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Old   November 11, 2009, 12:07
Default BL meshing airfoil with gap
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Hi,

I have a 2D airfoil that has a gap along the surface for blowing.

Gambit is unable to create a boundary layer on the surface, stating it is not the correct entity when I tried to select the edge. I assumed this is due to the gap.

Can I have some advice how I should create the BL or mesh this airfoil?

Thank you.
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Old   November 11, 2009, 16:37
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Quote:
Originally Posted by ghost View Post
Hi,

I have a 2D airfoil that has a gap along the surface for blowing.

Gambit is unable to create a boundary layer on the surface, stating it is not the correct entity when I tried to select the edge. I assumed this is due to the gap.

Can I have some advice how I should create the BL or mesh this airfoil?

Thank you.
Would something like this work? (See attachment.)

airfoil gap.JPG
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Old   November 11, 2009, 16:59
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I think Chris's suggestion is the right way to do it. split the wall, assign inlet BC to the blowing part
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Old   November 11, 2009, 21:40
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sorry, but i do not quite understand the image. how do i actually split the wall?

my airfoil is 2D, and there is a gap at the top wetted surface of the airfoil, but no gap at the bottom of airfoil.

and the blowing is tangential to the airfoil surface, not normal.
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Old   November 12, 2009, 19:25
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Quote:
Originally Posted by ghost View Post
sorry, but i do not quite understand the image. how do i actually split the wall?

my airfoil is 2D, and there is a gap at the top wetted surface of the airfoil, but no gap at the bottom of airfoil.

and the blowing is tangential to the airfoil surface, not normal.
I'm not sure of what your configuration actually is. Is it something like what I have attached below? If not, could you please upload a sketch so we know what's going on?

airfoil gap 2.jpgairfoil gap 3.JPG
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Old   November 12, 2009, 21:39
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this is an image of the leading edge of the aerofoil. there is a gap where there will be blowing.

i would like to add BL along the surface of the aerofoil.
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File Type: jpg mesh.jpg (13.6 KB, 35 views)
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Old   November 13, 2009, 09:31
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Quote:
Originally Posted by ghost View Post
this is an image of the leading edge of the aerofoil. there is a gap where there will be blowing.

i would like to add BL along the surface of the aerofoil.
This is how I would mesh this:

Start off with an airfoil without at gap. Split the edge of the airfoil with verticies to define the gap, but don't delete the new edge that is formed from the split.

Next, offset the vertex at the origin some distance in the negative x direction and create an edge between this new vertex and the vertex at the origin. Then, under the face geometry toolbar, select "Sweep Edges" under "Form Faces", pick the edge you just created as the edge to be swept, pick the edge between the leading edge vertex of the airfoil and the start of the gap as the path edge, and under type select "Perpendicular", and click "Apply." Do this two more times, next using the edge that defines the gap as the path edge, and then using the edge after the end of the gap. After attaching a boundary layer to the edges that define the airfoil AND the edge that defines the gap, you should have something like this:

airfoil gap 4.JPG

Meshing the inside of the airfoil with a structured grid is tough due to the singularity at the edge of the gap, so I used an unstructured quad mesh as a first pass. Also, I made sure to cluster grid around the gap. This is what I got (the second image below is a close-up view of the gap):

airfoil gap 5.jpgairfoil gap 6.jpg

For boundary conditions, I would set "Wall" for the airfoil and "Interior" for the edges in the interior of the domain and the gap edge. I'm not really sure what to set for the inlet to the interior of the airfoil (maybe "Mass Flow Inlet"?)

