Analysis of the Effect of Porous Wall Interference on Transonic Airfoil Flutter

Abstract

A numerical investigation of the influence of the wind tunnel walls on the transonic, two-degree-of-freedom, bending/torsion, flutter characteristics of the NLR7301 section is carried out in the time domain. A parallelized, implicit, time-accurate, two-dimensional, compressible, thin-layer Navier-Stokes flow-solver is coupled with a two-degree-of-freedom structural model. A three-block deforming grid is used for the discretization of the domain including the airfoil and wind tunnel walls. Two types of porous-wall boundary-conditions are implemented and tested for the boundaries representing the tunnel walls. The first applies a porous, inviscid boundary condition and the second applies a porous, viscous one. The type of porous boundary condition is found to significantly influence both steady and unsteady solutions. Computed pressure distribution for steady, transonic flow show better agreement with the experimental results when a viscous, porous boundary condition is applied at the wind tunnel walls. Results show that the free-flight flutter behavior may differ significantly from the behavior found in a porous wind tunnel because of the strong dependence on the tunnel porosity parameter and the proximity of the walls.