Effect of flow direction on calibration of hot-film anemometers at low velocities

A constant-temperature anemometer with a quartz-coated cylindrical hot-film probe is considered. Essentially, the probe is moved in a water tank at a constant velocity. Calibration results are presented at a resistance ratio of 1.1 in water for imposed flow angles of 0, 15, 30, 45, 60, 75, and 90 deg. It is found that there is no appreciable difference between calibrations at flow angles between 0 deg (vertically up) and 30 deg. The effect is appreciable for angles greater than 30 deg, and for flows in the vicinity of horizontal there is a lower limit of velocity below which the output from a single sensor does not unambiguously determine velocity, even if the direction is otherwise known.