Instrumentation for Measuring Supercooled Large Droplet Cloud Distributions in Icing Wind Tunnels

Abstract

Supercooled large droplet (SLD) icing poses a threat to aviation. To study the characteristics of SLD icing and to facilitate testing of SLD detection technologies, the Braunschweig Icing Wind Tunnel (BIWT) is upgraded to generate Freezing Drizzle (FZDZ) conditions specified in Appendix O of CS-25 / 14 CFR Part 25. The bimodal nature of the FZDZ cloud and its wide range of DSD (3 – 500µm) is achieved from the superposition of sprays from two separate atomizer arrays producing smaller and larger droplet modes. For a better characterization of the DSD in BIWT, a Phase Doppler Interferometer (PDI) is used. The limited dynamic range of PDI precludes the measurement of the complete DSD of FZDZ in a single optical configuration. The static measurement range is extended without changing transmitter and receiver lenses by using the inbuilt receiving aperture masks. The three different mask configurations used reliably cover the range of FZDZ, and the complete DSD of FZDZ is obtained by combining the partial spectra measured with different masks. The masks designed for larger droplets showed a lower sensitivity to smaller droplets within the overlap regions. Three methods for combining the partial DSDs are investigated, the method of combining the distributions based on the maximum number density of the size class is found to be more reliable than the methods based on averaging and blending of volume fluxes. The DSDs retrieved from the maximum number density method are compared with distributions obtained with a well-characterized Cloud Combination Probe (CCP). A good agreement is found in the large droplet mode. A consistent trend is observed in the small droplet mode, however, the CCP reports higher number of densities and stronger peaks at a diameter of 10µm. The spurious large droplets in PDI affecting the quality are effectively filtered with the inherent velocity slip of large droplets computed from Lagrangian droplet tracking. The calibration validated the successful actualisation of bimodal FZDZ conditions with low liquid water content. The present work demonstrates the effectiveness of receiving aperture masks in extending standard PDI to measure the App. O conditions, the proposed droplet size-velocity validation filter improves the quality of the measurements.