
[Sponsors] 
April 22, 2013, 09:43 
Hydrodynamic force?

#1 
Member
Arun Krishnan.L.H
Join Date: Jan 2013
Posts: 75
Rep Power: 13 
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 nondimensionalizing 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 xdirection 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 

April 22, 2013, 18:10 

#2 
Member
Arun Krishnan.L.H
Join Date: Jan 2013
Posts: 75
Rep Power: 13 
Can any one please give me some reply.?
regards Arun 

April 28, 2013, 13:55 

#3 
Member
Arun Krishnan.L.H
Join Date: Jan 2013
Posts: 75
Rep Power: 13 
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 Regards Arun 

May 14, 2014, 06:26 

#4 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
hey bro, I am giving you a reply.


May 14, 2014, 07:21 

#5 
Senior Member
Ping
Join Date: Mar 2009
Posts: 556
Rep Power: 20 
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 

May 14, 2014, 07:31 

#6 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
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?


May 14, 2014, 07:34 

#7 
Senior Member
Ping
Join Date: Mar 2009
Posts: 556
Rep Power: 20 
the hydrostatic components should balance out around your cylinder unless i dont understand your meaning


May 14, 2014, 07:48 

#8 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
sorry, forget to say that if we want to calculate the added mass in heave direction, we need the force in heave direction only.


May 14, 2014, 09:25 

#9 
Senior Member
Ping
Join Date: Mar 2009
Posts: 556
Rep Power: 20 
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 hydrostaticpressure * $$Area[0] 

May 17, 2014, 11:39 

#10 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
The problem has free surface problem


May 18, 2014, 21:57 

#11 
New Member
Philhellene Ithaca
Join Date: Mar 2014
Posts: 10
Rep Power: 12 
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 

June 2, 2014, 07:37 
Hi Arun

#12 
New Member
Join Date: Mar 2014
Posts: 4
Rep Power: 12 
Are you trying to get the coefficients from the generated force time history you obtained from your simulation


July 15, 2014, 10:27 

#13 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
Hi Pinto
Do you know how to do that? 

July 15, 2014, 10:38 

#14 
New Member
Join Date: Mar 2014
Posts: 4
Rep Power: 12 
Yes, I've figured out a way to do it


July 15, 2014, 10:41 

#15 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
Hi Pinto
Could you please share the method? 

July 15, 2014, 10:46 

#16 
New Member
Join Date: Mar 2014
Posts: 4
Rep Power: 12 
My apologies, I just read the thread properly and it's for star ccm. I only know how to do it for fluent


July 15, 2014, 11:02 

#17 
Member
David
Join Date: Nov 2011
Posts: 39
Rep Power: 14 
Thanks anyway


September 17, 2014, 04:38 
Sloshing Damping Rate

#18 
Member
Nazim
Join Date: Oct 2013
Posts: 93
Rep Power: 13 
dear friends,
can anyone tell me how to find damping rate, frequency, longitudinal force for a sloshing problem in fluent 

August 29, 2017, 06:55 

#19 
New Member
Can
Join Date: Aug 2017
Posts: 12
Rep Power: 9 
Dear friends,
I ve just found this thread which discusses about getting hydrodynamic forces in Star CCM. I m working on a multiphase maneuvering problem and trying to obtain forces and moment acting on the ship with a drift angle. I just want to get only hydrodynamic component of the forces. As you see in the attachment, the forces and moment due to fluid flow are calculated with using the static pressure. Is it possible to change it to dynamic pressure? Thanks in advance. Lazarus 

August 30, 2017, 09:37 

#20 
Senior Member
Ping
Join Date: Mar 2009
Posts: 556
Rep Power: 20 
you need to create your own force and moment reports by creating a special vector field function that multiply cell face area by dynamic pressure ( calculated as shown above by yannaos or your own subtraction of g * water depth from the static pressure)
then sum those face forces to get the total force and for moments create another field fuction and multiply the face force by the radial distance from each face to the center of a coordinate system and sum that field 

Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
ActuatorDiskExplicitForce in OF2.1. Help  be_inspired  OpenFOAM Programming & Development  10  September 14, 2018 12:12 
New sixDoFRigidBody BC working with laplaceFaceDecomposition  Ya_Squall2010  OpenFOAM Running, Solving & CFD  13  April 17, 2013 03:04 
calculation of Hydrodynamic force on cylinder  kamrul029  FLUENT  0  March 27, 2012 13:38 
Force can not converge  colopolo  CFX  13  October 4, 2011 23:03 
DEFINE_CG_MOTION and pressure force  Teo Fumagalli  FLUENT  0  April 11, 2008 11:25 