Particle Imaging Velocimetry (PIV)

How to use a Photron FASTCAM Camera for PIV Applications

This application note will give a brief overview of Particle Imaging Velocimetry (PIV) for flow visualization using Photon high speed FASTCAM cameras. The technique is used to obtain the time dependent velocity distributions in a Field-Of-View (FOV) of single and multi-phase flows which are very fast, turbulent and complex. PIV is very useful for analysis of turbulent flow, transient flow, micro flow and 3D volumetric flow studies. As an example, the high temporal rate allows visualization of the laminar separation bubble found in the flow over an airfoil, turbulent flow near an object or complex flows found in bioreactors.

To facilitate visualization of a flow, most applications require seeding the flow with tracer particles. These tracer particles will be very visible in the flow when pulsed with a sheet or spot laser illumination.

The laser illumination is very short in duration, easily stopping all motion. When the laser illumination is off, the image is completely dark. The short intense laser pulse provides an excellent contrast to the tracer particles after capture. The tracer particles can then be tracked for movement in the flow where each image is analyzed for the position of these particles as a function of time. Knowing the precise time (Dt) between the double laser pulse provides a reference for the spacing and direction change between these particles in the image sequence. The particles are plotted as a series of velocity vectors representing the flow at a given instant in time (shown to the right).

A technique called frame straddling is timed & produced by a pulse laser and our FASTCAM cameras to capture image pairs that have an extremely short time interval between images. Using either a diode-pumped Nd:YLF or Nd:YAG laser, the laser is pulsed twice, straddling the two frames as shown below.

To setup the Photron FASTCAM camera for PIV, input the Sync Generator signal to the camera Sync In and select the External Sync mode. The camera Sync Out should be set to .5 which provides one TTL pulse for every other frame and use this pulse to trigger the laser. Use the camera built-in delay function to delay the Sync Out pulse so that the laser pulses straddle the frame as shown above.