[sandy_1982]

friends ,

i have a wing body geometry which i have meshed in ICEM-CFD - unstructured mesh with a prism layer having y+ as unity

i have done two cases ,
(a) with tetra size ratio (tsr) = 1.25 (in the Part Mesh Setup)

(b) with tetra size ratio = 0

i am using CFX solver and solving for Angle of attack 18deg,
what i notice is the following :

Mesh of case (a) shows the wing stalling at the root

Mesh of case (b) shows the wing stalling at the tip

i am confused now ..which one is correct ? which Mesh is showing the true physical picture od flow over the wing ? whether to give 'tsr' or not ?


Please share your experience on this

Thanks

Sandeeo

[Far]

Originally Posted by sandy_1982

friends ,

i have a wing body geometry which i have meshed in ICEM-CFD - unstructured mesh with a prism layer having y+ as unity

i have done two cases ,
(a) with tetra size ratio (tsr) = 1.25 (in the Part Mesh Setup)

(b) with tetra size ratio = 0

i am using CFX solver and solving for Angle of attack 18deg,
what i notice is the following :

Mesh of case (a) shows the wing stalling at the root

Mesh of case (b) shows the wing stalling at the tip

i am confused now ..which one is correct ? which Mesh is showing the true physical picture od flow over the wing ? whether to give 'tsr' or not ?


Please share your experience on this

Thanks

Sandeeo

Here is something from help:

Tetra Size Ratio
controls the growth (edge length) of tetra mesh as it moves away from the surface.

For Octree Tetra mesh, size transitions are 2 to 1 locally, but you can still affect the transition rate over a larger number of transitions. For example, if the surface size is 2 and the volume size is 64, you may see something size transitions like the following:

Delaunay 2 3 4.5 6.75 10.13 15.19 22.78 34.17 51.26 76.89
Octree 2 2 4 4 8 8 16 32 32 64
If the ratio was 1.5, the big picture would show both the Octree and Delaunay sizes growing at a rate of 1.5, but the Octree method has to fit it to powers of 2 times the smallest size in the model. Also, you can imagine that changing this to 1.4 or 1.6 might not make that much difference to the Octree mesher, at least not for the first few layers, but would directly change the Delaunay progression.

So I don't see they should make any difference. Please do the mesh independence study before drawing any conclusions!

[sandy_1982]

@Far : sorry i forgot to mention that i am using the Octree Mesh for both cases

i am studying the effect of 'tetra size ratio' on the cfd solution , so this is the only parameter that i am changing in to generate case (b) Mesh from case (a) Mesh

also what i have observed is that at AOA = 14 , 16 deg , 'tsr' is not having much effect with flow pattern over the wing more of less same , and separating over the root region..it is only at AOA = 18deg. that the change comes as i have mentioned in my first post.

as per your answer , what i understand is that i should not trust the solution which mesh with tsr = 1.25 gives untill i do a mesh independence - or in other words you;re suggeting that i try with tsr 1.3 or say 1.5 and then see ?

[Far]

Octree will only grow with the ratio of 2. So if you set it 0 or 1.5 it does not make any difference.

[sandy_1982]

"Octree will only grow with the ratio of 2. So if you set it 0 or 1.5 it does not make any difference"

mesh with tsr = 1.25 has 6.5 Million nodes
mesh with tsr = 0 has 5.1 Million nodes

and also the way cell growth happens is quite different specially in the vicinity of the body - which i have visually observed by taking a section plane.

so i think 'tsr' does make a difference in the mesh.

[Far]

yes you are correct it is making difference but in octree way...

Here is what I just got from TSR 1.2, 1.5 and 2 (0 is same as 2)

For TSR = 0 and 2
No of elements 33346 No of nodes 5837
For TSR = 1.5
No of elements 66457 and no of nodes 11409
For TSR = 1.2
No of elements 234322 and no of nodes 39710




PS:- Geometry dimensions are: Inner cube of size 1 and outer cube of size 10
Mesh sizes : Inner cube : 0.1, Outer cube : 2. Maximum seed size = 2

[sandy_1982]

@Far : thanks for the effort you've made for this small research into tsr

i am now inspired to do something like this for 'built topology/tolerance' concept
and the part sizing to be given..what i have observed is that for built topo with a topo tolerance 1 mm , i am still not able to mesh the flap of the wing (this one being a little more complicated geometry) which shows 'hole' in the geometry thought all the curves have turned red


shall update soon on any progress/ problem i face

Thanks

[Far]

there are many reasons for the hole problem. Recently i have defined two material points : Fluid and Solid . Solid was not closed on one side and even was not required as output mesh. But ICEM was giving error "hole in mesh"


You may give smaller value for the topo-tolerance. By default it is 1/2500 of diagonal of box around your geometry or domain.

Please use these documents as reference (By Simon): http://www.cfd-online.com/Forums/ans...efinement.html