“…Restriction methods for density based topology optimization problems can roughly be divided into three categories: 1) mesh-independent filtering methods, constituting sensitivity filters (Sigmund, 1994(Sigmund, , 1997Sigmund and Petersson, 1998) and density filters (Bruns and Tortorelli, 2001;Bourdin, 2001); 2) constraint methods such as perimeter control (Ambrosio and Buttazzo, 1993;Haber et al, 1994), global gradient control (Bendsøe, 1995;Borrvall, 2001), local gradient control (Niordson, 1983;Petersson and Sigmund, 1998;Zhou et al, 2001), regularized penalty methods (Borrvall and Petersson, 2001) and integral filtering (Poulsen, 2003); and 3) other methods like wavelet parameterizations (Kim and Yoon, 2000;Poulsen, 2002), phase-field approaches (Bourdin and Chambolle, 2003;Wang and Zhou, 2004) and level-set methods (Sethian and Wiegmann, 2000;Wang et al, 2003;Allaire et al, 2004) 1 . The filtering methods in group 1 are probably the most popular ones due to their ease of implementation and their efficiency.…”