Volumetric velocity field measurements in the wake of a co-axial, counter-rotating helicopter rotor hub

The unsteady loading and turbulent wake shed by co-axial, counter-rotating helicopter rotor hubs are examined using tomographic Particle Image Velocimetry. Experiments were conducted in the 12-inch water tunnel at the Applied Research Laboratory at Penn Statue using a circular test section 30 inches in length. Four high-speed cameras were used to record the motion of glass microspheres in addition to 3-component laser Doppler velocimetry (LDV) measurements of the rotor wake. These two measurements provide independent comparisons of the mean and fluctuating statistics of the velocity in the rotor wake. Good statistical agreement between the two velocity measurement techniques was achieved and large scale vortical regions in the wake are identified, relating to physical processes of the counter-rotating rotor flow.