Boundary inflation for improve y+ values and boundary conditions

Hi there,

I have been recently trying to improve the y+ value for my simulation.

I have two boundary inflation layers which are shown below, along with the images of the inflation i have included the calculation for the wall spacing (calculated as total height). The calculator link is HERE

For the first image it shows the floor of the flow domain, which has a total length of 2100m, for the ABL i have defined I used the free stream velocity at a height of 0.01m. The boundary conditions applied here is a no-slip conditions with a wall function. I used a y+ value of 1 in the calculation.

The second shows the building surface, for this i used a slip condition, the building width is 30m and is used in the calculation of wall spacing as shown. The velocity at total building height was used to calculate the cell height. Here i used a y+ value of 50, this was to attain a y+ between 30 and 300.

In the above picture i have shown that for the floor the surface layer thickness input is half the calculator value and in the building surface i have shown i did not do this. This is done because i am unsure as to what method is correct.

I have checked if the inflation has been applied and got the following, my question regarding this part is that is this acceptable?

Any help with what I can do what be appreciated, the link to the project is below.

The image below shows the y+ values,


Please refer to the following documentation:

I am not sure that your choice of the length of the flow domain as characteristic length is the correct one, have you considered other alternatives?

Hi @ggiraldof,

I have read the documentation many times and have not managed to improve my results, with regards to the characteristic length I have not used any other values. What value would you recommend to use?

Doing some testing to get good settings… ill let you know when i have the results

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Thank you for putting your time into helping me, it is really appreciated!

Hey @ss02150

I was able to get some good inflation after a couple attempts. Use the following settings as a baseline and make changes from there.



Setting some of the values equal will force a certain size. This saves cells in un-needed areas like your corners

I tried to get inflation using 0.5 surface layer cells with no luck. maybe you can figure out a way, 0.25 worked though

I always like using the distance region refinement because you can specify gradually larger cells from the zone / surface. I just made a box the size of your building. be careful because region refinements will override surface so if I had set the surface to 0.5 for example it would have still resulted in 0.25 at a distance of 1 meter

Your region refinements i left pretty much the same.

For both the building and floor i used the same settings. I set a 300 Y+ value resulting in 0.003 meters. This is quite small considering the scale of the simulation and i believe that your inflation problems arose from trying to get the larger cells to scale down to this level.

With 0.003m 1st layer Y+ target we devide 0.003 by the local surface cell size which is 0.25m. This gives us 0.012. This is what you use for the Surface layer RELATIVE thickness input. 0.012 x "given expansion ratio is
1st layer = 0.003
2nd layer = 0.009
3rd layer = 0.0027
4th layer = 0.081

Play around with expansion ration & number of layers until something works for you.



Thank you for the work you put it, I will take a look through in the morning with a fresh mind.

Take care

Hi @dschroeder,

I have implemented some settings you suggested in a half domain model I created, as seen the inflation was applied by visual inspection of the mesh looks rather strange.

Does the visual inspection show error?


The visual inspection can be deceiving because slices can sometimes show wierd cell connections like on the right. The left circle shows that layers may be getting deleted as the cell size get smaller, not super sure on this though. A gradual cell size reduction is needed but its always a balance to having a massive mesh. Also, having good inflation on the building is more important then the ground if you are measuring the building as a whole.