An Assessment of LEWICE Roughness and Convection Enhancement Models 2019-01-1977

During aircraft design and certification, in-flight ice accretions are simulated using ice prediction codes. LEWICE, the ice accretion prediction code developed by NASA, employs a time-stepping procedure coupled with a thermodynamic model to calculate the location, size and shape of an ice accretion. LEWICE has been extensively validated for a wide range of icing conditions. However, continuing improvements to LEWICE predictive capabilities require better understandings of 1) the fundamental physics of turbulent flow generated by ice accretion roughness during an icing event and 2) the mechanisms responsible for convective enhancement of real ice accretion roughness. Recent experiments in the Icing Research Tunnel (IRT) at NASA Glenn Research Center have enabled significant insights into the nature of ice accretion roughness spatial and temporal variations. Other recent investigations have employed scans from the IRT to generate scaled test surfaces to investigate convection enhancement and skin friction interactions of flow over the surfaces with real ice roughness. For this investigation, the measurements of ice roughness characteristics, skin friction, and convection heat transfer enhancement are compared directly to predictions calculated using the LEWICE models. Issues associated with the LEWICE model for predicting laminar-to-turbulent transition are also addressed. The comparisons show that while the LEWICE models significantly overestimate equivalent sand-grain roughness heights and the local skin friction, the resulting LEWICE predictions of convection heat transfer generally agree within 25% of the experimental values for surfaces constructed with low thermal conductivity materials. At each step of the LEWICE model, methods to improve the predictions are provided.