Abstract
The electrochemical method has been used to study the wall turbulence in a rectangular channel flow and to assess the role of high polymers in the drag reduction phenomenon. Experiments were performed with aqueous solutions of two high molecular weight polymers: polyethylene oxide (PEO) and polyacrylamide (PAA). High levels of the drag reduction (up to 75%) were observed for small concentrations of these polymers under turbulent flow conditions ( Re = 5000–35,000). The photolithography technique of the probe fabrication was improved to turn out very accurate and complicate patterned electrodes. Double semi-circular gold electrodes of very small diameters ( d = 100 μm) located close to each other on the channel wall (the minimum center-to-center distance was 125 μm) were simultaneously working in the regime of the limiting diffusion current. Eight electrodes provided information on two components of the fluctuating wall shear rate at the wall. The frequency and the amplitude of flow fluctuations were greatly reduced by addition of polymer and the corresponding power spectra exhibited a shift towards low frequencies. Spatial correlations were calculated from the probe signals and an increase of the size of the coherent flow structures due to the polymer addition was observed.
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