The solution to your question might be consecutive mesh refinement.

So you can start with a rather coarse mesh and a best estimate for achieving - let's say - a value of y+~16.

From here you generate consecutively refined meshes, where you increase volumetric resolution step by step and where you subdivide the height of the near-wall layer in your prism layer from one mesh to the next level refined mesh by a factor of around 4. With that approach in 2 steps you should arrive at a y+~1 mesh with reasonable volumetric resolution.

The solution time on the very coarse first mesh should not be very substantial. The next two CFD simulations could be re-initialized from the CFD solutions on the next coarser mesh level, consequently no longer taking 8 hours to converge, since the initial guess for your flow of interest is already quite good (if CCM+ would nevertheless need 8 hours to converge, something is probably wrong).

As a side affect of this approach you can make a serious judgement on the mesh independence of your obtained CFD solution, by just comparing e.g. a couple of sensitive pressure or velocity profiles for the different mesh levels. Are the results showing a reasonable asymptotic convergence on the finest mesh towards a no longer changing CFD result? If yes, you know from the results difference between the last two mesh levels, how large your remaining solution error approximately might be. If no, discard the results and seek for some still existing systematic error in your solution approach.

Best regards,

Dr. Th. Frank.

The thickness of the first prism layer Y can be estimated by Y=(Y+)*L/0.172/power(Re,0.9), where Y+ is what you want to set, for example 50; L is the ship length; Re is Reynolds number. The number of layers and strech factor can be chose according to the best practice provided by STAR-CCM+ or by ITTC CFD guide, such as 6~10 layers and 1.2 respectively for high Y+ model. Generally, some difference still exists between Y+ what you want and Y+ what you read. However, their ratio seems to be constant in my experience. In other words, if you want to set Y+=50, what you read will be 25. So you can use Y+=100 in the formula above to get Y+=50 in the software. You can do some tests to find similar relation in your settings. I hope this can help you.

Hello Qiang Liu , i did what you said. But i can't obtain the results.

-The formula is Y= ((Y+)*L)/(.172*Re^0.9) right?

where L is the Water line or the entire lenght of the boat?

-When i obtain Y, i have to double it to obtain the height of the first cell, since Y is the distance between the wall and the centroid of the first cell...am i right?

a. The formula is right and L is the water line length. The formula is just for estimation, and your can find other types of formulas in fact. Besides, you can directly use online estimation, such as https://geolab.larc.nasa.gov/APPS/YPlus/

b. Acorrding to your data, L of your boat is about 12.75m. When the velocity of your boat is 13.8888m/s, its Froude number Fr is 1.242 as shown in the following picture, which means that the boat is very fast. Is there any mistakes or is the boat itself a high speed ship?

c. I directly used Y as the height of the first prism layer. A webbage https://www.cfd-online.com/Wiki/Dimensionless_wall_distance_(y_plus) may help you.

hello Qiang Liu ....okay. I understand your explication and i did it with a lenght of Lwl=13 m

When you settle the mesh settings, the minimun height of the cell of the core mesh should be near the height of the last cell in prism layer..right?

like here i obtain 1.6e-03m. So i set the minimun surface size of the core mesh to 2e-03 m. Do you use to do it like that or no?

Now when i realise the initial post processing, i obtain the value as you seen. (all y+ are under 5)

1- That is not the values of y+ that i expect. So should i wait the simulation to be done to observe the value that i wanted?

2- The value are under 5. I have settled 2 weeks ago a simulation like that(i mean the same initial values of Y+)....but during the simulation, these values of y+ have grown until they riched values like 3000 to 5000!....I have no idea how to keep these values under 100.

3- Concerning you preview question , i'am working with Hihg speed boat.

There seems to be some misunderstanding between the size of surface mesh and the height of prism layer. The former is a surface mesh, and the latter is a volume mesh. Number of Prism Layers is 6; Prism Layer Total Thickness->Absolute 6.42e-3m; Prism Layer Stretching is 1.2. Did you use these values? If yes, you can run simulation for some seconds to check Y+, such as 20s.

As for your questions, can you give me your sim.file or setting in detail? I am not sure whether there are other problems in your setting. My e-mail is [email protected].

Hello, have your resolved your problem? I am sorry to reply you so late, as my work was too busy in the last week. Unfortunately, I cannot open your sim.file because of my older version of STAR-CCM+ (10.04.008). Can you give me the ship geometry in IGES or CCM? So I can import it in my version to do some test.