twoPhaseEulerFoam - weird behaviour

grjmell · February 23, 2012, 07:13

To check some funcitonality of twoPhaseEulerFoam, I went back to basics and tried to do something really "simple". What i wanted to do is a box, with a layer of solids at the bottom, and above that just water. No inflow/outflow, just fluid at rest with sediment at the bottom of the box. I was expecting to see nothing happening. But velocities developed in the flow, out of nowhere.
I then tried removing the sediment, having just water in a box, no inflow/outflow. And again, some weird velocities develop. I have turned all turbulence etc.. off. I have tried buoyantPressure and zeroGradient BCs for pressure, and although the results are different, both result in some velocity in the box (see image, for t=1s). Have also tried an open boundary at the top but that didnt help either.
I have attached the case without sediment (however you can use the setFields file to add solids to the domain). I don't understand why the model is performing that way

, I couldnt find a problem in my set-up, but if someone could check or explain i'd be very grateful.

robbirobocop · February 23, 2012, 09:10

Having a look at your boundary conditions it can be seen that the top is opened by assigning a pressure with a fixedValue there.

Thus, a pressure field is calculated as can be seen in the second picture of the first line. Consequently, a flow movement occurs and velocity is build.

In a nutshell, with your assigned fixedValue for the pressure at the top you initialise the velocity field and thus, it does not come out of the blue for no reason.

In order to change that, you might work with a reference pressure inside your box and assign buoyantPressure to the top. I do not know if it will change anything since I usually do not simulate closed boxes

grjmell · February 23, 2012, 09:47

Ok I understand what you mean now. Can you explain what in a little more detail the bit with reference pressure please, and how id try to set it up...?

robbirobocop · February 23, 2012, 10:07

Inside the fvSolution file that is inside your case/system directory, there are two entries for the reference pressure.

pRefCell and
pRefValue

For the first entry you must enter coordinates in x,y and z direction, the second one specifies the value, e.g. 1e05 for ambient pressure.

Thus, the entries would be like:

pRefCell (0 1 0);
pRefValue 1e05;

grjmell · February 28, 2012, 07:42

Hi Rob,
Thanks for your help so far. I have implemented the pressure as suggested (i've attached the case with updated bcs), and it works (see 1st pic) probably as good as it ever will, only very small velocities is developed. it also works well for a flow-condition, e.g. moving lid. the same set-up however does not work once i add solids fraction, again weird things happen (see pic 2).
I'm not sure whether the BCs with a solid fraction need to be different then when you run it without solids? but it doesnt seem logical for that to be the case. anyway, i have tried with various different boundary conditions and i cant get it to work. any ideas?

Lydia · May 22, 2012, 05:07

Hello Foamers,

I just found this post, which is pretty similar to the one i recently started. If someone has similar problems, i recommend to have a look at the following post:

http://www.cfd-online.com/Forums/ope...behaviour.html

Alberto Passalacqua has given some good references for this topic here.

Best regards,
Lydia

May 22, 2012, 05:07	reference to similar post	#6
Lydia New Member Lydia Schulze Join Date: Jan 2012 Location: Karlsruhe, Germany Posts: 20 Rep Power: 14	Hello Foamers, I just found this post, which is pretty similar to the one i recently started. If someone has similar problems, i recommend to have a look at the following post: http://www.cfd-online.com/Forums/ope...behaviour.html Alberto Passalacqua has given some good references for this topic here. Best regards, Lydia

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February 23, 2012, 09:10		#2
robbirobocop Member Rob Join Date: Sep 2011 Posts: 55 Rep Power: 15	Having a look at your boundary conditions it can be seen that the top is opened by assigning a pressure with a fixedValue there. Thus, a pressure field is calculated as can be seen in the second picture of the first line. Consequently, a flow movement occurs and velocity is build. In a nutshell, with your assigned fixedValue for the pressure at the top you initialise the velocity field and thus, it does not come out of the blue for no reason. In order to change that, you might work with a reference pressure inside your box and assign buoyantPressure to the top. I do not know if it will change anything since I usually do not simulate closed boxes

February 23, 2012, 09:47		#3
grjmell Member Join Date: Nov 2010 Posts: 41 Rep Power: 16	Ok I understand what you mean now. Can you explain what in a little more detail the bit with reference pressure please, and how id try to set it up...?

February 23, 2012, 10:07		#4
robbirobocop Member Rob Join Date: Sep 2011 Posts: 55 Rep Power: 15	Inside the fvSolution file that is inside your case/system directory, there are two entries for the reference pressure. pRefCell and pRefValue For the first entry you must enter coordinates in x,y and z direction, the second one specifies the value, e.g. 1e05 for ambient pressure. Thus, the entries would be like: pRefCell (0 1 0); pRefValue 1e05;