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Influence of a back-flow flap on the dynamic stall flow topology

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Abstract

Dynamic stall is a major concern for highly loaded helicopter rotors in fast forward flight. The potential of a back-flow flap for dynamic stall reduction is investigated. The flap assembly is mounted on the suction side of a helicopter main rotor-blade airfoil undergoing deep-stall pitch oscillations. Wind-tunnel experiments using high-speed particle image velocimetry were conducted to identify the flow topology and to investigate the flap’s method of operation. A phase-averaged proper orthogonal decomposition (POD) is used to identify relevant flow events and to compare test cases with and without flap. The evolution of the large-scale dynamic stall vortex in the initial phases of flow separation is analyzed in detail. The back-flow flap splits the vortex into two smaller vortices and thereby reduces the pitching-moment peak. This effect can be described through the eigenmode coefficients of the POD. The study closes with an analysis of different pitching frequencies, which do not affect the flap’s method of operation.

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Abbreviations

a :

Temporal POD coefficient

c :

Airfoil model chord (m)

\(C_d\) :

Drag coefficient

\(C_l\) :

Lift coefficient

\(C_m\) :

Pitching moment coefficient

\(C_p\) :

Pressure coefficient

f :

Frequency of pitching (Hz)

k :

Reduced frequency, \(k = \pi f c / V_\infty\)

m :

POD mode number

M :

Mach number

N :

Number of samples

\(N^*\) :

Reduced-order cut-off

Re :

Reynolds number based on c

t :

Time (s)

u, w :

Velocity in x direction and z direction (m/s)

\(\varvec{u}\) :

Vector of 2-D velocity components, \(\varvec{u} = (u,w)\)

\(V_{\infty }\) :

Freestream velocity (m/s)

\(V_p\) :

In-plane velocity, \(V_p = \sqrt{u^2+w^2}\) (m/s)

x, z :

Cartesian coordinates (m)

\(\varvec{x}\) :

Vector of 2-D coordinates, \(\varvec{x} = (x,z)\)

\(\alpha\) :

Angle of attack (\(^\circ\))

\(\beta\) :

Opening angle (\(^\circ\))

\(\Delta\) :

Difference of two values

\(\lambda\) :

Eigenvalue

\(\varvec{\Phi }\) :

Eigenmode, eigen flow field

1MG:

‘1-meter’ wind tunnel at DLR Göttingen

DLR:

Deutsches Zentrum für Luft- und Raumfahrt

PIV:

Particle image velocimetry

POD:

Proper orthogonal decomposition

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Author notes

  1. C. B. Merz

    Present address: Daimler AG, Mercedesstrasse 132, 70327, Stuttgart, Germany

Authors and Affiliations

  1. DLR Institute of Aerodynamics and Flow Technology, Bunsenstrasse 10, 37073, Göttingen, Germany

    C. C. Wolf, A. D. Gardner & C. B. Merz

  2. DLR Institute of Composite Structures and Adaptive Systems, Lilienthalplatz 7, 38108, Braunschweig, Germany

    S. Opitz

Authors
  1. C. C. Wolf
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  2. A. D. Gardner
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  3. C. B. Merz
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  4. S. Opitz
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Corresponding author

Correspondence to C. C. Wolf.

Additional information

A version of this paper was presented at the European Rotorcraft Forum (ERF), Lille, France, September 5–8, 2016.

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Wolf, C.C., Gardner, A.D., Merz, C.B. et al. Influence of a back-flow flap on the dynamic stall flow topology. CEAS Aeronaut J 9, 39–51 (2018). https://doi.org/10.1007/s13272-017-0274-z

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