Measurement of instantaneous acceleration fields using particle-image velocimetry

Acceleration is a fundamental quantity in fluid mechanics because it embodies the pressure and viscous forces present within the flow. However, measurements of acceleration have been difficult to achieve heretofore, partially owing to the relative ease by which the fluid velocity can be characterized experimentally. In this work, the particle-image velocimetry (PIV) technique of measuring fluid velocity fields is extended to allow the measurement of temporal derivatives of velocity (in both fixed and convective frames). A two-CCD-camera particle-image accelerometer has been developed for this purpose. The cameras are focused on the same field of view; however, the acquisition of the second is delayed in time, facilitating the calculation of the associated instantaneous time-derivative and convective-derivative fields. Polarization filtering is utilized to separate the particle images viewed by each camera. The polarization filtering is achieved using cross-polarized light-sheets and a polarization filter just upstream of the imaging optics of the cameras. In this manner, PIV measurements can easily be achieved at time delays a few orders of magnitude smaller than the shutter-time of the CCD cameras.