Boundary Layer and Body Curvature

Jeff P. · June 18, 2016, 19:54

Hi,

I have been working with CFX as part of my undergraduate studies so please help my ignorance.

I am modeling the flow around a blunt, kidney shaped, disk as part of a project with an unstructured tetra mesh with SST. My issue arises in modeling the boundary layer and estimating the detachment point into the wake region. The area is of high curvature and traditional inflation seems to create nonphysical results as the normal velocity gradient seems high. I'm under the impression that the high aspect ratio doesn't do great with non-streamwise motion.

What would be the best way to model the boundary layer? Is an extremely fine mesh required to capture detachment or can I approximate?

Thank You!

ghorrocks · June 19, 2016, 06:23

I recommend you have a look into separation modelling with turbulence models. Many turbulence models have problems with getting separation points right. Having said that, SST is about as good as any of them so I do not recommend you change models. But it is important background information to know.

Please show some images of your mesh. Also show the y+ you have around your body, and any mesh sensitivity analysis you have done.

Jeff P. · June 19, 2016, 10:50

Glenn, I've looked into the ANSYS documentation on the SST model a bit. I'll see what else I can find.

As for my mesh, my y+ around the body is ~ 1 around the separation region and varies around 0.5 either way, elsewhere. I haven't done a sensitivity analysis yet as I was hoping to get a good model of the separation first. I have some tests which show it detaching ~75% up the body but that was for lower RE but still turbulent. I'm trying to match that. Below is my mesh in the detachment region. Its fine but the gradient is steep.

I'm just curious as to if there are any "tricks" or something I may be overlooking.

ghorrocks · June 19, 2016, 20:10

There is a big problem with your mesh - the jump in mesh size from the inflation layers to the bulk mesh is too big. You need to add more inflation layers and transition them to the bulk mesh when the element volumes are approximately equal.

Regarding turbulence models: Read a textbook here, as I was trying to suggest you become aware of the well known limitations of turbulence models in things like separations, swirling flows, reattachments, bulk separations etc.

Jeff P. · June 19, 2016, 20:57

Ah. That makes sense. Thank you very much.

June 18, 2016, 19:54	Boundary Layer and Body Curvature	#1
Jeff P. New Member Jeff Join Date: Jun 2016 Posts: 20 Rep Power: 9	Hi, I have been working with CFX as part of my undergraduate studies so please help my ignorance. I am modeling the flow around a blunt, kidney shaped, disk as part of a project with an unstructured tetra mesh with SST. My issue arises in modeling the boundary layer and estimating the detachment point into the wake region. The area is of high curvature and traditional inflation seems to create nonphysical results as the normal velocity gradient seems high. I'm under the impression that the high aspect ratio doesn't do great with non-streamwise motion. What would be the best way to model the boundary layer? Is an extremely fine mesh required to capture detachment or can I approximate? Thank You!

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June 19, 2016, 06:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	I recommend you have a look into separation modelling with turbulence models. Many turbulence models have problems with getting separation points right. Having said that, SST is about as good as any of them so I do not recommend you change models. But it is important background information to know. Please show some images of your mesh. Also show the y+ you have around your body, and any mesh sensitivity analysis you have done.

June 19, 2016, 10:50		#3
Jeff P. New Member Jeff Join Date: Jun 2016 Posts: 20 Rep Power: 9	Glenn, I've looked into the ANSYS documentation on the SST model a bit. I'll see what else I can find. As for my mesh, my y+ around the body is ~ 1 around the separation region and varies around 0.5 either way, elsewhere. I haven't done a sensitivity analysis yet as I was hoping to get a good model of the separation first. I have some tests which show it detaching ~75% up the body but that was for lower RE but still turbulent. I'm trying to match that. Below is my mesh in the detachment region. Its fine but the gradient is steep. I'm just curious as to if there are any "tricks" or something I may be overlooking.

June 19, 2016, 20:10		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	There is a big problem with your mesh - the jump in mesh size from the inflation layers to the bulk mesh is too big. You need to add more inflation layers and transition them to the bulk mesh when the element volumes are approximately equal. Regarding turbulence models: Read a textbook here, as I was trying to suggest you become aware of the well known limitations of turbulence models in things like separations, swirling flows, reattachments, bulk separations etc.

June 19, 2016, 20:57		#5
Jeff P. New Member Jeff Join Date: Jun 2016 Posts: 20 Rep Power: 9	Ah. That makes sense. Thank you very much.