Jamie
So far i see two things that could be causing your problems.
- Since you are using absolute layering the bounding box layer addition Final Thickness should be relatively small. A good estimate is less then the area you have under the car. This is currently set to 0.2m which is double the space you have.
This is under the car
In the first picture, it seems you have boundary layer addition cells then they go away closer to the car, Probably because they are failing and cannot be added.
- In the second pic you have an 8 cell boundary layer on the diffuser next to the 8 cell BL on the car, also near the 5 cell bounding box layers. This is most likely too many layers and is not really necessary, with how the k-omega SST uses wall functions, and especially for your Y+ goals.
I would highly recommend running only 2-4 layers for your boundary layer and bounding box layers, then run a simulation after meshing and check Your Y+ values using the program created by Dale Kramer here
Good luck!
Dan