Infrared thermography as a tool for wall shear stress measurements

Auteurs

• Ilka Rudolph

Paramètres

• Manuels
• Manuels universitaires
• Aerospace engineering

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A variety of flow phenomena, such as flow separation or reattachment and boundary layer transition from laminar to turbulent can be detected using the wall shear stress distribution, as they directly influence the viscous forces at the wall. Therefore, a number of wall shear stress measurement techniques have been developed and tested in the last decades. However, the focus has been mainly aimed at sensor-based techniques, resulting in a scarcity of spatial methods. This lack of spatial methods also extends to purely visualizing techniques. To date, only one technique exists for measurements of the wall shear stress vector in magnitude and direction. The present work introduces two novel infrared-based wall shear stress measurement techniques. The first technique is sensor based and uses the thermal footprint around a heated spot to determine the wall shear stress vector. The technique was investigated in laminar and turbulent flow across a flat plate using a skin friction balance as a reference technique. Well suited correlation parameters for the measurement of shear stress magnitude and direction were identified, which also allow for a distinction between laminar and turbulent flow conditions. The second technique enables spatial wall shear stress visualizations and quantifications using a flexible heated structure that can be integrated into any surface of interest. It is based on the analogy between heat and mass transfer in the boundary layer, correlating the convective heat transfer and thus the surface temperature with the wall shear stress. Wall shear stress visualizations were carried out for the flow around a wall mounted cylinder and the results compared with oil flow visualizations as well as numerical and experimental reference data. The spatial quantitative technique was investigated numerically and experimentally for a laminar and turbulent plate flow. Suitable correlation parameters were found and tested numerically before they were applied to the experimental investigations. The techniques presented here successfully enhance the range of steady state measurement techniques, as they close the gap in existing techniques by providing a way to obtain spatial information as well as measure the wall shear stress vector.

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• Année de publication: 2012