Wind Tunnel Measurements of Simulated Glaciated Cloud Conditions to Evaluate Newly Developed 2D Imaging Probes

Kurzfassung

Instrumentation that has been used for characterization of mixed-phase and glaciated conditions in the past, like the OAP probes, are subject to errors caused by variations in diffraction on the images away from the object plane and by the discrete nature of their particle detection and sizing. Correction methods are necessary to consider their measurements adequate for high ice water content (IWC) environments judged to represent a significant safety hazard to propellers and turbofan engine operability and performance. For this reason, within the frame of EU FP7 HAIC project, instrumentation characterization and validation is considered a major element need for successful execution of flight tests campaigns. Clearly, instrumentation must be sufficiently reliable to assess the reproducibility of artificial clouds with high ice water content generated in icing tunnels. Instruments are required to measure these conditions with a sufficient level of accuracy for the purposes of the testing. Currently, there is an anticipated basic uncertainty of a factor of 2-5 when measuring clouds in-situ. This may be worse for thunderstorm core regions, because of the poorly measured ice particle sizes below 100 μm when using legacy instruments. Measurements below 100 μm are especially difficult for ice particles. In order to mitigate this measurement limitation, an innovative approach for imaging droplets and ice crystals was considered in this work. This method is based on Artium Technologies Inc. High Speed Imaging (HSI) instrument. The method utilizes a unique multi-beam illumination approach to control the depth of field, minimize out-of-focus image noise effects, and limit obscuration of particles in the sample volume produced by out-of-focus particles in the optical path. Rather than utilizing a linear array to acquire the images, a modern CMOS two-dimensional array imaging system is used. The lasers used for producing bright field shadow images are pulsed synchronously and have pulse duration of as little as 12.5 ns to “freeze” the particle motion. The response of the HSI will be reviewed on the basis of comparison with SPEC 2D-S by means of wind tunnel measurements for glaciated cloud conditions.