Mean flow features around the inline wheels of four-wheel landing gear

Landing gear are noted to be a significant, sometimes dominant, airframe noise source for commercial aircraft. Aerodynamic noise is a direct result of the fluctuating flow and its interaction with surface components. Knowledge of the mean flow, however, can be used to aid in the determination of noise sources. The complex geometry of multiple-wheel-set landing gear has thus far precluded even this basic information. In this study the mean flow-field is determined in a streamwise plane surrounding the inline wheels of a generic four-wheel landing-gear configuration using digital particle image velocimetry. The velocity and vorticity fields highlight a vortex that persists between the wheels on the ground side of the axle midplane. The formation of this vortex is believed to result from the geometric asymmetry caused by the presence of the center support strut. Evidence is also presented that shows the vortex does not remain stationary, but oscillates between the fore and aft wheels. Its position is hypothesized to depend on the state of an unstable vorticity layer that develops on the ground side of the fore wheel.