Learning Forum

[Simon]

Hello,

I am using Fluent to simulate a spherical particle impacting vertically air-water interface. The partcle experiences forces like gravity, buoyancy, drag force, and surface tension. I am using VOF and 6DOF to simulate the air and water flow fields and track the motion of the particle. I am wondering:

1. if surface tension force on the sphere can be reported at each time step and how to do that. 
2. Is surface tension included in the drag force and drag coefficient? I tried to save the drag and the force in the vertical direction ((1,0, 0) direction as shown in the second figure from the force report definition), and I found that they were exactly the same at each time step. I thougth the force in X direction meant the total force, which included drag and surface tension, etc. So, is the force in the report definition just drag?

[Vishesh Aggarwal]

1. There is no direct variable to report the surface tension forces. However, you can write an expression for surface tension forces using the "smoothed VOF gradient components" available under Variables > Phases... category. You may refer Eq. 14-24 for CSF model under Fluent Theory Guide 2025R1 > Sec. 14.3.9.  for reference of the surface tension force formulation based on VOF gradients.
2. Forces reported from the report panel do not account for surface tension forces explicitly. They are based on pressure and shear stresses on the walls.

[Simon]

Hello Vishesh,

Thank you for answering those two questions.

1. Just to confirm, for force balance of the sphere in the simulation, is buoyancy already included in the drag reported in Fluent? The motion of the sphere is of interest using Newton's second law, i.e., ma=-G+Fd+Fs, where G is gravity, Fd is drag, and Fs is surface tension. Is this equation correct?
2. In Eq.14-24 of CSF model, there is no contact angle in it. If contact angle needs to be considered, is wall adhesion required to be enabled? 

Thanks,

Simon

[Vishesh Aggarwal]

Hi Simon,

1. The force from Fluent on walls will only be "drag" components based on pressure and shear. G and Fs are not included in the Force report from Fluent.
2. Yes, you need to enable wall adhesion to consider contact angles. Contact angle will affect the shape of the interface where it meets the walls. It will not directly impact the surface tension forces.

Regards,

Vishesh