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Higher value of Cl and Cd

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Old   February 16, 2024, 05:23
Default Higher value of Cl and Cd
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aravind
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Hello everyone, I am trying to validate my result with experiments for Naca0008 airfoil at Re NO 6000. My results agree with experiments for lower angle of attack say 0,1,2 but after 5deg my simulated values is way off from experiments. I am using Cl and Cd to validate. PisoFoam is the solver i am using, for simulation type laminar i am using and mesh spacing is made with yplus values below 1. My mesh is made with domain spacing of 8.7C in the upstream of airfoil and 20C in downstream direction, 10C over top and bottom of airfoil. Below attached file of my case.
Please do share your thought on this.

I checked mesh quality with checkMesh and i shared it below for reference.

Time = 0

Mesh stats
points: 569562
internal points: 0
faces: 1134399
internal faces: 564837
cells: 283206
faces per cell: 6
boundary patches: 6
point zones: 0
face zones: 2
cell zones: 0

Overall number of cells of each type:
hexahedra: 283206
prisms: 0
wedges: 0
pyramids: 0
tet wedges: 0
tetrahedra: 0
polyhedra: 0

Checking topology...
Boundary definition OK.
Cell to face addressing OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
Patch Faces Points Surface topology
inlet 1075 2152 ok (non-closed singly connected)
lower 501 1004 ok (non-closed singly connected)
outlet 308 618 ok (non-closed singly connected)
topAndBottom 764 1532 ok (non-closed singly connected)
upper 502 1006 ok (non-closed singly connected)
BaseAndTop 566412 569562 ok (non-closed singly connected)

Checking faceZone topology for multiply connected surfaces...
FaceZone Faces Points Surface topology
vc-2-interiorFaces 564837 569558 multiply connected (shared edge)
vc-2-boundaryFaces 569562 569562 ok (closed singly connected)
<<Writing 563264 conflicting points to set nonManifoldPoints

Checking basic cellZone addressing...
No cellZones found.

Checking geometry...
Overall domain bounding box (-1.32506 -1.50093 0) (3 1.50029 0.00664576)
Mesh has 2 geometric (non-empty/wedge) directions (1 1 0)
Mesh has 2 solution (non-empty) directions (1 1 0)
All edges aligned with or perpendicular to non-empty directions.
Boundary openness (-8.74733e-19 -3.39235e-19 4.24076e-14) OK.
Max cell openness = 7.66769e-16 OK.
Max aspect ratio = 85.027 OK.
Minimum face area = 3.27005e-08. Maximum face area = 0.00304928. Face area magnitudes OK.
Min volume = 2.1732e-10. Max volume = 2.02648e-05. Total volume = 0.0798046. Cell volumes OK.
Mesh non-orthogonality Max: 35.3512 average: 13.2646
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.643822 OK.
Coupled point location match (average 0) OK.

Mesh OK.

End
Attached Images
File Type: jpg Screenshot 2024-02-16 154954.jpg (180.3 KB, 9 views)
File Type: jpg Screenshot 2024-02-16 155156.jpg (195.5 KB, 10 views)
Attached Files
File Type: zip naca0008_5deg_casefile.zip (5.5 KB, 2 views)
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Old   March 15, 2024, 10:10
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do you have vortex shedding/wake phenomena that you do not resolve for higher angles of attack?
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Old   March 16, 2024, 11:42
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First of all thank you for your response, yes sir i am able to resolve vortex shedding.
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Old   March 17, 2024, 09:13
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Could you please show a plot of simulated drag/lift versus experimental drag/lift?
For the turbulent case i know that after stall the flow becomes 3D and 2D simulations fail bc of that. Is the same phenomena maybe also for the laminar flow valid?
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Old   March 17, 2024, 12:23
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Sir , another issue that i am facing is there is some little region of separation near leading edge of the airfoil, i don't know whether it is numerical instability or not. This is leading edge separation is present for angle of attack over 6deg less than that no such error occurred. I am using the same mesh topology and node points over the airfoil irrespective of angle of attack.
Attached Files
File Type: pdf Problem_airfoil.pdf (187.6 KB, 4 views)
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Old   April 12, 2024, 22:45
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Quote:
Originally Posted by arauv View Post
Sir , another issue that i am facing is there is some little region of separation near leading edge of the airfoil, i don't know whether it is numerical instability or not. This is leading edge separation is present for angle of attack over 6deg less than that no such error occurred. I am using the same mesh topology and node points over the airfoil irrespective of angle of attack.
I looked at your pdf. Nice plots by the way. From the visible vortex street, the CL should be unsteady. Could it be that you are plotting maximum CL, while the experiment is more an average CL?

This is a laminar flow condition. Laminar flow will always have a separation bubble where the flow meets an adverse pressure gradient. So I would expect one on the leading edge at higher angles of attack.
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