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Experiments in Fluids

, 60:19 | Cite as

Optimization of differential infrared thermography for unsteady boundary layer transition measurement

  • C. Christian WolfEmail author
  • Christoph Mertens
  • Anthony D. Gardner
  • Christoph Dollinger
  • Andreas Fischer
Research Article
  • 88 Downloads

Abstract

Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar–turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short-time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady aerodynamics of helicopter rotors in forward flight. The current study investigates a pitch-oscillating airfoil and proposes several optimizations of the original concept. These include the extension of DIT to steady test cases, a temperature compensation for long-term measurements, and a discussion of the proper infrared image separation distance. The current results also provide a deeper insight into the working principles of the technique. The results compare well to reference data acquired by unsteady pressure transducers, but at least for the current setup DIT results in an additional measurement-related lag for relevant pitching frequencies.

Graphical abstract

Abbreviations

\(1\text{MG}\)

One-meter wind-tunnel Göttingen

DIT

Differential infrared thermography

DLR

German Aerospace Center

IT

Infrared thermography

List of symbols

c

Chord length, \(c={0.3\,\hbox {m}}\)

\(c_\mathrm{f}\)

Skin-friction coefficient

\(c_\mathrm{l}\)

Lift coefficient

\(c_\mathrm{p}\)

Pressure coefficient

C

Fluid specific heat capacity, J/m\(^3\)/K

f

Pitching frequency, Hz

k

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

\(\text{M}_\infty\)

Freestream Mach number

\({\dot{q}}_\mathrm{c}\)

Convective heat flux, W/m\(^3\)

Re

Reynolds number

t

Time, s

T

Airfoil surface temperature, K or counts

\(T_\infty\)

Freestream temperature, K

\(V_\infty\)

Freestream velocity, m/s

x

Coordinate along the airfoil’s chord line, m

\(x_{\text{tr}}\)

Transition position, m

Greek symbols

\(\alpha\)

Geometric angle of attack, deg

\({\overline{\alpha }}\)

Mean value of the angle of attack, deg

\({\widehat{\alpha }}\)

Amplitude of the angle of attack, deg

\(\varDelta\)

Difference between two values

\(\varDelta T_\mathrm{p}\)

DIT peak height, counts

\(\rho\)

Density, kg/m\(^3\)

\(\sigma C_\mathrm{p}\)

Standard deviation of the pressure coefficient

Notes

Acknowledgements

The studies were conducted in the framework of the DLR project “FAST-Rescue”.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.German Aerospace Center (DLR)Institute of Aerodynamics and Flow TechnologyGöttingenGermany
  2. 2.Bremen Institute for Metrology, Automation and Quality Science (BIMAQ)University of BremenBremenGermany

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