We needed a quick validation of where the vortex that was shedding off the barge board was indeed going, so we whipped out the tufts. These are particularly long and we also had a string-on-a-long-stick in play (you can see it poking in from the bottom of the frame). This was 2007 when developing the Super League Formula single make series car. Only time I've ever done wool tufts on a scale program.
For on surface flow visualization our preferred method is flow-viz fluid. Flow vis is a liquid concoction and the recipe is typically a highly guarded secret between outfits. Ours was a mixture of oleic acid, tempra paint (glow in the dark if I remember correctly), and paraffin oil, though for the life of me I can't find the exact ratios. 2006, Panoz DP01 Champ Car development:
