Transient level-set topology optimization of a planar acoustic lens working with short-duration pulse

With the advance of additive manufacturing, many researchers are increasingly interested in planar acoustic lenses that are not only easier to fabricate than typical convex/concave lenses, but also have excellent imaging performance. However, the planar acoustic lenses reported so far cannot work for a short-duration pulse used in conventional imaging systems due to their inherent dispersive characteristics. This study addresses the challenge by devising a transient topology optimization formulation to design a planar acoustic lens that works effectively for a short-duration pulse. A planar lens consists of two materials where optimal combination and distribution are obtained with a crisp interface via the level-set method. Design is based on the transient acoustic responses, which are calculated from a time-dependent acoustic model solved by the Newmark method. The proposed method uses the area-fraction approach to compute the acoustic properties of a cut element by the interface. A localizing time-window function is introduced so that acoustic energy can be focused within the desired time range as much as possible. We obtain optimum design solutions designed with the proposed method and verify its effectiveness through the numerical investigations.