I hope this helped!
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Old   November 29, 2009, 22:45
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after creating an edge over the gap, and creating the BL, how do I remove the edge? it cannot be deleted since it is being referenced by the BL now.
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Old   November 30, 2009, 11:25
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Quote:
Originally Posted by ghost View Post
after creating an edge over the gap, and creating the BL, how do I remove the edge? it cannot be deleted since it is being referenced by the BL now.
You don't need to delete it. Make it an "interior" boundary.
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Old   November 30, 2009, 13:15
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thanks for the help.

now i have another problem. the BL mesh does not attach to the surface of the aerofoil, n there is a small gap. may i know what is causing this? (first pic)

also, what is the usual method of meshing the area around an aerofoil? i used a semi circle with a rectangle at the trailing edge.

n now i have matching problem because the curve of the top quadrant is not the same as the curve of the aerofoil. if i used the sweeping edge method, the mesh will not be of similar sizes. the leading edge mesh will be larger than the trailing edge.
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File Type: jpg mesh2.jpg (83.7 KB, 19 views)
File Type: jpg mesh3.jpg (21.6 KB, 15 views)
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Old   November 30, 2009, 16:58
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Quote:
Originally Posted by ghost View Post
thanks for the help.

now i have another problem. the BL mesh does not attach to the surface of the aerofoil, n there is a small gap. may i know what is causing this? (first pic)

also, what is the usual method of meshing the area around an aerofoil? i used a semi circle with a rectangle at the trailing edge.

n now i have matching problem because the curve of the top quadrant is not the same as the curve of the aerofoil. if i used the sweeping edge method, the mesh will not be of similar sizes. the leading edge mesh will be larger than the trailing edge.
The edge mesh does not perfectly match the contour of the body. This is not a problem, as long as you have enough grid points to get an accurate approximation of the geometry. When you export the mesh, the geometry does not get exported along with it. Notice that the boundary layer mesh is attached to the dark blue edge, which is the grid, and not the light blue edge, which is the geometry. Therefore, the "gap" is only due to the mismatch between the continuous curve and the descrete grid that represents it, and there will not be a problem when you export the grid.

As far as meshing the domain around the airfoil, check out the grid in the FLUENT External Compressible Flow tutorial. In the tutorial, the airfoil is meshed with a C grid, which is one common way of meshing an airfoil.
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Old   December 1, 2009, 01:07
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I successfully mesh the aerofoil and set the gap to a mass-flow-inlet BC.

However when i read the mesh file in fluent, i got an "Error: mass-flow-inlet zone 3 has two adjacent cell zones" during the grid check.

I tried searching the forum n found that fluent cannot have blowing BC. Is that true?

If so, how else can I set a blowing boundary condition, with fluid coming out of the gap?
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Old   December 1, 2009, 02:26
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Quote:
Originally Posted by ghost View Post
I successfully mesh the aerofoil and set the gap to a mass-flow-inlet BC.

However when i read the mesh file in fluent, i got an "Error: mass-flow-inlet zone 3 has two adjacent cell zones" during the grid check.

I tried searching the forum n found that fluent cannot have blowing BC. Is that true?

If so, how else can I set a blowing boundary condition, with fluid coming out of the gap?
Can you post a picture of your grid with the boundary zones labeled? It sounds like you have two fluid zones separated by a mass flow inlet boundary condition, which is not allowed. The mass flow inlet boundary can only be adjacent to one fluid zone, as it is the boundary of your domain.
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Old   December 1, 2009, 02:42
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It is exactly the same as the above when I asked how to model a gap with BL over it. The gap is supposed to be the fluid blowing hole.

And yes, there are two mesh regions on both sides of the gap, but they are of the same fluid, which is air.
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Old   December 1, 2009, 10:51
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Quote:
Originally Posted by ghost View Post
It is exactly the same as the above when I asked how to model a gap with BL over it. The gap is supposed to be the fluid blowing hole.

And yes, there are two mesh regions on both sides of the gap, but they are of the same fluid, which is air.
You can't define the gap as a mass flow inlet if there is a fluid zone on both sides. If you want the gap to be the mass flow inlet boundary, delete the fluid zone in the interior of the airfoil. When you do this, the gap will only have one adjacent fluid zone (required), mass will enter the domain at the gap, and you will not be solving for the flow inside the airfoil.

