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Old   April 22, 2013, 08:43
Default Hydrodynamic force?
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Arun Krishnan.L.H
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Hello All,

I have worked on this problem for a long time and would be happy if someone could guide me regarding my doubts.

I have been working on swaying and heaving of cylinder on free surface to find out the added mass and damping. The motion of the cylinder in both cases is sinusoidal with an amplitude of 0.03 and frequency ranging from 1 to 6 rad/sec. This was chosen to compare with the experimental values. Rather than straight away calculating the added mass and mass and non-dimensionalizing it, I have compared the forces obtained from the CFD simulation to the experimental values of forces.

After a lot of trail and error and playing the right mesh, I managed to get very good results in swaying when compared with experiments. In this case I created a force report in the x-direction and compared with the horizontal force from the experiments.I have used morphing for both cases.(x is longitudinal, y is vertical and z is a small 3D width(3m)).

I have now tried to compare results for heaving. The problem I am facing is to extract the hydrodynamic component. I have defined a number of reports and wanted to see the results from each.

1. I defined a static pressure field function. My cylinder center lies at 0,0,0. So a simple field function was defined as
($$Centroid[1] < 0) ? -$Densitywater*(Centroid[1])*9.807 : 0.

This would give me the staic pressure about the mean free surface line. I created a surface integral of this pressure field function to get the force component.

2. I defined a surface integral report of the total pressure field function(default). According to the definition in STAR CCM+, total pressure is the pressure when the flow is isoentropically brought to rest? Then it has both static and velocity components.

3. I defined a surface integral of the static pressure field function(default).

4. I also defined a force report in the y direction excluding shear(only pressure).

When i compared all values I get no relevance . The static and total seems to be very close with the difference only about 50 N which is wrong as per the experiments.

The static pressure defined by me shows values greater than the total pressure.

When i check the vertical force it oscillates about a mean which is not zero. actually ,none oscillate with 0 mean, which is also understandable. So when i check the vertical force and initialize it to 0 mean and compare, it is the same as the hydrodynamic heaving force as per the experiments. I am unable to explain the results and the values of pressure and forces that STAR CCM+ gives me.

Please any one there could explain. The force report as per STAR is the surface force when the body is not defined as rigid? When using rigid body simulation it includes body forces.

Why am I not getting the dynamic component when i subtract the total and static? Why am I getting the dynamic component in the vertical force report?


Please any reply would help me.

The problem is inviscid. I have not yet applied a turbulent model. The domain is large enough not to have any wave reflection and I have also given wave damping. The cylinder oscillates on still water and waves are created only due to the oscillations.

I can share the model if you need.

Thanks
Regards
Arun
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Old   April 22, 2013, 17:10
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Arun Krishnan.L.H
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Can any one please give me some reply.?
regards
Arun
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Old   April 28, 2013, 12:55
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Arun Krishnan.L.H
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I did not find any thread also which discusses about hydrodynamic force. So please advice if you know the answer. I have completed the simulation for all 6 frequencies and even the vertical force is not what i want.


Thanks
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Arun
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Old   May 14, 2014, 05:26
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hey bro, I am giving you a reply.
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Old   May 14, 2014, 06:21
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without knowing too much about your case and experiements I suggest that a standard force report on each axis should be all you need and forget about field functions etc

not sure why your static pressure field function has the if statement unless your cylinder is only half in the water at y=0
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Old   May 14, 2014, 06:31
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Hi ping, the reason why we create a filed function to calculate the static pressure is when calculating hydrodynamic coeifficients we need hydrodynamic force only. The standard force report gives a force that include the hydrostatic force. Therefore, we need to subtract the hydrostatic force from the standard force report. Do you have any better method to get hydrodynamic force?
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Old   May 14, 2014, 06:34
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the hydrostatic components should balance out around your cylinder unless i dont understand your meaning
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Old   May 14, 2014, 06:48
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sorry, forget to say that if we want to calculate the added mass in heave direction, we need the force in heave direction only.
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Old   May 14, 2014, 08:25
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so it is very handy that simple force reports need to be specified in one direction in the chosen coordinate system so i think it is still very simple

also if you do really want to prove that the net hydrostatic force is zero in the horizontal axis your field function formula was incorrect since it fails to take account of the face area of each cell since force = pressure * area

and you need the component of area in the heave direction too so it is hydrostatic-pressure * $$Area[0]
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Old   May 17, 2014, 10:39
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The problem has free surface problem
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Old   May 18, 2014, 20:57
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Hi Arun,

Not sure if I've well understood your problem but I would say that you just want to extract properly the water pressure head from pressure. Assuming you have use a FlatVOF Wave for initialization and boundaries then you can do it by creating a field function as follows:
$Pressure - $HydrostaticPressureFlatVOFWave.
This is close to your original field function but only identical at water surface. (Assuming your Y=0 corresponds to water surface).

Best Regards,
Yannaos
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Old   June 2, 2014, 06:37
Default Hi Arun
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Are you trying to get the coefficients from the generated force time history you obtained from your simulation
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Old   July 15, 2014, 09:27
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Hi Pinto

Do you know how to do that?
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Old   July 15, 2014, 09:38
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Yes, I've figured out a way to do it
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Old   July 15, 2014, 09:41
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Hi Pinto

Could you please share the method?
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Old   July 15, 2014, 09:46
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My apologies, I just read the thread properly and it's for star ccm. I only know how to do it for fluent
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Old   July 15, 2014, 10:02
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Thanks anyway
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