Development of a digital inline holographic system for aircraft icing studies

While aircraft icing poses a critical safety risk and can degrade the aerodynamic performance of aircraft significantly, accurate prediction of aircraft icing events requires precise characterization of atmospheric icing conditions, particularly the properties of airborne supercooled water droplets (SWDs) and ice crystals in icing clouds. This study introduces the development of, to our knowledge, a novel digital inline holography (DIH) system for simultaneous characterization and classification of airborne SWDs and ice crystals for aircraft icing studies. The measurement accuracy of the DIH system was analyzed based on a quantitative comparison of the DIH measurements against the nominal values of the standard particles certificated by the National Institute of Standards and Technology (NIST). By leveraging the unique Icing Research Tunnel of Iowa State University (ISU-IRT), the unique capacity of the DIH system is demonstrated to achieve <i>in</i><i>situ</i> measurements to characterize the airborne SWDs and ice crystals in terms of median volume diameter (MVD), liquid water content (LWC), ice water content (IWC), and their size and shape distributions under typical atmospheric icing conditions within aircraft inflight icing envelopes. A shape-based nondimensional parameter (i.e., circularity) is demonstrated for the first time to successfully differentiate the airborne SWDs from ice crystals in mixed-phase icing clouds for better monitoring and forecasting icing events, thus ensuring safer and more efficient operations of aircraft in cold climates.