Recursive expressions in CFX

Viento · August 25, 2019, 16:46

For example, I am modeling a free piston, accelerated with pressure. So I need to set a changing speed.

I can easily find acceleration a = PS/m и ΔU = a*timestep. The piston has inertia, and its speed increases (or maybe decreases) with each timestep.

So, I need "cumulative" expression like U = U+ΔU, to change the speed per each step. But CFX does not support self-referencing expressions. How to do this correctly?

Gert-Jan · August 27, 2019, 04:52

You should not set the velocity on a free piston. The piston needs to find its position by itself. Probably by deforming mesh. Please look for the tutorial #23 on the ANSYS support site.

August 25, 2019, 16:46	Recursive expressions in CFX	#1
Viento New Member Alex Join Date: Jan 2018 Posts: 9 Rep Power: 8	For example, I am modeling a free piston, accelerated with pressure. So I need to set a changing speed. I can easily find acceleration a = PS/m и ΔU = a*timestep. The piston has inertia, and its speed increases (or maybe decreases) with each timestep. So, I need "cumulative" expression like U = U+ΔU, to change the speed per each step. But CFX does not support self-referencing expressions. How to do this correctly?

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August 27, 2019, 04:52		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,929 Rep Power: 28	You should not set the velocity on a free piston. The piston needs to find its position by itself. Probably by deforming mesh. Please look for the tutorial #23 on the ANSYS support site.