Wall functions vs. boundary layers

kandelabr · May 18, 2017, 03:26

Hello,

I am a bit confused about wall functions and meshes with layers on walls.

If I understand correctly, wall functions model boundary layer of the flow near the wall so it applies to first layer of cells in mesh. Those cells must be thicker than y+.

If I make a mesh with layers on walls thinner than y+ there is no need for wall functions because conditions in boundary layer are calculated directly.

Suppose I have a mesh from the latter case. What boundary conditions should I set for k and omega? What's the 'omega' value on the wall? Infinity?

How do I estimate y+ in a pipe?
The calculator (https://www.cfd-online.com/Tools/yplus.php) requires boundary layer length. What length should I use, say, for a very long pipe?

Thanks!

piu58 · May 18, 2017, 03:51

> What boundary conditions should I set for k and omega?

There is no need to set any wall functions in your case. Simply set a no-slip boundary condition at the walls.

kandelabr · May 18, 2017, 04:09

I can't do that. I can set a noSlip BC for velocity but there's no such thing for k and omega. There is an omega BC called 'slip' but that's definitely not 'noSlip'

kandelabr · June 12, 2017, 08:05

A few more questions.

When calculating y+, I need boundary layer length. What does that even mean? I have fully developed flow at the inlet, so what length should I choose?
I'm trying to simulate a centrifugal pump.

May 18, 2017, 03:26	Wall functions vs. boundary layers	#1
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	Hello, I am a bit confused about wall functions and meshes with layers on walls. If I understand correctly, wall functions model boundary layer of the flow near the wall so it applies to first layer of cells in mesh. Those cells must be thicker than y+. If I make a mesh with layers on walls thinner than y+ there is no need for wall functions because conditions in boundary layer are calculated directly. Suppose I have a mesh from the latter case. What boundary conditions should I set for k and omega? What's the 'omega' value on the wall? Infinity? How do I estimate y+ in a pipe? The calculator (https://www.cfd-online.com/Tools/yplus.php) requires boundary layer length. What length should I use, say, for a very long pipe? Thanks!

May 18, 2017, 03:51		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	> What boundary conditions should I set for k and omega? There is no need to set any wall functions in your case. Simply set a no-slip boundary condition at the walls. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

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May 18, 2017, 04:09		#3
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	I can't do that. I can set a noSlip BC for velocity but there's no such thing for k and omega. There is an omega BC called 'slip' but that's definitely not 'noSlip'

June 12, 2017, 08:05		#4
kandelabr Senior Member Nejc Join Date: Feb 2017 Location: Slovenia Posts: 196 Rep Power: 9	A few more questions. When calculating y+, I need boundary layer length. What does that even mean? I have fully developed flow at the inlet, so what length should I choose? I'm trying to simulate a centrifugal pump.