Negative pressure in hydroforming process

Shahab Khakpour · March 7, 2019, 02:54

Hello everyone,
I am going to simulate hydroforming process of a close-end pipe. I need to apply velocity as the inlet and all other surfaces should be considered as the wall. I did it through two approaches: 1- one-way FSI: I solved the fluid model via fluent and linked the solution to the transient structural. I considered the fluid pressure on the wall as the external force for structural analysis and 2- I tested two ways FSI via system-coupling and in both cases, I faced with the same problem:
At the beginning, only the positive pressure exists at the close-end region and all of the tother regions have negative pressure. By keep going, the positive pressure moves toward the inlet, but after 5 mins of simulation, still, one third of the pipe has negative pressure. The negative pressure causes shrinkage of the pipe, while the aim of the hydroforming is inflation of the subject. I think it could be due to that there is not any outlet. I use k-e turbulent model and I have tested multiphase flow, but it did not work.
I would greatly appreciate it if you could help me that.

VSass · March 8, 2019, 04:41

What boundary and initial conditions do you apply for the pressure? Does your solver solve for the absolute or relative pressure fields? Mesh specifications are also important.

March 7, 2019, 02:54	Negative pressure in hydroforming process	#1
Shahab Khakpour New Member Shahab Khakpour Join Date: Mar 2019 Posts: 2 Rep Power: 0	Hello everyone, I am going to simulate hydroforming process of a close-end pipe. I need to apply velocity as the inlet and all other surfaces should be considered as the wall. I did it through two approaches: 1- one-way FSI: I solved the fluid model via fluent and linked the solution to the transient structural. I considered the fluid pressure on the wall as the external force for structural analysis and 2- I tested two ways FSI via system-coupling and in both cases, I faced with the same problem: At the beginning, only the positive pressure exists at the close-end region and all of the tother regions have negative pressure. By keep going, the positive pressure moves toward the inlet, but after 5 mins of simulation, still, one third of the pipe has negative pressure. The negative pressure causes shrinkage of the pipe, while the aim of the hydroforming is inflation of the subject. I think it could be due to that there is not any outlet. I use k-e turbulent model and I have tested multiphase flow, but it did not work. I would greatly appreciate it if you could help me that.

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March 8, 2019, 04:41		#2
VSass Member VS Join Date: Nov 2012 Posts: 86 Rep Power: 14	What boundary and initial conditions do you apply for the pressure? Does your solver solve for the absolute or relative pressure fields? Mesh specifications are also important.