It is well known that the Galerkin approximation of the incompressible Navier-Stokes equations leads to an ODE with quadratic non-linearity. The SOS approach can be applied to such a system since the dynamics are represented by polynomials, and this application is one of the topics of the proposed research. The proposers have already used SOS in a study of a hydrodynamic-type ODE system. In this study the method of exploiting in the SOS approach the energy-invariance property of the bilinear component of the systems of hydrodynamic type was developed. This allowed the construction of a Lyapunov function for the value of R about seven times larger than Re, thus demonstrating the strong potential of applying SOS in fluid dynamics.
Although these results were very promising, in the sense that very substantial improvements in the computed stability bounds for the flow were achieved, significant technical hurdles remain if this approach is to gain widespread acceptance. In particular, the method should be able to handle systems with significantly larger numbers of states, since accurate approximation of a fluid flow requires considering systems with a large number of Galerkin modes. However, application of conventional SOS methods to very large systems is problematic, since the SDP problems to be solved in such cases are prohibitively large. Therefore, the proposers developed a method of reducing the order of the system in SOS studies, based on representing the higher modes with their energy only. The method is guaranteed always to give results at least as good as the standard energy stability analysis. Moreover, it has also been proven mathematically that if the flow remains globally stable for R > Re given by the energy stability analysis, then there exists a polynomial Lyapunov function(al), which could be sought for using a SOS approach.
An alternative would be to search for a Lyapunov functional for a fluid system directly without recourse to a finite-dimensional approximation. This is more difficult since in this case the arising polynomials contain powers of the partial derivatives of the flow variables in addition to the variables themselves. Some preliminary results in this direction have been published, and it was shown that SOS optimization methods can be applied directly to analysis of infinite dimensional systems characterized by PDEs, and in particular to stability of 2D channel flow.