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Future compliance to FAA 14 CFR Part 25 and EASA CS-25 Appendix O conditions has required icing wind tunnels to expand their cloud simulation envelope, and demonstrate accurate calibration of liquid water content and droplet particle size distributions under these conditions. This has led to a renewed community interest in the accuracy of these calibrations, and the potential inter-facility bias due to the choice of instrumentation and processing methods. This article provides a comparison of the response of various hot-wire liquid water content instruments under Appendix C and supercooled large droplet conditions, after an independent similar analysis at other wind tunnel facilities. The instruments are being used, or are under consideration for use, by facilities collaborating in the ICE GENESIS program.

A new optical array imaging probe, called the 1D2D probe, has been developed by Science Engineering Associates, with features added to improve the real-time and post-analysis measurements of particle spectra, particularly in the Supercooled Large Droplet size range. The probe uses optical fibers and avalanche photodiodes to achieve a very high frequency response, and a Field-Programmable Gate Array that performs real-time particle rejection and processing of accepted particles with negligible inter-particle dead time. The probe records monochromatic two-dimensional images, while also recording the number of individual particle pixels at a second grey scale level. The probe implements flexible features to filter recording of highly out of focus particles to improve the accuracy of particle size determination, or to reject small particles to improve the statistics of measurements of larger particles.

An ice shape database has been created to document ice accretions on a 21-inch chord NACA0012 model and a 72-inch chord NACA 23012 airfoil model resulting from an exposure to a Supercooled Large Drop (SLD) icing cloud with a bimodal drop size distribution. The ice shapes created were documented with photographs, laser scanned surface measurements over a section of the model span, and measurement of the ice mass over the same section of each accretion. The icing conditions used in the test matrix were based upon previously used conditions on the same models but with an alternate approach to evaluation of drop distribution effects. Ice shapes resulting from the bimodal distribution as well as from equivalent monomodal drop size distributions were obtained and compared.

High Ice Water Content (HIWC) has been identified as a primary causal factor in numerous engine events over the past two decades. Previous attempts to develop a remote detection process utilizing modern commercial radars have failed to produce reliable results. This paper discusses the reasons for previous failures and describes a new technique that has shown very encouraging accuracy and range performance without the need for any modifications to industry’s current radar design(s). The performance of this new process was evaluated during the joint NASA/FAA HIWC RADAR II Flight Campaign in August of 2018. Results from that evaluation are discussed, along with the potential for commercial application, and development of minimum operational performance standards for future radar products.

Aerodynamic assessment of icing effects on swept wings is an important component of a larger effort to improve three-dimensional icing simulation capabilities. An understanding of ice-shape geometric fidelity and Reynolds and Mach number effects on iced-wing aerodynamics is needed to guide the development and validation of ice-accretion simulation tools. To this end, wind-tunnel testing was carried out for 8.9% and 13.3% scale semispan wing models based upon the Common Research Model airplane configuration. Various levels of geometric fidelity of an artificial ice shape representing a realistic glaze-ice accretion on a swept wing were investigated. The highest fidelity artificial ice shape reproduced all of the three-dimensional features associated with the glaze ice accretion. The lowest fidelity artificial ice shapes were simple, spanwise-varying horn ice geometries intended to represent the maximum ice thickness on the wing upper surface.

This paper describes a numerical model that simulates the thermal interaction between ice particles, water droplets, and the flowing air applicable during icing wind tunnel tests where there is significant phase-change of the cloud. It has been previously observed that test conditions, most notably temperature and humidity, change when the icing cloud is activated. It is hypothesized that the ice particles and water droplets thermally interact with the flowing air causing the air temperature and humidity to change by the time it reaches the test section. Unlike previous models where the air and particles are uncoupled, this model attempts to explain the observed changes in test conditions by coupling the conservation of mass and energy equations. The model is compared to measurements taken during wind tunnel tests simulating ice-crystal and mixed-phase icing that relate to ice accretions within turbofan engines.

Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice-crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations.

A total water content probe for flight- and ground-based testing is being completed. During operation across a range of altitudes and water content conditions, the probe has to maintain isokinetic flow, vaporize the solid and liquid water content and maintain the inlet ice free to ensure isokinetic flow. Despite achieving isokinetic operation, the collection efficiency of particles less than 30 μm can be less than 100%. A correlation of collection efficiency to Stokes number has been determined to correct the results for this effect. In preparation for flight testing an integrated data acquisition, control and power supply unit was developed and successfully tested. Results from testing at the NASA Glenn Icing Research Tunnel are presented covering both ice crystals and super-cooled liquid conditions. The results correspond well to previously published work and problems encountered during previous testing of this probe are shown to have been resolved.

In this work, a newly developed iced-aircraft modeling tool is applied to wings, engine inlets, and helicopter rotors. The tool is based on a multiscale-physics, unstructured finite-volume CFD approach and is applicable to general purpose aircraft icing applications. The present approach combines an Eulerian-based droplet-trajectory solver that is loosely coupled, in a time-accurate manner, to a surface-film and ice-evolution model. The goal of the model is to improve the fidelity of ice accretion modeling on dynamic geometries and for three-dimensional ice shapes typical of helicopter rotors. The numerical formulation is discussed and presented alongside 2D and 3D static validation cases, and dynamic helicopter rotors. The present results display good validation for predicting ice shape on a variety of geometries, and a strong initial capability of modeling ice forming on helicopters in forward flight.

To improve the understanding of the flow field within the NASA Glenn Icing Research Tunnel (IRT) with three different tunnel configurations, three-dimensional Reynold-Average Navier-Stokes (RANS) simulations were performed using the Menter-SST turbulence model. The 2000 tunnel configuration was simulated in the settling chamber from the spray bars to the test section. The 2009 tunnel configuration was simulated with vertical struts and multiple Mod-1 air jets implemented using embedded velocity profiles. The 2012 tunnel configuration has a new heat exchanger which was modeled starting from the exit of the heat exchanger to the test section. The results described herein focus on the flow turbulence since this defines test section performance but also is used to improve uniformity of the Liquid Water Content (LWC).