By E. Ower, R. C. Pankhurst
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Effects of Viscosity and Wall Proximity The readings of a very small pitot tube are subject to errors due to viscosity (see p. 13). Although their magnitude has been established (see Fig. 15) for both circular and flattened pitot tubes, (24) whenever possible such small tubes are best avoided because of their extremely slow rate of response. By using a tube large enough to avoid excessive lag, it will still usually be possible to keep the tube Reynolds number above that at which viscosity effects begin to be appreciable (typically about 250 based on internal diameter (25) or height; (24) see also ref.
6. Prandtl's pitot-static tube. The slit has the advantage of symmetry, and, in a dust-laden airstream, it is less likely than are small holes to become blocked ; but its sharp edges are a mechanical disadvantage and lead to difficulties of exact reproducibility in manufacture. The factor for this instrument also is very near to 1. Later work, discussed below, has shown that an ellipsoidal nose is preferable to both the tapered and hemispherical types. L. e. they were moved through initially still air, at measured speeds between 0-6 and 20 m/sec for the tapered nose and 0-6 and 7 m/sec for the hemispherical/ 2 ' 3) At all speeds from 6 m/sec upwards the factor for the tapered-nose tube was found to be 1-000 ±0-001, and the factor for the hemispherical-nose instrument, obtained from comparative tests in a wind tunnel with the tapered-nose instrument, was found to be the same within the accuracy of observation.
If the value of AT in (1) is to be 1, the static orifices must be placed in a region where the pressure acting on the surface of the head is equal to the stream static pressure. The pressure distribution, and hence the correct position of the static orifices, will depend partly on the shape of the front of the head (the nose), and partly on the position of the stem, which produces a pressure upstream of itself and so affects the pressure at the static orifices. (1) Observations were made with four differently shaped noses, three of them tapered as in Fig.