Correlation of skin friction and convective heat transfer on surfaces with realistic roughness variations.

Ice accretions can considerably degrade the in-flight performance and safety of an aircraft. Increases in aerodynamic drag and decreases in lift and stall angle of attack limit aircraft maneuverability, and can destabilize an aircraft during all phases of flight. During aircraft design and certification, in-flight ice accretions are simulated using analytical ice prediction codes. LEWICE, the ice accretion prediction code developed by NASA, employs a time-stepping procedure coupled with a thermodynamic model to determine the location, size, and shape of the ice that will form on a geometry of interest. LEWICE has been extensively validated for a number of ice accretion cases over a wide range of icing conditions, however, continuing improvements to its predictive capabilities requires a better understanding of 1) the fundamental physics of turbulent flow generated by ice accretion roughness during an icing event and 2) how those physics are accounted for in the LEWICE analytical models. Velocity boundary layer measurements were performed to characterize the skin friction and turbulent length scale development over ten surfaces with ice accretion roughness. Four of the surfaces were created from laser scans of real ice accretion roughness on a 21-in. NACA 0012 airfoil, and six of the surfaces were created with semi-deterministic roughness distributions and were meant to model various aspects of ice accretion roughness. The resulting skin friction and turbulent length scale data are presented and discussed. Additionally, the LEWICE skin friction and convection models were evaluated for each of the rough surfaces. The values predicted by the models are compared to experimental skin friction measurements presented in this investigation, and to experimental convection measurements presented in prior studies.