Effects of the proximity of a wall
The correction to the reading of a Pitot tube placed near a wall are less immediate than the corrections for the gradient of stagnation pressure since the error depends on the whole speed profile between the tube and the wall. A process of successive approximations could be developed to deduce the true profile from the profile measured but this procedure would require a large amount of experimental work. As said above, for
Pitot probes for boundary layer
measurements in thin boundary layers very small tubes are used for which the correction for the effect of proximity to a wall can be ignored; for cylindrical tubes the effect tends to zero at distances greater than 2 diameters from the wall (Figure 2.9); the correction for a flattened tube is more uncertain since the effect of the wall is to prevent the deviation of the streamlines induced by the presence of the Pitot tube and a flattened tube deforms the streamlines more than a circular pipe.