Effect of Aft Rotor on Forward Rotor Blade Wakes in Open Rotor Propulsion Systems

NASA’s Environmentally Responsible Aviation (ERA) project supported the collection of a large stereoscopic Particle Image Velocimetry (PIV) dataset of the Open Rotor Propulsion Rig (ORPR) in the 9 × 15 Low Speed Wind Tunnel at the NASA Glenn Research Center (GRC). The data collection effort acquired a volume of three component velocity measurements composed of 30 planes from near the hub radially outward towards the tip. The PIV cameras and laser were mounted to a traverse that allowed the entire data acquisition system to move from plane to plane. The PIV data acquisition was triggered on the front rotor such that the front rotor was always in the same position for each acquisition event. The aft rotor position was not recorded and varied randomly during the acquisition. Because the position of the aft rotor was not synchronized to either the forward rotor or the camera it was necessary to separate individual PIV images based off of the phase of the aft rotor before they could be processed. The phase of the aft rotor was determined by locating the outline of the rotor in the PIV images and determining its position relative to a known point. This process was conducted by an image processing algorithm. Previous algorithms were able to make a relatively accurate 3D model of the wake between the forward and aft rotors, however some small inaccuracies were present. Improvements to this algorithm allowed for more accurate phase averaging, which yielded an improved PIV dataset. Both rotors were set to the same nominal rotational speed, however variations in motor control and other physical mechanisms allowed for some differences in the true RPM of the two rotors. The effects of the aft rotor on the front rotor blade wakes within the inter-rotor flow field of the ORPR were examined. The aft rotor potential field was shown to have significant upstream impact on the front rotor wakes, altering their topology approaching the aft rotor. The wake strength was quantified through determination of the minimum velocities that occurred within a wake while a defined velocity deficit from the free stream velocity was used to determine the width of the wake. This work has potential applications to the radiated acoustics and efficiency of open rotor propulsion systems.