[hellsblade.91]

I am trying to simulate the flow of air at various pressures and also modifying my no of inlets.

I am simulating the max velocity and mass out flow at my outlet

My goal is not to check the pressure and vel gradient losses at the walls however I see that when I change my mesh I get different values of Wall y+ ( from 900 max to 150 max ) the velocity at outlet reduces by 8 m/s

In both the sims at diff meshes I have converging monitor residual- ie are weighted avg vell at outlet and mass flow at outlet which converges from 100 to 200 iterations

However my question Do I always need to take care of wall y + value (It should be in the range of 30-300 for Standard Wall Function) . I am using K epsilon realizable turbulence model .

[LuckyTran]

y+ will change with the mesh because it depends on the wall shear stress and the cell size of the wall adjacent cells.

I point you to the work of Salem & Cheah. Generally, the standard wall function likes to either be globally y+<10 or globally y+>30 so that you avoid the buffer region. So it's a good idea to check both the max y+ but also you need to check the min y+ and probably the average y+.

I also recommend y+< 300 if you want reasonable wall shear values.

[hellsblade.91]

Hi thanks for your reply so is wall y+ really an important criteria everytime while using turbulence modelling and do achieve reasonable range of values with increase of pressure and vel i should use a finer mesh and vice versa ?

[hellsblade.91]

I am using std wall function my min wall fn is 0.09 and max is 270 area weighted avg is 20

I am getting these results after using a very coarse mesh nw getting a difference of 2 m/s at outlet ie the veloicty now is lesser the mass flow is gone done by 0.004 kg /s

The circular dia is my inlet and bottom face of the rectangular domain is my outlet at 1 bar

Inlet pressure is 4 bar I am using K epsilon realizable model

[Far]

either use salable wall function or enhanced wall treatment.

standard wall functions are tricky to use. As you go below the Y+ 30, you will get into trouble.

[hellsblade.91]

Thanks for your reply

Yes if wall y+ along the elemental wall is below 30 it means we are underpredicting turbulence

What is the difference bettween enhanced and non equilibrium wall treatement

[hellsblade.91]

Thanks for your reply

Yes if wall y+ along the elemental wall is below 30 it means we are underpredicting turbulence

What do you mean that standard wall function is tricky to use ?

What is the difference bettween enhanced and non equilibrium wall treatement

[Far]

Yes if wall y+ along the elemental wall is below 30 it means we are underpredicting turbulence

Wall shear stress (turbulent) = Laminar shear + turbulent stresses arising from the average of NS equations.

In fact near the wall, we have no turbulence as there is no velocity

We are predicting the shear stress terms incorrectly.

Means if the mesh has Y+ = 5 and we are using the standard wall functions, we are providing (input) wrong value of velocity /shear stress to the law of the wall. Coz when we have Y+ below 5 (or 11.06) we should be solving it as the viscous sub-layer not the log layer.

What do you mean that standard wall function is tricky to use ?

They have strict requirment of having Y+>30. If you use the salable wall function, it will set the Y* = max [Y+, 11.06]. So even if the Y+ is less than 11.06 (up to 11.06 we have viscous sub layer also known as linear profile, above 11.06 we can assume it to be buffer layer. but better method would be to use weighted average of both layers in region of Y+ from 5-30), salable wall function will mathematical enforce the value of Y+ to be 11.06. which will make the mathematics correct.

What is the difference between enhanced and non equilibrium wall treatment

Enhancement wall treatment is two layer model used to solve the all the way up to wall. It will use low Reynolds number formulation when Y+ < 2 (or may be 5, i dont recall correctly) or so, wall functions when it is above 30 and weighted average when it is between 5 and 30.

I cannot comment about the non equilibrium wall treatment at the moment. For this I have go back to books