Particle Image Velocimetry (PIV) is our main measurement technique. It uses two laser sheet pulses to illuminate a slice of a flow. We add particles to the flow (theater fog, in our case), and the laser pulses illuminate these particles. Think of the lasers as a camera flash, but only a slice is illuminated, which makes the picture much clearer. The video below depicts this process.
We use a computer algorithm to determine which direction and how far particles in different parts of the picture are moving by comparing the first picture to the second picture, which was taken a short time later. We can get a velocity vector for every 16 pixels, so in 2018, our cameras can provide hundreds of vectors in the vertical and horizontal direction.
The video below shows the setup used at Utah State University. The ball is “pitched” (by a machine) on the other side of the wall. It crosses two red laser sensors, and if it trips them both (indicating the ball is where we want it to be) the PIV system is triggered and takes two pictures as the ball passes by.
The still image below is from the instant the lasers fire. There are actually 2 pairs of lasers here, one pair on top and another on bottom. Having them above and below allows us it fill in the shadow that the ball makes. They are pairs because one flashes for the first picture and the other for the second picture. It happens much too fast to see (the laser shots are 20 microseconds apart). The (very special) camera can be seen on the right.
The raw data (which are 2 images) are shown below.
