Boundary Layer in Bounding Box/ Moving Ground

[dschroeder]

Hey, @TheDound26

@TheDound26

I have another question brewing in mind and that is how can we calculate the thickness of boundary layer?

The answer to this is entirely dependent on the air speed you are trying to measure as well as your target Y+ values. (either measuring the viscous sub-layer or the log-law region) I have written an article covering some of this as well as other useful topics here: FSAE Tutorial - Preparing Simulations For Success - Boundary Inflation Techniques And Y+ Validation

@TheDound26

As for the moving ground, we don’t have a reference length required to calculate thickness.

My opinion on this is that there is not definitive answer because there must always be a balance in simulation between accuracy and speed. You obviously dont want, and most of the time cant, run a massive 30 million cell mesh quickly. So you need to reduce mesh quality in some areas so that the simulation doesnt take 30hours or fails altogether.

My advice is to just use the length of the car for the reference. that is what i use for the reference length in the coefficient of friction result control. Also i feel that its best to put more cells where you are trying to measure. If you want to look at the impact of ground effect and diffuser function → then add more boundary layers to the floor to make it more accurate. If you are looking at the rear wing, then you can reduce the floor BL cells because it wont have a big impact on rear wing results.

As Retsam said:

@Retsam

I will definitely use BLs on moving wall (or rotating wall), as with car wheels we have high pressure gradients on or near interfaces. For that reason we need better mesh in those zones.

Also for your other question

@TheDound26

Thank you for showing interest in this topic. I understood what you are trying to explain apart from one thing,

@Retsam

Other part of moving wall could have ‘extended’ mesh cells as pressure gradients are low (and we can save simulation time by reducing mesh).

If you can explain this, it would be helpful.

I think Retsam means that Boundary layer cells for the moving floor in unimportant areas can be thicker or ‘extended’ to reduce the overall amount of cells. For example: The areas further in front or behind the car can be bigger and less accurate because these areas are not important to measure, then approaching the car you want more cells to increase the accuracy.

Just remember that everything in simulation is a trade off between accuracy and speed .

Dan