The integral invariant of the equations of motion of a viscous gas

An expression for the velocity of motion of a simple vortex contour for which the circulation of the fluid velocity is preserved in it is obtained in the general three-dimensional case for the flow of a viscous gas or liquid. The velocity of motion of the contour at each point is calculated from the values of the flow parameters and their derivatives at the same point. This result extends Thomson's theorem, which is well known for an ideal barotropic fluid. A previously unknown conservation property is found, which consists of the fact that the circumferential circulation of the swirling axisymmetric flow in the potential field of mass forces is the first integral of the equations of unsteady flow of a non-barotropic ideal gas. (C) 2016 Elsevier Ltd. All rights reserved.