Split-Film Anemometer Sensors for Three-Dimensional Velocity-Vector Measurement
A probe for the fast-response measurement of the total air velocity vector is described. In the past, velocity-vector measurement has been achieved with a probe consisting of three, orthogonal, hot-film sensors. However, this measurement requires an a priori knowledge of the octant of the velocity vector. The new probe has a similar configuration but uses three split-film sensors to provide automatic octant indication. Each hot-film sensor consists of a 6-mil diameter, 80-mil long cylindrical quartz rod coated with a 1000 Å platinum film. The film of this new sensor is axially segmented, or split, with two splits 180° apart. Each split film element is electrically heated to the same constant temperature by a separate electronic constant-temperature anemometer control system. The non-uniformity of heat flux around the sensor is detected by comparing the individual heat flux to each split-film element. For Reynolds numbers less than approximately 5 × 104, the heat flux to the upstream split film is greater than the downstream. This is used to detect the sense of the velocity component normal to the sensor. Such information from all three orthogonal sensors yields the octant of the air velocity vector. The sum of the heat fluxes to the two split-film elements of a sensor is invariant with azimuthal angle. This permits the use of standard techniques to measure the magnitude and direction of the velocity vector within the detected octant.
Experimental data and empirical correlations are presented for the variation of heat flux with velocity magnitude and two directional angles — the yaw and azimuthal angles to the sensor. The heat-flux ratio of the downstream to the upstream split-film element has a simple quadratic dependence on azimuthal angle. The sum of the heat fluxes to the two elements of a sensor is correlated in the typical fashion with an effective cooling velocity. Other probe configurations using split-film sensors are described — the rugged three-dimensional probe, the boundary-layer probe, and the horizontal wind-vector probe.
KeywordsHeat Flux Azimuthal Angle Total Heat Flux Film Element Thermal Anemometer
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