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Optimization of differential infrared thermography for unsteady boundary layer transition measurement

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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.

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Abbreviations

\(1\text{MG}\) :

One-meter wind-tunnel Göttingen

DIT:

Differential infrared thermography

DLR:

German Aerospace Center

IT:

Infrared thermography

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

\(\alpha\) :

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

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Acknowledgements

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

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Authors and Affiliations

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

    C. Christian Wolf, Christoph Mertens & Anthony D. Gardner

  2. Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), University of Bremen, Linzer Strasse 13, 28359, Bremen, Germany

    Christoph Dollinger & Andreas Fischer

Authors
  1. C. Christian Wolf
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  2. Christoph Mertens
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  3. Anthony D. Gardner
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  5. Andreas Fischer
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Correspondence to C. Christian Wolf.

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Wolf, C.C., Mertens, C., Gardner, A.D. et al. Optimization of differential infrared thermography for unsteady boundary layer transition measurement. Exp Fluids 60, 19 (2019). https://doi.org/10.1007/s00348-018-2667-0

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