Prediction of the Wind Tunnel Sidewall Effect for the iGREEN Wing-Tailplane Interference Experiment in the TWG

Abstract

The DLR project iGREEN (Integrated Green Aircraft; 2007-2010) investigates the effects of increased aircraft elasticity from optimised structures or new aerodynamic configurations. One work package concerns the wing-tailplane interference through buffet processes and includes experiments in the Transonic Wind Tunnel Goettingen (TWG) adaptive test section in 2010. An NLR7301 airfoil is to be attached to a flutter rig providing high-frequency pitching oscillations upstream of an elastic swept wing. In support of the experimental work, RANS computations with the DLR-TAU code were started in 2007 to quantify the effects of the TWG side-wall boundary layer on the experiment, and to identify potential problems. Computations for the NLR7301 comparing viscous and symmetry sidewalls identified significant changes due to the side-wall boundary layer at high Mach number and angle of attack. Here 3D separation bubbles form at the join between the wall and both the upper and lower sides of the airfoil. These separations act as displacing bodies which accelerate the flow, giving an effect vaguely similar to a reduction in angle of attack. Further investigation using 2D computations with a corrected angle of attack could only approximately reproduce the effect shown.