Spherical five-holes probe
Another type of tool for subsonic 3D fields, shown in Figure 2.28, consists of a sphere supported on a stem parallel or orthogonal to the stream with 4 holes on the front located at 90° from each other and a central hole for the measurement of p0. The sensitivity of the instrument is about 0.08 per degree.
Figure 2.2 shows the theoretical and measured pressure around a sphere in laminar and turbulent flows. Assuming a potential flow maximum sensitivity is obtained at 42°, experimentally the maximum sensitivity is obtained around 50° and depends on the position of separation and thus on the Reynolds number. The transition point may move randomly producing asymmetry and indeterminate results. The maximum pitch angle detected is about 60°.
The presence of velocity gradients and the proximity of a wall restrict the use of this probe as they do for all probes with well-separated holes.