A calibration system for low-velocity flows at stratospheric conditions

In order to calibrate balloon-borne fine-wire anemometers to the conditions of the middle and upper stratosphere, a novel small-scale open-circuit wind tunnel was developed. The system allows for calibration of hot-wire anemometers and cold-wire thermometers at pressures between approximately 275 and 5,500 Pa with air temperatures tested as low as 210 K. To represent typical high-altitude balloon rise rates, the facility was designed to generate test section centerline velocities between 2 and 10 m/s. The flow field at the test section was predicted analytically for fully-developed laminar circular Poiseuille flow and verified using a commercial hot-wire anemometer. Numerical simulations of the flow field were used to quantify any distortions of the velocity profile due to blockage effects. The accuracy and stability of the system with respect to how it will be used as a calibration facility is discussed.