Reducing uncertainty in low flows for bench‐top wind tunnels

Bench‐top wind tunnels are used extensively by the US Air Force for calibrating anemometers. As anemometers have improved, the need for reduced uncertainties in the bench‐top wind tunnels was required. A three‐pronged approach was used to reduce low velocity uncertainties by a factor of 2‐3.Design/methodology/approach – The reduction in velocity uncertainties was achieved by upgrading the wind tunnel instrumentation that measured the pressure and differential pressure and by improving the velocity calibration of the bench‐top wind tunnel. A detailed uncertainty analysis was performed to determine how much the instrumentation needed to improve. A laser Doppler velocimetry (LDV) was used to calibrate each wind tunnel at low velocities.Findings – The uncertainty analysis indicated that the main contributors to the velocity uncertainty were the differential pressure and the pressure measurements. These two process instruments were upgraded to reduce their individual uncertainties by a factor of 2. Additionally each bench‐top wind tunnel was calibrated using the LDV with special emphasis on flows from 0.15‐3.0 m/s. In all, nine wind tunnels were calibrated and the upgraded systems exhibited a reduction in uncertainties in the low flow region of a factor of 2‐3.Originality/value – A need to reduce velocity uncertainties in bench‐top wind tunnels was a requirement for the US Air Force calibration program. Upgraded instrumentation and individual calibration with an LDV provided the needed reduction. In the low flow region of 0.15 to 3.0 m/s, uncertainties were reduced by a factor of 2‐3.