## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### FLUENT – Trouble Finding Flow Separation Point using Wall Shear Stress

• dtmaidenmueller
Subscriber

For a class assignment, we have been directed to find the location of flow separation from a FLUENT simulation of flow around an airfoil (uniform free-stream velocity, Re=50k). We were directed to look at the wall shear stress and find the point where it changes sign.

From the simulation results, flow separation appears to occur a little over half-way along the airfoil.

• DrAmine
Ansys Employee
Wall Shear Stress is the magnitude of the wall shear stress component with X,Y and Z (if 3D) the vector components of the wall shear stress. The formal definitions to be found in the Manual.
So the methodology you are using is correct you can even make it more elegant by using X-wise based friction factor (C_f). Whenever that C_f changes the sign (really gets first zero then becomes negative) then separation occurs.
Looking into your geometry it is clear that x direction is not tangential to it: so you can think about some coordinate transformation if you want much more accurate representation.
• Rob
Ansys Employee
If you look https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v221/en/flu_ug/flu_ug_fvdefs.html you'll find the field definitions.
Wall Shear Stress won't switch sign as it's a function of flow speed, which is never negative. I suspect the minimum is what you're after which is the zero velocity point where the flow detaches.
As an aside, the mesh is horrible. You really want more resolution along the wing if you're after the separation position.
• dtmaidenmueller
Subscriber
Where can I find the manual? I can't access Rob's link because it keeps asking me to add an "account number" to my account when I sign in, but I just have the free student version.
• dtmaidenmueller
Subscriber
Noted on the x-direction mesh density. Thank you.
• DrAmine
Ansys Employee
Wherever you are in Fluent Hit F1 or click on Help. After a browser opens you can then click on the Link provided by Prof. .
• dtmaidenmueller
Subscriber
I can open the workbench help from within workbench (and I can log in fine), but when I try to navigate (from that browser) to Prof Rob's link, it goes back to the page where it wants me to "complete my registration" with an account number.
• DrAmine
Ansys Employee
Forget for now about the workbench: if you just launch Fluent and click on F1 what does it happen?
• dtmaidenmueller
Subscriber
When I press F1 in Fluent, nothing happens. When I use the menu to access "Online Technical Resources", I end up back at the "complete customer registration" page that wants an account number. The menu option for "User's Guide Contents" opens a page successfully, but I can't seem to find a section for the field definitions. Navigating some links on that page (Home, API Docs) takes me back to the "complete registration" page. I tried entering "Wall Shear Stress" in the search box, and it just spun for about 5 minutes without bringing anything up.
• Rob
Ansys Employee
Don't access "Online....." go to the User Manual as you did. Then put in the link I sent. Ch 37.4 in the Solver section of the User's Guide.