Level Set Topology and Shape Optimization by Density Methods Using Cut Elements with Length Scale Control
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Noname manuscript No. (will be inserted by the editor) Level set topology and shape optimization by density methods using cut elements with length scale control Casper Schousboe Andreasen · Martin Ohrt Elingaard · Niels Aage Received: date / Accepted: date / DOI: https://doi.org/10.1007/s00158-020-02527-1 Abstract The level set and density methods for topol- Keywords Topology optimization · Cut elements · ogy optimization are often perceived as two very dif- Level set methods · Density methods · Length scale ferent approaches. This has to some extent led to two control competing research directions working in parallel with only little overlap and knowledge exchange. In this pa- per we conjecture that this is a misconception and that 1 Introduction the overlap and similarities are far greater than the dif- ferences. To verify this claim, we employ, without signif- Since its introduction in the late 1980's (Bendsøe and icant modifications, many of the base ingredients from Kikuchi 1988) the material distribution method, known the density method to construct a crisp interface level as topology optimization, has shifted from an academic set optimization approach using a simple cut element research discipline to an important and commonly used method. That is, we use the same design field repre- tool in many industries. In engineering it is used for the sentation, the same projection filters, the same opti- design of structural components e.g. in aerospace and mizer and the same so-called robust approach as used automotive industries (Bendsøe and Sigmund 2004), in density based optimization for length-scale control. while many architects and industrial designers use it The only noticeable difference lies in the finite element as inspiration (Beghini et al. 2014). The success of the and sensitivity analysis, here based on a cut element design methodology has led to its application in several method, which provides an accurate tool to model arbi- other physical disciplines such as fluid dynamics (Bor- trary, crisp interfaces on a structured mesh based on the rvall and Petersson 2003; Andreasen et al. 2009; Dilgen thresholding of a level set - or density - field. The pre- et al. 2018), acoustics (D¨uhring et al. 2008; Christiansen sented work includes a heuristic hole generation scheme and Sigmund 2016), MEMS devices (Larsen et al. 1997; and we demonstrate the design approach on several Sigmund 2001b), large-scale and giga-resolution design numerical examples covering compliance minimization (Evgrafov et al. 2008; Aage et al. 2017) and even as and a compliant force inverter. Finally, we provide our interactive educational tools (Aage et al. 2013; Nobel- Matlab code, downloadable from www.topopt.dtu.dk, Jørgensen et al. 2016). As a consequence of the vast to facilitate further extension of the proposed method popularity of the design method, a large number of to e.g. multiphysics problems. variants exist in the literature, of which, the two dom- inant ones are the density based (Bendsøe 1989; Zhou and Rozvany 1991; Wang et al. 2011; Sigmund and The authors thank the Villum Foundation for support Maute 2013) and the level set based approaches (Os- through the Villum Investigator project InnoTop. her and Sethian 1988; Sethian and Wiegmann 2000; Casper Schousboe Andreasen Wang et al. 2003; Van Dijk et al. 2013). Within each Department of Mechanical Engineering, Section for Solid Me- chanics, Technical University of Denmark, Nils Koppels All´e, of the two major approaches there exist a multitude Building 404, DK-2800 Kgs. Lyngby of different directions. For example, the term level set Tel.: +45 45254262 topology optimization is used for methods that employ E-mail: [email protected] an ersatz material model c.f. Fig 1(c) (Allaire et al. 2 Casper Schousboe Andreasen et al. 2004; Wang and Wang 2006); methods that use exten- introducing a contrast parameter to the void domain, sive remeshing and exact boundary tracking, c.f. Fig. similar to standard SIMP topology optimization and, 1(b) (Allaire et al. 2013); methods that perform the to some extend, the Finite Cell method(D¨usteret al. design update using mathematical programming (Van 2008). Miegroet and Duysinx 2007; Kreissl and Maute 2012) Within the field of level set based structural opti- and those that update the design by time-stepping an mization, several works have been published using a advection-diffusion type equation (Allaire et al. 2004; similar modelling technique to that presented here. For Chen et al. 2008). For density methods, both nodal and example, in Van Miegroet and Duysinx (2007) the au- element-wise design representations are employed and thors present a similar XFEM level set method, with there exist several approaches to performing the design the major difference that the void domain is omitted update, e.g. BESO (Querin et al. 1998) and mathemat- from