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Old   July 8, 2014, 16:20
Default Unstructured surface mesh
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Lefteris
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Hello all!

I am trying to mesh an aircraft geometry and I have a problem with the surface mesh on two surfaces. Those surfaces are the left and right side of the engine's pylon.

I have assembled the models for this surface, created the connectors, dimensioned them and finally I assembled the surface mesh. The result is illustrated in the first figure. There is an anomaly near the corner where the pylon meets the wing.

However, if I select the domain and go to Grid --> Solve --> Attributes and chance the "Surface Shape" from "Database" to "Free", I get a much better domain which I'm not sure if it follows the underlying geometry (figure 2).

My question is why the domain that follows the database has such a behaviour and how can I make it be like the "free" one?

I would appreciate it if someone could give me an insight on this.


Thanks in advance.

Lefteris



PS

I experimented changing some parameters in the attributes panel and the boundary decay seemed to have an effect up to a certain point. The boundary conditions did not seem to have any effect whatsoever.
Attached Images
File Type: jpg pylon_mesh_on_database.jpg (46.4 KB, 131 views)
File Type: jpg pylon_mesh_free.jpg (45.6 KB, 91 views)
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Old   July 9, 2014, 08:25
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Chris Sideroff
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Quote:
Originally Posted by Aeronautics El. K. View Post
However, if I select the domain and go to Grid --> Solve --> Attributes and chance the "Surface Shape" from "Database" to "Free", I get a much better domain which I'm not sure if it follows the underlying geometry (figure 2).

My question is why the domain that follows the database has such a behaviour and how can I make it be like the "free" one?
By default, when Pointwise creates a domain on a database entity, it will constrain the interior points to the surface using a parametric projection, i.e. using UV coordinates of underlying surfaces. Your clue this happened is the domain is colored purple when you first create the domain.

Now sometimes, possibly due to a strange or corrupted underlying surface parametrization, Pointwise may have difficulty creating a "nice" domain.

When you change the surface shape to Free it doesn't try to project the points (so problem averted). However, the interior points no longer lie on the surface. To get them back on, select the domain and Edit>Project. Closest Point should work. [I believe] Edit>Project uses a 3D Cartesian-based projection which generally isn't affected by a poor surface parameterization.

That's the general gist. My explanation maybe somewhat imcomplete so if someone from Pointwise wants to clarify, please feel free.

Let me know if that helps.
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Old   July 9, 2014, 14:57
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Quote:
Originally Posted by cnsidero View Post
To get them back on, select the domain and Edit>Project. Closest Point should work. [I believe] Edit>Project uses a 3D Cartesian-based projection which generally isn't affected by a poor surface parameterization.

...

Let me know if that helps.

Hi Chris!!

Thanks for your help. I tried what you suggested and it worked. You saved the day with this one!

I do have another problem though again with the nacelle and the pylon. More specifically, I made a structured domain for the nacelle which then I diagonalised (and deleted the structured domain). Now the problem is that I cannot remove the inner part of that domain where the pylon meets the nacelle (figure 1). Trimming the two models (nacelle and pylon base lets say) did not work and that's where I figured that the database geometry is bad. The pylon does not actually touch the nacelle.
So, what I did was to create the domain defined by the pylon's base (figure 2) and (remembering Chris' suggestion) I projected it on the nacelle. Then I removed the interior edges and I was left with what is demonstrated in figure 3.

As you can see, there is a small area at the back of the domain which shouldn't really be there. Does anyone have any idea why this happens and how it could be fixed?

Update
I don't know if it is extremely important or not but when I use the "merge" command there is again a strange behaviour (figure 4).
Attached Images
File Type: jpg nacelle_pylon_junction.jpg (37.0 KB, 69 views)
File Type: jpg projection_1.jpg (54.8 KB, 55 views)
File Type: jpg removed_edge_2.jpg (44.1 KB, 44 views)
File Type: jpg after_merge.jpg (55.0 KB, 40 views)
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Last edited by Aeronautics El. K.; July 9, 2014 at 16:14.
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Old   July 10, 2014, 08:18
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First, confirm with me whether or not I understand your steps so far:

- created a diagonalized domain on nacelle without hole for pylon base
- created domain for pylon base and project it onto nacelle
- added an internal edge to nacelle domain using connectors from pylon base

Assuming this is case, I think I understand the problem. The back edge of the nacelle is/should be coincident with the pylon base - meaning there should be no connector in the domain's definition. However, when you add the edge to the nacelle domain, it will be a closed loop. It will not modify the first (outer) edge of the nacelle and thus at the back edge a tiny, sliver exists between the outer and inner edges of the domain.

