desired y+ value

lukas' · June 10, 2020, 06:01

Hey everybody!

I'm relativly new to Ansys...
For my current simulation I want to calculate the first layer thickness (https://www.cfd-online.com/Tools/yplus.php). I use the SST-model.
During my reseach I red different things on what desired y+ value I have to choose. Is it 1?

Thank you for help!

mCiFlDk · June 11, 2020, 04:23

Quote:

Originally Posted by lukas'

Hey everybody!

I'm relativly new to Ansys...
For my current simulation I want to calculate the first layer thickness (https://www.cfd-online.com/Tools/yplus.php). I use the SST-model.
During my reseach I red different things on what desired y+ value I have to choose. Is it 1?

Thank you for help!

Hi Lukas,

I don't know how you want your analysis to be, but within CFD problems there are two ways of simulating boundary layers:

Simulating the whole boundary layer
Using wall functions

SIMULATING BL --> If you want to simulate the whole BL, then you're right and a height of yPlus=1 should be used. This decision for quite big geometries could lead in unaffordable computational resources, but it is the best option if what you want is precision and certainty.

WALL FUNCTIONS --> On the other hand, if your interest in simulating the BL or studying it is not that big, the usage of wall functions to approximate the viscous, buffer and log layers is advisable. It allows to reduce the number of elements a lot and in this case, the first layer height should be comprised between yPlus=30 and yPlus=300. I'd strongly recommend you to choose between 30 to 100 to achieve a better precision, but 300 is also suitable following ANSYS' literature.

I hope I've helped you!

Regards

lukas' · June 11, 2020, 06:59

Hey mCiFlDk,

thanks a lot for your answers! It helped to get better understanding.

I recognized another problem: The parameters to calculate y+ are not 100% known in my case (because of unknown temperatur due to the heater and and unknown freestream velocity due to free convection). Therefore I can only estimate the values but there might be some mistake then.

How do I deal best with such a situation?

Thanks a lot and best regards!

mCiFlDk · June 11, 2020, 07:16

Quote:

Originally Posted by lukas'

Hey mCiFlDk,

thanks a lot for your answers! It helped to get better understanding.

I recognized another problem: The parameters to calculate y+ are not 100% known in my case (because of unknown temperatur due to the heater and and unknown freestream velocity due to free convection). Therefore I can only estimate the values but there might be some mistake then.

How do I deal best with such a situation?

Thanks a lot and best regards!

Hi Lukas,

What range of temperatures and velocities are we talking about?

Regards.

lukas' · June 11, 2020, 07:38

Hey mCiFlDk,

the inner part of the installation space is heatet up to 500°C while the external part has room temperature. The fluid inbetween has therefore a big temperature variity.

The installation space is nearly splid up into two parts. In the one part there is a stream with 8 m/s. In the other part there is no external stream. Just free convention and its effects.

Thanks a lot for your help!
Best regards!

mCiFlDk · June 11, 2020, 08:07

Quote:

Originally Posted by lukas'

Hey mCiFlDk,

the inner part of the installation space is heatet up to 500°C while the external part has room temperature. The fluid inbetween has therefore a big temperature variity.

The installation space is nearly splid up into two parts. In the one part there is a stream with 8 m/s. In the other part there is no external stream. Just free convention and its effects.

Thanks a lot for your help!
Best regards!

Well, I've been looking for info and anything appears as regards to temperature, so here's my guess:

In yPlus calculation, the only variable dependent on temperature is Reynolds number by means of the kinematic viscosity. Hence, I'll calculate the Re number correspondent to 500ºC (bigger than the previous one).

On the other hand, the velocity I'll apply would be the bigger one of 8 m/s.

To sum up, I'll do yPlus calculation with Re at 8 m/s and 500ºC to avoid major errors.

I hope it helped you,

Regards

vinerm · June 15, 2020, 08:47

Quote:

Originally Posted by lukas'

Hey everybody!

I'm relativly new to Ansys...
For my current simulation I want to calculate the first layer thickness (https://www.cfd-online.com/Tools/yplus.php). I use the SST-model.
During my reseach I red different things on what desired y+ value I have to choose. Is it 1?

Thank you for help!

values are directly available under Turbulence. Why do you want to calculate it yourself? It makes sense before doing the simulation, but once you have results for a simulation with some realistic field values, you can report

directly from Fluent.

lukas' · June 15, 2020, 09:46

Hi vinerm,

actually I don't want to calculate y+. I want to calculate the first layer thickness. It was misleading how I phrased it.

Best regards!

karachun · June 15, 2020, 09:55

Quote:

Originally Posted by lukas'

Hi vinerm,

actually I don't want to calculate y+. I want to calculate the first layer thickness. It was misleading how I phrased it.

Best regards!

https://www.pointwise.com/yplus/

vinerm · June 15, 2020, 10:08

Quote:

Originally Posted by lukas'

Hi vinerm,

actually I don't want to calculate y+. I want to calculate the first layer thickness. It was misleading how I phrased it.

Best regards!

First cell height is also available under Mesh. You can also back calculate it using

since $u_\tau$ is available as well.

June 10, 2020, 06:01	desired y+ value	#1
lukas' New Member Lukas Join Date: Jun 2020 Posts: 13 Rep Power: 5	Hey everybody! I'm relativly new to Ansys... For my current simulation I want to calculate the first layer thickness (https://www.cfd-online.com/Tools/yplus.php). I use the SST-model. During my reseach I red different things on what desired y+ value I have to choose. Is it 1? Thank you for help!

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June 11, 2020, 06:59		#3
lukas' New Member Lukas Join Date: Jun 2020 Posts: 13 Rep Power: 5	Hey mCiFlDk, thanks a lot for your answers! It helped to get better understanding. I recognized another problem: The parameters to calculate y+ are not 100% known in my case (because of unknown temperatur due to the heater and and unknown freestream velocity due to free convection). Therefore I can only estimate the values but there might be some mistake then. How do I deal best with such a situation? Thanks a lot and best regards!

June 11, 2020, 07:38		#5
lukas' New Member Lukas Join Date: Jun 2020 Posts: 13 Rep Power: 5	Hey mCiFlDk, the inner part of the installation space is heatet up to 500°C while the external part has room temperature. The fluid inbetween has therefore a big temperature variity. The installation space is nearly splid up into two parts. In the one part there is a stream with 8 m/s. In the other part there is no external stream. Just free convention and its effects. Thanks a lot for your help! Best regards!

June 15, 2020, 09:46		#8
lukas' New Member Lukas Join Date: Jun 2020 Posts: 13 Rep Power: 5	Hi vinerm, actually I don't want to calculate y+. I want to calculate the first layer thickness. It was misleading how I phrased it. Best regards!