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The dynamic forces which a turbulent flow exerts on the individual protrusions of a rough wall have been measured. It was found that even in the case of an extremely high relative roughness, the drag force on the protrusions may be determined from the logarithmic friction laws. A very significant lift force which tends to pull the protrusions away from the wall and into the flow was measured. This lift force was divided into two parts: A constant average value and a random fluctuation superimposed over the average. A careful statistical analysis showed that the frequency of different pressures or lift forces at the wall follows very accurately the normal error law. This fact seems to indicate that in the description of turbulence near a rough wall, the pressures must be regarded as the primary influence, not the velocities.
Einstein et al. (Fri,) studied this question.