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Infrared thermography techniques for boundary layer state visualisation

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The rapid decarbonisation of the power genera- tion and aviation sectors will require a move away from in- cremental development, exposing designers and researchers to the risk of unexpected results from uncertainty in bound- ary layer state. This problem already exists for parts devel- oped with fully turbulent assumptions, but in novel design spaces the risk increases for both real components, where previous knowledge of similar designs may be inapplica- ble, and particularly in experimental testing of scaled mod- els, where reducing Reynolds number can result in a dras- tic change in flow topology that skews the conclusions of a test. Computational methods struggle to reliably predict boundary layer state so experimental techniques for diag- nosing boundary layer state are needed. Infrared thermography (IR) is a non-invasive technique that offers simple, fast visualisation of boundary layer state with no additional instrumentation. IR is relatively uncom- mon in the literature and there is minimal information avail- able on the best practices for its use. This paper aims to en- courage the adoption of IR as a diagnostic tool by demon- strating routes for optimisation and pointing out pitfalls to avoid. A low-order model is developed and used to predict how the signal-to-noise ratio (SNR) of an IR visualisation changes depending on the thermal design of the test piece. It is shown that in low-speed flows with active heating from the surface the SNR is maximised through a suitable choice of surface insulation, while in high-speed flows, where pas- sive temperature differences are used, there is a crossover between heat transfer and recovery temperature effects that results in an SNR of zero, an effect that can arise in both steady-state and transient experiments. Experimental valida- tion of the 1D model in both flow regimes is shown along- side two case studies on the use of IR in sub-scale testing where uncertainty in boundary layer state results in critical differences from the full-scale flow.


Acknowledgements: The authors would like to thank Mitsubishi Heavy Industries for their generous financial support, with particular thanks to Yoshiyuki Okabe. Thanks also go to Dr Will Playford for his discussion, Bram Hulhoven for the use of his high-speed rig, Dr Chris Clark for his help with roughness trips and Liam Cohen and Oliver Wadsworth for their manufacturing work. Finally, the authors are grateful to George Hawkswell (distributed propulsor aerofoil), Dr Kshitij Sabnis and Prof. Holger Babinsky (transonic nacelle) for their collaboration on the case studies in this paper.

Funder: Mitsubishi Heavy Industries; doi:


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Experiments in Fluids

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EPSRC (2447999)
Mitsubishi Heavy Industries and the Engineering and Physical Sciences Research Council (EPSRC reference EP/S023003/1)