If you want to solve for the flow inside the airfoil, which (I think) is the configuration that you described before, then define the gap as an interior boundary. In this case, the gap will be the "outlet" of the fluid zone inside the airfoil to the fluid zone outside of the airfoil. Use the mass flow inlet boundary condition for the inlet of the fluid zone inside of the airfoil.
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Old   December 1, 2009, 11:06
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Quote:
Originally Posted by Chris D View Post
You can't define the gap as a mass flow inlet if there is a fluid zone on both sides. If you want the gap to be the mass flow inlet boundary, delete the fluid zone in the interior of the airfoil. When you do this, the gap will only have one adjacent fluid zone (required), mass will enter the domain at the gap, and you will not be solving for the flow inside the airfoil.

If you want to solve for the flow inside the airfoil, which (I think) is the configuration that you described before, then define the gap as an interior boundary. In this case, the gap will be the "outlet" of the fluid zone inside the airfoil to the fluid zone outside of the airfoil. Use the mass flow inlet boundary condition for the inlet of the fluid zone inside of the airfoil.
if i use the first method, n remove the internal fluid zone, means there will be no flow inside, but just a gap for mass flow outwards?

for the second method, if the gap is set as interior, do u mean that the internal region of the airfoil BC be set as "mass flow inlet" instead of "wall"? is the total mass flow inlet now supposed to be equal to what i need at the gap?
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Old   December 1, 2009, 13:29
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Quote:
Originally Posted by ghost View Post
if i use the first method, n remove the internal fluid zone, means there will be no flow inside, but just a gap for mass flow outwards?

for the second method, if the gap is set as interior, do u mean that the internal region of the airfoil BC be set as "mass flow inlet" instead of "wall"? is the total mass flow inlet now supposed to be equal to what i need at the gap?
Yes, if you remove the fluid inside the airfoil, then there is no flow inside. I've attached a picture to show you what I mean for the second method.

airfoil gap 7.jpg

The "internal region" is a fluid zone. In 2D, the edges that define the zone are where you apply boundary conditions. So you don't define the region as a mass flow inlet, you define it as a fluid zone. Define the edge marked in the picture as a mass flow inlet.

To your question about the mass flow rate at the gap, if it's steady flow, then the mass flow rate at the inlet should be equal to the mass flow rate at the gap.
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Old   December 1, 2009, 22:18
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thanks for the help. i used the second method n the grid check is successful.

but i have a question regarding meshing. below is a pic of my meshing. due to the shape of the aerofoil, the mesh is highly skewed. does it matter?

the top n bottom half are not symmetrical because for the leading edge of the top half, i made it perpendicular to the contour of the aerofoil. n it becomes very sparse as it moves radially outwards. whereas for the bottom half, i just made it follow the contour of the quadrant, hence it becomes highly skewed.
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File Type: jpg mesh2.jpg (76.0 KB, 18 views)
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Old   December 1, 2009, 23:47
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Quote:
Originally Posted by ghost View Post
thanks for the help. i used the second method n the grid check is successful.

but i have a question regarding meshing. below is a pic of my meshing. due to the shape of the aerofoil, the mesh is highly skewed. does it matter?

the top n bottom half are not symmetrical because for the leading edge of the top half, i made it perpendicular to the contour of the aerofoil. n it becomes very sparse as it moves radially outwards. whereas for the bottom half, i just made it follow the contour of the quadrant, hence it becomes highly skewed.
The skewed mesh at the edge of the boundary layer might give you some problems. Not to rant, but this is honestly one of my biggest problems with Gambit; in my experience, it seems like you need to fight the software a gird that is orthogonal to the airfoil and smoothly transitions to the far field. In your case, the grid inside the boundary layer is good, but it gets bad outside of the boundary layer.

If you have the resources, you should try your grid to see if it works. Ultimately, though, you'll probably need to play around with how you are blocking the domain and meshing the edges until you get a grid that is good enough. Beyond that, I'm sorry to say that I'm not really sure what else to do. Hopefully, someone can chime in and discuss how they have managed to solve a problem like this.

Also, you'll probably want to increase the density of the grid at the gap inside the airfoil (the unstructured region.) As you have it, there is a large change in cell volume at that point, which might give you some problems.

Last edited by Chris D; December 1, 2009 at 23:49. Reason: added some more...
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