Applications Of Digital Holography Particle Image Velocimetry(DHPIV) On Measurement Of Rotating Flow Fields

Digital holography particle image velocimetry(DHPIV)， combined by digitalholography(DH) and particle image velocimetry(PIV)，is applied for measurement ofcomplex fluid field (such as: fuel spray field, automotive aerodynamics flow field, et al.).This technology, instantaneously capturing the three-dimension information of themeasurement fields and measuring the dynamic flow field in time by using the simpledevice, is an extraordinary hot technology. However, it is difficult to provide any criterionthat digital holography particle image velocimetry is applied to actual project measurement.Central to the technology is these issues as following: un-highly accuracy of focal planedetermination, influence of noise to image quality, particles pairing with low accuracy.To locate the focal plane of object in digital holography, the overall-sharpness [Os]method, which is the supplement of the edge-sharpness [Es] method, was presented.Combing above methods, the integrated gray-level gradient method was obtained. Thecriterion with Es was confirmed to be appropriate for large particles or small objectdistance. In contrast, the criterion with Os was determined to be appropriate for smallparticles or larger object distance. The conditions of above criterions were discussed byusing numerical simulation holograms of particles. At last, the integrated gray-levelgradient method was applied to the real holograms of dot array target and calibration target.The experiment results indicate that this method can effectively extract the focal plane ofparticle.In order to further accurately determine the focal plane of the object, the constrainedleast squares filter which can effectively eliminate the effect of noise on the focal planedetermination，is introduced for digital holography. It not only overcomes the limitationsof average filter (losses of details), min filter (excessive noise amplification) and wienerfilter (estimation of the noise-to-signal power ratio) but also improves image’s visibilityand perceptibility. Finally, this filter is applied to the real holograms of calibration targetand sprays. The experiment results manifest that this filter is an effective tool forenhancing the quality of hologram and improving the precision of focal planedetermination and works well in the sprays.To further demonstrate the research results discussed above, they are applied tomeasure the rotating fluid field. By using the3D cross-correlation pairing algorithm, therotating flow fields’3D visual velocities fields, corresponded with the theoretical modeling, are obtained by DHPIV based on the integrated gray-level gradient methodand the3D cross-correlation pairing algorithm. The experiment results demonstrate thatthis technology can be effectively applied for measurement of the complex flow fields.