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why fluent 6dof rotational mass cannot be changed??

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Old   April 4, 2020, 09:07
Question why fluent 6dof rotational mass cannot be changed??
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hello dears,
I'm using fluent ansys 19.2, and working on a swing check valve simulation, it is a flapper/door hinged in the top of a pipe, and the flow pushes and moves that flapper up.
i've defined the solid material of the flapper as plastic PLA (density=1240 kg/m^3),
the fluid is liquid water,
then i defined the 6dof properties :
name: flapper_rotation
mass: 0.015 kg,
one dof rotation
the rotating axis : (1,0,0)
center of rotation (0,22.5,0)
center of gravity location (0,12,23)
constrained angle: min=-60 , max = 60
inertia: 5.2e-6
then I pressed OK, and I closed the 6dof property dialogue
>>>>> and here is the proplem
I reopen the "flapper_rotation" and found the mass has changed to 0 kg !!!
I re-changed the mass value to 0.015 kg, closed the 6dof property dialogue, then reopened it again and found the mass again has changed to 0 kg !!!!!!!!!!!!!!!!!!!!!!!!
it cannot keep the value I assign to it !!,
but though i started the simulation with inlet velocity of 0.13 m/s which i have experimentally investigated the flapper with, this velocity value is very sufficient to push the plastic PLA flapper up, but in the simulation it just pushed it a tiny little bit, which means that the mass of it has really changed to 0 kg and not as 0.015 as i have feed the solver.

can any one help with this bug that always changes the mass value in the 6dof property dialogue to 0 kg???

n.b. i've thought of using a UDF file, but I cannot find an option to restrict the angular motion in the UDF manual.

thanks a lot ,

Last edited by EngGhost; April 4, 2020 at 14:04.
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Old   April 6, 2020, 06:21
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Fluent does not use the mass based on the density provided for the material. In fact, solid object does not need to be there at all. However, as you stated that you defined the mass as 0.015 kg, it should work with that. Are you using UDF to specify this or are you directly entering this value in the GUI? To restrict motions within UDF, there are options available, such as, to restrict translation and rotation about x, set the following to true, SDOF_ZERO_TRANS_X and SDOF_ZERO_ROT_X
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Old   April 7, 2020, 11:26
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thanks for your reply Vinerm,
I know that fluent does not care about the solid properties, but I just added it just in case,
but I mainly entered the mass of 0.015 g in the user interface pannel of the 6dof property,
but when I click ok and go back I found it zero again,
I may try using the udf,
but what I mean if udf restriction of motion is that I will have a 1dof rotational motion about x axis plus I want this 1dof rotation to work only between 0 and 60 degrees only, which I cannot find a way to do that in udf codes..
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Old   April 7, 2020, 13:08
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That's strange. Never observed such behavior. Are you using Fluent in single precision or double precision?
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Old   April 8, 2020, 16:17
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I use single precision,
I've read that doube precision is effective only in very small meshes (nano meter) while my mesh minimum cell size is 2 mm
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Old   April 9, 2020, 03:36
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You have heard it wrong then. At nano-meter scale, Navier-Stokes equation is not even valid. Double precision is to be meant for use if value of any field variable varies by a very small amount from cell to cell. For all industrial problems, it is recommended to use double precision.
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Old   April 10, 2020, 07:29
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thanks for the info,
I'll try it for sure..
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Old   January 31, 2023, 23:40
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Quote:
Originally Posted by EngGhost View Post
hello dears,
I'm using fluent ansys 19.2, and working on a swing check valve simulation, it is a flapper/door hinged in the top of a pipe, and the flow pushes and moves that flapper up.
i've defined the solid material of the flapper as plastic PLA (density=1240 kg/m^3),
the fluid is liquid water,
then i defined the 6dof properties :
name: flapper_rotation
mass: 0.015 kg,
one dof rotation
the rotating axis : (1,0,0)
center of rotation (0,22.5,0)
center of gravity location (0,12,23)
constrained angle: min=-60 , max = 60
inertia: 5.2e-6
then I pressed OK, and I closed the 6dof property dialogue
>>>>> and here is the proplem
I reopen the "flapper_rotation" and found the mass has changed to 0 kg !!!
I re-changed the mass value to 0.015 kg, closed the 6dof property dialogue, then reopened it again and found the mass again has changed to 0 kg !!!!!!!!!!!!!!!!!!!!!!!!
it cannot keep the value I assign to it !!,
but though i started the simulation with inlet velocity of 0.13 m/s which i have experimentally investigated the flapper with, this velocity value is very sufficient to push the plastic PLA flapper up, but in the simulation it just pushed it a tiny little bit, which means that the mass of it has really changed to 0 kg and not as 0.015 as i have feed the solver.

can any one help with this bug that always changes the mass value in the 6dof property dialogue to 0 kg???

n.b. i've thought of using a UDF file, but I cannot find an option to restrict the angular motion in the UDF manual.

thanks a lot ,


I just had this issue, it's a bug in the Fluent GUI. Simply switch to single DOF translation, change the mass value, and click ok and close the prompt. Then, reopen the prompt, change from single DOF translation to single DOF rotation, and then click ok. The mass should now be correct
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