NIST is developing techniques to calibrate 2- and 3-dimensional anemometer systems (such as multi-hole pitot tubes) with a resolution of 0.3 %. The pitch and yaw response of multi-hole pitot tubes is complicated; therefore, calibrations require hundreds of measurements. Until accurate models become available, NIST will disseminate the calibration data as tables. Many National Metrology Institutes (including NIST) calibrate anemometers in low-turbulence wind tunnels. During NISTs first calibration of a commercially manufactured multi-hole pitot tube, we observed hysteresis in the pressure differences between particular pairs of holes in particular ranges of airspeed, pitch angle, and yaw angle. In the worst case, the pressure difference for increasing air speed was 30 % larger than the pressure difference for decreasing air speed. By visualizing and mapping the flow near the pitot tubes surface, we demonstrated that this hysteresis was caused by boundary layer separation. The hysteresis disappeared when we added turbulence to the flow in NISTs wind tunnel.
2012 Measurement Science Conference
March 19-23, 2012
multi-hole pitot tube, 3-D airspeed calibration, detached flows, flow visualization