Characterisation of blade tip vortices by simultaneous time-resolved velocity and density field measurements

The flow field around a helicopter is dominated by strong coherent vortices originating from the blade tip as a result of the pressure difference between suction and pressure side and a high radial velocity at the tip (Leishman, 2001; Green and Acosta, 1991). These vortices have a strong impact on the rotorcraft acoustics and performance due to its interactions with the rotor blades and the helicopter fuselage (Conlisk, 2001). Detailed knowledge of the vortex development, in particular of the formation process, and the scaling of characteristic parameters, e.g. vortex size and strength, are necessary for the design and the optimisation of vortex control measures. Due to the complexity and the inherent unsteady character of the vortex dynamics, this can only be achieved by time-resolved measurements of the velocity and density field simultaneously.

Aperiodicity in the near field of full-scale rotor blade tip vortices

Karen Ann J Mulleners

Blade tip vortices are the dominant vortical structures of the helicopter flow field. The inherent complexity of the vortex dynamics has led to an increasing interest in full-scale in situ experiments, where the near field, closely behind the blade, is of particular interest, since measures of vortex control mostly target this initial stage of development. To examine the near field, three-component particle image velocimetry (PIV) measurements of blade tip vortices of a full-scale helicopter in simulated hover flight in ground effect were conducted. A feasible and robust evaluation procedure was developed to minimise the shortcomings of full-scale PIV applications, such as a moderate spatial resolution and an elevated measurement noise level. At vortex ages ranging from

\psi-\rm v=1\circ

to 30°, a pronounced aperiodicity and asymmetry of the vortex were observed in -sections perpendicular to the vortex axes. At

\psi-\rm v=1\circ

, a preferential orientation of the vortex was observed. For increasing wake age, vortex wandering increased while the asymmetry of the vortex cores decreased. The high level of aperiodicity and core asymmetry must be taken into account when considering phase-averaged vortex characteristics in the near wake region. © The Author(s) 2010. This article is published with open access at Springerlink.com.

2011

Aperiodicity in the near field of full-scale rotor blade tip vortices

Karen Ann J Mulleners

\psi-\rm v=1\circ

to 30°, a pronounced aperiodicity and asymmetry of the vortex were observed in -sections perpendicular to the vortex axes. At

\psi-\rm v=1\circ

2011