One-dimensional reductions and functional separable solutions to unsteady plane and axisymmetric boundary-layer equations for non-Newtonian fluids

Abstract

The paper deals with equations describing the unsteady axisymmetric boundary layer of power-law non-Newtonian fluids on a body of revolution. The axisymmetric boundary-layer equation for the stream function is shown to reduce to a single third-order PDE of the formwtz+wzwxz−wxwzz=κrn+1(x)wzzn−1wzzz+F(t,x),where n is a rheological parameter of the fluid; the function r(x), determining the shape of the body, is assumed arbitrary. For this non-linear PDE, we describe one-dimensional reductions as well as a number of new generalized and functional separable solutions, which depend on two to five arbitrary functions. The solutions are obtained with the direct method of functional separation of variables by using particular solutions to an auxiliary ODE and systems of first-order PDEs. Many of the solutions are expressed in terms of elementary functions, which, together with significant arbitrariness, makes them especially useful for solving certain model problems and testing numerical and approximate analytical methods in fluid dynamics. Apart from power-law fluids, the paper looks at three-parameter polynomial and generalized Sisko models of non-Newtonian fluids. The unsteady plane boundary-layer equations for the general non-Newtonian fluid model are shown to admit a reduction to an ODE. The paper presents new exact solutions to the plane boundary-layer equations for power-law fluids as well as some other rheologically complex fluids.