The idea is not for absolute precision - but more to get a stream of air about 5 - 25Kmh (walking speed to bicycle speed), and to see how the air moves around the object; or as the stationary object, moves through the moving air.
Consider a plain cardboard box - a good container shape, compares to a torpedo or boat tailed bullet shape - a good at moving through liquid shape.
The flip side is the box moves through air like a parachute, and the really good air penetrating shape of the torpedo or spire pointed, boat tailed bullet, is an incredibly **** shape to get things in and out of, and transport things in.
So by seeing where all the power chewing features of the box are, with a wind tunnel, where the air flow separates from the shape and creates all the turbulence from, that is where you can add the aerodynamic (or hydrodynamic) properties of the torpedo's shape, to the box shape of the trailer.
And it's not much of an effort to turn a totally **** aerodynamic shape, into one that is a remarkably good improvement.
At the end of the day, you can't make a shoe box operate like a javelin, and you can't make a javelin as useful as a box - but you can make a very aerodynamic shoe box.
Basically what your after is the general shape of an aircraft wing, with rounded over ends - for the sides.
Have a look at the curves and blended tapers on the bus and the nascar.
The bus is technically a house brick, but at it's slow speed, it's aerodynamic profile basically eliminates all significant sources of drag - making that component, a significant part in reducing it's fuel consumption.
And the NASCAR, although these run at 320Kmh, and not 100Kmh, like a typical trailer would, all the principles of shape, blended curves, tapers that you need in your trailer design.
https://en.wikipedia.org/wiki/File:Leading_edge.jpg
https://en.wikipedia.org/wiki/File:Tempo_SR.jpg
https://en.wikipedia.org/wiki/File:K...ge_Charger.jpg
Get something like a shoe box, or smaller, and make it from clay or blocks of watery plaster (easy to shape) and start working on that - an incense stick for your smoke generator and even an open window with a bathroom fan sucking the air into the room, through the window.
You now have all the info you need, study the subject, do your tests, and then make it work.