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Need Help-Residence time contours

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Old   April 11, 2007, 20:47
Default Need Help-Residence time contours
  #1
Garth Truesdale
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I am a fluent newbie and I need some help to accomplish something I am trying to do in Fluent. I am not sure if their is an easy way to do this. I want to plot contours of residence time. I am using fluent on a school design project. I am modeling airflow patterns inside of an enclosed geometry (3-D) with multiple velocity inlets and pressure outlets. The only species I have is air. A good metric to compare the different models I have ran would be to determine which models evacuate air from the geometry the fastest, and where my stale or old air is collecting, to aid in redesign of my vent placement.

I am pretty sure that this will involve solving a transient solution and writing some type of code to insert into Fluent as a user defined function. I have no experience with this so if anyone could lend some advice or knew of a good tutorial I would be very grateful. Thanks, Garth Truesdale
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Old   April 12, 2007, 09:32
Default Re: Need Help-Residence time contours
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bashu
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I assume you have tried Display-Pathlines and color the paths by particle_variables-time and that's not what you want, right?
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Old   April 12, 2007, 13:23
Default Re: Need Help-Residence time contours
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Garth
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Thank you for your response. Yes I have done that, but I am trying to mature my analysis a bit. The pathlines worked well when I was only doing a 2-D model. It is not what I want to do as it looks very messy in 3-D (hard to visualize and tell my 'story' of what is happening). What I want is filled contours (similar to velocity or pressure contours) where I can slice up the geometry and show where I have regions of stale air. Thanks in advance for your guidance. Garth
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Old   April 12, 2007, 14:20
Default Re: Need Help-Residence time contours
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bashu
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OK, here is a crazy solution: - write out the pathlines data (fluent 6.3, standard format) - this will give you the x,y,z and time for your injection - write out an interpolate file with one variable - a code can read the x,y,z information for each cell from the interpolate file, look in the pathline file and within a certain tolerance (cell size or ...) find the match data, average the residence time for all the data that matched the cell coordinate, write a new interpolate file with the residence time as the dependent varaible. - read back the interpolate file and contour plot it.

Problem is that there maybe some cells that may not get a hit from your search in the pathline file and the question is that what value you want to assign for those. Other problem is that the residence time is not a point value it can only be meaningful related to a start point. Another problem is that you will calculate some sort of average residence time for each cell. I am not sure what that means.

OK, if you ever decided to try this please let us know.
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Old   April 12, 2007, 15:53
Default Re: Need Help-Residence time contours
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Garth
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Before I get too involved with this method let me pose a question: Will this method work for an unstructured grid? I have been using an unstructured grid and I am thinking that if the cell size is the tolerance, that this may be a bit of trouble to determine. I have seen this done before, but I think that another program was used, for example maybe polyflow.

The problems that you mentioned will be difficult to overcome, as I know that every cell will not be getting a hit. The geometry is around 400 cubic feet, my mesh is just under 1 million nodes, etc...and the path lines do not go 'everywhere'. Thanks again for your insights!! I appreciate it a lot.
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Old   April 12, 2007, 23:58
Default Re: Need Help-Residence time contours
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bashu
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Cell size was just a suggesion for the tolerance. I am not exactly sure how much involved this method is though.

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Old   April 13, 2007, 03:24
Default Re: Need Help-Residence time contours
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Dr. Flow Squad
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Hi Garth. I'm not to sure how this could be done in FLUENT. But I have done it with CFX, easily. Have a look at http://www.cfd-online.com/Forum/cfx_...cgi/read/11571 Maybe you could get the necessary information from that. Or try CFX if you have access.
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Old   April 25, 2007, 13:20
Default Re: Need Help-Residence time contours
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Andy R
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See if you can get your hands on a UDF that fluent had available a while ago.

fanger.c

It literally defines a passive scalar which at convergence tells you the "age" of the air in a cell in seconds. Then just use path lines and color by that scalar.

- Andy R
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