An electrochemical method, which is the mass transfer analog of the hot film anemometer, has been developed to study flow fluctuations in the immediate vicinity of a wall. Electrodes mounted flush with the wall are the cathode of an electrolysis cell. At high enough voltages, the electric current is controlled by the rate of mass transfer to the cathode and the concentration of the reacting species at the surface is zero. These techniques allow the measurement of two components of the fluctuating velocity gradient at the wall. Results have been obtained on the structure of turbulence close to a wall and on the influence of drag reducing polymers on this structure.
A model for the kinematics of a turbulent flow close to a solid boundary is explored. The field is assumed to be homogeneous in the direction of mean flow. The equations of motion are solved numerically for a flow which is periodic in time and in a direction transverse to the direction of mean flow. The period is taken to be the time interval between ‘bursts’ and the wavelength, the spacing of the streaky structure close to the wall observed by a number of investigators. Good agreement is obtained between the calculated flow field and experimental results, especially for y+ < 15. This agreement suggests that the flow oriented eddies in the viscous wall region can be represented by a model which views the flow in this region to be coherent and to be associated with spanwise flow deviations in a well-mixed outer region. The model allows for the periodic movement of low momentum fluid from the wall, which, because of the assumption of a well mixed outer region, gives rise to a shear layer. This seems to correspond to the observed ‘bursting’ phenomenon. The calculations confirm the suggestion by Fortuna and Hanratty (Fortuna 1970; Hanratty, Chorn & Hatziavramidis 1977) that the secondary flow in the viscous wall region generated by these spanwise flow deviations gives rise to the development of large velocity fluctuations in the direction of mean flow and accounts for the experimentally observed maximum in the velocity fluctuations close to the wall. Also, the comparison of calculations with measurements of the average velocity and with an experimental quadrant analysis of the Reynolds stress suggests that the secondary flow is making a major contribution to the Reynolds stress.
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