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Navin March 9, 2009 15:42

Units for Boundary Conditions

I am trying to find out the Input units for Boundary conditions, specially for Velocity Inlet, Can I change the input units, if yes How...??

I also want to display the Units in my plot and animation and I am not successful can anyone throw some light on it...

I appreciate your kind help

Thank you


MC March 9, 2009 16:35

Re: Units for Boundary Conditions
Hi~ FLOW-3D(R) doesn't know units so you have to stay consistent throughout. If you are using v931 you can set the units in the General Tab. These are the simulation units. You can view you results in the different units according to the conversions given by FLOW-3D(R). You can also modify the plot data to give you units not given by FLOW-3D(R). This however may not be necesssay. What version are you using so I can help you with displaying units in plots and animations and setting simulation units. MC

Navin March 9, 2009 17:08

Re: Units for Boundary Conditions

Thanks for your response

I am using the V9.31,

When I am setting the units as CGS in the genral tab and after starting the simulation I noticed that it changes itself to custom.

I am having a velocity input (2.75 m/s) so I tried to give 2750 in the boundary condition is that right...???

I assume the results were almost good at this value.

Am trying to simulate a water flow in a pipe of 40mm diameter and has a orifice action near the inlet (sudden reduation in the area dia reduced to 30mm) and nozzle action at the outllet with outlet dia being nearly 20mm...

then I want the water to flow out of pipe and opposite to gravity (up) and then come down due to gravity.

the flow enters at x axis and comes out with an angle of 153deg from x axis in XY plane.. I'm simulating the problem in 3D. Can you please help on this. Right now am getting flow outside the pipe... but the domain need to be big... but i run out of memory due to no of cells,

So am reducing the cell count by building cubic steps, do u have any suggestions for me in this regard.

Thank you once again


Navin March 9, 2009 17:14

Re: Units for Boundary Conditions
And one more thing I am not able to get the Flow outside the pipe into atmosphere as continous, its disrupted like its travelling in lumps and its never coming down even after 10-15metres.

MC March 9, 2009 18:08

Re: Units for Boundary Conditions
You are correct that the velocity boundary should be set at 2750 cm/s.

You can view the velocity in meters by Analyze tab>units then select SI and check the box to show units on graph.

How are you modeling the pipe? To reduce the number of active cells you can make the pipe a complement if you are using stl files. Then use a hole to cut for the domain outside of the pipe.

MC March 9, 2009 18:10

Re: Units for Boundary Conditions
Could you send me your prepin file?

Navin March 9, 2009 20:43

Re: Units for Boundary Conditions
Yes I can, I used solid works to model the pipe and then imported the STL file.. I tried to use to compliment but i was not sucessfull.. I will post the prep file in any filetransfer forum and will mail you, can you plese give me your mail id so that i can send the link.

Thank you


MC March 10, 2009 09:35

Re: Units for Boundary Conditions
Actually can you send the prepin file to support along with a copy of your license. I can look at it there. MC

Navin March 10, 2009 22:48

Re: Units for Boundary Conditions
Thank you for the suggestions,

I had almost solved the problem, expect for few doubts,

How to get the animation in 3D, I tried to take the output 3D I was not getting proper play, because everytime i tried to delete the images. One more thing was that though I selected the proper boxes the units were not displayed.

Thank you for your time

Regards navin

Kaushik B March 11, 2009 13:16

Re: Units for Boundary Conditions

Please correct me if I'm wrong and if I have missed any details to be considered.

Since you mentioned the inlet velocity is 2.75 m/s, should it not read 275 cm/s instead of 2750 cm/s in CGS system?

srbbd March 12, 2009 20:31

Re: Units for Boundary Conditions
Navin, your email is not working, could u pls have a check. I tried so many times to send an email but it bounce back. my address is

I am seeking your help to solve my CFD problem. Could you please help me?

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