Multiple Light Sheet Particle Holography for 3-D Flow Velocimetry

Abstract

Holographic recording promises the ability to extend Particle Image Velocimetry (PIV) to a fully 3-D measuring method, where PIV is no longer limited to the instantaneous registration of 2-D velocity vectors within a single sheet. The recording of small particles in a 3-D volume introduces, however, additional noise in the interrogation step. This is investigated in more detail. First the holographic recording of a continuous deep volume is considered. Sampling the depth by spaced lightsheets allows a first improvement of the signal to noise ratio and provides a higher validation rate in the interrogation step. In addition, the light for illumination of the particles is used in a more economic way. If dense sampling is required the concept of simultaneous recording of all light sheets, but reconstruction of single sheets each can be used. This is possible by taking advantage of the limited coherence of the laser. A demonstration of this method is included. The validation characteristics of typical evaluation methods like autocorrelation and crosscorrelation techniques are estimated by Monte-Carlo-simulations. This gives the ability to find the method best suited for the fluid mechanical problem under investigation.