[warhammer920]

Greetings!

I am a university student and am currently trying to find out the added mass matrix for an AUV that we are developing. I am following this article as a guide and using Ansys Fluent 18.2.

https://www.researchgate.net/publica...Fluid_Dynamics

Following the theory I'm running two simulations, one simulation with a constant flow velocity equal to 1m/s and one simulation with a constant acceleration of the flow at 1m/s2.

I have done the first simulation and it gave me a drag force of around 70 N.

Now while running the second simulation(with an acceleration of 1m/s^2), the solution diverges pretty fast and a floating point exception is raised as the continuity residual blows up really fast.

The details of setup are-:
1. Flow along -ve Z axis, gravity along -ve Y axis.
2. Udf for inlet boundary condition(attached below)
3. Transient state
4. Models- Viscous realizable k-e, scalable wall function
5. Pressure outlet
6. Solution method PISO (attached below)

While the calculation is running I get the following message in the console:-
"turbulent viscosity limited to viscosity ratio of in cells" after which the residuals begin to blow up.

Note: my inlet has 5 faces of the enclosure. Excluding the one along Z axis, all the other four faces will have velocity tangential to them and it would be equal to the freestream velocity.

The result and the setup can be found in this URL.

https://imgur.com/a/kn6KT6v

If anyone can guide me then it would a great help!! Also, I'm new to fluent so If I have committed a stupid mistake then please forgive me.

Thanks,
Prajwal