Effects of the Mach number
By increasing the Mach number of the undisturbed stream, a critical value is reached (Mcr = 0.7) for which M > 1 is generated on the tube, and upstream of the holes the first shock waves change the static pressure; with the growing Mach number the supersonic zone extends and the shock waves moving downstream step over the holes which are therefore in a supersonic flow. When this happens, reading the pressure becomes independent of the position of the support and the distribution of velocity on the front of the tube differs appreciably from that which occurs at low speeds.
Under these conditions, therefore, a static tube designed to operate at low speed can result in appreciable errors. This is illustrated in Figure 2.15 which shows the results obtained with a tube in which the phenomena are accentuated by the blunt shape of the head. By using tubes with a very slender head, a critical Mach number can be increased up to values close to 1.
At supersonic speeds, the head of the tube should be conical: the opening angle must be less than that for which a detached shock wave is
Influence of compressibility on the readings of a blunt static probe at different angles of attack
Effect of position of holes in a static pressure supersonic probe
formed and static holes should be placed at least ten diameters after the beginning of the cylindrical tube (Figure 2.16) where the effects of shock waves are offset by the expansion that occurs at the beginning of the cylinder.