On the Rotational Invariance and Hyperbolicity of Shallow Water Moment Equations in Two Dimensions
Matthew Bauerle, Andrew J. Christlieb, Mingchang Ding, Juntao Huang
Source abstract
Abstract. In this paper, we investigate the two-dimensional extension of a recently introduced set of shallow water models based on a regularized moment expansion of the incompressible Navier–Stokes equations [J. Kowalski and M. Torrilhon, Commun. Comput. Phys., 25 (2018), pp. 669–702; J. Koellermeier and M. Rominger, Commun. Comput. Phys., 28 (2020), pp. 1038–1084]. We show the rotational invariance of the proposed moment models with two different approaches. The first proof involves the split of the coefficient matrix into the conservative and nonconservative parts and proves the rotational invariance for each part, while the second one relies on the special block structure of the coefficient matrices. With the aid of rotational invariance, the analysis of the hyperbolicity for the moment model in two dimensions is reduced to the real diagonalizability of the coefficient matrix in one dimension. Then we analyze the real diagonalizability by deriving the analytical form of the characteristic polynomial. We find that the moment model in two dimensions is hyperbolic in most cases and weakly hyperbolic in a degenerate edge case. With a simple modification to the coefficient matrices, we fix this weakly hyperbolicity and propose a new global hyperbolic model. Furthermore, we extend the model to include a more general class of closure relations than the original model and establish that this set of general closure relations retains both rotational invariance and hyperbolicity.
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