How to simulate a flow in a 3D Pipe

Changli · May 11, 2017, 16:53

1.I get scenario with a 3D pipe with a diameter of 5 meters, and with a length of 200 meters, also its bended shape in space likes a snake. I only know the water velocity inside of pipe is around 0.18m/s(at least at the entrance), no information about the pressure. How should I set the boundary conditions regarding the velocity, and also the pressure. Totally no clue about what the zeroGradient thing is doing.
2.And also want to know if I set the inlet velocity using the surfaceNormalFixedValue, how should I set the inlet pressure together with the outlet's velocity and outlet's pressure?

piu58 · May 12, 2017, 01:23

I recommend to set
inlet:
- U: fixed value, your velocity
- p: zerogradient
outlet:
- U: zero gradient
- p: fixed value 0 (reference pressure)
wall:
- U: uniform (0 0 0 )
- p: zero gradient

Depending on your model you need boundary conditions for the auxiliary values like k, epsilon or omega and/or others.

Beside this I recommend to avoid 3D at least in the first step. Use the cylinder symmetry instead. You have to mesh a 5° slice which is 1 element wide. Use the wedge boundary conditions at the free sides.

Changli · May 31, 2017, 17:44

Thank you for your suggestions, everything works fine with laminar case, and right now I need to consider some cases with turbulence model like k-w SST model. Then how should I set the boundary conditions in this case? Usually what kind of values can be used as default? And also I want to ask which turbulence model are the most common in reality? Is it k-w SST? Looking forward for your reply!

piu58 · June 1, 2017, 07:38

After "laminar" the next step is RANS. There are two models wide accepted: k-epsilon and k-omega.
I recommend to start with k-epsilon which is easier to understand and to implement (you don't need to implement it, it is ready available). It si numerical stable and does not lean toward instability.
k-omega has advantages in mixed flow (steep pressure gradients) where laminar and fully turbulent regions of the flow are in one model. k-omega tends to overestimate the turbulence form my experience. All in all k-epsilon gives a better solution far away from the walls, where k-omega is too turbulent. k-omega gives a better answer in the Prandtl layer and describes the evolution of turbulence better.

I am sure there exist more advanced models, which try to combine pros and cons. In most cases they are not as universal applicable as k-epsilon and k-omega.

Changli · June 2, 2017, 05:04

why the next step is to have transient + k-epsilon instead of steady + k-epislon? And can I consider I do steady + k-epislon then we get the mean field directly?

May 11, 2017, 16:53	How to simulate a flow in a 3D Pipe	#1
Changli New Member Changli Han Join Date: May 2017 Posts: 3 Rep Power: 9	1.I get scenario with a 3D pipe with a diameter of 5 meters, and with a length of 200 meters, also its bended shape in space likes a snake. I only know the water velocity inside of pipe is around 0.18m/s(at least at the entrance), no information about the pressure. How should I set the boundary conditions regarding the velocity, and also the pressure. Totally no clue about what the zeroGradient thing is doing. 2.And also want to know if I set the inlet velocity using the surfaceNormalFixedValue, how should I set the inlet pressure together with the outlet's velocity and outlet's pressure?

May 12, 2017, 01:23		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	I recommend to set inlet: - U: fixed value, your velocity - p: zerogradient outlet: - U: zero gradient - p: fixed value 0 (reference pressure) wall: - U: uniform (0 0 0 ) - p: zero gradient Depending on your model you need boundary conditions for the auxiliary values like k, epsilon or omega and/or others. Beside this I recommend to avoid 3D at least in the first step. Use the cylinder symmetry instead. You have to mesh a 5° slice which is 1 element wide. Use the wedge boundary conditions at the free sides. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

June 1, 2017, 07:38		#4
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	After "laminar" the next step is RANS. There are two models wide accepted: k-epsilon and k-omega. I recommend to start with k-epsilon which is easier to understand and to implement (you don't need to implement it, it is ready available). It si numerical stable and does not lean toward instability. k-omega has advantages in mixed flow (steep pressure gradients) where laminar and fully turbulent regions of the flow are in one model. k-omega tends to overestimate the turbulence form my experience. All in all k-epsilon gives a better solution far away from the walls, where k-omega is too turbulent. k-omega gives a better answer in the Prandtl layer and describes the evolution of turbulence better. I am sure there exist more advanced models, which try to combine pros and cons. In most cases they are not as universal applicable as k-epsilon and k-omega. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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May 31, 2017, 17:44		#3
Changli New Member Changli Han Join Date: May 2017 Posts: 3 Rep Power: 9	Thank you for your suggestions, everything works fine with laminar case, and right now I need to consider some cases with turbulence model like k-w SST model. Then how should I set the boundary conditions in this case? Usually what kind of values can be used as default? And also I want to ask which turbulence model are the most common in reality? Is it k-w SST? Looking forward for your reply!

June 2, 2017, 05:04		#5
Changli New Member Changli Han Join Date: May 2017 Posts: 3 Rep Power: 9	why the next step is to have transient + k-epsilon instead of steady + k-epislon? And can I consider I do steady + k-epislon then we get the mean field directly?