# Virtual wave problem at ship calm water simulation

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March 20, 2020, 08:38
Virtual wave problem at ship calm water simulation
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Dong-Hyun, Kim
Join Date: Jul 2017
Posts: 2
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Hi guys

I studied ship resistance simulation on clam water during 3 years.

But, problem of virtual wave seen on figure (a) in attached file has not been solved yet.

This is not Calvin wave clearly.

This virtual wave cause big pressure vibration, so pressure resistance(drag) can't be calculated.

Virtual wave is generated on below conditions
* VOF free-surface
* only calm water
* low velocity
* fine x-y mesh (x->advanced direction, y->side)
* rough z mesh (z->up-down direction)
* fine(low) time-step
* high-order scheme (convection and implicit)

At low speed, wave height due to ship shape is very low (about 3mm in 8m model ship).
Rough z mesh can't demonstrate this, so high elevation and short period virtual wave is generated to satisfy conservation law.

One solution that I found is very fine z mesh (figure (b)). But we can't use this method due to 30 millions grid number.

At most precedent studies with KCS ship, this problem was not taken because velocity of KCS is very high.

But recently, most ship is operated with low speed due to EEDI reduction. So, virtual wave is big problem at resistance CFD simulation.

Any of you have any ideas for solving this problem?
Attached Images
 figure a.jpg (71.2 KB, 20 views) figure b.jpg (46.9 KB, 16 views)

 March 20, 2020, 23:19 #2 New Member   Dong-Hyun, Kim Join Date: Jul 2017 Posts: 2 Rep Power: 0 I have many experience about resistance simulation at graduate school and shipbuilding company with colleagues. We also have calculated many ship type with high accuracy resistance and wake, wave pattern prediction. But this isn't problem at 'normal' resistance simulation. This is specific case at very low speed where normal Calvin wave is not exist. So, this problem is just numerical error at low velocity problem. Only solution is high grid resolution. But I want find other way which has low grid point by some numerical damping method. I want to hear idea about that from good expert in this forums.

 Tags cfd, resistance, ship, star-ccm+, wave

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