[cyln]

Hello,

I have been working on ICEM to generate a mesh for an engine exhaust mixer and I am very close to finishing the mesh. However, I have a few questions to ask.

1. how can I determine the total height of the prism layers?
you can see what my geometry is like below. The mixer is surrounded by tetra mesh; however, there are prism layers near the walls in order to resolve the boundary layer.

2. How can I determine a desired y+ value? Then I will calculate the first cell height. I know that y+ value has different ranges for different turbulence models. I will be using SST turbulence model.

3. In some of the papers (with similar geometries), in order to define the spatial resolution around the walls of the mixer, there are some Deltax+ ,rDeltaTheta+ values and r+ value for the first node above the wall. But I couldnt find these on any book. I guess they are related to y+ value. What are these values?


Thanks a lot in advance.

[babri]

Originally Posted by cyln

Hello,

I have been working on ICEM to generate a mesh for an engine exhaust mixer and I am very close to finishing the mesh. However, I have a few questions to ask.

1. how can I determine the total height of the prism layers?
you can see what my geometry is like below. The mixer is surrounded by tetra mesh; however, there are prism layers near the walls in order to resolve the boundary layer.

2. How can I determine a desired y+ value? Then I will calculate the first cell height. I know that y+ value has different ranges for different turbulence models. I will be using SST turbulence model.

3. In some of the papers (with similar geometries), in order to define the spatial resolution around the walls of the mixer, there are some Deltax+ ,rDeltaTheta+ values and r+ value for the first node above the wall. But I couldnt find these on any book. I guess they are related to y+ value. What are these values?


Thanks a lot in advance.

1)for total height of the prism layers certainly you have first layer thickness and grow rate from your mesh , so calculate it with total number of boundary layers.
2)there are several online y+ calculator in web so you can search on google and use one of them like:http://www.cfd-online.com/Tools/yplus.php , As far as I know if you want to use sst model your y+ value should less than 5 and greater than 1 and because of that your first layer prism should very small...
if you use models with wall functions like k-e then y+ should greater than 30 and your first layer thickness is greater than sst model

[niablis]

3) deltax is just change along the x axis. it is important b/c boundary layers usually grow in height along the x axis. deltaTheta would most likely be change in the momentum thickness. r values most likely have to do with the asymmetric shape. I cant find my notes on it, so I can't say for sure. Hopefully someone else can confirm or refute.

As a rule of thumb, to make sure the entire boundary layer is captured, you should add an additional 20% to your calculated boundary layer thickness. Also, be careful when using the online y+ calculators; the majority of them use the equations derived for flows over flat plates. Your geometry looks quite different compared to a flat plate.

[cyln]

Thank you guys.. I still have some question marks on the last question. They look like non-dimensional values like y+ value because there is no dimension stated. See the attachment below.

The length of this mixer is about 300 mm. How should I interpret these delta+, rDelta_theta+ and r+ values?