An infinite element is the one that can handle a domain of infinity. Itis a special finite element, and can have aseamless connection with convertional finite elements. It can be a mapped infiniteelement or a non-mapped infinite element. The former, such as Bettess element and Astley element, needsgeometry mapping and shape functions in terms of local coordinates,while for the latter, the shape functions are directly expressed in terms of global coordinates. This paper reviews thestate-of-the-art and recent advances of the infinite element methodfor unbounded domains. First, the concept and features of the infinite element method are introduced. Then, taking thewave problems governed by the Helmholtz equation as an example, severalconventional infinite elements such as the Bettess element, the Astleyelement and the Burnett element are compared and reviewed. Next, weintroduce the generalized infinite element and its relation to the conventional infinite elements. Finally, the applications of theinfinite element to various problems are summarized.

The micro air vehicles (MAVs) are not simple scale-downs of common aircraft, but face many technical challenges. One ofthe most serious bottlenecks is low Reynolds number aerodynamics,which is also one of the most attractive research fields. This paperdiscusses the aerodynamic problems realted with the MAVs at lowReynolds numbers, and the latest progresses in China in this field. Theseproblems are classified according to the flight style and the structurecharacteristics of MAVs in this paper. Firstly, the low Reynolds number aerodynamic problems of 2-D and 3-Drigid wing MAVs are described, including the laminarseparation bubbles, nonlinear effects of lift coefficient at small angle ofattack, the effects of static hystereses at middle angle of attack, and theaerodynamic characteristics of the low-aspect-ratio wing. Secondly, theflapping wing low Reynolds number aerodynamic problems, which involve manyunsteady mechanisms of low Reynolds number flapping flight for the insects, such as theWeis-Fogh (clap-and-fling), the delayed stall, the Kramer(rotationalforces), the wake capture, the added mass, where some studies in Chinain recent years are also included. Thirdly, the low Reynolds number aerodynamics of the flexible wing. It is shown that the flexible wing could effectively improve the ability ofresisting gust for the rigid wing MAVs, and have significant effectsfor producing enough lift and thrust for the flapping wing MAVs.Finally, some studies on the morphing wing MAVs are briefly discussed. Itis indicated that many bottlenecks of MAVs design could be brokenthrough the morphing wing. On the other hand, the concept of morphing wing technologycould be tested through flight experiments on this low speed and cheap air vehicles.