Hello abenhadj and thank you for your answer ! 

When you tell me to calculate the k+, do you mean ks+ or is it another variable ? If it is can you just tell me quickly how can I calculate it or where can I find documentation ?

By High Roughness model do you mean the Icing models ? Can I use it even if I don't have icing on my wall ?

Also, do I have to enable the near wall mesh insensitive treatment or is it by default with the k-w SST model ?

Best regards, Hugo