[neda2014]

Hi, I'm new using cfd softwares. I'm using ansys cfx to model a floating body in water. my buoy is a rectangle (1m Length*.5m Height) in a (8m Length*4m Height) container. I determine depth below the water so that gravity force balance buoyancy force and buoy is in equilibrium and I define one degree
of freedom for rigid body(heave motion)
I use laminar flow and free surface for my model but when I run simulation with cfx solver manager and cfx plot the displacement of buoy is not fixed and the buoy move in y direction(heave), cfx give error that the flow is not laminar!!
please help me , if i use ansys fluent to model floating body can I plot displacement of buoy in water????

[mahdimazloom]

If your object moves and the movement is significant then you have to define a remesh state.

http://www.edr.se/blogg/blogg/ansys_...cfx_re_meshing

[ghorrocks]

The solver message about the flow not being laminar is a warning, not an error. It is up to you to decide whether you need to change solver settings or not. If you are convinced that your flow is laminar then you can ignore it and proceed.

[neda2014]

Thank you for your reply. I do remesh same as tutorial said but when I solve it with solver manager I get this error:
'Run remesh 003': No data source for Workspace, cannot monitor Solver data.
.
.
and, C:\Program Files\ANSYS Inc\v145\CFX\bin\winnt\cclsetup.exe was interrupted
by signal HUP (1)
is the problem about installing ansys????

[JuPa]

I'm sure it's preferential to switch to a turbulence model if it's borderline laminar/turbulent, since the turbulence model will add numerical diffusion and aid convergence.

I doubt a floating body in water is laminar, unless it's a small particle which will follow the streamlines of your flow - this isn't the case. You have so many sources of turbulence:
- Wave turbulence (and subgrid wave turbulence if using RANS)
- Buoyancy turbulence production and dissipation
- Possible splashing and entrainment

[ghorrocks]

Yes, the additional diffusion of the turbulence model will aid convergence but if you have excessive diffusion already then adding more is going to make it worse. But I agree that if it is borderline transitional flow then it is usually better to err on the side of caution and model it as turbulent.

Many water flows are laminar - my field is MEMS and I can assure you I have never modelled a turbulent flow in a MEMS device Be careful generalising. You have to look at the Reynolds number of the key aspects of the flow to make an informed decision as to whether the flow is laminar or not.

[JuPa]

I didn't even know what MEMS was until I just Googled it!

I agree regarding Reylonds, and other dimensional numbers.

[neda2014]

do you mean when a buoy is in water the flow is not laminar???
I change dimension of tank but cfx solver error turbulent flow , and i change laminar flow to turbulent but the buoy move!
Is there any way to model a buoy in still water when the buoy is in equilibrium position (when buoyancy force and gravity force is equal) in cfx??
THanks for your help...

[mahdimazloom]

How about defining a weak spring for the buoy to help the buoy remain in its equilibrium position (Of course if the lift force causing the movement of the buoy is low)

[ghorrocks]

do you mean when a buoy is in water the flow is not laminar???

That suggests you missed the point of the post. You need to work out the relevant non-dimensional numbers (such as Reynolds Number) of the motion using a typical time, velocity and length scale and work out in your case whether it is likely to be laminar or not.