Doug asks a good question - what is being looked for with airflow and string movement?
I should say up front that I'm not an expert and don't want to put myself out there as one. I've read a few books, studied a good bit of video and consider myself to be a curious guy and for the most part this serves me pretty well.
With that said - I was looking for smooth and attached airflow along the clam shell just below the headlight and above the canard primarily. If you get the shape or angle wrong the strings do crazy things and can whirl in circles or even start to face forward if you get it way wrong. But get it right...........or closer to right..........and the strings on the canard lay pretty flat and point up and back on the canard and don't just flail to the side. The strings on the clam just below the headlight stay stuck to the clam and run more or less parallel to the canard indicating that the air is rising in that area. I did not cover the entire side of the car with strings and see how the canard affected airflow over the front wheel well and back toward the rear wing. I think that would be fun and interesting.
This is a bit off topic at this point but one place I found really lacking in the original Lotus design was the rear diffuser. All the reading I did said that the angle of the roof of the diffuser was really critical and that the typical effective angle was somewhere between 7° and 14°. The stock Lotus diffuser is made of of three sections with the outer two having a roof angle much steeper than the center section.......and much steeper than the accepted 'norm'.
Using the same strings and GoPro technique I tested the airflow on the center section of the diffuser and the steeper outer sections. The airflow on the center section looked to be smooth and attached..........the strings stayed against the roof of the diffuser and pointed back. I tested this at low speeds as I am most interested in solo events so I was testing at 55 mph. At higher speeds the strings were quieter and almost looked glued or stuck to the roof.
The outer sections of the diffuser were not this way. The roof angle is very steep and the strings pointed mostly down and whirled in circles. This made me think that the air is in stall here...........it certainly wasn't attached airflow. Common sense lead me to believe that the air would need to move back and up, just like in the center section, to best make a low pressure area under the car.
So I next made a fake cardboard roof for the side pod of the diffuser that I could adjust the angle of. I then tested the airflow at various angles and found that much like the angle of a rear wing - a very small difference can have a profound effect on how the air moves. I changed the roof angle about 2° at a time until I got good attached flow at my relatively low speeds. It was very cool to drop from 11° to 9° and watch the string action totally change. I settled on 9° which coincidentally or not was the same angle as the large center section of the diffuser.
Once I established the angle I wanted took a stock diffuser and removed the side sections and replaced them with sections that I made at the new lower roof angle. I made them a bit longer in an effort to make them as effective as I could.
I did all this stuff for two reasons - I race solo (XP) and want every advantage I can get and I find learning new stuff to be fun. I've had many question why I would bother with aero stuff for solo as the speeds are so low. Well I'm lucky to have a place were my SCCA club and the others in Montana use a large facility that allows for sustained slalom speeds of 80+ mph. There is also a section of road (the place is a police and fire training facility) that has a long radius right/left curve combo that can be taken at well over 80 mph.
Before I made the aero stuff and bought the wing the car was unstable in these high speed sections and if I had gone in too hard or deep it was a real wrestling match to keep the ass end of the car behind me - lift and spin. With the aero stuff in place I can now flat foot it though this high speed road section and if I'm going too hard can lift a fraction and not fear the car swapping ends. My times dropped and my placings went up.
I feel completely confident in saying that someone with more smarts and experience could set the aero stuff up better than I could and that the car would be faster still.........and at the same time I am 100% sure I am faster, and that the car sticks better, due to the fine tuning of things. If I had just bolted stuff on it would probably improve some but I feel good knowing that I did what I could to get the most out of the car. And it's just gravy that the race results have been very good.
Fun stuff to mess with.