the analysis. More closely related to this study is ical programming. As a result, the number of variations the work of Wei et al. (2010) which employs an XFEM are numerous, and mentioning all is outside the scope of finite element technique similar to that presented here. this paper. Despite their different mathematical formu- However, their method differs in the design field repre- lations and numerical implementations, we argue that sentation, which uses radial basis functions, and in the they share more similarities than differences (as also way the regularization is introduced to the design prob- pointed out in e.g. Sigmund and Maute (2013)). The lem. More recent publications include the work of Vil- main similarity, and a common trade for all successful lanueva and Maute (2017) which utilize classical XFEM methods, both level set and density based, is that they with enrichments. Furthermore, the ghost penalty sta- are based on sensitivity information (Sigmund 2011). In bilized CutFEM method by Burman et al. (2014) has this work we focus solely on the transition from a den- been used for level set shape optimization using an sity representation to a level set based formulation, and advection-diffusion design update in Burman et al. (2018) show that this only requires an update of the underly- and Bernland et al. (2018), and based on an optimality ing finite element analysis and minor modification to criteria in Burman et al. (2019). the sensitivity computations. All other aspects remain In this paper we present a generalized shape and exactly as those used in density methods. topology optimization approach, in which, we utilize Depending on the physics at hand, the need for well-known and thoroughly tested tools from the den- an explicit boundary representation may be crucial for sity method to construct a crisp interface level set type the successful application of structural optimization. design method. That is, from the density methods we For example, the works (Sigmund and Clausen 2007; employ the design field representation, image process- Yoon et al. 2007; Lundgaard et al. 2018) present density ing filters for regularization (Sigmund and Torquato based methods in which the interface is of primary im- 1996; Bourdin 2001; Guest et al. 2004) and the so- portance and all illustrate the difficulties in introducing called robust approach (Sigmund 2009; Wang et al. the appropriate interface conditions using a density rep- 2011) to ensure a minimum feature size. The result- resentation. Therefore, methods capable of an accurate ing optimization problem is solved by mathematical representation of such conditions are of high interest to programming using the Method of Moving Asymptotes the community. To realize the accurate modelling of a (MMA) (Svanberg 1987). The mathematical design field crisp boundary we employ a cut element method. The is introduced as a nodal field, which after a series of fil- proposed method can, in general terms, be viewed as ters and projections is mapped to a level set field. Thus, a subset of XFEM (Daux et al. 2000; Belytschko et al. there is a clear link between the between the nodal den- 2003), CutFEM (Hansbo and Hansbo 2002; Burman sity representation and the level set interpretation. This et al. 2014) or the Finite Cell method (D¨usteret al. approach allows us to introduce length scale control, 2008) if no enrichments or stabilization is used, and if and hence robustness, to the design problem using the the entire background domain is modelled at all times. well-known robust formulation from Wang et al. (2011). In other words, the proposed cut element approach al- We remark that other length-scale approaches orig- lows for a crisp boundary representation while working inally developed for density methods already have been on a fixed structured background mesh with constant employed for level set optimization, e.g. the geomet- degree of freedom (dof) numbering, c.f. Fig. 1(d). As the ric constraint of Zhou et al. (2015) is used by Jansen proposed cut element approach does not require the in- (2018). It is also important to mention that other fea- troduction of any additional enrichment dofs, it is well ture size approaches exist which are especially devel- suited for large-scale topology optimization in a paral- oped for level set methods c.f. Chen et al. (2008), Da- lel environment, since the domain decomposition and pogny et al. (2017) and Yamada (2019), which uses an dof numbering remains constant. This is achieved by energy method, a distance penalization and a fictitious Level set topology and shape optimization by density methods using cut elements with length scale control 3 (a) (b) (c) (d) Fig. 1 Illustration of different ways to represent an hourglass type geometry in structural optimization. Plot (a) show the true geometry and (b) shows a body fitted mesh of the hourglass. In (c) the standard ersatz material / density model is shown, while (d) shows the cut element approach where the cuts are highlighted in blue.