When McLaren ran Flow Viz paint on their car in testing last year, the world took notice. Teams have been in fact running this solution for years and years, before that wool tufts have been used to show surface aerodynamic flows.
Flow Viz is a fluorescent powder suspended in a light oil (often paraffin). This is liberally applied by a spray gun to bodywork up stream of the area you want to investigate. As the car laps, the airflow spreads the non drying liquid back along the car, following the same path over the surface as the airflow. By then analysing the patterns the flow Viz leaves, you can see the behaviour of the airflow over the car. Details like direction and attached\non-attached flow can be observed. But this is only surface airflow and this is not the entire story when it comes to aerodynamics, so Flow Viz is only one solution for measuring aero on track.
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