Finite Element Analysis of Transonic Flows over Thin Airfoils. Volume 1

Abstract : A finite element algorithm is described for computing steady and unsteady (oscillatory and transient) transonic flows over thin airfoils by solving directly the unsteady, nonlinear transonic potential equation based on small disturbance theory. The numerical algorithm is developed using the concept of finite elements in conjunction with the least squares method of weighted residuals applied to both space and time. The basic element presently used is a product of an element in space and an element in time. The former has a cubic expansion inside each element, while the latter is a quadratic Lagrange element. The embedded shocks are smeared and, in computing flow over lifting airfoils, use is made of the far field asymptotic solution to increase computational efficiency. For each time step, the finite element discretization in both space and time results in a recurrence relationship in the form of a banded system of algebraic equations, which is solved by Gaussian eliminations. Sample problems of steady flow over lifting airfoils and unsteady flow over airfoils executing harmonic motion are calculated to demonstrate the applicability and validity of the present approach. The solution procedures are found to be adequately accurate and very efficient, with unsteady solution obtainable in less than ten minutes CPU time on a CDC 6600 computer.