What you need to do is recreate the nacelle domain but include the pylon base connectors in the definition of the outer edge. You may need to make sure the connector on the back edge of the nacelle and intersected/split with the pylon base connectors.

If you're not following, let me know and I'll put together an example.
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Old   July 10, 2014, 09:34
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Hi Chris!

I can confirm that I followed those steps so you understood correctly.
I think I get it and I'll try it right now. So, I start over the engine-pylon following your suggestions. There is one point for which I'm not sure.
I do have a model for the nacelle but I create the model for the pylon's base. Should I assemble the two models together before doing anything or work with two separate models?
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Old   July 10, 2014, 14:24
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Quote:
Originally Posted by Aeronautics El. K. View Post
Hi Chris!
There is one point for which I'm not sure.
I do have a model for the nacelle but I create the model for the pylon's base. Should I assemble the two models together before doing anything or work with two separate models?
You are not required to create a model joining the nacelle and pylon. You can simply use the connectors you've already created to make the domain on the nacelle with the notch for the pylon base removed. As I mentioned in my last reply, you may need to split the nacelle and pylon base connectors where they intersect to make them join at a common node.
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Old   July 13, 2014, 18:14
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Hello!

I'm sorry I didn't reply sooner but I started over meshing the whole geometry.

Chris, your advice was invaluable. I think I got the hang of it! Thank you!

There is a point though that I can't understand. I'll start a new thread though as it's irrelevant with the unstructured mesh in this one.
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Old   July 13, 2014, 19:21
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Hi,

Sorry if I am interrupting your discussion, I have just followed up this thread, and thought maybe it became useful to mention a point.

If I understand the Chrisí point correctly, the problem is your connectors which belongs to the nacelle domain in the back does not belong to the pylon domain.

From this point, I think there is two solutions, first you can use the grid > merge in order to unify the connectors around the pylon base which belong to the nacelle domain and the connectors around the pylon base which belong to the pylon domain. To this end, you can change the tolerance in the merge section to make it convenient.

Second, there is a tutorial in Pointwise, if remember it correctly, wing with the store, second or third case, which explain how to trim a geometry. Particularly, this tutorial has a similar geometry like yours, a store with several winglets. You can refer to this tutorial and see how you can trim the pylon surface with the nacelle surface in order to have a unify connectors when you are creating the connectors on the geometry, which make it possible to evade this problem. Please note, don't forget to create surface of the pylon base a little bit into the nacelle surface for trimming.

I hope it becomes helpful.

PDP
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Old   July 13, 2014, 22:17
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You're not interrupting anything of course! Thanks for your reply.

Trimming the two surfaces was the first thing I did as it also seemed to be the most reasonable step. However, the inside area wasn't removed but I believe this was due to the fact that the nacelle and pylon where not touching each other. What I can make from the CAD is that the pylon's base actually intersects the nacelle rather than touching it. (I had the same problem at the wing-pylon interface.)

So, I created the connectors on the base of the pylon and projected them on the nacelle's database (Chris actually made me realise the importance of the project command). After that, I just created the domain by defining the edges as shown in the figure (two figures actually, one with the edges and one with the resulting domain)

I used the merge commands (auto, node-connector and node-node mainly) in other occasions and I fully appreciate their capabilities!!They're very useful.

Thanks for your help!!
Attached Images
File Type: jpg nacelle_pylon_edge.jpg (21.9 KB, 27 views)
File Type: jpg nacelle_pylon_domain.jpg (29.2 KB, 31 views)
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Old   July 28, 2014, 11:13
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Sorry for my delayed reply. Okay, you could remove the inside area. Also, you could merge the connectors by picking and changing the tolerance. I am afraid not, it doesn't suppose to touch the nacelle.

I am just using something similar (a blade with its hub) to your case (nacelle and pylon) to illustrate the point. First lets trim the geometry.

