## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Fluid flows upwards despite the fact that gravity is enabled

• Ivy.s
Subscriber

I want to simulate a screw through which cement (with a viscosity of 1000 Pa*s) flows. In the following pdf document you can see what i want to simulate.

My problem is, that the cement flows through the holes of the screw (which is correct) but after that it flows upwards on the outside of the screw. I enabled gravity, but that did not help. I don't know if i used the wrong model for that or what's wrong with my simulation. Please help me.

In the pictures you can see the settings of my simulation, i hope you can help me with this.

phase 1 = air

phase 2 = high viscosity liquid

• Ivy.s
Subscriber

• raul.raghav
Subscriber

Based on your coordinate system, the gravity should be +9.81 m/s^2. And edit gravity in the "General" tab. See screenshot below.

• peteroznewman
Subscriber

@Rahul, in the Structural Mechanics side of ANSYS, I use an acceleration of x = -9.81 m/s^2 to create the effect of a gravity force that pulls mass in the +x direction as shown in the last snapshot. I'm not an expert at Fluent, but I would assume it would be the same.

• raul.raghav
Subscriber

@peteroznewman, I'm quite sure in Fluent and CFX its not the same way. The definition of gravity depends on the coordinate system.

Please see the tutorial below and the attached screenshot to get an idea of how gravity is accounted for (under Step 2: Models).

Fluent Tutorial: Using the Eulerian Granular Multiphase Model with Heat Transfer

• Ivy.s
Subscriber

I've already tried to Change the gravity into +9.81 but the result is the same. And I also edited it in the "General" tab. I just forgot to make a screenshot of it.

• raul.raghav
Subscriber

Not sure what could have gone wrong. Would you be able to share your mesh file by any chance?

• Ivy.s
Subscriber

I can not upload it here, so that's the link to the mesh file.

https://www.dropbox.com/mesh_file

• Raef.Kobeissi
Subscriber
Just a quick comment. You need to have your acceleration as +9.81m/s2 in the X dirextion otherwise your gravitational acceleration is wrong.
From the picture you posted it seems the flow is reaching a wall and being squeezed upward or seeping through a hole. You need to check your geometery and mesh for any anomalies.
• Ivy.s
Subscriber

I changed the gravity now to +9.81 m/s²

The result is the same. The liquid flows upwards outside of the screw.

It is correct, that the liquid is being squeezed through the holes of the screw, as you can see in the following picture.

But I don't understand why it flows upwards after that. In reality it flows a bit upwards as well, because of the pressure that squeezes the liquid out of the holes, but not that much. Most of the liquid must flow downwards because of the gravity.

• Raef.Kobeissi
Subscriber
It depends on your geometry, when you rectified your gravity acceleration have you restarted the simulation? Can you share the pressure and velocity contour in your simulation please.
Cheers
• Ivy.s
Subscriber

Yes I restarted the simulation after it.

• Raef.Kobeissi
Subscriber
Hello, from the contours you just showed it looks like you have high differential pressure and I believe the flow behaviour upward despite gravity is perfectly normal due to high pressure and flow deflection.
Regards
• Ivy.s
Subscriber

Do you know how I can set the pressure down, to test if the result is better with a lower pressure?

• Raef.Kobeissi
Subscriber
You can use inlet pressure instead of velocity inlet, this way you can set the pressure at whatever value you want but you wont be able to control the velocity.