目前,电子显微镜技术(electron microscopy)已成为研究机体微细结构的重要手段。常用的有透射电镜 (transmission electron microscope,TEM)和扫描电子显微镜(scanning electron microscope,SEM)。与光镜相比电镜用电子束代替了可见光,用电磁透镜代替了光学透镜并使用荧光屏将肉眼不可见电子束成像。
Geostrophic adjustment of an isolated axisymmetric lens was examined to better understand thedependence of radial displacements and the adjusted velocity on Burger number and the geometryof initial conditions. The behavior of the adjustment was examined using laboratory experimentsand numerical simulations, which were in turn compared to published analytical solutions. Threedefining length scales of the initial conditions were used to distinguish between various asymptoticbehaviors for large and small Burger number: the Rossby radius of deformation, the horizontallength scale of the initial density defect, and the horizontal length scale of the initial pressuregradient. Numerical simulations for the fully nonlinear time dependent adjustment agreed bothqualitatively and quantitatively with analogous analytical solutions. For large Burger number,similar agreement was found in laboratory experiments. Results show that a broad range of finalstates can result from different initial geometries, depending on the values of the relevant lengthscales, and the Burger number computed from initial conditions. For Burger number much largeror smaller than unity, differences between different initial geometries can readily exceed an order ofmagnitude for both displacement and velocity.
A coaxial jet was actively controlled by a MEMS-fabricated micro flap actuator nozzle. The effect ofdifferent control modes on secondary azimuthal instabilities and the evolution of streamwise vorticeswere investigated by applying stereoscopic PIV to the cross-stream plane of the jet. Forcing withnon-symmetric modes, in particular the least-stable helical mode, accelerates the evolution of thestreamwise vortices through the enhancement of azimuthal instabilities. Although forcing is appliedto the outer shear layer of the outer jet, the control effect is most pronounced in the inner shear layerof the inner jet. Unlike in the natural jet, streamwise vortices appear in the inner shear layer of thecontrolled jet. For forcing with the fundamental axisymmetric mode, a Strouhal number of the orderof unity maximise the azimuthal instabilities and hence the counts of the streamwise vortices. Thepresent result is in accordance with our previous experimental findings in the longitudinal plane,where the evolution of the primary vortices and mixing between the inner and the outer jets wereexamined through 2D-PIV and PLIF (Kurimoto et al., 2004, Active control of coaxial jet mixing witharrayed micro actuators. Transactions of the Japanese Society of Mechanical Engineers, pp. 31–38.)This emphasises the connection between primary and streamwise vortices and their significance in themixing enhancement process. It is also found that the azimuthal wavelength under the present controlscheme is almost the same as that of the natural jet and independent of the streamwise position.