[aja1345]

Hi,

I am running an axial turbine with RNG K-e turbulence model. Wall Function is scalable and Y+ contours are as follows: (please see range of Y+-I enable clip to range)

1. Is there any problem in these contours?
2. How much Y+ is enough in K-e model? What range is Y+ in K-e model?
3. If there is a problem in terms of Y+, What is your suggestion for solving this problem?



Y+ On the blade:











Y+ On the Hub



more explanation about hub position:




Thanks.

[ghorrocks]

You need to check the sensitivity to y+ for your case. Run a simulation with a coarser or finer y+ and see if it makes a difference to parameters you care about.

You can't tell if the y+ is OK from a contour plot.

[aja1345]

Thanks,

You can't tell if the y+ is OK from a contour plot.

Why?

I think that Y+ value have to be match with turbulence model.

For example, Y+ should be equal to about 1 in K-W SST.

Am I right? What is your idea?

I am grateful that guide me more in detail.

[aja1345]

Is there difference between "grid sensitivity" and "grid independence"?

Could you please explain me the difference between the two terminologies, 'grid Independence' and 'grid Sensitivity '

[ghorrocks]

You can't tell if the y+ is OK from a contour plot.
Why?

Different simulations have different sensitivities to wall modelling. So some simulations are OK with very large y+, some require y+ to be closely controlled.

For example, Y+ should be equal to about 1 in K-W SST.
Am I right? What is your idea?

This is a very common misconception with SST. You do not need y+=1 for SST. Two reasons:
1) If the result is not sensitive to wall resolution
2) If the wall function approach works well then you want y+>11

You ONLY want y+=1 when you know you are sensitive to near wall resolution AND you know you need to integrate to the wall as the wall function approach will not work.

Is there difference between "grid sensitivity" and "grid independence"?

Grid sensitivity means that when you change your grid your results change. This is bad and means your simulation is going to be inaccurate.

Grid independence means that when you change your grid your results do not change. This is what you want and is a necessary requirement for validation of a simulation.

[aja1345]

Thanks for your clear reply.

2) If the wall function approach works well then you want y+>11

What is maximum of Y+ value here?

How do i find this maximum value?

[ghorrocks]

What is maximum of Y+ value here?

It depends on your application - there is no universal answer. You have to do a sensitivity analysis.

[aja1345]

Thanks.

Y+=11.225 is the transition to the log-law region.

Now I have one question:

We know that buffer layer is between 5 to 30. So it seems somehow wrong to place first layer in buffer layer!

Can you explain in this case?

[ghorrocks]

So it seems somehow wrong to place first layer in buffer layer!

In some cases that might be the case. But if your simulation is not sensitive to near wall boundary layer effects then you can stick the first cell where ever you like and it won't make any difference. So you need to determine for your case what level of sensitivity you have and act appropriately. There are no universal answers.