Hello,

I'm analyzing the increase of friction coefficient due to the growth of marine fouling on ship hull. Currently, I've been doing simulations using ANSYS Fluent on a flat plate with five different velocities (3 m/s, 6 m/s, 9 m/s, 12 m/s, and 15 m/s) and four different types of roughness for each velocity. I still need to do simulations for the ship model, but right now I'm still stuck on this problem with grid-independent mesh.

The simulation used a single-phase fluid, the air, as its purpose is to validate my former colleague's experiment of a similar analysis that was done by using a wind tunnel. I've done the grid-independence study for the smallest velocity with the smooth condition of the plate and found that the suitable grid consists of around 1.1 million cells, with a 3.84% error from the experiment result. I've used the k-w SST turbulent model with y+ = 1 for the near-wall treatment. I've used the tetrahedral patch conforming method for the mesh.

The problem is that when I tried to do a simulation for 6 m/s using the same meshing configuration as the previous grid-independent mesh for V = 3 m/s, the result turned out to be 9% different from the wind tunnel experiment result. I only changed the first layer thickness value. Up to this day, I'm still clueless about what has gone wrong. I also tried using cut cell and multizone methods to no avail. I also have asked for help from my seniors and supervisor, but I am still stuck here. If you can help me find out what's wrong with my simulation, it'd be very appreciated.

Thank you so much, I hope you all stay healthy and safe in the middle of this pandemic situation.

# Trouble Using Grid Independent Mesh for Other Velocity

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- Last Post 19 May 2020

Hello,

For different velocities, does the flow field change? Also, when you are changing the y+ mesh, does it change the overall mesh? What happens to the 9% case when you refine the mesh further? Do you get a better result?

Also, what variable are you really comparing with experiments?

Thanks.

Best,

karthik

Hello, thank you for answering.

I haven't checked the flow field. I'm gonna check it soon and let you know. No, I used the same y+ value, which equals to 1, for every velocity. So, I only changed the first layer thickness value for every velocity, and I didn't change the rest of the mesh configuration. I did try to refine the mesh further until it reached 2 million cells, but it didn't work.

I am comparing the total drag results between my CFD simulations and my colleague's wind tunnel experiment. Both are done for a flat plate.

Thank you, Karthik.

I am using full viscous simulations and I am not sure how to model the flow field as one would model the flow field for free surface simulation

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