Directional sensitivity of unplated normal-wire probes

This paper describes an experimental study of the response to yaw of normal-wire probes oriented at various roll angles with respect to the yaw plane. The results obtained show that unplated normal wires are most sensitive to yaw when the plane formed by the wire element and the prongs is aligned with the yaw plane. When the wire-prongs plane is normal to the yaw plane, the sensitivity of these probes is reduced by 30 to 33%. When the wire-prongs plane is inclined at 45° to the yaw plane, the probe response is almost insensitive to yaw within a large range of flow angles. The current experimental results are compared qualitatively with the ``effective cooling velocity'' model. It is shown that this model is too oversimplified to be correct; the tangential and the binormal cooling parameters are Re number and yaw dependent and, under certain flow conditions, the tangential cooling parameter k² may even attain negative values. It is concluded, therefore, that much better signal-interpretation expressions should be sought.