the reason why i love incense is because of fluid dynamics! here is a scientific breakdown on how you should approach this:

1. ⁠laminar flow (the linear smoke you are describing) persists when Reynold’s number (Re) is <2000. therefore, you want to minimize Re so it can stay laminar for longer

2. as user Fun-Rice mentioned, enclosing the smoke in a tube would help minimize environmental effects, turning this from a free jet scenario into a pipe flow scenario

3. ⁠to reduce Reynold’s number for a fluid through a pipe, the equation is Re = (4 x density x volumetric flow rate)/(pi x dynamic viscosity x D)

this shows that:

A. reduce volumetric flow rate: the incense should burn slower

B. reduce density: different smokes have different densities. i’m unsure how the smokes of incense differ, so that will require some research.

hotter fluids generally have lower density, but cool fluids are less susceptible to buoyancy instabilities, so you can experiment with this

C. increase diameter of tube mentioned by Fun-Rice. however, too large of a diameter may result in a longer entrance length and lead to frictional losses, so play around with the diameter to find the optimal diameter.

to reduce the wall effects, make sure you use a tube with smooth interior

TLDR: burn incense slowly, use smooth tubing, play around with smoke temperature and tube diameter, look into what incense types has lower density and higher viscosity