Studies in Panel Flutter at Higher Mach Numbers: I. Flat and Slightly Curved Panels at Mach number 2.81. II. Cylindrical Shells with Boundary Layer

Abstract

Part I. Two series of panel flutter tests were carried out in the Jet Propulsion Laboratory's 12 inch supersonic wind tunnel. Flat and slightly curved panels were tested at Mach number 2.81. The flat, rectangular panels were designed to study two-dimensional flutter. They were clamped at front and rear with free sides which extended into the boundary layer at the sides of the tunnel. These panels fluttered in a two-dimensional mode which occurred at a thickness ratio approximately 15 per cent different from the predictions of existing theory. One of the panels exhibited a three-dimensional rocking flutter which has not been observed or discussed before. A theory is developed for this type of flutter. The slightly curved panels were shallow circular cylindrical shells with the generators perpendicular to the flow direction. These panels were all of aspect ratio one. It was found that the effect of curvature was destabilizing and that the effect of internal pressurization was stabilizing. Part II. The effect of a boundary layer on the flutter of a cylindrical shell is studied. The aerodynamic forces are developed for a shell of infinite length. The boundary layer is idealized as an annular region of uniform subsonic flow surrounding the shell. This boundary layer is of constant thickness along the shell and has a constant velocity distribution through its thickness. The external supersonic flow is also taken to be of uniform velocity, resulting in a stepped velocity profile through the boundary layer. Small perturbation theory is used in the boundary layer region and linear piston theory is used for the supersonic flow. In order to replace a physical boundary layer with an idealization for calculations, a procedure is developed for choosing the boundary layer parameters of velocity, pressure, etc. in a consistent way. The forces which are found through this boundary layer theory are compared with those obtained using piston theory directly. It is found that the forces on a mode with many circumferential waves are much smaller than the forces given by piston theory - - a reduction in amplitude of 95 per cent is possible. Phase changes also occur. The effect of the boundary layer on axisymmetric modes is not so great. Flutter boundaries are obtained for axisymmetric flutter under several conditions and illustrate the effect of boundary layer thickness and structural damping.