The numerical algorithm for solving nonlinear boundary problem of thin rod's dynamic deformations

At this paper the algorithm of the subprogram for solving a two-point boundary value problem for system of the nonlinear differential equations of the first order is presented. The new algorithm of the subprogram named KLPALG united in itself the main ideas of the subprograms BVPFD (DD14AD, PASVA3) and PASSIN realizing the technique of continuation of solution by parameter. Besides, the generalized results of works of authors in a problem of nonlinear dynamic deformation of a thin spatial curvilinear rod calculated by its differential model are presented. The unknown functions in the equations of motion are calculated at discrete mesh points. The methods of direct integration allow us to express time derivatives by the current coordinates and coordinates and velocities calculated in the previous time steps. The first derivative of coordinate is replaced by finite difference; boundary conditions are added. The obtained system of nonlinear algebraic equations is solved by Newton method with the step length control of the convergence conditions. The Jacobi matrix of this system is of the block-tridiagonal structure which lends itself to efficient LU-decomposition. This decoupling of the Jacobi matrix allows you to quickly solve the corresponding system of linear algebraic equations of the big sizes. If the condition of convergence of Newton's method gives too small step, then used the technique of continuation of the solution on the parameter a (pseudo arc-length). As soon as the system of nonlinear equations is solved, to refine the nodal values of the calculated functions we use the deferred correction method. This method subtracts from the received solution the mistakes made by the approximation derived by the method of finite differences in the initial phase of the numerical solution. Thus obtained numerical solution is of accuracy appointed by user. This method is implemented in KLPALG subroutine which algorithm is presented in this paper.