Learning Forum

[Jafar]

Hello, 

Could anyone please advice on the above mentioned problem please. I'm trying to obtain the structural responses of the boat in the heave and pitch directions (translational motion in the y axis and rotational motion in the z if the screen is the x-y plane). Please see below the details of my simulation:

 

 

The above mesh is created with min orthogonality of 0.57 and max skewness of 0.603. Edge sizing is added for the Hull. the geometry is sliced so that the hull is moving through water.

UDF file is below:

 

Models used are Multiphase VOF with open channel flow, open channel wave BC and implicit body force enabled and viscous model of k-omega with SST.

Boundary conditions are as follow:

water_inlet: velocity inlet with open channel wave BC enabled and details of wave inserted.

air_inlet: velocity inlet

water_outlet: pressure outlet with open channel wave BC enabled and details of wave inserted.

air_outlet: pressure outlet

except the interface line is defined as a wall and given the same udf as the hull but with passive enabled.

moving mesh is with 0.5 spring factor and remeshing details are from mesh scale info.

time step is 0.01.

I keep getting negative cell volume detected error. 

could you please advice.

 

 

 

[Rob]

Try reducing the time step size. Negative volume errors are typically caused by parts of the model moving too quickly for the remeshing.  You want the hull to move by less than one cell length in a time step. Depending on the amount of motion remeshing as well as smoothing may be required. 

[Jafar]

I reduced the time step to 0.0001 and it still runs for 5 iterations and gives the error. I use remeshing and smoothing. 

is the udf fine?

[Rob]

I'd missed this as I've been working away: if we don't answer feel free to bump the thread after a couple of working days.

If the model runs for 5 iterations have a look at the flow field after 4. It means the solver is struggling, so could mean the time step is still too big: you need to check the flow speed & cell size. 

[Jafar]

Thank you for your reply. I tried smaller time step with no hope. lets start from the mesh. do I have to create two surfaces, one around the boat as inner domain and other for outer domain to refine the mesh around the boat?

what will be the dynamic mesh setup?

Numerical beach should be enabled? as I'm interested in predicting the heaving and pitching motions over time (translation in the y axis and rotation about z as shown below)




Is my UDF shown below OK?



what should I define as a moving zone? just the edges of the hull as a rigid body or should I also define the inner zone as a deforming/rigid body?

please see the below pic for boundary conditions and zones:



your help is very much appreciated

[Rob]

Have a look at this   https://www.youtube.com/watch?v=T92HnTKv1-I  to get some ideas. 

 

Also if you update to 2019R2 you'll have access to help videos & a couple of tutorials through the ANSYS Help system. 

[Jafar]

Thanks very much for the video.

The video is for a 3D case with no head sea waves. I have tried the dynamic mesh setup used in the video but still gets negative cell volume. 

I have access to 19.2 version and looked at the tutorials but still not managing.

[Rob]

Remesh with a coarse mesh & try that with the same time step etc. This model is to check the set up, don't worry about accuracy. 

[Jafar]

I have done it before but it did not help. I'm currently trying to do an overset mesh similar to the video you posted. It kept running for the first time ever for more than 1600 iterations (time step of 0.005 s and # of time steps 3000). I will update you with results once its done. 

[Jafar]

No negative cell volume with overset mesh but the hull sinks down after a few second. What could be the problem?

[Jafar]

so with overset mesh the solution completes normally with no problems but this is how it looks like at the final time step???
what could be the reason for this??
Is there a way to upload a video for the simulation ?

[Rob]

Video upload can be done via YouTube, we can't download files anyway. If the boat is sinking it means you've got the forces and/or mass calculation wrong: it's doing what you told it to do. 

[Jafar]

the simulation now runs for a few more iterations and then stops due to negative cell volume again. I tried to reduce the time step but still get negative cell volume.