What enclosure shapes I could have for best structure-meshing these modelling?
 

One is sharp edged 3D conical-like shape, the other one is blunt edge 3D conical-like shape. The first three pictures are for the blunt leading edge model, the last two are for the sharp edged model.

I am planning mesh and analyse each of them in half so as to save the time. what kinds of 3D enclosures you guys may reckon to be put around the models? since I am a ICEM CFD beginner, I only came up with cylinder and rectangular shapes. Looking forward to your replies.

Personally I think the shape of the 3D enclosure would depend on the conditions of the simulation. For instance, if you are modelling a low speed flow where the information in the flowfield can proporgate upstream then a large (you need to assess how large is suitable) hemisphere / hemicylinder / box should make no difference. I usually go with the sphere to save on a few elements, but it all depends on the simulation I'm working on. However, if the flow is fast and a strong shockwave is at the tip of the body then no flowfield information can flow upstream so maybe consider a conical enclosure based on the shock angle - again that should save on elements. Ultimately, I think that so long as the enclosure shape does not influence the simulation result all is good.