[Onurozcan]

Hi,

I am working on 3D heat exchanger (gas-gas).As you know system consist of a lot of tubes flowing through air and outer region ( that is exterior of tubes) I define '' waste heat fluid domain''.(https://imageshack.com/i/pboyWh5bp)

I know density of both fluid besides inlet-outlet temperature and mass flow rate in both fluid (waste and air)

Hence I am to calculate value y+. But the problem is I have valued Y+ and in consequence of coming out of boundary layer length.

Both fluid are in turbulence regime ( 30<y+<300).But as I dont know reel value y+ , boundary layer length is defined false value.Because I can give different value y+ ( 30,40 etc.).How can I define real y+ value?


Calculator Y+ : http://www.cfd-online.com/Tools/yplus.php

[DarylMusashi]

If you use a turbulence model with wall functions (like k-epsilon) your y+ value should be between 30 and 300. If you use a turbulence model without wall-functions ("low Reynolds approach") your target value is y+ = 1.

Maybe there is a misunderstanding in the meaning of y+? It defines the distance of your wall-nearest cell normal to the wall. As a first guess you can try the calculator you mentioned very well.
The value "boundary layer length" you do not know in detail. Just choose a value, which is reasonable in relation to your whole geometry. As you vary this "boundary layer length" in the calculator you can see, that the influence on the y+ value is limited.

It is normal to do a first mesh, run a simulation, check y+ in the result and refine/coarse the mesh - it is an iterative process.