Trim:

1- Please see a picture in the below. There is hub which is intersected by a blade. After importing the geometry, I assembled base of the blade.
trim1.JPG

2- Then, I will select the assembled base quilt of blade, and go to the Edit, Trim by surface. I will select two quilts which belong to the hub, they are going to participate in the trim. Choose both, Trim A with B, and Trim B with A, and check both, Keep Inside and Keep Outside. Then, I will press Imprint to see which quilts suppose to be removed. Quilts that are colored will remain, and quilts that aren't colored will remove. Therefore, I am removing the tick for Keep Inside to remove the surface trapping between the hub and blade. Finally, click on OK. See the following picture.
trim2.JPG

3- Now, I have the blade, hub, and extended part of the blade inside the hub which needed to be removed. For this purpose, I am selecting the assembled base quilt of the blade again. Then, going to the Edit, Trim by Curve. Select a curve which indicate a border between inside extended part of the blade and the outside part of blade. Press the imprint, and then press Apply and Select. See the following picture.
trim3.JPG

4- In the Apply and Select, I will click on the outside part of the blade, and then on OK. See the following picture.
trim4.JPG

5- Finally, the extended part of blade which was inside the hub has been removed. See the following picture.
trim5.JPG

Please, follow a similar instruction to trim your pylon surface with the nacelle surface.

There is a limitation for number of pictures that could be attached at one post, so follow the merge in the next post.
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Old   July 28, 2014, 11:59
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After that you trimmed your pylon surface with nacelle, and creating the connectors on the database, if you need to merge the connectors, in particular in the interconnection between the pylon base the nacelle surface, see the following to figure out.

Again, I used a similar problem to show the points. You are seeing a corner of geometry which is between a extruded boundary layer block and a outside unstructured domain. The outside unstructured domain obtained through using a create, periodic, rotational. What is matter is we are using the merge options to unify the connectors at the corner, which some of them belong to the outside unstructured domain and some of them belong to outside boundary of the extruded boundary layer block.

Merge :

1- As you are seeing in the following picture, there is a gap between the connectors that belongs to the unstructured periodic domain, indicated by red in the picture, and the connectors which belongs to the boundary layer block. After going to the Grid, Merge..., I am selecting the Merge Pairs of Entities.., then select the Node-Node, to unify the node at the corner. For this purpose, I am changing the tolerance until getting the green color on the nodes. Then, I will select pair of nodes on the screen, and click on Apply.

merge1.JPG

2- Then, I will select the Connector-Connector, and change the tolerance until getting the green color on right side of the connectors, select them through screen, and click on Apply. See the following picture.

merge2.JPG

3- As you are seen in the following picture, the pair of connectors in left side had a pretty big gap, which may cause the domains to disturb after merging. Therefore, first thing that I am going to do is matching their node distribution.

merge3.JPG

4- For unifying the node distribution on the left side connectors, I am going to Grid, Distribute, Function tab, Copy From option. Then, I am selecting the outside connector of boundary layer block, and click on Copy Scaled Distribution. Then, press the OK. Find the result in the following picture.

merge4.JPG

5- Still there is huge gap between the two connectors on the left side. See the following picture.

merge5.JPG

See the next post for the next three steps.
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Old   July 28, 2014, 12:08
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6- Therefore, I will put the corresponding node on the outside boundary layer connector. To do this, I am selecting the connector that belong to the outside periodic domain, then going to the Edit, Smooth, and increasing the Fit Tolerance a little bit for example 0.01. Clicking on OK, then, going to the Edit, Curve. There, I am selecting exactly the node that had the maximum distance from the outside boundary layer connectors, dragging and dropping it on the outside boundary layer block. See the following picture.

merge6.JPG

7- Finally, I am going back to the Grid, Merge. Select the Merge Pairs of Entities, then select the Connector-Connector. Next, changing the Tolerance until getting green color for the connectors on the left side. Finally, click on OK. see the following picture for the results.

merge7.JPG
merge8.JPG

For you problem which had a interconnection between the nacelle surface and pylon surface you simply could follow a similar instruction to merge the connectors.

Bests,
PDP
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Old   August 3, 2014, 20:11
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Hi Payam!

Thanks for your detailed explanation and the example you set up!
The truth is that I hadn't really understood how the trim by surface and trim by curve options work because after trying them, I couldn't see any difference. I think though that I wasn't doing it correctly so I'll follow your instructions and I'll practice a little bit! As a matter of fact, I'll try to do what you just illustrated and see what happens!

Thanks a lot!!!

Lefteris
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