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## Simulation Mechanical and Multiphysics

Active Member
Posts: 6
Registered: ‎09-06-2011

# Problem with Simulation of an "Open Channel Flow"

1583 Views, 10 Replies
09-06-2011 06:02 AM

Hi,

my name is Johannes. I come from Germany and i have a Student Version of Autodesk Simulation 2012. First...sorry for my writing...i am not the best in english.

My Problem is the simulation of an open channel flow.

I have a Tube witch has 4 open connections. That means the first 3 connections are inlets. the last is the outlet of all 3 inlets.

It looks like a straight tube. the 3 inlets come from the side and the outlet is in line. I hope you understand...if not please tell me what i should describe better.

The Main Problem is the configuration of the "initial fluid volume". I can build a big cube for example over the tube but what should i do with that? How can i define my tube model with 2 materials (gas and water)??? And how can i define that every inlet of the tube will take 6,5litres/min.?in case of that i would not need the initial fluid volume because the 6,5l/min come constantly...

I also tried quite easier simulations but that also went wrong. The easier description would be a tube with a lenght of 1m. At the first side is the inlet witch a fauced and at the other side is nothing and the water can leave the tube without any resistance. And for open channel flow the diameter of the tube is much much bigger than the diameter of the faucet. do you understand my problem? I just have no idea what i should do with my tube.

(Of couse i tried the ball valve tutorial and some other normal simulations of CFD...Everything is working but i cant simulate a fluid in an open channel. And i try that simulation now for about one week with about 5-7 hours every day...so frustrating)

i hope i didnt make toooooooo many mistakes.

Johannes

Employee
Posts: 492
Registered: ‎03-25-2010

# Re: Problem with Simulation of an "Open Channel Flow"

09-06-2011 08:35 AM in reply to: JF16625

Hi Johannes,

If I understand your description, the pipe will not always be filled with water. My guess is you want to know what volume of the pipe is filled when the three inlets provide 6.5 liters/min. Naturally, the volume inside the pipe and inlets are the model that needs to be meshed; the solid pipe is not needed for the simulation. (Are there pipes at the inlet? If so, you should include a short section of inlet pipe in the simulation. Maybe 10 diameters or so.)

It would be best to start the simulation with the pipes completely filled, So the "initial fluid volume" is a big box that encloses your entire model.

You do not need to designate where the air is in the model. The absence of water is equivalent to air, so the solution will calculate where that will be.

The inlet flow rates can be applied to the inlet surfaces as a velocity (you would need to calculate it) or as a fan (and choose the option to specify the flow rate).

Good luck.

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
Active Member
Posts: 6
Registered: ‎09-06-2011

# Re: Problem with Simulation of an "Open Channel Flow"

09-06-2011 11:48 AM in reply to: INACTIVE_AstroJohn

Hi, thank you for the quick answer,

I made a picture of the complete part I want to simulate.

In reality the 3 inlet tubes are not directly upwards...they have different degrees to horizon. but i just want to learn it

and if i can manage this easier made model I think I can manage the "original" situation.

so....

all 3 tubes should deliver 6,5l/min. In this case...from above

The lower accumulative tube is only opended to the front. the back is closed. The front end is opened and the Water falls down...to the ground.

The simulation should show the water level and speed inside the accumulative tube. But this tube is not filled completly

I hope this description is better than the last

I try it tomorrow with fans and your description. I hope you can help me with my "little" problem.

thanks a lot

Johannes

Active Member
Posts: 6
Registered: ‎09-06-2011

# Re: Problem with Simulation of an "Open Channel Flow"

09-07-2011 12:30 AM in reply to: JF16625

the next problem is that I cant mesh the hole part....not as a tube out of one part and not as a tube out of several parts...the meshing method i chose was Tetrahedra and wedges (boundary layer)﻿. it doesnt work...even at the smalest mesh size...

the method cuboid and tetrahedra works but only with several warnings...the calculation is shutting down after the second step...

i dont know what i have to do.

Employee
Posts: 492
Registered: ‎03-25-2010

# Re: Problem with Simulation of an "Open Channel Flow"

09-07-2011 07:28 AM in reply to: JF16625

Johannes,

Are you trying to mesh the tubes or the fluid inside the tubes? Because you are doing a fluid flow analysis, you need to mesh the fluid inside the tubes.

The fluid can be generated in Autodesk Simulation (Mesh > CAD Additions > Internal Fluid), or your CAD application should be able to create it, too.

If you are still having problems, you can create an archive of the model and post it to your reply. Please read the new post that I am about to make which describes how to create and post an archive to the discussion group.

