I have the problem with the mesh file in GAMBIT. Now I have the 3D model of the aircraft used for testing in the Fluent, but I do not know the way to estimate the wind tunnel size that producing the best solution.

My project objective is to simulate the wind tunnel test for aircraft model by using Fluent.

My aircraft size as follow aircraft length : 2.575 m semi span : 1.875 m aircraft height : 0.431 m (exclude the landing gears)

What is the Mach number that you are going to use? If it is subsonic/transonic, you might have to put your boundaries at 10-15 times your chord length. If supersonic, I think 2-3 times your chord length might suffice.

I use subsonic analyze and now I dont know what shape that I have to create in 3D

and in your suggestion that put boundaries at 10-15 times my chord, which chord of my A/C just 0.4 meters so I have to create wind tunnel domain about 4~6 meters right?

furthermore, I split A/C in symmetry and calculate for a half side of A/C so now I have to create in 4~6 meters or 2~3 meters?

Yeah, your domain extents are quite big. But you need that range because for the subsonic flow problem, the values that you compute near the region of interest will travel forward and influence the solution for the next iteration.

I suppose you will be using an unstructured mesh? If so, your domain can be in the shape of a box. As to the spanwise dimension, I am not really sure how large that must be.

I don't think symmetry influences the domain extent. Your symmetry is in the axis along the length of the aircraft, so your domain would use the 4-6m length.

Hope this helps.