We present an experimental study of the spatial structure of the velocity field in the boundary layer region of a Rayleigh-B\'enard convection cell, using water as the working fluid. Our results show that the mean flow, the shear rate, and the viscous boundary layer thickness all change significantly across the conducting horizontal surface of the cell. Moreover, the measurements reveal that the spatial structure of the velocity field in the boundary layer region does not change with the Rayleigh number, in sharp contrast with those found for the thermal boundary layers [S.-L. Lui and K.-Q. Xia, Phys. Rev. E 57, 5494 (1998)]. The normalized velocity profiles measured at various positions in the direction of the mean flow and for different Rayleigh number are also found to have an invariant form.
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Qiu et al. (1998) studied this question.