Quantitative infrared-thermography for wall-shear stress measurement in laminar flow

The hot film is a common technique to measure wall-shear stress in boundary-layer flows. A new technique to measure the wall-shear stress, referred to as quantitative infrared-thermography, has been developed. It replaces the internal heating and the temperature detection using the hot film with the external heating using a laser and the external temperature measurement using an infrared camera, respectively. First the laser creates a hot spot on a substrate of polycarbonate covered with a plastic foil; the conductivity of the chosen substrate is small and the emissivity is large, giving the required small, but clearly detectable spot. Then the laser is switched off and the temperature decay is monitored. The measured unsteady wall temperature is used as a boundary condition to numerically solve the heat transfer in the solid, which gives, through the total heat balance, the heat transfer from the solid to the fluid. A local similarity relation, which applies for small spots, is used to relate the wall-heat transfer to the wall-shear stress. The technique is demonstrated for the Blasius boundary layer in a wind-tunnel experiment, where an accuracy of about 10% has been achieved. (C) 1998 Elsevier Science Ltd. All rights reserved.