The flow reference system-an air data system for wind tunnels

The paper describes an integrated system specifically developed for establishing the elementary flow quantities Total Pressure (P_t ), Static Pressure (P_s), and Total Temperature (T_t ) in a windtunnel environment with the highest presently attainable accuracy. These quantities are required for calculating quintessential aerodynamic parameters like Mach number (Ma), Dynamic Pressure (Q), Reynolds number (Re), and Flow Velocity (V). Up to four extremely accurate (0.015% Full Scale) absolute or differential Pressure Sensing Units can be accommodated on a plug-in basis in this “Flow Reference System”. Temperature measurement is based on the use of a Platinum 100 ohm RTD sensor; accuracy of signal conditioning and linearization are better than 0.05°C over a range of -200 to +100°C. Data output for pressures and temperature is in fully corrected Engineering Units over two independent output links at a high sampling rate of 10 Hz. A status word output allows an effective monitoring of system integrity. The concept was originally developed by the European Transonic Windtunnel GmbH for application in their High-Reynolds number cryogenic facility, but the flexible, modular approach makes it an attractive proposition for any windtunnel. The implementation is based on the 5400 System Processor (SP) of Pressure Systems Inc. (PSI). The paper discusses the design considerations including an accuracy analysis, and presents details of the practical realization. Some results are given based on two years of operational experience