[surajp92]

I am modelling KA 4 70 propeller using SST k-w gamma transition modeling. I am trying to do meshing such that I get value of Y+=1 so that boundary layer separation can be predicted. I tried adjusting the prism layer thickness but I am still get Y+ value greater than 1 at few places.

My prism layer thickness is 0.25, prism layer growth rate 1.2 and number of layers 10. I changed prism layer thickness from 0.5 to 0.25. Can you please guide me which parameters should I change to get smaller value of Y+?

I am not able to get sufficient prism layers at trailing edge as the edge is sharp (wedge shape).My leading edge is also sharp but I was able to get prism layers at Leading edge.

Thank you.

[Fiedde1887]

The y+ value depends on the thickness of the cell next to the wall. You have to reduce the thickness of that cell to achive your y+ value.
If you want to create a thinner layer in some regions of the blades, you can use Surface controll for meshing. Use the Surface repair tool, to create different surfaces for the Surface control.

[fluid23]

A few things:
1. Not only do you need to have sufficiently sized cells at the wall to get a good y+, you also need your prism layers to grow from small to large to match your core mesh. The mesh you show in your images does not have a transition. You go from your outer prism layer to a much, much larger core cell. You need to play around with prism layer thickness, number of cells AND layer ratio to get a mesh that grows naturally away from your surface. An ideal mesh will have NO noticeable demarcation between prism layers and core mesh.

2. You do not need y+ exactly equal to 1 to adequately model the boundary layer separation. The near wall model which "requires" y+~=1 is still very accurate for 1< y+ <=5. and 0<<y+<1.

3. Sharp trailing edges are difficult to do with unstructured mesh and unfortunately Star-CCM does not have very strong structured mesh capabilities. I would recommend either modifying the trailing edge to be slightly blunted (will have minimal impact on results but will allow you to get a better trailing edge mesh) or using a 3rd party mesher to get a structured mesh and importing that into Star-CCM.

4. Somewhat off topic... your turbulence model is good for adverse pressure gradients (separated flow) but has some problems with swirling flow that will be in your wake. I recommend turning on curvature correction which will help with this. It's important to remember that your wake has a significant influence on the flow around your body so it's important to accurately model this. I would also recommend adding some kind of wake refinement to your mesh, if you haven't already.

[surajp92]

Thank you very much for quick reply.

I would also like to tell that with my current mesh I am getting torque coefficient within 5% of experimental results. But my thrust coefficient is around 25% less than the experimental value. I think this maybe due to the size of domain downstream of propeller. Currently my domain is 3D upstream and 5D downstream from propeller center.

What do you think can lower thrust be due to domain size or maybe my CFD mode is not predicting the flow around propeller correctly?

Thank you.

[Fiedde1887]

Have a look at the pressure Gradient at the leading edge. That´s the Region, where the thrust is generated. The Resolution of that Gradient must be nice

[surajp92]

Originally Posted by Fiedde1887

Have a look at the pressure Gradient at the leading edge. That´s the Region, where the thrust is generated. The Resolution of that Gradient must be nice

Actually, the information which is available in literature does not say anything about the shape of airfoil at leading edge (like leading edge radius). I have the upper and lower surface coordinates. I do not have information about shape of airfoil from x=0 to 5% C (x=0 is leading edge). So I am closing leading edge like sharp edge. I have attached the screenshot of 2d airfoil for more information.

Currently, I am having stagnation pressure at suction side of propeller at leading edge and low pressure on pressure side. I think it should be other way. Please correct me if I am wrong. Maybe change in leading edge shape can help in getting correct value of thrust.

Thank you.

[surajp92]

Originally Posted by MBdonCFD

A few things:
4. Somewhat off topic... your turbulence model is good for adverse pressure gradients (separated flow) but has some problems with swirling flow that will be in your wake. I recommend turning on curvature correction which will help with this. It's important to remember that your wake has a significant influence on the flow around your body so it's important to accurately model this. I would also recommend adding some kind of wake refinement to your mesh, if you haven't already.

I am creating the mesh in Continua, so I guess I am using region based meshing. I have added volumetric control downstream of the propeller. I have not used part based meshing till now. So, is wake refinement in PBM different from volumetric controls in RBM? Also, how to add wake refinement when I am using PBM? Shall I create wake refinement for propeller surface or create another part like cylinder and then add wake refinement to the cylinder?

Thank you very much.

[fluid23]

Yes, if you are applying mesh properties to a region you are using RBM. PBM is definitely more user friendly for most things. You can do a volumetric control to define wake refinement, however, there is also an option to add wake refinement to your parts based mesher controls. It is just an option you need to select when you set up your PBM custom controls. You will need to specify surfaces for the wake to grow from, a vector to follow, a spread angle and length and a mesh size. The other option if you want to stick with RBM is to use a wake refinement table that splits cells based on some criteria you can choose (like pressure gradient).

[surajp92]

I think I can use wake refinement in region based meshing only if I am using trimmed mesher?

[Fiedde1887]

The wake-refinement is also available for the poly mesher. Have a look at the Manual.