Hexa Meshing of Ventilation Air Duct

  • Last Post 03 June 2019
ayodele1 posted this 21 May 2019

Hello everyone,

I am generating hexahedral mesh in ANSYS ICEM CFD 16.1 for flow analysis in ventilation air duct. My pre-mesh is not looking alright - mesh only cover a larger section of the geometry (see below images). I used the native 3D hexa meshing approach i.e. blocking, association, application of mesh parameters (Curve Mesh Setup) with the blocking carried out with a T-shaped topology. I also tried creating O-grid Splitting method, no improvement. Below are the images of my pre-mesh (3D, X-Z view, Y-Z and X-Y respectively). Kindly assist me with ideas on getting a proper pre-mesh for this geometry. 

Many thanks in anticipation.



Image from native 3D meshing


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raul.raghav posted this 21 May 2019

Looks like there might have gone wrong with the association. Could you post a picture of the Blocks from the Mesh?


ayodele1 posted this 21 May 2019

Thanks Rahul.

The following are the 3D and X-Z views of the Blocks from the mesh. I look forward to your thoughts ...

Thanks a ton


raul.raghav posted this 22 May 2019

Ayodele, can check the minimum value for the Determinant from the Pre-Mesh Quality. If its negative, you might have an inverted block (which might be the case).


ayodele1 posted this 22 May 2019

Dear Rahul,

Thanks for your response and suggestion. I just checked the Pre-mesh Quality Histogram and found the minimum to be "0", a total of 1,992 elements at the section having the inappropriate meshing have determinant values of <0.4. What could I be doing wrong?

Kind Regards,



raul.raghav posted this 23 May 2019

If you notice carefully, the minimum determinant is shown as -1 on the graph (just not visible as the X-axis value is set from 0 to 1). To see the negative elements, set MinX value to -1 to a MaxX Value to 1 in the Histogram Options. Refer to attached image for reference:

To fix the problem, you need to fix the inverted blocks. See the following screenshot for reference:


ayodele1 posted this 25 May 2019

Hi Rahul,

You are indeed correct, I found the minimum determinant to be -1, thanks. I however tried to Fix Inverted Blocks, but no change observed, the determinant remains -1 and the section without mesh remains the same. Is there anymore step I could take to address this?

I am grateful for your time and assistance - thank you.


rwoolhou posted this 03 June 2019

I've not used ICEM CFD in about 20 years so am slightly rusty: we used to add all of the blocking manually back then. Can you delete that block & recreate it? 

Alternatively, ANSYS Meshing or Fluent Meshing with the Clean CAD Workflow (2019R1) will give you a mesh with much less manual intervention.