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Vortex detection criteria assessment for PIV data in rotorcraft applications

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Abstract

The aim of the article is to propose a simple engineering method for identifying and characterizing vortical structures within a flow field measured with a classic two-component PIV measurement system. Some of the most popular vortex detection systems are briefly presented. Of these, many fail if spurious vectors are present within the flow field due to poor PIV image quality or due to particle voids in the vortex core. The chosen method is the Γ2-criterion. The investigated method is robust and reliable, because it is based on the velocity field topology without using velocity derivatives sensitive to the spurious vectors. The method is tested on synthetic velocity fields of ideal vortices and on real PIV velocity field of a four-bladed rotor wake. The synthetic velocity fields reproduce single and multiple vortices to investigate the mutual interaction and the effect on the vortex detection criteria. The synthetic velocity fields have different spatial resolution, noise level, and different particle void to perform a parametric assessment. Other vortex identification schemes are applied for comparison. Information on how to use the criterion for different noise conditions, particle void, and presence of multiple vortices is provided.

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Abbreviations

Ω :

Antisymmetric part of \(\nabla {\varvec{v}}\), [1/s]

σ:

Rotor solidity, σ = 0.116

Ω:

Blade rotational speed, [rad/s]

\(\nabla {\varvec{v}}\) :

Velocity gradient tensor, [1/s]

c :

Blade chord, c = 0.0327 m

D :

Γ2 Domain radius, [−]

d :

Distance between vortices, [m]

d c :

Distance from the vortex centre, [m]

Q :

Q-Criterion, [1/s2]

R :

Blade radius, R = 0.36 m

r :

Radial distance from the centre, [m]

r c :

Vortex core radius, [m]

r v :

Void radius, [m]

S :

Symmetric part of \(\nabla {\varvec{v}}\), [1/s]

\({\varvec{v}}\) :

Velocity vector

u, v, w :

Cartesian velocity components, [m/s]

V θ :

Swirl velocity, [m/s]

x, y, z :

Cartesian coordinate system, [m]

Δ:

Δ-Criterion

ΔL :

Spatial resolution, [m]

ε(dc):

Vortex centre error, %

ε(rc):

Radius core error, %

ε(Vθ):

Swirl velocity error, %

γ:

Circulation, [m2/s]

ω :

\(\overrightarrow{\omega }=\nabla \times \overrightarrow{u}\), [1/S]

Γ2 :

Vortex detection scalar value, [−]

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Acknowledgements

The authors would like to acknowledge the contribution of Manuela Coletta in developing the first release of the software for vortex detection and prof. Gaetano Iuso for the valued support. The authors also appreciate the beneficial discussion with Anthony Gardner during the 45th European Rotorcraft Forum about data void limitation for detection methods that drove to additional investigations.

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Authors and Affiliations

  1. Aerodynamic and Icing Measurement Lab., Italian Aerospace Research Centre - CIRA, Capua, Italy

    Fabrizio De Gregorio

  2. Fluid Mechanics Lab., Italian Aerospace Research Centre - CIRA, Capua, Italy

    Antonio Visingardi

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  1. Fabrizio De Gregorio
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  2. Antonio Visingardi
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Correspondence to Fabrizio De Gregorio.

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De Gregorio, F., Visingardi, A. Vortex detection criteria assessment for PIV data in rotorcraft applications. Exp Fluids 61, 179 (2020). https://doi.org/10.1007/s00348-020-03012-7

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