Unsteady aerodynamic effects experienced by aerofoils during acceleration and retardation

Abstract

This investigation focuses on the aerodynamic effects caused by acceleration and retardation. It shows that, when an object is accelerated in a compressible fluid, the aerodynamic forces and moments experienced by the object at any instantaneous Mach number are different to those, experienced at the same Mach number, when the object moves at constant velocity. A supersonic biconvex aerofoil was chosen, with Fluent as the computational fluid dynamic (CFD) software. Constant velocity (steady state) simulations were conducted at Mach numbers ranging from 0.1 to 1.6 for the aerofoil. The aerofoil was then accelerated at 1041 m/s2 (106 g) and 86.77 m/s2 (8.845 g), starting at Mach 0.1, and decelerated at −1041 and −86.77 m/s2, starting at Mach 1.6, through the same range of Mach numbers using time-dependent (unsteady) simulations. Significant differences were shown between the steady and the unsteady cases with the greatest differences observed in the transonic region. In this region, for each specific Mach number, acceleration-dependant variations in the position of the shock wave on the upper surface of the aerofoil were also observed. This was used to explain the great differences in the aerodynamic forces and moments between the steady and the unsteady cases in the transonic region.