# Modelling Contact and Frictional Force in 6DOF

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 February 17, 2021, 12:31 Modelling Contact and Frictional Force in 6DOF #1 Member   Will Crawford-Jones Join Date: Jan 2021 Posts: 43 Rep Power: 4 Hi, Let's say I have a 6DOF model and I would like this to start in 'contact' with the wall, hence, it will begin such that the edge of my 6DOF object is at the position of the proximity threshold. Based on this, contact is occurring. Now, I have little understanding of UDFs, however, I think it should be possible for me to write a UDF that calculates the normal force from the 6DOF object to the wall. This would consist of the component of all shear stresses, pressures and the weight of the object in the direction of the wall's normal. Rather than writing a UDF that simply reflects the velocity at contact, I would like to write one that models the contact force from wall to the object. I think this may be possible by defining a thread that is in 'contact' with the wall (there are contact macros for this) and overwriting the pressure on these cell faces and making this equal and opposite to the shear stresses, pressures and weight I previously mentioned. Is there a way I can overwrite pressures in this way? I thought that DEFINE_ADJUST may work but it seems to only work across the whole domain. Presumably there is an equivalent to DEFINE_ADJUST for a given thread, in this case the contact thread? This would balance the forces on the 6DOF object and prevent motion. As an added point I would also like to do the same with friction. If I can solve this problem with the contact force then it can't be much of a leap to do the same by overwriting the shear force on the 6DOF object as well (again, at the point of 'contact'). Thanks a lot for any advice you can provide!

March 12, 2021, 02:41
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Alexander
Join Date: Apr 2013
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Quote:
 possible by defining a thread that is in 'contact' with the wall (there are contact macros for this)
you've told there is macro to find cells in contact zone, I don't know this marco, but if you know you should use it

DEFINE_ADJUST is not related to loop over domain
its a macro which executes your code before each iteration/timestep, that's it, nothing else

so if you have macro to check if the cell is in contact that's what you need, loop over all cells and check the criteria
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 March 12, 2021, 04:17 #3 Member   Will Crawford-Jones Join Date: Jan 2021 Posts: 43 Rep Power: 4 Thanks for the response. I think then it is in theory possible to create a UDF that simulates normal contact forces as your body moves in contact with boundaries. What I'm less sure about is frictional (tangential) forces. For now, I have switched to Star CCM+, however, I may return to Fluent in the future. Thanks again!

 Tags contact, define_adjust macro, sdof, udf

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