Ahmed body simulation on a straight line (Steady-State)

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AMR96
Posts: 2
Joined: Tue Mar 21, 2023 11:11 am

Ahmed body simulation on a straight line (Steady-State)

Post by AMR96 »

Hello everyone,

I am new to the OpenFoam and FreeCAD CfdOF community. I am working on a project where I have to study the aerodynamics of cornering of an Ahmed body but since I am just starting with OpenFoam I decided to first simulate a straight line case to familiarize myself with OF.

The problem I am facing right now is with the residuals. After some time my residuals become flat and don't decrease at all, this is especially the case for the pressure residual as you can see in the photo below. I am aware that residuals are not the sole indicator of accuracy of the results and I have also monitored lift and drag coefficients but they also seem to be somewhat off from the experimental results in the literature. I have used cfmesh for the case which is attached to the post.
Below are the BCs and initial conditions that have used:
Inlet: Velocity inlet v=40 m/s
Outlet: Pressure outlet P=0 Pa
Domain walls (top and left side): Slip
Ground: No slip
Right wall: Symmetry
Car body: No-slip

Initial conditions for velocity and pressure are set to potential flow. The flow is turbulent and RANS SST k-omega is used.

I would appreciate any help that would node me in the right direction.
Residual curves
Residual curves
residual_ahmedbody.JPG (57.75 KiB) Viewed 824 times
Lift and drag coefficients
Lift and drag coefficients
cd_cl_ahmedbody.JPG (57.97 KiB) Viewed 824 times
Attachments
ahmed_body_sym.FCStd
Simulation file
(73 KiB) Downloaded 50 times
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Raedchen_im_System
Posts: 95
Joined: Sun Oct 27, 2019 5:22 pm
Location: In der Nähe von Stuttgart
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Re: Ahmed body simulation on a straight line (Steady-State)

Post by Raedchen_im_System »

hi @AMR96,

after I meshed your model, I ran the function "check mesh":
check_mesh.jpg
check_mesh.jpg (39.51 KiB) Viewed 786 times
The "check mesh" function gave me 4 areas where mesh was not OK. The area where problems with meshing occured is the area under the body:
area not meshed.JPG
area not meshed.JPG (116.79 KiB) Viewed 786 times
The root cause of the mesh error is that the meshing function has changed. Currently, you define a mesh refinement of 0.5 on the surface which is equal to the mesh refinement via the extra body. You should change the mesh refinement on the surface to 0.25. In this post, you will please find a solution for your problem: viewtopic.php?p=652814#p652814
AMR96
Posts: 2
Joined: Tue Mar 21, 2023 11:11 am

Re: Ahmed body simulation on a straight line (Steady-State)

Post by AMR96 »

Raedchen_im_System wrote: Tue Mar 21, 2023 9:27 pm hi @AMR96,

after I meshed your model, I ran the function "check mesh":

check_mesh.jpg

The "check mesh" function gave me 4 areas where mesh was not OK. The area where problems with meshing occured is the area under the body:

area not meshed.JPG

The root cause of the mesh error is that the meshing function has changed. Currently, you define a mesh refinement of 0.5 on the surface which is equal to the mesh refinement via the extra body. You should change the mesh refinement on the surface to 0.25. In this post, you will please find a solution for your problem: viewtopic.php?p=652814#p652814
Thanks for your reply @Raedchen_im_System. I tried what you suggested but when I click on check mesh, I get the following error at the end ''3 mesh checks failed''. However, the ground is now meshed as in the picture. Could this be due to the contact between the underside cylinders and the enclosure?
check_mesh.JPG
check_mesh.JPG (231.03 KiB) Viewed 676 times
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Raedchen_im_System
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Joined: Sun Oct 27, 2019 5:22 pm
Location: In der Nähe von Stuttgart
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Re: Ahmed body simulation on a straight line (Steady-State)

Post by Raedchen_im_System »

Hello @AMR96

when I look into into your model, it looks like CfMesh has an issue with the 2 intersecting fillets:
Problems fillet.jpg
Problems fillet.jpg (181.53 KiB) Viewed 603 times
As far as I know, the "checkMesh" - function in the mesher of CfdOf indicates that your mesh has issues, but it does not show you these issues in the 3D-view. My proposal would be to change the fillets
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