流体中速度矢量场,应力,应力张量检测方案(CCD相机)

13"Int. Symp on Appl. Laser Techniques to Fluid Mechanics， Lisbon， Portugal， June 26-29， 200613.1 Fat-sheet PIV with computation of full 3D-strain tensor using tomographic reconstruction Bernhard Wieneke ， Simon Taylor 1： LaVision GmbH， Gottingen， Germany， bwieneke@lavision.de 2： Loughborough University， UK， simon.taylor@web.de Keywords： tomographic-PIV，3D-velocity gradient tensor， Stereo-PIV A new technique (fat-sheet ‘Tomo-PIV’) ispresented to measure instantaneously the full 3D-velocitygradient tensor in a light sheet plane， which is an importantquantity e.g. for turbulent research. So far this tensor hasbeen measured by dual-plane Stereo-PIV， fast scanning PIV(only for slow flows)， and with less spatial resolution by 3D-PTV and digital holographic PIV. The experimental setup ofTomo-PIV is similar to Stereo-PIV but using three camerasand a somewhat thicker sheet. 3D-straintensorusinggaathicker lighttsheet. Usingsynthetically generated images the out-of-plane gradients aremeasured within 5-10%， for real images within 10-20%.This is similar to what has been reported for dual-planeStereo-PIV requiring 4 cameras and two double-pulse lasers，where the errors depend strongly on accurate laser sheetprofile and position determination. Firsitt tlthe1intensitiess offallvoxels within the light sheet arereconstructed using a MARTtomographic reconstructionalgorithm (Elsinga et.al. 2005).Then the volume is split intotwo parts， z<0 and z> 0， andfor each part a three-dimensional cross-correlation isdone to compute two planes of3C-vectorsand subsequentlyall components oftthevelocity gradient tensor. Firstthe accuracy of the proposedmethod computingg onlyasingle vector plane is comparedto standard Stereo-PIV usingsynthetic and real data. It isquantified ]how theesspatial resolution and accuracy of Stereo-PIV islimited by the laser sheet thickness andin-plane and out-of-plane flow gradientsdue to non-identicalinterrogationvolumes for both cameras. Tomo-PIV does not suffer from thislimitation， but has other error sourcesmainly from imperfect reconstructiondue to optical defects. For synthetic dataTomo-PIV has less errors and betterspatial resolution thanStereo-PIVespecially for thicker light sheet needede.g. for cross-flow measurements. Forrealimages Stereo-PIVwas labout afactor of 2 better in 10Wgradientregions. Then it is shown for synthetic and realdata how Tomo-PIV can compute twoclosely-spaced vectors fields and the Fig. 1. Computation of dual-plane vector fields by fat-sheet Tomo-PIV. Comparison between 2D2C-， 2D3C-， and Tomo-PlV Fig. 2. Reduction in measured amplitude as a function of wavelength L and lightsheet thickness d for 2D-PIV (green lines)， Stereo-PIV (blue)， and Tomo-PIV (red).