Aerodynamicists use wind tunnels to test models of aircraft and spacecraft, and their engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model providing diagnostic information. Such a test does not provide overall performance, but helps engineers better understand how the fluid moves around and through the model. This photo shows the operation of the laser Doppler velocimetry, or LDV, system which uses a laser that sits outside the tunnel. Optics split a laser beam into two parallel beams that are sent into the tunnel and focused on a small test volume near the model. Because of wave interference, a fringe pattern is generated. Seed particles are introduced upstream in the flow and as the seeds pass through the fringes they are illuminated. A photodetector measures the frequency of the pulses as the particle passes through the fringes. Credit: NASA/Glenn Research Center #nasa #nasaglenn #windtunnels #nasatech #tech...