Ice accretion is a primary operational flight hazard for which all FAA certified aircraft are evaluated. Due to the geometric limitation of icing research tunnels and the required icing studies performed prior to flight tests, a recommended scaling method by Anderson and Tsao is utilized to adapt icing tunnel parameters (such as the flow temperature, the test section flow velocity, the water droplet median volume diameter (MVD), the liquid water content (LWC), and the ice accretion time, τ, etc.), along with model scaling to ensure the ice accretion collected is representative of the full-scale ice formation. Previous results have indicated good geometric agreement between the ice accretions formed on a full-scale model and the ice accretions on a scaled model, using the recommended scaling method under Title 14 of the Code of Federal Regulations (CFR), Part 25, Appendix C envelope. However, with the addition of the Appendix O envelope, which includes a large range of Supercooled Large Drop (SLD) conditions, the scaling method needed to be revised. SLD conditions refer to large median volume diameter (MVD) droplet conditions ranging from 50μm to greater than 500μm in the Appendix O envelope and generate larger frontal ice shape variation along with larger ice accretion further aft on a model.
Prior work by Anderson and Tsao evaluating the use of the ice shape scaling method for a limited range of SLD conditions, up to 190μm, was based on geometric similitude of the frontal ice shape between the full-scale and scale collected ice accretions within allowable tolerances. The geometries of the full-scale and scale model collected ice accretions show significantly greater geometric variation under SLD conditions than Appendix C conditions, especially in the feather region, the ice accretions aft of the main formation on the leading edge. This brings into question the scaling methods ability to preserve the aerodynamics associated with the full-scale ice shape including changes in aerodynamic coefficients and its associated trends, and the mean and fluctuating velocity components in the flow field. The presented investigation provides insight into the current scaling method’s effective use for SLD conditions based on aerodynamics through experimental evaluation of the lift and drag coefficients, the mean and fluctuating components of the leading edge and near wake flow fields, and the coherent structures present in these same flow field regions.
The computation software, LEWICE, predicts the ice formation on aircraft surfaces under a wide range of icing conditions. Literature exists on evaluation of LEWICE simulated ice accretions based on geometric similitude. However, minimal research has been performed to evaluate and verify the preservation of aerodynamics by the LEWICE predicted ice accretions, especially under SLD conditions. The LEWICE software does not predict any feather region ice accretions. As such, significant differences are seen between the ice accretions from experimental investigations and the ones predicted by LEWICE under specified SLD conditions. The research presented in this thesis provides insight into the LEWICE software’s ability to predict an ice shape which is capable of recreating the aerodynamics and aerodynamic penalties associated with icing under SLD conditions. This includes experimental evaluation of the lift and drag coefficients, the mean and fluctuating components of the leading edge and near wake flow fields, and the coherent structures present in these same flow field regions.
The experimentation performed for aerodynamic evaluation was conducted at the University of Dayton Low Speed Wind Tunnel (UD-LSWT). Force-based experimentation was performed to study the lift and drag coefficients of each ice shape across a range of angles of attack. This allowed for deviations in stall angle, zero-lift angle of attack, and drag increase from a non-iced baseline NACA0012 to be compared across the three different ice accretions: the LEWICE ice shape, the full-scale ice shape, and the scaled ice shape. Ice shapes collected with two different stagnation freezing fractions (n_0=0.3 and n_0=0.5) were compared through the experimentation in the UD-LSWT. The stagnation freezing factor has a large effect on the geometry of the ice shape. Therefore, evaluating ice shapes collected at different stagnation freezing fractions allows for significantly different horn shapes to be studied for verification.
Particle Image Velocimetry (PIV) was performed on the leading edge and near wake regions of the iced airfoils to evaluate the flow dynamics of the ice accretions. The flow field dynamics were compared based on evaluation of the mean and fluctuating components of velocity. The mean component was studied through evaluating the momentum deficit in the wake, the flow separation on the upper and lower surfaces of the airfoil, the mean streamwise velocity, and the vorticity. The fluctuating component was studied through Reynolds stress. Along with these flow field parameters, a modal analysis of the flow fields was performed using Proper Orthogonal Decomposition (POD) to evaluate the presence of coherent structures in the flow and their corresponding length scales. This allowed for the LEWICE software and ice shape scaling method to be evaluated for the preservation of the aerodynamics associated with the full-scale ice shape to be evaluated.
The results of the low Reynolds number aerodynamic evaluation showed that ice accretions gathered on the scaled model inconsistently replicates the aerodynamics created by the ice accretions in the full-scale model for the given conditions. The stagnation freezing fraction was the primary difference between the scaled ice accretion that successfully replicated the full-scale aerodynamics, as in the case of n_0=0.3, and the scaled ice accretion that did not, as in the case of n_0=0.5. This suggests a possible stagnation freezing fraction constraint of the scaling method’s use for SLD conditions. The LEWICE software consistently predicted an ice shape that was aerodynamically uncharacteristic of the full-scale ice accretion, therefore failing aerodynamic verification for the SLD conditions studied.