Reduction of Aerodynamic Noise in a Flight Deck by Use of Vortex Generators

Navier-Stokes CFD was used to define the traits of the boundary layer and separation bubble. Following ESDU and Boeing guidelines, the VG's were designed to improve mixing at the separation line. The VG's were placed in counter-rotating pairs, 1-cm high and represented by body forces in the grid. The CFD solutions benefited from a curvature correction in the turbulence model. The solutions predicted a narrowing of the separation bubble by about 67%, essentially shrinking it away and off of the flight-deck windows. CFD does not provide estimates of the pressure fluctuations, but the effect on separation was promising enough to proceed directly to a full-scale verification. Flight tests were conducted, with extensive sound pressure level measurements in the flight deck. Fourteen VG pairs were bonded to the body, about 30cm ahead of the main Window. No external measurements were taken, so that the separation control was not confirmed directly. However, the noise reduction at the pilots' heads ranged from 1-dB to 3-dB, depending on the ear, which noise metric was chosen, the density, and whether the Environmental Control System (ECS) was operating. The noise reduction was highest in the frequency range 600-700Hz, reaching 6-dB at the outboard pilot's ear. Accelerometers attached to the windows indicated a significant vibration reduction, even for the side windows, suggesting that highly-turbulent fluid streams past them in the baseline (no VG's) configuration.