  Similar to the notion of h-adaptivity, where the discretization resolution is
adaptively changed, I propose the notion of model adaptivity, where the
underlying model (the governing equations) is adaptively changed in space and
time. Specifically, this work introduces a hybrid and adaptive coupling of a 3D
bulk fluid flow model with a 2D thin film flow model. As a result, this work
extends the applicability of existing thin film flow models to complex
scenarios where, for example, bulk flow develops into thin films after striking
a surface. At each location in space and time, the proposed framework
automatically decides whether a 3D model or a 2D model must be applied. Using a
meshless approach for both 3D and 2D models, at each particle, the decision to
apply a 2D or 3D model is based on the user-prescribed resolution and a local
principal component analysis. When a particle needs to be changed from a 3D
model to 2D, or vice versa, the discretization is changed, and all relevant
data mapping is done on-the-fly. Appropriate two-way coupling conditions and
mass conservation considerations between the 3D and 2D models are also
developed. Numerical results show that this model adaptive framework shows
higher flexibility and compares well against finely resolved 3D simulations. In
an actual application scenario, a 3 factor speed up is obtained, while
maintaining the accuracy of the solution.
