Velocity measurements in unseeded air flows by microwave scattering from a laser generated micro volume plasma

Velocity measurements that rely on Particle Imaging Velocimetry (PIV) or Laser Doppler Velocimetry (LDV) require flow seeding, which for some test facilities is very difficult and for flight applications is often impossible. Even when particles are present in the flow, measurements can only be made when they pass through the region of interest, which is a problem for measurements transient phenomena where the timing is critical. These factors have motivated the development of a new laser/microwave approach to point velocity measurements that is reported here. A pulsed laser is focused to a point in the flow and generates a small region of ionization, so no seeding is required. The laser can be used to select a particular location in the flow and interrogate that location at a specified time. The region is illuminated with an unfocused, low power microwave beam, and microwave scattering from this point occurs because of the presence of unbound electrons. As the flow moves, the small region of ionization is convected with it, and the microwave scattering is Doppler shifted by the motion. Quadrature detection maps the phase of the scattering as a function of time, providing microsecond resolved, single shot measurements of velocity.