SIMULTANEOUS WALL SHEAR RATE MEASUREMENTS BY A 3 SEGMENTED ELECTRODIFFUSION PROBE AND LASER-DOPPLER-ANEMOMETRY

In a fully developed turbulent channel flow, the instantaneous wall sh ear gradient was measured simultaneously by an electrodiffusion (ED) p robe and by laser-Doppler anemometry (LDA). The LDA measurements were done in the viscous sublayer and the linear velocity profile was used to calculate the instantaneous shear gradient at the wall. The electro diffusion probe requires presence of a suitable electrolyte (e.g. an i odine and potassium-iodine aqueous solution) for reduction of the spec ies on the working electrode (cathode). This results in a concentratio n gradient of the ions due to the convective diffusion of the [I-3](-) to the cathode. Due to the limited molecular diffusion rate and due t o the dimension of the electrodiffusion probe its frequency resolution is limited. It is shown in which limits the correction of the signal on the concentration boundary layer inertia is not necessary and the p robe can be used for measuring instantaneous wall shear gradients in t urbulent hows. The results are compared with those obtained by LDA and elucidate the capability of the ED probe for measuring instantaneous shear rates at low Reynolds numbers.