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Blade tip vortex measurements on actively twisted rotor blades

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Abstract

Active rotor control concepts, such as active twist actuation, have the potential to effectively reduce the noise and vibrations of helicopter rotors. The present study focuses on the experimental investigation of active twist for the reduction of blade–vortex interaction (BVI) effects on a model rotor. Results of a large-scale smart-twisting active rotor test under hover conditions are described. This test investigated the effects of individual blade twist control on the blade tip vortices. The rotor blades were actuated with peak torsion amplitudes of up to \(2^\circ\) and harmonic frequencies of 1–5/rev with different phase angles. Time-resolved stereoscopic particle image velocimetry was carried out to study the effects of active twist on the strength and trajectories of the tip vortices between \(\psi _\text {v}= 3.6^\circ\) and \(45.7^\circ\) of vortex age. The analysis of the vortex trajectories revealed that the 1/rev active twist actuation mainly caused a vertical deflection of the blade tip and the corresponding vortex trajectories of up to \(1.3\%\) of the rotor radius R above and \(-1\%R\) below the unactuated condition. An actuation with frequencies of 2 and 3/rev significantly affected the shapes of the vortex trajectories and caused negative vertical displacements of the vortices relative to the unactuated case of up to \(2\%R\) within the first \(35^\circ\) of wake age. The 2 and 3/rev actuation also had the most significant effects on the vortex strength and altered the initial peak swirl velocity by up to \(-34\) and \(+31\%\) relative to the unactuated value. The present aerodynamic investigation reveals a high control authority of the active twist actuation on the strength and trajectories of the trailing blade tip vortices. The magnitude of the evoked changes indicates that the active twist actuation constitutes an effective measure for the mitigation of BVI-induced noise on helicopters.

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Notes

  1. Average diameters according to the probability density function (PDF) of the distributions of length \(0.21\,\upmu \text {m}\) and volume \(0.77\,\upmu \text {m}\).

Abbreviations

a :

Velocity fit parameter

c :

Blade chord length, \(\text {m}\)

\(C_\text {T}\) :

Thrust coefficient, \(C_\text {T}=T/(\rho \pi \Omega ^2 R^4)\)

i :

Blade index

k :

Control frequency

\(L_\text {m}\) :

Measurement resolution, \(\text {m}\)

\(M_\text {tip}\) :

Tip Mach number

n :

Vatistas swirl shape parameter

\(N_\text {b}\) :

Number of blades

r :

Radial coordinate, \(\text {m}\)

\(r_\text {c}\) :

Vortex core radius, \(\text {m}\)

\(r_\text {void}\) :

Particle void radius, \(\text {m}\)

R :

Rotor radius, \(\text {m}\)

t :

Time, \(\text {s}\)

\(t_\text {0}\) :

Time of quarter chord passing through light sheet, \(\text {s}\)

T :

Rotor thrust, \(\text {N}\)

u, v :

Velocity components, \(\text {m}/\text {s}\)

\(U_n\) :

Maximum control amplitude, \(\text {V}\)

\(U_i\) :

Control voltage signal, \(\text {V}\)

\(V_{\text {tip}}\) :

Blade tip speed, \(V_{\text {tip}}=\Omega R\), \(\text {m}/\text {s}\)

\(V_{z}\) :

Vortex induced axial velocity, \(\text {m}/\text {s}\)

\(V_{\theta }\) :

Vortex induced swirl velocity, \(\text {m}/\text {s}\)

x, y, z :

Coordinates in PIV image plane, \(\text {m}\)

\(\alpha _{\text {e}}\) :

Effective angle of attack, \(\text {deg}\)

\(\Gamma _\text {v}\) :

Vortex circulation, \(\text {m}^2/\text {s}\)

\(\rho\) :

Air density, \(\text {kg}/\text {m}^3\)

\(\sigma\) :

Rotor solidity, \(\sigma =N_\text {b} c /(\pi R)\)

\(\varphi\) :

Control phase, \(\text {rad}\)

\(\psi _\text {v}\) :

Vortex age, \(\text {deg}\)

\(\Psi\) :

Azimuth, \(\Psi =\Omega t\), \(\text {deg}\)

\(\omega _z\) :

Vorticity normal to x–y plane, \(1/\text {s}\)

\(\Omega\) :

Rotor rotational frequency, \(\text {rad}/\text {s}\)

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Acknowledgements

The technical and financial support of the STAR partners for the hover test is highly appreciated. The authors are indebted to the rotor test team, especially F. Hoffmann, B. G. van der Wall, R. Keimer, S. Kalow, and M. Krebs.

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Authors and Affiliations

  1. Helicopter Department, Institute of Aerodynamics and Flow Technology, German Aerospace Center (DLR), Bunsenstr. 10, 37073, Göttingen, Germany

    André Bauknecht, Benjamin Ewers & Markus Raffel

  2. Rotorcraft Department, Institute of Flight Systems, German Aerospace Center (DLR), Lilienthalplatz 7, 38108, Braunschweig, Germany

    Oliver Schneider

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  1. André Bauknecht
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  2. Benjamin Ewers
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  3. Oliver Schneider
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  4. Markus Raffel
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Correspondence to André Bauknecht.

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Bauknecht, A., Ewers, B., Schneider, O. et al. Blade tip vortex measurements on actively twisted rotor blades. Exp Fluids 58, 49 (2017). https://doi.org/10.1007/s00348-017-2312-3

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