Die Genauigkeit optischer Bewegungsverfolgung eines Modell-Targets im HEG

Kurzfassung

Optical movement tracking is a non-intrusive method to determine the acting forces and moments on a wind tunnel model. The elemental tracking process is based on a series of images showing the model-contour during the movement. By fitting a given model geometry to each picture, data of the object’s position and orientation over time can be gained. Differentiated twice, its acceleration and thus the forces applied to it are obtained. In high-enthalpy shock tunnels, the short testing time doesn’t allow classic force measuring techniques based on stress-equilibrium. When simulating a Mach 8 current in 30km altitude, the High Enthalpy Shock Tunnel Goettingen (HEG) achieves a test time of 3 ms to 6 ms [5], making optical tracking a promising method. Different force measurement techniques have been used at the HEG prior to the recent optical tracking method. The Stress Wave Force Measurement Technique was a method to obtain lift, drag and pitching moment by observing the model’s dynamic response to flow arrival. Strain gauges monitor the propagation of stress waves through the model and the supporting structure and allow to draw conclusions about the acting aerodynamic forces. Besides the necessarily careful calibration, also the possible interference of the supporting sting with the current accounts for the downsides of the method. As a result, a method based on optical tracking was developed, inspired by similar techniques used at the T5 shock tunnel of GALCIT [7–9]. The model was supported by an exterior structure and filmed during its movement in the flow. To enable this movement, the support structure was flexible to a certain extent. Later, a free flight of the model during test time was achieved by suspending it from Kevlar threads which were cut off by integrated razor blades upon flow arrival. This way the probable influence of the support sting is avoided leaving only minor interferences.