Learning Forum

[P_Heng]

Hello everyone,

I make the mesh with the quadratic element order. But I check the value of velocity after I run the calculation, it seems that my mesh is linear element order.

you can check the picture below.

My regards,

P. Heng.

[P_Heng]

Can you tell me the reasons?

[P_Heng]

As you see my sent pictures, my mesh is used quadratic element order. But when I show the nodes in setup after I run the calculation, it shows linear element order.

can you tell me the reason? How to solve it?

[Rob]

Fluent uses the cell centre so only takes the cell shape from Meshing: you have a zero velocity on the wall and two flow values. As with your other models you need more mesh: repeat the test with 10 cells across the channel.

[P_Heng]

I cannot increase mesh because I am not allowed to do that.

My problem now is that, I use quadratic element order but why the mesh in setup show the linear element order?

can I you it in the case of quadratic element order?

My regards P. Heng.

[Rob]

Fluent uses quad & tri cells: it's a shape not an order. Mechanical uses "order" in it's cell definition and I think that's linked to mid-side nodes and the like.
Run with more nodes and see how the result is.

[P_Heng]

I add more meshes or not, the results show that they are linear, not quadratic. As you see in the picture.
You can see this result:
V = 1.6E-04 and V = 3.1E-04
this result shows that this is linear.

V = 1.6E-04 should be greater than 2E-04 ( hand calculation).

My prof. doesnÔÇÖt accept it.

My regards.
P. Heng.

[Rob]

Run with a good mesh: ie inflation and 10 or so nodes across the channel. It's linear as you currently have too few data points for it to be anything but.

[P_Heng]

I am doing for four case of mesh, coarse mesh, medium mesh, fine mesh, and finer mesh.

In fine mesh and finer mesh, I see the velocity vector with parabolic shape.

The problem is that the value in the middle side of cell is different to my hand calculation. The value on the edge node of cell, they are right with my hand calculation. You send you the formula:
k1 is velocity.
k1 = 1/2 y (y - d)

My regards P. Heng.

[Rob]

How different? Is the centre of the domain a mesh cell, or does it fall at a cell boundary?

[P_Heng]

As you see the picture, the velocity is equal to 1.6E-04, should be equal to 2.34E-04 (according to the hand calculation above).

if it is equal to 1.6E-04, it seems that I use linear element because you the shape of the vector is not look like parabolic shape.

I use quadratic element.

My regards P. Heng.

[Rob]

In the fine mesh what is the result? Please show the contours of velocity (node values off) too.

[P_Heng]

These pictures in the case of fine mesh.

[P_Heng]

Are there any solutions for my case?

My regards P. Heng.

[Rob]

Is the graph & table the hand calc and the lower image Fluent with 8 cells across the channel? What's the y+ like?

[P_Heng]

both pictures above, I use quadratic mesh but the result is like I am using linear mesh. For the hand calculation, I use the formula above to calculate it and it is given by my prof.

Sorry Rob, I donÔÇÖt know y+. What is it? Can you explain me?

My regards P. Heng.

[Rob]

A quad mesh in Fluent means the cells are square-ish. The profile is then a function of the flow and whether you've resolved it.

[P_Heng]

In meshing, I see quadratic mesh showing 8 nodes but why do I see only 4 nodes when I export the file to Tecplot?

this is the problem which my prof. think that my calculation is wrong.

how can I know that what am I doing is quadratic mesh?

as you see my picture, all the velocity on the edge node is right with hand calculation but the velocity value on the middle between those edge node is all wrong.
For the list above, I use quadratic mesh. There are 16 elements.
My regards P. Heng.

[Rob]

With the exception of the cell that's adjacent to the wall you're within around 1% : do you really expect to get an exact agreement with the theoretical calculation? Have you used exactly the same density, viscosity values etc?
Re the number of nodes on a 2d hex (quad) element. Read how CFD (Fluent) uses cells, and then read how Mechanical makes use of mid-side nodes.

[P_Heng]

My prof. doesnÔÇÖt agree because the result should show the vector of velocity in case of using quadratic element.

In Ansys Fluent provide me the vector of velocity in the case of linear element.

the result of velocity should be the same as the picture below:
But in Ansys, the velocity at d/4 is equal to 1.563E-04. this value is really different to hand calculation. ThatÔÇÖs why, my prof. doesnÔÇÖt agree.

Moreover, it is just the a simple shape geometry. He wants everything is like hand calculation.

My regards P. Heng.

[Rob]

The two columns above for velocity look identical to me.

[P_Heng]

Now, I donÔÇÖt know how to negotiate with my prof because I am using the quadratic mesh element but the velocity value is like I am using linear element.

Do you have any solutions?

My regards P. Heng.

[Rob]

Not really, my MEng and PhD supervisor explained the problems clearly, and usually in the pub so we didn't have this problem.

[P_Heng]

Now, I get stuck with this problem.
What should I explain him about this problem? How should I explain him about the process of Ansys Fluent ?
He usually wants me do 4 cases, coarse mesh, medium mesh, fine mesh, and finer mesh.
If the first case, coarse mesh, is wrong, he doesnÔÇÖt want to see the other cases.

Regards P. Heng.

[Rob]

Try asking how a code is supposed to give an accurate answer at discrete points if there aren't enough to resolve the flow? Fluent is an unstructured solver even on a cartesian mesh: he may not realise that.

[P_Heng]

Sorry Rob, Can you explain me more easy to understand? Because I want to take the reference from you to explain him.

Thank you, Rob.

My regards P. Heng.

[Rob]

Look in the Fluent theory. Also look into any text books on CFD for the numerical schemes. I don't have those references as I never needed to explain solver theory at that level.

[P_Heng]

Thank you, Rob, for your response.

I want to know more that.
How does Ansys Fluent do the calculation with finite element?

Now, talking about 1 element only.
if we use the linear element, 4 nodes, for the mesh, the software should use the whole element to calculate. Is it right?

if we use the quadratic element, 8 nodes, for the mesh, does the software use the whole element to calculate or the software separate a element into 4 parts and then start to calculate because it has 8 nodes? (See picture below)
I hard to explain you in English, I am sorry. I hope you understand me.

My regards P. Heng.

[Rob]

I do, and you need to read this https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v212/en/flu_th/flu_th_chp_solve.html but also remember that Fluent uses the finite VOLUME method. There are no mid side notes.