# Strange flow partern (Reverse Flow) in fluid past circular cylinder problem at exit

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June 6, 2016, 17:27
Strange flow partern (Reverse Flow) in fluid past circular cylinder problem at exit
#1
Senior Member

Hector Redal
Join Date: Aug 2010
Posts: 243
Rep Power: 15
Hi,

I would like to comment some strange pattern I am observing when simulating a flow past a circular cylinder.
I have detected that at the exit I see that the flow enters the domain at the upper side of the exit and exits at the lower side of the exit. I would expect that the fluid exits the domain accross the exit.

I am attaching an image where this strange pattern happens.

The boundary conditions I am using at the exit are du/dx = 0, dv/dx = 0 and dp/dx=0.

Reynolds number = 10
Richardson number = 1.0
gravitiy force downwards (negative value of y-direction).
Fluid is air.
The cylinder is hotter than the air.
I am trying to analyze the bouyancy effects caused by the hot cylinder.

If needed I can provide more details of the simulation.

Best regards,
Hector.
Attached Images
 ReverseFlowPastCylinder.jpg (71.8 KB, 30 views)

 June 6, 2016, 17:32 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,401 Rep Power: 68 The flow pattern appears too strange everywhere, not only at the exit... Are you using a 2D case? Because if you fix du/dx=0 at the exit, then the continuity constraint at the exit becomes dv/dy=0, therefore the v component must be constant (zero if it is zero at some points)

 June 6, 2016, 17:54 #3 Senior Member   Hector Redal Join Date: Aug 2010 Location: Madrid, Spain Posts: 243 Rep Power: 15 Hi Filippo You are right. That's a good point. I will check it. Thanks,

 June 6, 2016, 18:05 #4 Senior Member   Hector Redal Join Date: Aug 2010 Location: Madrid, Spain Posts: 243 Rep Power: 15 By the way, two questions: Are the boundary conditions du/dx = dv/dy = dp/dx = 0 the correct ones to impose at the exit? Or should I impose different boundary conditions?

 June 7, 2016, 00:30 #5 Senior Member   david Join Date: Oct 2012 Posts: 142 Rep Power: 12 I would move the outlet further away. You are imposing a boundary condition near a region where there is a lot of fluid interaction.

 June 7, 2016, 17:34 #6 Senior Member   Hector Redal Join Date: Aug 2010 Location: Madrid, Spain Posts: 243 Rep Power: 15 I have a question in regards, with the boundary constrains at the exit. In this type of simulations, the velocity vy at the exit usually shows a periodic pattern, which oscilates around a mean value which is zero. The usually constraint imposed at the exit is du/dx = dv / dx = 0. According to your comment if du/dx= 0, due to the continuity equation, the dv/dy must be also 0, which means constant value. But this is not the case since the velocity profile at the exit oscilates. How can this be?

June 7, 2016, 17:48
#7
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,401
Rep Power: 68
Quote:
 Originally Posted by HectorRedal I have a question in regards, with the boundary constrains at the exit. In this type of simulations, the velocity vy at the exit usually shows a periodic pattern, which oscilates around a mean value which is zero. The usually constraint imposed at the exit is du/dx = dv / dx = 0. According to your comment if du/dx= 0, due to the continuity equation, the dv/dy must be also 0, which means constant value. But this is not the case since the velocity profile at the exit oscilates. How can this be?

but the fixed condition du/dx= 0 (that drives to dv/dy=0) is your choice, that is an arbitrary boundary condition, therefore if you have at the exit an oscillating flow condition you cannot use du/dx=0. However, I suppose that if you increease the lenght of the domain sufficiently, the physical flow oscillations will be damped and the condition du/dx=0 could be acceptable. Note that the consequence that v=constant along y let such constant to vary in time.

 June 9, 2016, 17:04 #8 Senior Member   Hector Redal Join Date: Aug 2010 Location: Madrid, Spain Posts: 243 Rep Power: 15 Yes, you are right, the boundary condition at the exit is my choice. But it is the usually boundary conditions imposed in this kind of problems (Flow past circular cylinder). What I think is that one thing is dv/dy = 0 and other different thing is dv/dt = 0. In my opinion, dv/dt is different of zero because v oscilates among a mid point. But, if dv/dy = 0, I understand that all the points at the exit should have the same v value, but oscillating all them together. Could it be possible? I think so.

June 9, 2016, 17:24
#9
Senior Member

Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,401
Rep Power: 68
Quote:
 Originally Posted by HectorRedal Yes, you are right, the boundary condition at the exit is my choice. But it is the usually boundary conditions imposed in this kind of problems (Flow past circular cylinder). What I think is that one thing is dv/dy = 0 and other different thing is dv/dt = 0. In my opinion, dv/dt is different of zero because v oscilates among a mid point. But, if dv/dy = 0, I understand that all the points at the exit should have the same v value, but oscillating all them together. Could it be possible? I think so.