Ice Accretion Prediction for Small Unmanned Aircraft Conditions

While the icing problem has been considered extensively for manned aircraft, the key physical parameters that define ice accretion are vastly different in the UAS realm. The trajectory of droplets are moving in a significantly lower velocity, the wing is at a smaller scale, and the heat flux properties do not follow the assumptions in established icing models. The need for greater understanding of accretion physics at low speeds and low altitudes is obvious when considering the ways in which icing models for manned aircraft are unsuited for small UAS. Cylinder models are incredibly useful in that the stagnation region has the highest amount of geometry change due to ice and has the highest rate of heat transfer. SUAS are usually incapable of supporting anti-icing systems and may be required to fly in hazardous icing regions. This presents a straightforward subject for investigation. Ice accretion prediction for UAS scale airfoil and UAS flight conditions. In order to further focus the problem, the icing problem considered here will be limited to the stagnation region.

Numerical tools for icing research are vastly important, simply because of the fact that icing conditions are difficult to reproduce experimentally. Simulation methods contribute to icing knowledge as well as aid in test design and experimental validation. Current numerical tools are not verified under low velocities. The icing model developed is be suited for UAS using analytical methods suited for low velocities and empirically derived heat flux relations.

”The icing of a cylinder is a unifying theme in icing research because it represents a simple, well-defined icing problem for which a solution can be used to predict icing in more complex situations [Lozowski et al.].” Along this theme, cylinder icing will be considered for the low velocity, small airfoil, low altitude conditions expected for UAS. A numerical model and LEWICE simulations will be compared in conjunction with past experimental studies.

In order to further the understanding of the ice accretion at the range of Reynolds numbers, a simple model was written in MATLab specifically suited to the conditions expected. The code will calculate heat flux based on its atmospheric conditions and use empirical equations to get collection efficiency based on the flow’s amount of deviation from Stoke’s Law in equation below, where C_D is drag coefficient, and Re is Reynolds number. Though LEWICE is robust in its ability to adapt to large accretions and a large range of meteorological conditions, it is unsuited for first level SUAS icing investigation. The software is not open and it is difficult to manipulate the physical drivers with the changeable namelist files. Cylinder investigation specifically is unsuited for LEWICE because of the substantial difference in flow.

Because of some of the simplifications made in the ice thickness calculation, the resultant geometries are expected to have some error. The work done with collection efficiency and heat transfer advocate an accurate determination of the amount wet vs. dry icing on each simulation. The results of the simulations show that in low velocities a low level of accretion is likely. Wet icing will only form at temperatures close to zero and relatively high liquid water contents. Even when wet icing is present notable horn shapes are unlikely.

There are some immediate goals for this research that will take place in the next few months. These include extending the heat transfer tests on a vehicle with a robust setup. Additionally, but this apparatus and the wind tunnel will be characterized for free-stream turbulence. Once the environments are fully characterized, tests can be completed under a finer range of angles past stagnation line.