ABL and roughness height in custom boundary

If I have as Velocity inlet an ABL (with aerodynamic roughness length [zo] is 0.5m) and I had assumed that the air speed would stay constant over a non-slip Wall boundary condition in a free field. But thinking about this, I can understand I was wrong. I see the air speed increasing; perhaps because the roughness of a slip Wall is smaller than what aerodynamic roughness length of the ABL?

So I though if I make a Custom boundary with roughness height of 0.5, and thus I thought that I might have the same aerodynamic roughness length as the ABL. But the air speed is still increasing in the free field.
So what Custom roughness height is needed such that the air speed is not increasing in a free field, with a certain aerodynamic roughness length of the ABL?

In this project Rigde height by jjansman | SimScale I have two simulations (one with non-slip Wall 4 boundary and another one with Custom 4 with a roughness height of 0.5). The free fields are on the left and right side of the x-axii.

So my question is what roughness height in a Custom 4 do I need to use so that an ABL with z0 stays constant in a free field?

Thanks for your help.


The intention of setting up a wall roughness to the ground is mostly to maintain the ABL profile that comes from the inlet before it eventually reaches the buildings.

So to me the wall roughness seems to be doing its job.

The actual ABL formulation that you use will determine the velocity profile. This is what you get if you plot Z versus velocity for the formula that you defined.


Thanks Ricardo for your clear picture! I had been looking at a z plane (15m height) and not close to the inlet but more at the end and there I saw higher speeds (at x=-280m and y=280m). But now I realise that is thus due the influence of the buildings far (80m) away from the measurement point. Thanks for pointing this out!

Is it correct that for simulation the ground say in a town environment to use a Custom boundary with roughness height of 0.5m? Thanks.

No worries. Just keep in mind that the “Roughness height” under the custom BC definition is sand-grain roughness, meanwhile the ABL profile uses aerodynamic roughness instead.

There is a 32.62x conversion factor between them. i.e Ks = 32.62 x Z_0


THANKS for your help!

Sorry, if I read the link you gave (https://roadmap.simscale.com/c/134-aerodynamic-roughness-for-surface-assignment-in-pedestrian-wind-comfort) (grain) roughness height = 32.62 times Z_0.
Or do I read it wrongly?

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Oh sorry, yes - It looks like I inverted the numbers. I’ll edit the other post.