# Get Mesh Acceleration

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 July 22, 2015, 10:48 Get Mesh Acceleration #1 New Member   Pietro Join Date: Jul 2015 Posts: 9 Rep Power: 3 Hi everyone, i am looking for mesh acceleration values in ansys cfx. I have searched in help and in forums but no one seem to know how can i compute this value. My problem is: i need compute the add mass due the heave motion of a ship's transversal section. For this, i need know the mesh accelaration.

 July 22, 2015, 12:31 #2 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 Is this a 6DOF problem (rigid body or immersed solid)? You can use rbstate to get this.

July 22, 2015, 13:03
#3
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Pietro
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Quote:
 Originally Posted by singer1812 Is this a 6DOF problem (rigid body or immersed solid)? You can use rbstate to get this.
My friend, i thank you response. I dont set a rigid body. As can be seen by the figure that i have attached, i set the surfaces of the hull as wall boundary conditions and forced her to make an oscillatory movement in the Z axis, which is defined by a function that I called "Osc=A.sen(w.t)".
I already have the acceleration of the surface(hull), and get this only by deriving the expression "Osc", the concept of additional mass implies look for fluid particles that have a close enough acceleration if compared with hull acceleration, for that, I need the vertical acceleration information of fluid particles throughout the domain.
I tried to get the velocity field in the previous time step, and could make a linear approximation of the vertical acceleration just subtracting the speed at the current time step less speed in the previous time step divided by time step. But I failed to get this data as well. Do you have any idea?
Attached Images
 osc.jpg (29.2 KB, 13 views)

 July 22, 2015, 13:38 #4 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 You are doing it the way I would suggest. What do you mean by you are not able to get this data? Do you mean you dont know how to take the previous time step data and subtract it from the current timestep? I can help you with this, if that is the problem. Pietro Giorgio likes this.

July 22, 2015, 13:53
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Pietro
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Quote:
 Originally Posted by singer1812 You are doing it the way I would suggest. What do you mean by you are not able to get this data? Do you mean you dont know how to take the previous time step data and subtract it from the current timestep? I can help you with this, if that is the problem.
Yes my friend, I really tried and I could not do it. Could you teach me? I am deeply grateful!

 July 22, 2015, 14:01 #6 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 You need to load in the previous time step and create a new variable. That variable will be a FROZEN copy of the velocity field you want to get the accelerations from (you can create a separate new variable for each u,v,w). That variable will retain the old time steps values and not change with a new time step. Load in the new time step. Create a new variable of your acceleration. It will be an expression variable of (velocity-old_velocity_variable)/timestep. Now if you want to do this over a lot of time steps you will either have to create the FROZEN copy by hand at each time step, or use Power Syntax to do this for you. Power Syntax is a whole another topic that I will allow you to look into, but for now, you know the process. Pietro Giorgio likes this.

 July 22, 2015, 14:26 #7 New Member   Pietro Join Date: Jul 2015 Posts: 9 Rep Power: 3 My friend, really Thanks! I will have a look at it. You saved my project haha !

July 23, 2015, 17:50
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Pietro
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I have a problem about it. The frozen copy is not really freezing the previous time step. I can not see what I did wrong, i consulted the tutorial, but apparently the method is correct. I went to the previous time step and created a new variable using the "Frozen Copy", called it "Vel Old" (Figure 1). Went to the next time step. I made expression called "Acc Mesh" and made the linear approximation to the acceleration (Figure 2), have created a variable called "Mesh Acceleration Z" and I related with "Acc Mesh" expression, however, it is always giving 0 (Figure 3), this means that the Frozen copy is updating his value, because as can be seen from figures 4 and 5, the speed varies greatly around the hull over time. Could anyone tell me what i am doing wrong?
Attached Images
 a1.JPG (21.7 KB, 8 views) a2.JPG (23.8 KB, 8 views) a4.JPG (23.5 KB, 7 views) a5.JPG (40.0 KB, 10 views) a6.JPG (38.0 KB, 6 views)

Last edited by Pietro Giorgio; July 23, 2015 at 18:57.

 July 24, 2015, 00:04 #9 New Member   Pietro Join Date: Jul 2015 Posts: 9 Rep Power: 3 Hi! I found the problem, the CFX 13.0 does not work well with frozen copy . I tested the version 15.0 and it worked perfectly.