Features

The formation of ice over lifting surfaces can affect aerodynamic performance. In the case of helicopters, this loss in lift and the increase in sectional drag forces will have a dramatic effect on vehicle performance. The simulation of rotorcraft flow fields is a challenging multidisciplinary problem that lags in development over its counterpart in the fixed wing world by more than a decade. Successful aerodynamic simulation of a rotor/fuselage system requires the modeling of unsteady three-dimensional flows that include transonic shocks, dynamic stall with boundary layer separation, vortical wakes, blade/wake and wake/wake interactions, rigid body motion, blade deformations and the loss of performance caused by ice accretion.

(Photo Courtesy U.S. Army)

Stand-alone ice accretion prediction tools, as well as ice accretion fully integrated with aerodynamics, currently exist for 2D airfoils and 3D aircraft configurations. Ice accretion predictions are typically two-dimensional in nature and based on the classical Messinger model. The analysis consists of four critical steps: flow-field calculation, water droplet impingement calculation, heat transfer prediction, and ice accumulation normal to the surface.

LEWICE and LEWICE3D

LEWICE is NASA's flagship code for 2D ice accretion prediction, and it is the core of the 3D ice accretion tools as well. LEWICE development was initiated in the early 1980s, with the first general release (Version 1.0) in 1991. Four major updates to the code followed, in 1993 (Version 1.3), 1995 (Version 1.6), 1999 (Version 2.0) and 2002 (Version 2.2). Recent updates were released in 2005 (Version 3.0) and 2006 (Version 3.2), and a mixed-phase modeling capability was added in 2008.

The code uses a potential panel method to determine the flow field about a clean surface, then calculates water droplet trajectories from some upstream location until they impact the surface or until the body is bypassed. Collection efficiency is then determined from the water droplet impact location pattern between the impingement limits. A quasi-steady analysis of the control volume mass and energy balance is performed next using a time-stepping routine. Density correlations are used to convert ice growth mass into volume. LEWICE also features multiple drop size distributions, multiple airfoil elements, thermal models for anti-icing/deicing systems, and an interface with structured grid codes, allowing the use of viscous Navier-Stokes flow solutions.

The thermal models in LEWICE combine the features of previous codes, LEWICE/Thermal and ANTICE, to simulate de-icing and anti-icing with electrothermal or hot air systems. Features are included to allow determination of optimized heater sequencing (for electrothermal analysis) and multiple boundary conditions (for bleed air analysis).

LEWICE has been thoroughly validated for a wide range of conditions, with a database of over 3,000 ice shapes on 9 different geometries. Validation has been documented in numerous papers as well as NASA reports. The validation database lies mostly within the Appendix C continuous maximum or intermittent maximum envelopes, but there are some exceedance and supercooled large droplet conditions for comparison as well. This validation, along with significant research into recommended test methods and advanced component models, has led to a degree of acceptance for use in reducing the cost of development and certification programs.

However, this acceptance does not exist for rotary-wing applications.

LEWICE does not simulate a fully rotational system, but does allow the user to input a number of simple parameters-distance from the hub to the 2D section of interest, rotation speed, and orientation of the plane of rotation (vertical for propellers, horizontal for rotors). LEWICE performs three additional calculations in rotating body cases. The rotation speed is used to calculate an increase in the aerodynamic heating term in the energy balance, the rotational force is included in the ice shedding determination, and the rotational force is used to find the resultant force of the shed ice particle, which is then used to track the particle after it is shed. The rotating body information is not used by the potential flow solver in LEWICE, nor is the rotating body information used by the trajectory equation.

Figure 1. Methodology for Developing a 3D Ice Shape.

LEWICE3D is a suite of codes, developed by NASA and used widely by industry, to determine the amount and location of ice accretion on an aircraft. It is used to calculate water loading on aircraft surfaces so that the size and location of ice protection systems can be determined, to optimize the placement of icing sensors, and to determine ice shapes used in failed ice protection system tests. It is also used to determine corrections for cloud measurement instruments, such as droplet size probes or liquid water content probes on NASA research aircraft.

LEWICE3D uses a Monte Carlo-based collection efficiency calculation using droplet impact counts. Trajectories are calculated using an Adams-type predictor- corrector method. Tangent trajectories and collection efficiencies for simple 2D or 3D regions can also be calculated using a modified version of the 2D LEWICE method. Streamlines are calculated using a 4th-order Runge-Kutta integration scheme.

The ice growth methodology in LEWICE3D uses a single time step strip approach and requires a steady or time-averaged flow solution, supplied by the user. The strip approach is based on the classical Messinger energy balance procedure with an integral boundary layer technique used to generate heat transfer coefficients, and is a modified version of the method used in the 2D LEWICE code applied along streamlines. LEWICE3D supports multi-block structured grids, adaptive Cartesian grids and unstructured grids, as well as panel-based binary-tree grids.

LEWICE3D includes extensions which allow generation of a full 3D ice accretion for surfaces and generation of a new iced surface, calculation of off-body concentration factors, and determination of shadow zones. The program has been parallelized using OpenMP and MPI (message passing interface) to complete jobs faster on parallel machines. The parallel version has been ported to SGI and Linux machines.

However, the current NASA icing codes, LEWICE and LEWICE3D, cannot be applied directly to the ice accretion of rotorcraft flows for several reasons. These codes are acceptable for the majority of fixed-wing applications, such as general aviation, business jets or commercial transports, but there are still shortcomings for some vehicle types, notably rotorcraft.

Past Research in Rotorcraft Icing Codes

The importance of rotorcraft airfoil oscillation during ice accretion was recognized in the early 1980s and NASA sponsored an icing wind tunnel test of a 2D six-inch chord oscillating airfoil. This Sikorsky-run test confirmed that the variation in angle of attack altered the ice shape and produced changes in the drag coefficient. About this same period of time, Sikorsky and the United Technologies Research Center designed, fabricated, and tested in dry air 2D and 3D models with a chord of 17 inches. Meanwhile, as part of an earlier project, Sikorsky designed and fabricated an airfoil test rig to span the NASA Icing Research Tunnel (IRT) test section. This apparatus had a chord of 15 inches and mounted to the upper and lower IRT turntable. Maximum angle of attack for the airfoil was 10 degrees at the maximum operating speed of the IRT of 250 knots (with the model installed and with ice on the model leading edge), with an angle of attack of at least 20 degrees at 150 knots in the IRT.