Boundary Layer around Motorcycle

[mgiannios]

Hi everyone,

I have a really hard time creating a nice mesh around a motorcycle.

I would like to investigate the temperature and velocity fields around a motorcycle, for wind speed equals to 6.5 m/s. The characteristic length of the geometry is 1.9m.

I want to use wall function on the geometry, so I need y+>30. According to my calculations the right thickness for the total boundary layer thickness is around 48 mm.

However each combination of “inflate boundary layer” parameters, that I have tried, in order to create a boundary layer like that, leads to a bad mesh like the following.

Could anyone help me to improve the boundary layer?

This is the link of the project: https://www.simscale.com/workbench/?pid=1727361331641938870&mi=spec%3A38ff3f1e-a017-44e2-b2e6-4bfdae2a57ec%2Cservice%3ASIMULATION%2Cstrategy%3A5

Thanks!

[tsite]

Hi there, thank you for using the forum!

You are using a total boundary layer height of 1 m at the moment, which is of course large. If you wish the first layer to be 0.048m, then the total bl height for 5 layers with 1.1 expansion ration is: H=50.0481.1=0.264m.
However, that also seems large, have you tried this calculator?
https://www.pointwise.com/yplus/index.html

Best regards,
Fillia

[mgiannios]

Thanks for your reply.

I think that the total boundary layer height is not 1 m, at the moment.
The only parameter that is set to 1 is the “Final layer thickness”, which, as I can understand, means that the last layer height is 100% of the size of the first undistored cell, according to the following screenshot.

Additionally I don’t want the first layer to be 0.048m. I want the total boundary layer height to be around 0.048m.

The boundary layer that I really want to generate is the following:
Total boundary layer height: 0.050 m
7 layers
Growth ratio: 1.2
1st layer thickness: 0.003886 m
2nd layer thickness: 0.004664 m
3rd layer thickness: 0.005597 m
4th layer thickness: 0.006716 m
5th layer thickness: 0.008059 m
6th layer thickness: 0.009671 m
7th layer thickness: 0.011605 m

The size of the cells upon the geometry is 0.0039 m or 0.0078 m.
However in this case the Final layer thickness, I think that should take the value 0.011605/0.0039=2.98
and every mesh that I generated with Final layer thickness>1 has bad boundary layers.

Please tell me if I am wrong, or if you can help me generate the aforementioned boundary layer.

Thank you very much.

[tsite]

Yes you are right, the final layer thickness is set to 1m. However when setting a boundary layer, we are mostly focused on the near wall layer. How did you calculate the input for your case (the 0.048m of total height)? Did you use a calculator?

[mgiannios]

I have done the calculations myself.

To calculate the Total boundary layer height I used the formula which is presented in the following screenshot.

And to calculate the first layer height I used the formula which is presented in the following screenshot.

In any case the calculator gives a similar result for the first layer height:

How can I generate the boundary layer that I mentioned on the previous comment?

[tsite]

Hey there, your boundary layer is generated nicely now with this new final layer thickness input, maybe you consider reducing the number of layers though? 3 would be also good for this analysis.

[mgiannios]

I will try it. Thank you for your replies!