Over the past few years, the FAST wind turbine simulation tool has undergone a major restructuring. FAST is now, at its core, an algorithm and software framework for coupling time-dependent multi-physics modules relevant to computer-aided engineering (CAE) of wind turbines. Each module, which represents one or more turbine components or physics control volumes, is constituted by a mathematical model composed of time-dependent constraint and/or differential equations that are typically nonlinear. Under this new modular form, modules can interact through matching or non-matching spatial meshes and can be time advanced with different time steps and different time integrators. Sharing of data between modules is accomplished with a predictor-corrector approach, which allows for either implicit or explicit time integration within each module. This new modularity positions FAST as a backbone for coupling both high-fidelity and engineering-level wind turbine physics models. In this paper, we describe new features of the FAST modular framework. In particular, we describe a new mixed-time-step algorithm, sparse-matrix storage, a direct solver for sparse linear systems, and interpolation of rotation fields in space for mesh mapping and in time for time advancement. We also show several numerical examples that demonstrate the performance and flexibility of the FAST framework, and we use those results to provide modeling guidance to users.
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