The Higher-Order CESE Method for Two-dimensional Shallow Water Magnetohydrodynamics Equations

The numerical solution of two-dimensional shallow water magnetohydrodynamics model is obtained using the $4^{th}$-order conservation element solution element method (CESE). The method is based on unified treatment of spatial and temporal dimensions contrary to the finite difference and finite volume methods. The higher-order CESE scheme is constructed using same definitions of conservation and solution elements that are used for $2^{nd}$-order CESE scheme formulation. Hence it is more convenient to increase accuracy of CESE methods as compared to the finite difference and finite volume methods. Moreover the scheme is developed using the conservative formulation and do not require change in the source term for treating the degenerate hyperbolic nature of shallow water magnetohydrodynamics system due to divergence constraint. The spatial and temporal derivatives have been obtained by incorporating $4^{th}$-order Taylor expansion and the projection method is used to handle the divergence constraint. The accuracy and robustness of the extended method is tested by performing a benchmark numerical test taken from the literature. Numerical experiment revealed the accuracy and computational efficiency of the scheme.

Shear flow instabilities in shallow-water magnetohydrodynamics

Within the framework of shallow-water magnetohydrodynamics, we investigate the linear instability of horizontal shear flows, influenced by an aligned magnetic field and stratification. Various classical instability results, such as Høiland’s growth-rate bound and Howard’s semi-circle theorem, are extended to this shallow-water system for quite general flow and field profiles. In the limit of long-wavelength disturbances, a generalisation of the asymptotic analysis of Drazin & Howard (J. Fluid Mech., vol. 14, 1962, pp. 257–283) is performed, establishing that flows can be distinguished as either shear layers or jets. These possess contrasting instabilities, which are shown to be analogous to those of certain piecewise-constant velocity profiles (the vortex sheet and the rectangular jet). In both cases it is found that the magnetic field and stratification (as measured by the Froude number) are generally each stabilising, but weak instabilities can be found at arbitrarily large Froude number. With this distinction between shear layers and jets in mind, the results are extended numerically to finite wavenumber for two particular flows: the hyperbolic-tangent shear layer and the Bickley jet. For the shear layer, the instability mechanism is interpreted in terms of counter-propagating Rossby waves, thereby allowing an explication of the stabilising effects of the magnetic field and stratification. For the jet, the competition between even and odd modes is discussed, together with the existence at large Froude number of multiple modes of instability.