kutta condition and separated flow in transient simulation

Nick R · April 19, 2011, 01:40

Hi,

In my incompressible simulation of flow over naca 0021 ( using the GAMMA-Theta steady solver first and then switching to transient) I notice that the flow from under the trailing edge whips around the foil there and enters the separated region. It looks like this is a violation of the kutta condition. Has anyone observed anything like that?
The AOA is 4deg.

ghorrocks · April 19, 2011, 07:41

What is a violation of the kutta condition?

Nick R · April 19, 2011, 08:12

The kutta condition dictates that the flow should leave the trailing edge smoothly but if the flow from underneath goes around the trailing edge to the top doesnt that negate the Kutta condition?

siw · April 19, 2011, 13:56

Surely, that is how the airflow separates on the upper surface anyway at some angle of attack? By the airflow traversing from the lower surface and effectively "peeling" the boundary layer off the upper surface. So in order to do that the airflow must flow around the trailing edge and in the upstream direction to seaparte the downstream airflow. Keep this going and it stalls.

Nick R · April 20, 2011, 00:13

Thanks. I let the flow evolve and the flow under the trailing edge is leaving it smoothly and there's a separation zone above. Now I have another question (I think you guys are quite experienced and can help ) This is with regard to the currant (Courrant number- I hear a joke coming) . Is this number only important for explicit schemes? In my simulation I use the backward Euler which is an implicit scheme so does this mean this number is not important?
Thanks in advance.

ghorrocks · April 20, 2011, 00:37

Well, before you ask about any strange flow behaviour please check your simulation is accurate, otherwise you are wasting everybody's time. Here are some tips: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Here is something with a high currant number:
images.jpg

I suspect you really mean Courant number. As CFX is an implicit solver it is not limited by Courant number. The Courant number can give you a guide as to what time step to use but nothing more. You need to do a sensitivity check on the time step size to find what time step size your model really needs.

April 19, 2011, 01:40	kutta condition and separated flow in transient simulation	#1
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Hi, In my incompressible simulation of flow over naca 0021 ( using the GAMMA-Theta steady solver first and then switching to transient) I notice that the flow from under the trailing edge whips around the foil there and enters the separated region. It looks like this is a violation of the kutta condition. Has anyone observed anything like that? The AOA is 4deg.

April 20, 2011, 00:13		#5
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Thanks. I let the flow evolve and the flow under the trailing edge is leaving it smoothly and there's a separation zone above. Now I have another question (I think you guys are quite experienced and can help ) This is with regard to the currant (Courrant number- I hear a joke coming) . Is this number only important for explicit schemes? In my simulation I use the backward Euler which is an implicit scheme so does this mean this number is not important? Thanks in advance. Last edited by Nick R; April 20, 2011 at 00:57.

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April 19, 2011, 07:41		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	What is a violation of the kutta condition?

April 19, 2011, 08:12		#3
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	The kutta condition dictates that the flow should leave the trailing edge smoothly but if the flow from underneath goes around the trailing edge to the top doesnt that negate the Kutta condition?

April 19, 2011, 13:56		#4
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 739 Rep Power: 26	Surely, that is how the airflow separates on the upper surface anyway at some angle of attack? By the airflow traversing from the lower surface and effectively "peeling" the boundary layer off the upper surface. So in order to do that the airflow must flow around the trailing edge and in the upstream direction to seaparte the downstream airflow. Keep this going and it stalls.

April 20, 2011, 00:37		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Well, before you ask about any strange flow behaviour please check your simulation is accurate, otherwise you are wasting everybody's time. Here are some tips: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F Here is something with a high currant number: images.jpg I suspect you really mean Courant number. As CFX is an implicit solver it is not limited by Courant number. The Courant number can give you a guide as to what time step to use but nothing more. You need to do a sensitivity check on the time step size to find what time step size your model really needs.