An experimental study of wind-driven surface water transport process pertinent to aircraft icing
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Water transport behaviors will significantly influence the icing accretion process during glaze icing conditions. Many important micro-physical processes associated with water transport phenomena, such as film/rivulet formation on the flat and curve surface, surface waves generation, and interaction of runback liquid with local ice roughnesses, are still unclear. In order to elucidate the underlying physics of water transport behaviors under icing conditions, advanced experimental technique capable of providing accurate measurements on the wind-driven thin film/rivulet flows are highly desirable. A novel digital image projection (DIP) system is presented in this work. Using this new technique, a comprehensive experimental study was conducted to quantify the transient behaviors of the wind-driven surface water transport processes pertinent to aircraft icing problems.
DIP technique is a further development of digital fringe projection (DFP) technique. In contrast to project sinusoidal patterns, the digital projector projects a grid pattern with known characteristics onto test objects (i.e., water droplet/rivulet flows over icing accreting surfaces). The heights of 3D objects are linear dependent on the grid point displacements between the measurement images of a 3D shape and the reference image of a zero height substrate. Compared with typical DFP measurement system, the DIP technique can significantly reduce the measurement error as well as decrease the requirement of the measurement image quality.
After carefully calibrated and validated, the proposed DIP technique was applied to characterize the wind-driven water rivulet flows. Seen from measurement results, the transient motion of rivulet front was found to be significantly influenced by the surface waves' behaviors. The Force Balance (FB) rivulet breaking criteria is further refined and evaluated by the reconstructed tiny rivulet flow structures. Rivulet meandering phenomena and the water mass trapping induced by the meandered water-air contact line were observed. A model based on force balance analysis at the cross-section of meandering rivulet was built to illustrate the meandering instability of wind-driven rivulet flow.
In order to examine the effects of the roughness arrays on the surface film flow, i.e., trapped mass effects, which is pertinent to the surface water runback over airfoils/wings with ice roughness, the DIP technique was used to quantify the transient behavior of wind-driven film flow over a surface with roughness arrays. While surface water mass trapping was observed clearly right downstream of the roughness elements, some other interesting features about the water film flow within roughness elements were also revealed clearly from the quantitative DIP measurements, which were found to agree well with those previous numerical studies.
The water runback process on an airfoil surface was reconstructed by the DIP technique. The measurement results clearly revealed that, after impinged on the leading edge of the NACA0012 airfoil, the micro-sized water droplets would coalesce to form a thin water film in the region near the leading edge of the airfoil. The formation of rivulets was found to be time-dependent process and relies on the initial water runback flow structure. The film thickness scaling law is evaluated by the time-average measurements of the film thickness. The measurement results show good consistent with the analytical scaling predictions.