in FLuent, I start to see some trapped particles, however in that case I can only see the particles trapped to the corner because of the impaction. I tried to enter a UDF in Ansys which assumes the nodal velocity on the boundary is zero(for the 3um case), this time again I only see the particles trapped because of impaction. 

If you look at the result pictures carefully and compare them, you will see fluent particle paths are straight while CFX are not. 

Is there anyone who has an idea what is going on here?

 

[Prashanth]

Hi,

Is the particle per parcel count same in both the setups? 

[Emine Goktepe]

 

Hello,

 

Thank you for the reply. I tried to run these simulations in both coupled and one way. In both cases I tried to set everyting same. In both cases for particles, I only defined the total mass flow rate for particles and the number of the particles to be tracked.I also defined both uniform and random relesing points. In Ansys Fluent, I do not select unsteady particle tracking, so software interface do not let any change on “parcel” (please see the picture below, I am still making trials so the diameter is different in the picture). I didnt set in CFX anythink like this neither. 

I checked the reports but couldnt see any information about it. If you please describe how to find the parcel properties, I will try to check that. 

One other thing I tried during the time waiting for an answer, I set the boundary nodal velocity as zero, in that case I got some of the particles in fluent trapped. 

 

 

 

 

 

[Prashanth]

What about stochastic tracking? Are you using discrete random walk model?

[Emine Goktepe]

Hi,

 

No, I dont use probabilistic model. 

 

Lastly I enabled energy equation and brownian motion, but still no trapped particles unfortunately.  I re-checked the boundary conditon for wall. It is selected as trap, no problem.

What do you think should I employ stochastic model?