Explicit representation of the reduced Euler equations of a compressible fluid and the complete system of hydrodynamic equations in integral form

Various ways of reducing the complete system of hydrodynamic equations in volume to a system of equations on the surface are of great scientific interest. In this article, the unsteady Euler equations of a compressible fluid and the complete system of hydrodynamic equations are reduced to “stationary” integral-differential equations where time derivatives are absent. Such a procedure (3𝐷 → 2𝐷, 2𝐷 → 1𝐷) makes it possible to reduce the dimension of the problem by one, which significantly reduces the necessary computing power when modeling them. If to set the correct problem, then one can determine the entire unsteady flow in the volume without solving the unsteady problem. It is enough to set time-varying data only on some surface of this stream, which must be determined independently. The peculiarity of this work is that all equations reduced in dimension are obtained in explicit form, unlike previous works of the authors, where up to 200-500 equations with reduced dimension were proposed, which are very difficult to study and model. In previous authors’ work, dimensionality was reduced by reducing overdetermined systems of differential equations. In this method, in the case of a successful choice of an additional constraint equation for overdetermined systems of differential equations, there is a reduction to PDE systems of dimension less than that of the original PDE systems. The method that is used to reduce the dimension of the Euler equations and the complete system of hydrodynamic equations in this paper is actually a special case of the above. The technique of dimensional reduction in overdetermined systems of PDEs is obviously generalized to the case of integral-differential equations. The article also obtained integral equations in new variables (Lagrangian and pseudo-Lagrangian), which determine the evolution of the flow. Also, a new method is proposed for overdetermination of any system of PDEs using the general integral space relation, which is consequence of the Helmholtz decomposition theorem.