The formation and evolutionary characteristics of the blade tip vortices generated by a hovering rotor were studied using dual-plane stereoscopic digital particle image velocimetry (DPS-DPIV). The DPS-DPIV technique permitted non-invasive measurement of the three components of the velocity field and the nine-components of the velocity gradient tensor, a capability not possible with 'classical' DPIV. The DPS-DPIV method is based on coincident flow measurements made over two differentially spaced laser sheet planes. A polarization-based approach was used in which the two laser sheets are given orthogonal polarizations, with filters and beam-splitting optical cubes placed so that the cameras image Mie scattered light from only one or other of the laser sheets. The digital processing of the images used a deformation grid correlation algorithm optimized for the high velocity gradient and small-scale turbulent flows found inside blade tip vortices. High-resolution flow imaging of the vortex sheet in the wake behind the rotor blade combined with detailed turbulence measurements revealed the presence of several turbulent flow features during the vortex roll-up process that appear to play an important role in its final evolution. The DPS-DPIV measurements also included the fluctuating terms that would be involved in the Reynolds-averaged stress transport equations. The results have shown that an isotropic assumption of turbulence is invalid inside blade tip vortices, and that stress should not be represented as a linear function of strain. The DPS-DPIV measurements were also compared with velocity and turbulence measurements made using a laser Doppler velocimeter system, with good correlation between the two techniques.
Turbulent tip vortex measurements using dual-plane digital particle image velocimetry
Messung turbulenter Blattspitzenwirbel mittels digitaler PIV in zwei Ebenen
AHS Annual Forum, AHS International, 64 ; 1722-1744
2008
23 Seiten, 55 Quellen
Aufsatz (Konferenz)
Englisch
Korrelationsverfahren , Doppler-Effekt , Gradientenverfahren , Laser , optisches Filter , turbulente Strömung , Geschwindigkeitsmesser , Geschwindigkeitsmessung , Strömungsmessung , Streulicht , Geschwindigkeitsfeld , Aerodynamik , Teilchenabbildgeschwindigkeitsmessung , Hubschrauber , MIE-Streuung , Laser-Doppler-Geschwindigkeitsmessung , Strömungsvisualisierung
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