Help needed for CFX/Fluent analysis

Jaga · November 7, 2017, 01:19

Hi there guys. I am currently a student with 0 prior knowledge with Ansys, let alone CFX. I am currently designing a model of a raschel mesh net ( see attached images ) to do an analysis on the volume of water that is able to get trapped on the fibre of the mesh net.

I am currently looking to input a flow of fog ( simulated using water vapour ) at 2m/s flowing through the mesh net with approximately 300ml of water to be distributed over a period of 1 hour. ( * Sorry if i sound very ambitious with expectations of the simulation. I am unaware of the capabilities and limitations of the software still. )

My main goal is to calculate the rentention of water on the net fibre to calculate for the water harvesting capabilities. For example, 300 ml goes in over a period of 1 hour. After the 1 hour, maybe outlet has only 250 ml. Meaning to say supposedly 50 ml of water has been trapped/ bounced back by the mesh net.

May I know if the software is capable of fulfilling my requirements or if I need to take a chill pill on the ambitions I have. And if yes, could I request for some assistance to make this simulation work.

ghorrocks · November 7, 2017, 04:18

At a basic level, yes, CFX can model the basic physics going on here. You could model the fog as lagrangian particles and see how many impact the surface. There is an example of how to do this - the butterfly valve example looks at the impact of sand particles on the valve. Your application is similar, just the particles are now water. You can get the tutorial examples from the ANSYS customer website.

But when you start looking at the detailed physics going on you will find this is actually a challenging simulation. Some reasons include:

1) The water will build up and form droplets. This will affect the flow in the mesh and they way droplets impact the mesh. Modelling the build up of droplets on the surface will be very challenging.

2) When water droplets are very small and very close to the mesh some new forces come into play. These include electrostatics and even surface chemistry effects. These probably play an important role in determining whether a droplet hits or misses the mesh. Quantifying these effects and modelling them will be challenging.

3) Condensation and evaporation are also probably important. This is also a very challenging area.

So the basic simulation is quite straight forward and there are tutorials to get you started. If this is good enough for your purpose then fantastic. But be careful of the more complex factors I listed (and there may be more). If any of those are significant then this will be a very challenging model to get working accurately.

Jaga · November 7, 2017, 04:56

Thanks for the feedback. I have a feeling that you have completely understood what I am looking into. As I am studying the effects of fog on mesh nets, I actually am looking into the simulation of the water particles being retained at the surface level. Thus, like you said, I will be encountering the 3 issues which you have highlighted if I were to pursue a more in depth analysis study.

Based on what you have said, is it a situation whereby those issues will pop up during the running of the analysis or is it that they are of a whole different kinda analysis setting?

And much appreciated on the butterfly valve example. I will check it out as well.

ghorrocks · November 7, 2017, 05:35

At the student level I would just concentrate on what is possible, so look at the basic simulation method. Note that it is still a challenging simulation in itself and if you successfully complete it, it will demonstrate you understand how to conduct a CFD analysis.

It looks like the drop size of fog is well studied, so that helps a lot (eg http://journals.ametsoc.org/doi/pdf/...D%3E2.0.CO%3B2)

But I would look into whether there is also more complex physics at work. If you don't put them into the simulation then this will be a discrepancy between your model and the real physics, so this will be one explanation for any error in your simulation.

Jaga · November 8, 2017, 01:10

Thank you for the assistance. I will look into it and will update you on the progress.

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November 7, 2017, 04:18		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	At a basic level, yes, CFX can model the basic physics going on here. You could model the fog as lagrangian particles and see how many impact the surface. There is an example of how to do this - the butterfly valve example looks at the impact of sand particles on the valve. Your application is similar, just the particles are now water. You can get the tutorial examples from the ANSYS customer website. But when you start looking at the detailed physics going on you will find this is actually a challenging simulation. Some reasons include: 1) The water will build up and form droplets. This will affect the flow in the mesh and they way droplets impact the mesh. Modelling the build up of droplets on the surface will be very challenging. 2) When water droplets are very small and very close to the mesh some new forces come into play. These include electrostatics and even surface chemistry effects. These probably play an important role in determining whether a droplet hits or misses the mesh. Quantifying these effects and modelling them will be challenging. 3) Condensation and evaporation are also probably important. This is also a very challenging area. So the basic simulation is quite straight forward and there are tutorials to get you started. If this is good enough for your purpose then fantastic. But be careful of the more complex factors I listed (and there may be more). If any of those are significant then this will be a very challenging model to get working accurately.

November 7, 2017, 04:56		#3
Jaga New Member Singapore Join Date: Nov 2017 Posts: 3 Rep Power: 8	Thanks for the feedback. I have a feeling that you have completely understood what I am looking into. As I am studying the effects of fog on mesh nets, I actually am looking into the simulation of the water particles being retained at the surface level. Thus, like you said, I will be encountering the 3 issues which you have highlighted if I were to pursue a more in depth analysis study. Based on what you have said, is it a situation whereby those issues will pop up during the running of the analysis or is it that they are of a whole different kinda analysis setting? And much appreciated on the butterfly valve example. I will check it out as well.

November 7, 2017, 05:35		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,705 Rep Power: 143	At the student level I would just concentrate on what is possible, so look at the basic simulation method. Note that it is still a challenging simulation in itself and if you successfully complete it, it will demonstrate you understand how to conduct a CFD analysis. It looks like the drop size of fog is well studied, so that helps a lot (eg http://journals.ametsoc.org/doi/pdf/...D%3E2.0.CO%3B2) But I would look into whether there is also more complex physics at work. If you don't put them into the simulation then this will be a discrepancy between your model and the real physics, so this will be one explanation for any error in your simulation.

November 8, 2017, 01:10		#5
Jaga New Member Singapore Join Date: Nov 2017 Posts: 3 Rep Power: 8	Thank you for the assistance. I will look into it and will update you on the progress.