# compressible low speed channel flow

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 May 8, 2013, 12:27 compressible low speed channel flow #1 New Member   Ali Join Date: May 2013 Posts: 9 Rep Power: 5 Hi all I am trying to simulate the low Reynolds (say Re=5) mass driven compressible flow in a rectangular microchannel. I used rhoCentralFoam with the following BC: Pressure: inlet { type zeroGradient; } outlet { type fixedValue; value uniform 101315; } Velocity: inlet { type fixedValue; value uniform (0.79473 0 0); } outlet { type zeroGradient; } However, the results are not physical for example a reverse flow appears at the inlet! I am wondering if I should use another solver or other BC? Is there anyone who can help me? Thanks.

 May 9, 2013, 00:25 #2 New Member   Ali Join Date: May 2013 Posts: 9 Rep Power: 5 Hi I am really confused with the simple problem of straight rectangular channel. I need to solve the flow but really cant find out what the problem is with the setting of the openFoam. I would be greatly thankful if any body has a promising feedback.

 May 9, 2013, 01:26 #3 Senior Member   Nima Samkhaniani Join Date: Sep 2009 Location: Tehran, Iran Posts: 1,195 Blog Entries: 1 Rep Power: 16 Dear Ali @ inlet, fixed your pressure and @ outlet, assign a zeroGradient __________________ Telegram channel (https://telegram.me/openfoam4Iranian) My Weblog (http://openfoam.blogfa.com/) Training Course on OpenFOAM at (http://www.isme.ir/)

 May 9, 2013, 03:54 #4 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 9 The problem in your case is that the pressure gradient in your domain is too small. You should work with a solver using a "relative pressure". Your case reminds me this topic: heat transfer with RANS wall function, over a flat plate (validation with fluent) You can also try another solver. rhoSimplecFoam / rhoPimpleFoam Edit: put your full case here. Maybe your problem is not located in the boundary conidtion.

 May 9, 2013, 04:35 #5 New Member   Ali Join Date: May 2013 Posts: 9 Rep Power: 5 Dear Nima Thanks a lot for your kind reply. I changed the BC as you said but the solution is wrong again. I also set the inlet pressure somewhat higher that the interior to avoid any back flow but surprisingly this leads to a stronger reverse flow. can you please help me on this?

 May 9, 2013, 04:54 #6 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 9 By the way, why do you need a compressible flow ? And try to post your full case. Maybe you made a mistake somewhere else.

May 9, 2013, 05:11
#8
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HECKMANN Frédéric
Join Date: Jul 2010
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Instead of copying all the file here, you can actually upload them. There is a function "Attach Files" just below the submit button.

rhoSimplecFoam can be either turbulent or laminar. You can use k-epsilon, k-omega, Sparlat Allmaras, or laminar. And if you use rhoPimpleFoam, you can use all those model and even LES.

The official description is:
Quote:
 rhoSimplecFoam Steady-state SIMPLEC solver for laminar or turbulent RANS flow of compressible fluids
So yes, this solver can do the job...

 May 9, 2013, 05:15 #9 Senior Member   HECKMANN Frédéric Join Date: Jul 2010 Posts: 237 Rep Power: 9 I'm not familiar with rhoCentralFoam but I don't think it is suitable for you. rhoCentralFoam is a density based solver (which was primarily designed for supersonic flow) while rhoSimplecFoam is a pressure based solver (which was primarily designed for subsonic flow).

May 9, 2013, 05:17
#10
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Ali
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Quote:
 Originally Posted by fredo490 By the way, why do you need a compressible flow ? And try to post your full case. Maybe you made a mistake somewhere else.
Dear Frédéric
I have solved the flow with an incompressible CFD code but whereas the pressure drop in microchannels are considerable and also there is a considerable temperature gradient in my problem, I am curious to see what happen if I use a compressible solver. Actually I have to compare my results with those of compressible case.

 May 9, 2013, 05:29 #11 New Member   Ali Join Date: May 2013 Posts: 9 Rep Power: 5 Thanks for you comments dear Frédéric (I am a row new member in this site )OK; I will try the rhoSimplecFoam hope it work. Then I will tell you about the results. Again thanks for your kind informative comments.

May 9, 2013, 07:40
#12
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HECKMANN Frédéric
Join Date: Jul 2010
Posts: 237
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Quote:
 Originally Posted by -Ali Dear Frédéric I have solved the flow with an incompressible CFD code but whereas the pressure drop in microchannels are considerable and also there is a considerable temperature gradient in my problem, I am curious to see what happen if I use a compressible solver. Actually I have to compare my results with those of compressible case.
Well, you didn't mention that before... I'm not familiar with micro channels. Where does the heat come from ?

Also, are you sure your flow is really 100% laminar ? How is the flow at the inlet of your simulation ? Does it come from a "tube", how long ? If yes, you might need to consider the turbulence of the upstream flow.

The problem in micro channel, if I remember well, is that the boundary layers tend to meet at the middle of the channel. Maybe a fine mesh with a k-omega SST model can handle this problem. You might need to compare two simulations (laminar vs k-omega sst).

 Tags channel flow, compressible solver, low speed, mass driven, rhocentralfoam

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