Issue link: https://resources.system-analysis.cadence.com/i/1472078
Preparation of Geometry Models for Mesh Generation and CFD 7 www.cadence.com Implicit modeling is well-suited for use with design optimization in which models can be created that might otherwise be impossible or at least impractical to design, particularly in cases where the topology of the overall shape changes drastically. While implicit modeling is not currently available in most major MCAD software, it is available in specialized software such as the nTop platform from nTopology [29]. Figure 5: In this implicit geometry model of the union of a rod and sphere, the surface is defined as where the implicit function evaluates as zero, with negative values on the inside and positive values on the outside. Image from Ref. [30]. Originating Intent & Software The originating intent and software used (the two are often implicitly coupled) for creating a geometry model strongly influence its suitability for simulation. An organization that practices model-based engineering (MBE) creates a geometry model for product definition. A so-called "master model" is maintained that captures the product's complete architecture. Product definition models are typically created in mechanical CAD (MCAD) software applications. A geometry model may be created specifically for simulation. Whether or not this model is derived from a master model in MCAD software, it will typically include simplifications and abstractions that vary based on the simulation type (e.g., solid mechanics, fluid dynamics, electromagnetics). Depending on the organization's practices performing the design, access to MCAD software may be limited for cost or control reasons. Simulation models are often created using software other than MCAD, such as software developed in-house or commercial off-the-shelf (COTS) software. This is especially true during the conceptual design phase when software tools like these may be the predominant tool for generating OML. Software of this type has been termed "Pre-CAD" by Chawner et al. [35]. It is essential to note that the type of simulation is important even when a model has been generated for simulation. For example, CFD typically has mesh resolution requirements that are orders of magnitude finer than solid mechanics' and sometimes smaller than the tolerance used to assemble the model's topology. To circumvent the complexity associated with using geometry models created in MCAD software, many organizations utilize design software tailored to their specific application. Notable among these tools for aerospace applications are OpenVSP [36] and ESP [37].] Models derived from physical measurement, specifically 3D scanning, capture the object as-built versus as-designed and under operating load. These models are often generated specifically for simulation during the verification design stage to form the basis for seeking insight into an unexplained behavior. Geometry models may originate from synthetic environments such as flight simulators, where they are used to represent objects within the simulated environment, such as terrain and vehicles. Like an extant mesh, these models may be completely unsuitable (or unsuitably challenging).