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
Active Member
Posts: 6
Registered: ‎09-06-2011

# Re: Problem with Simulation of an "Open Channel Flow"

09-07-2011 11:29 PM in reply to: INACTIVE_AstroJohn

Hi,

off course i tried to mesh the inside of the part.

now i created a much more simple part for testing the open channel flow.

its a 90degree tube system with a smaller inlet than the rest of the tube system

i meshed it correct without any mistakes.

i also created a volume of 64m³ Start Volume.

i tried it to analyze with original parameters and with changed parameters. It always breakes of calculating after the first or the second step...

"time is too short to analyze...you should usw a time step of 4E-4" or something like that...

well when i make more steps to calculate it goes to step 3 for example and then it breaks of the simulation...

thats the mistake with the mesh like in ball valve tutorial

the other mistake is something with the mesh i createt...i dont know the correct sentence now but the programme says that it cant calculate the mesh

thats the mistake with the mesh with tetr. and quader...

well i simulatet tube systems with full filled tubes already. its working fine. but the open channel flow is really bad...

everybody can try it without a part of mine because ist now only a right angel of 2 tubes... from the top only drops a little water inside of about 5l/min and a tube diameter of about 25mm (for example inlet tube). at the bottom side the water comes out of the tube without any resistance and leaves it to the ground(outlet tube diameter of about 50mm)...

that shouldn´t be too hard to simulate but it doesnt work.

i would be very happy if someone can tell me how to set the parameters for simulation...i hope that there is the mistake...

I made a PDF File of the report. see the attachment please. i am sorry but it is in german...maybe someone can read or compare it.

thanks

Johannes

Employee
Posts: 492
Registered: ‎03-25-2010

# Re: Problem with Simulation of an "Open Channel Flow"

09-08-2011 07:18 AM in reply to: JF16625

Hi Johannes,

I suggest that you provide an archive of your model. It will be easier for you to know what's wrong. If someone provides a model that works, you still will not know what the problem is in your model.

See the post Create, Post, or Provide an Archive of your model if you need guidance.

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
Active Member
Posts: 6
Registered: ‎09-06-2011

# Re: Problem with Simulation of an "Open Channel Flow"

09-08-2011 11:27 PM in reply to: INACTIVE_AstroJohn

ok. i made an archive of my model.

its now an easier model...with only one inlet in 90° angle. i already set the inlet velocity and the outlet surface.

i didnt change any parameters or something else. i also didnt create the starting fluid volume.

its just the model with inlet and outlet and its meshed.

i hope i didnt do anything wrong and you can help me.

you can find the ach file there:

Johannes

also you can see the picture of my try yesterday...it cant be true that this small model will calculate 5 steps in about 3 hours... and its still filled and no air is inside it....well...i made something wrong. i had to set the "max. iterations" at 1000 because below that the calculation stopped with the message something like that: "couldnt find the free volume" not correct message but something like this.

Employee
Posts: 259
Registered: ‎06-14-2010

# Re: Problem with Simulation of an "Open Channel Flow"

09-12-2011 07:04 AM in reply to: JF16625

This was from a knowing software limitation that inlet/outlet boundary condition surface can't have pressure gradient due to gravity, this implies that the selected inlet/outlet boundary condition surface should be perpendicular to gravity direction so that gravity produces zero pressure gradient.

If above restriction is not satisfied, the solution will have poor convergence, the picture attached on 09-08-2011 11:27 PM typically showed the pattern of poor convergence.  This restriction impacts on (a) Open channel flow, (b) Coupled fluid/thermal analysis, both of them have gravity static pressure involved.

A workaround is that extending a section from existing inlet/outlet surface and make the new inlet/outlet boundary satisfy above restriction.

Jianhui Xie, Ph.D
Principal Engineer
MFG-Digital Simulation
Active Member
Posts: 6
Registered: ‎09-06-2011

# Re: Problem with Simulation of an "Open Channel Flow"

09-19-2011 01:26 AM in reply to: Joey.X

Sorry... I was away last week.

So...i read the answer a few times...and even with a translation into German I dont know what i have to to with my model.

If I make an extension of my model...where should I place the inlet and outlet surface? How should i extend the model...when i create a mesh of the inside of my tubes it would be just the same as now? Sorry but i really dont understand...

I tried it with small and big start volumes and I also tried it with putting or not putting the hole model into the starting volume...nothing changes the effect of calculation error...

If somebody can simulate my model and post it as an archieve file I can see what i have done wrong.