Pneumatic probes are still useful tools when investigating flow fields within turbomachines, cascades or any other aerodynamic facilities. Due to the fact that flow velocity and flow direction cannot be measured directly by the probes as well as due to inaccuracies when manufacturing the probe heads, a calibration process in flow fields with well known and constant flow conditions is necessary.
For this purpose a facility for probe calibration (SEG) has been designed and built at DLR in Göttingen, which uses existing auxiliary devices (compressors, vacuum pump etc.) of the Rotating Cascade Tunnel (RGG). A main advantage of this new calibration facility is the independent variation of Mach and Reynolds number within a wide range of interest.
The test section of the SEG - a long free jet downstream of the nozzle - is positioned inside a big chamber. The big cylindrical chamber can be opened at front and rear side, to enable easy access to the nozzle, the traverse mechanisms with the probe or to the diffusor. Various types of probes with stems up to 700 mm length can be mounted inside the test chamber. Different traverse mechanisms are available which allow for moving probes in three dimensions and two flow angles (pitch and yaw). In order to cover the desired Mach number range from subsonic to supersonic flow several axisymmetric nozzles with a mean exit diameter of 50 mm are easily exchangeable. The overall contraction ratio from the settling chamber to the nozzle exit is about 16. A flow straightener and three screens serving for a homogeneous flow field at a low level of turbulence are fixed in the settling chamber. The free jet downstream of the nozzle enters a diffusor with a movable insert enabling the reverse of kinetic energy into pressure rise. For flow field visualization the test section chamber is also equipped with windows for schlieren optics and an exchangeable infrared window to allow for planar temperature measurements by an infrared camera.
Main performance data of the SEG and a diagram showing the achieved Mach and Reynolds numbers are given in the following table and the figure on the right. The Reynolds numbers are calculated with a mean nozzle diameter of 50 mm.
The SEG is operating using two auxiliary compressors from the Rotating Cascade Wind Tunnel which can work either in single or parallel mode. Both compressors are running at constant speed, whereas the flow control in the test section (mass flow and therefore static pressure) is done by adjusting the flow through two bypass ducts parallel to these compressors. The Mach number is then obtained from the ratio of the static pressure inside the test section chamber to the total pressure, which is measured in the settling chamber. Additional the total pressure (overall pressure level) in the SEG can be adjusted by a vacuum pump and by feeding dry pressurized air from the DLR air supply into the circuit. Therefore, an independent variation of the Mach and Reynolds number is achieved. An air drier and two coolers for keeping the total temperature constant are also included in the circuit. Flow control, probe positioning and data acquisition are performed using a SIMATIC software from Siemens and a LabView software from National Instruments, installed into two personal computers.
For a more detailed description of the SEG see following publication:
Gieß, P.-A., Rehder, H.-J., Kost, F.:
A New Test Facility for Probe Calibration Offering Independent Variation of Mach and Reynolds Number
Proceedings of the 15th Symposium on "Measuring Techniques for Transonic and Supersonic Flows in Cascades and Turbomachines", Firenze, Italy, September, 21-22, 2000
Authors: Fritz Kost, Hans-Jürgen Rehder