# Lattice Structure Wind Load – STAR-CCM+

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 May 15, 2020, 21:19 Lattice Structure Wind Load – STAR-CCM+ #1 New Member   Nederland Join Date: May 2020 Posts: 3 Rep Power: 5 For my final year bachelor project, I need to make a CFD-model to calculate the wind load on a lattice structure/crane arm. As comparison material, I have the real wind tunnel measurements with a net force of around 4.05 N. I made the lattice structure in SW, saved it as Parasolid, imported it through 3D-CAD Models, and then made a new geometry part from it. Also, the model is made with part-based meshing. For the calculation of the number of prism layers and the prism layer thickness. The results after 1000 iterations are around 4.14 N which is great, however I am unsure if this is accurate or just pure luck because the mesh does not look great. I currently have a few problems/questions that I would like to have your insights on, thank you in advance! Parts: • Lattice Structure (only half with a symmetry plane) • Wind tunnel • Volume of Refinement – Wind Tunnel • Volume of Refinement – Wake • Volume of Refinement – Offset of Lattice Structure Models: • All y+ Wall Treatment • Constant Density • Gas (Air) • Gradients • K-Omega Turbulence (SST) • RANS • Segregated Flow • Solution Interpolation • Steady • Three Dimensional • Turbulent • Wall Distance Settings: https://imgur.com/bWuKP2q Volume Mesh Inside: https://imgur.com/7M4ug1u Volume Mesh Outside: https://imgur.com/2GQomkd Questions: 1. There are weird lines on the tubes of my mesh because of the prism layers. How can I solve this? https://imgur.com/Uv5alm0 2. I made an Excel table/calculation for the prism layer thickness and amount of prism layers and it works because if I measure the last prism layer it is exactly 1.3X smaller than the base size of the smallest volume of refinement (Offset). However, there is a smaller transition mesh between the last boundary layer and the volume of refinement mesh, how do I get rid of this? https://imgur.com/nSO67do 3. After reading online that for a trimmed mesh you need to have custom sizes that are n * 0.5 * base size, I changed the following controls, but it made the boundary layers have a weird cone-like shape. I expected the mesh to improve after this change, so I am wondering what I did do wrong? Old: https://imgur.com/LroApty New: https://imgur.com/W524HqY • Base size: 0.1 • Minimum Surface Size: 0.0002 -> 0.0001953125 • VoR - Offset: 0.001 -> 0.0015625 • VoR - Wake: 0.005 -> 0.00625 • VoR - Windtunnel: 0.01 -> 0.0125 • Number of Prism Layers: 17 -> 19 • Prism Layer Total Thickness: 4.41E-3 -> 7.49E-3 4.The y+ calculation is based on a reference length of 4mm, which is the diameter of the diagonal V-tubes on the side of the lattice structure. I used 4 mm because that is the diameter of the diagonal V-tubes in the lattice structure that are the most common and therefore probably the most critical. Is this method acceptable or do I need to calculate the reference length by calculating Total Volume/Effective Area? https://imgur.com/RockrMO

May 17, 2020, 13:33
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 Originally Posted by Fluidy For my final year bachelor project, I need to make a CFD-model to calculate the wind load on a lattice structure/crane arm. As comparison material, I have the real wind tunnel measurements with a net force of around 4.05 N. I made the lattice structure in SW, saved it as Parasolid, imported it through 3D-CAD Models, and then made a new geometry part from it. Also, the model is made with part-based meshing. For the calculation of the number of prism layers and the prism layer thickness. The results after 1000 iterations are around 4.14 N which is great, however I am unsure if this is accurate or just pure luck because the mesh does not look great. I currently have a few problems/questions that I would like to have your insights on, thank you in advance! Parts: • Lattice Structure (only half with a symmetry plane) • Wind tunnel • Volume of Refinement – Wind Tunnel • Volume of Refinement – Wake • Volume of Refinement – Offset of Lattice Structure Models: • All y+ Wall Treatment • Constant Density • Gas (Air) • Gradients • K-Omega Turbulence (SST) • RANS • Segregated Flow • Solution Interpolation • Steady • Three Dimensional • Turbulent • Wall Distance Settings: https://imgur.com/bWuKP2q Volume Mesh Inside: https://imgur.com/7M4ug1u Volume Mesh Outside: https://imgur.com/2GQomkd Questions: 1. There are weird lines on the tubes of my mesh because of the prism layers. How can I solve this? https://imgur.com/Uv5alm0 2. I made an Excel table/calculation for the prism layer thickness and amount of prism layers and it works because if I measure the last prism layer it is exactly 1.3X smaller than the base size of the smallest volume of refinement (Offset). However, there is a smaller transition mesh between the last boundary layer and the volume of refinement mesh, how do I get rid of this? https://imgur.com/nSO67do 3. After reading online that for a trimmed mesh you need to have custom sizes that are n * 0.5 * base size, I changed the following controls, but it made the boundary layers have a weird cone-like shape. I expected the mesh to improve after this change, so I am wondering what I did do wrong? Old: https://imgur.com/LroApty New: https://imgur.com/W524HqY • Base size: 0.1 • Minimum Surface Size: 0.0002 -> 0.0001953125 • VoR - Offset: 0.001 -> 0.0015625 • VoR - Wake: 0.005 -> 0.00625 • VoR - Windtunnel: 0.01 -> 0.0125 • Number of Prism Layers: 17 -> 19 • Prism Layer Total Thickness: 4.41E-3 -> 7.49E-3 4.The y+ calculation is based on a reference length of 4mm, which is the diameter of the diagonal V-tubes on the side of the lattice structure. I used 4 mm because that is the diameter of the diagonal V-tubes in the lattice structure that are the most common and therefore probably the most critical. Is this method acceptable or do I need to calculate the reference length by calculating Total Volume/Effective Area? https://imgur.com/RockrMO

What is the max. skewness angle, face validity and min. face quality for your mesh? Also, have you checked your imported geometry for errors before creating mesh?

 Tags boundary layer, mesh, star ccm, trimmer, volumetric control