Flow Characterization of Heavy-Lift Helicopter Rear Fuselage with Pulsed Jets for Separation Control

An experimental study on the flow over the rear body of a 1:7-scaled fuselage model equipped with pulsed jets (PJs) for separation control is conducted in wind-tunnel experiments at length-based Reynolds number of similar to 8x106 by means of planar particle image velocimetry. The model is representative of an existing heavy-lift helicopter fuselage. The flowfield in correspondence of the loading ramp is investigated along both the longitudinal and transversal directions either at fixed incidence angle alpha=-3 degrees, 0 degrees, and 4.5 degrees or monotonically varying it in the range between -5 degrees <=alpha <= 5 degrees. Two cases, that is, either without control or under the periodic excitation induced by PJs, have been considered. A statistical analysis of the case without control reveals the presence of a massive separation in the inspected region characterized by crosswise eddies, predominant at positive incidences, and streamwise vortices. Even though the separation mainly occurs slightly downstream the upsweep shape of the rear fuselage, two preferred reattachment regions are recognized either along the ramp or the boom tail for alpha=-3 degrees and 0 degrees. When the incidence angle is increased, the separation points move upstream, and the reattachment points are predominantly located at the ramp end. The effectiveness of the control is achieved through a momentum addition of c mu=0.012 at a reduced frequency F+=1.14 for the range of the investigated incidence angles. Benefits in aerodynamic performances result in a drag reduction from 5 to 24% for the incidence angle sweep. Under the periodic excitation, the separation is completely suppressed and the streamwise vortices move closer to the tail boom of the model. The vortical pattern preserves its symmetric spatial organization with respect to the longitudinal direction. A modal analysis using proper orthogonal decomposition (POD) indicates the dominance of periodic crosswise vortical structures evolving along the ramp of the fuselage in phase with the control input. A phase-average reconstruction, based on the POD analysis, highlights that these vortices travel along the ramp and then dissipate by the time they reach the tail boom of the helicopter fuselage.