A comprehensive PIV measurement campaign on a fully equipped helicopter model

Abstract

The flow field around a helicopter is characterised by its inherent complexity including effects of fluidstructure interference, shock-boundary layer interaction, and dynamic stall. Since the advancement of computational fluid dynamics and computing capabilities has led to an increasing demand for experimental validation data, a comprehensive wind tunnel test campaign of a fully equipped and motorised generic medium transport helicopter was conducted in the framework of the GOAHEAD project. Different model configurations (with or without main/tail rotor blades) and several flight conditions were investigated. In this paper, the results of the three-component velocity field measurements around the model are surveyed. The effect of the interaction between the main rotor wake and the fuselage for cruise/tail shake flight conditions was analysed based on the flow characteristics downstream from the rotor hub and the rear fuselage hatch. The results indicated a sensible increment of the intensity of the vortex shedding from the lower part of the fuselage and a strong interaction between the blade vortex filaments and the wakes shed by the rotor hub and by the engine exhaust areas. The pitch-up phenomenon was addressed, detecting the blade tip vortices impacting on the horizontal tail plane. For high-speed forward flight, the shock wave formation on the advancing blade was detected, measuring the location on the blade chord and the intensity. Furthermore, dynamic stall on the retreating main rotor blade in high-speed forward flight was observed at r/R = 0.5 and 0.6. The analysis of the substructures forming the dynamic stall vortex revealed an unexpected spatial concentration suggesting a rotational stabilisation of large-scale structures on the blade. Abbreviations CFDComputational fluid mechanics DEHS Di-ethyl-hexyl-sebacat DS Dynamic stall GOAHEAD Generation of advanced helicopter experimental aerodynamic database for CFD code validation PIV Particle image velocimetry WT Wind tunnel Symbols a Speed of sound, m/s c Blade chord, m L Fuselage length, m L m Measurement volume length, m M Mach number r Radial coordinate, m r v Vortex radius, m r c Vortex core radius, m R Rotor radius, m t Time, s

Advance ratio effects on the flow structure and unsteadiness of the dynamic-stall vortex of a rotating blade in steady forward flight

“…Studies investigating the velocity field on helicopter rotors [3][4][5] and wind turbine blades [6][7][8] have shown the existence of a three-dimensional DSV, while similar flow structures have also been observed in rotating insect wing investigations. 2,[9][10][11][12][13] A common theme among most of these investigations is the existence of a strong spanwise flow from wing root-to-tip (hereafter referred to as radial flow) on the associated rotating wing.…”

“…To the authors' knowledge, no studies have been directed towards the effect of advance ratio during forward flight conditions on the flow structure of a retreating side wing undergoing simultaneous rotation and pitching. Although the problem defined here is well known in the rotorcraft community, only a few studies investigate the flow structure on simultaneously rotating and pitching wings, [3][4][5] and none address the effect of advance ratio during forward flight.…”

Experimental investigation of the unsteady aerodynamics of FOWT through PIV and hot-wire wake measurements

“…The accuracy of the present PIV measurements can be estimated considering a maximum displacement error of 0.1 pixels [13]. Thus, taking into account the employed pulse-separation time and the optical magnification [14], the maximum in-plane velocity components error of the present measurements is about 0.07 m/s. The number of image pairs considered in the present PIV campaign for each measurement window was a compromise to limit the acquisition time due to the large number of test conditions and the amplitude of the investigation area.…”

Experimental investigation of a helicopter rotor with Gurney flaps

“…The accuracy of the present PIV measurement can be estimated considering a maximum displacement error of 0.1 px (25) . Thus, taking into account the employed pulse-separation time and the optical magnification (26) , the maximum in-plane velocity components error was about 0.2 m/s.…”