Jean-Louis Laforte

New Roughness Computation Method and Geometric Accretion Model for Airfoil Icing

This paper presents recent developments in wet and dry ice accretion simulation at AMIL (AntiIcing Materials International Laboratory), in a joint project with CIRA (Italian Aerospace Research Center). This paper introduces an analytical model to calculate the surface roughness and the remaining, runback, and shedding liquid water mass on an airfoil surface. Three analytical formulations are used to calculate the local roughness height based on the maximum height that a bead can reach before moving and the wave height on a water film. A mass balance is used to determine the remaining and runback water masses when the water state and the maximum bead height are known. The water shedding mass is determined using a simple mass model. A new method is used to build the accreted ice surface on the airfoils. It uses the bisection of the angle between adjacent panels to determine ice shape. The new roughness computation method and the geometric model generate the complex ice shapes observed experimentally and the results agree well with icing profiles obtained in wind tunnel experiments.

Deicing Strains and Stresses of Iced Substrates

Abstract The performance of low ice adhesion or icephobic coatings might be improved by adding a mechanical energy component via an underlying low powered electro-deformable substrate. The strain could be generated with many types of smart actuators consisting of piezoelectric devices, shape memory alloys (SMAs), conductive polymers, or ionic membrane polymer composites (IMPCs). An important step in designing a new electromechanical deicing system would consist of measuring the level of strain needed within an iced substrate to break and shed ice at the interface. In this paper are presented the results of an experimental investigation, in which new set-ups were built and used to simulate the behaviour of an active iced electromechanical substrate generating three types of strains: tensile, torsion, and bending. A total of 174 icing/deicing tests were conducted with aluminum and polyamide test specimens covered with hard rime ice deposits 2, 5 and 10 mm thick and stressed at −10°C at various strain rates in the brittle ice regime. Real time strains and forces were precisely monitored using strain gages and load cells. The stress was calculated from the deicing strain ϵ deicing and force measured at time of deicing corresponding to an interfacial failure between ice and substrate and/or cohesive failure of ice. Under test conditions used, strains were found to be very similar in torsion and in tension but about ten times lower in bending. Moreover, stresses and strains at deicing were found to increase with increasing substrate roughness and decrease with increasing ice thickness.

The influence of electro-freezing on ice formation on high-voltage DC transmission lines

Abstract Supercooled droplets ranging from 15 to 38 μm diameter are accumulated on an aluminium conductor of 1 cm in diameter installed in the working section of an open wind tunnel. Density and adhesive strength of the ice are measured as a function of the applied field strength at the surface of the conductor. All experiments are conducted in a cold chamber under constant atmospheric conditions. The results obtained with ice formed from droplets of 20 μm mean diameter show that DC negative and positive electric fields above −10 and +15 kV/cm respectively decrease considerably the density, and consequently the adhesive strength of ice. Alternating fields show weaker effects upon ice density when compared with DC fields. The effect of the electric field upon ice density decreases with increasing size of the supercooled droplets. Alternating fields have practically no effect upon the density of ice formed from 38 μm droplets. An increase in the conductivity of water by the addition of a small quantity of NaCl results in an increase in ice density even in the absence of electric field. Ice accretions under DC electric fields contain many heavily branched ice trees. These ice branches present a converging structure under a positive field and a diverging one under a negative field. From the similarity of the shapes of ice branches and corona streamers, it appears that supercooled droplets freeze along the corona streamers due to the corona wind, which is caused by the evacuation of ionised gas particles.

PREDICTION OF 2D AIRFOIL ICE ACCRETION BY BISECTION METHOD AND BY RIVULETS AND BEADS MODELING

The paper presents recent developments in wet and dry ice accretion simulation at AMIL (Anti-Icing Materials International Laboratory) in a joint project with CIRA (Italian Aerospace Research Center). The thermodynamic model of ice accretion is similar to existing ones developed by LEWICE in USA, DRA in British, ONERA in France and Ecole Polytechnique de Montreal in Canada. However, this paper introduces an analytical model to calculate the surface roughness in the wet regime based on the residual, runback and shedding liquid water mass on an airfoil surface. Also, a new geometric model to build the ice surface, based on panel angle bisections, is presented. In the wet regime, the empirical LEWICE correlation used to determine the equivalent sand-grain roughness is replaced by two analytical formulations to calculate the local roughness height. The first one considers the maximal height that the bead can reach before moving, while the second computes the wave height on the water film. The maximum bead height before moving is determined from the equilibrium between aerodynamic, gravitational and surface tension forces. The bead behavior in dry and wet regimes was described analytically Based on the work of Al-Khalil and Hansman, which led to the determination of a water surface state (film, rivulets or beads). A mass balance is used to determine the residual and runback mass of water when the water state and the maximal bead height before moving are known. The water shedding mass is equal to the runback water mass for the lower surface and is zero for the upper surface of the airfoil respectively. A geometrical model based on panel bisection allows the ice growth in normal direction to the surface. This method simulates the ice surface accretion continually without gaps between panels. The accretion model is validated with icing profiles obtained experimentally in wind tunnel by Shin and Bond for a NACA0012 wing profile with a 0.5334 m chord, a 20 µm median volume droplet diameter, a 1 g/m³ liquid water content and a 65 m/s airspeed. These results cover both ice accretion regimes in the -4.4° to -28.3°C temperature interval. The roughness calculated analytically is in the same order of magnitude as LEWICE correlation. The use of analytical models for roughness generated the complex icing shapes (horn) as the ones observed experimentally. However, in most cases, the accreted ice was slightly bigger than the measured.

Tensile, Torsional and Bending Strain at the Adhesive Rupture of an Iced Substrate

In order to develop an effective deicing device using mechanical deformation of substrates, the adhesive and/or cohesive strains of ice at rupture were measured for three different modes of solicitation: tensile, twisting and bending. A total of 108 icing/deicing tests were conducted with aluminum and nylon samples covered with hard rime ice deposits 2, 5, and 10 mm thick strained at various strains rates in brittle regime at −10°C. Real time deformation was precisely monitored using a strain gage fixed to the A1 interface, and force by means of load cells and a torque-meter. Deicing strain was determined at the time of ice detachment, which corresponds to a visible, instant change in the slope of stress-strain curves. The mean values of deicing strains, e %, measured in tensile, torsion and bending experiments are respectively, 0.037 ± 0.015%, 0.043 ± 0.023% and 0.004 ± 0.003% As for adhesion strength, the highest values were obtained in tension, 4 MPa ± 50%, and the lowest in bending, 0.014 MPa ± 36%. In torsion, the value was intermediary, at 1.26 MPa ± 67%. Measurements also showed that deicing stress and strain tended to increase with substrate roughness, whereas they decrease with increasing ice thicknesses. In summary, this work points out the effects of two major factors on ice adhesion strength, the solicitation mode and the ice thickness. Finally these results suggest that the first criteria for a mechanical deicing device has to satisfy to be effective is to have the capacity to generating a strain at around 0.04% ice/substrate interface.Copyright

Prediction of Ice Shapes on NACA0012 2D Airfoil

The objective of this communication is to present the new capability at AMIL in ice accretion simulation on 2D Airfoils at low speed. AMIL, in a joint project with CIRA (Italian Aerospace Research Center), has developed a numerical model called CIRAMIL. This model is able to predict ice shapes in wet and dry regimes. The thermodynamic model used is similar to existing ones. The major difference is in the approach of calculating the surface roughness and the residual, runback and shedding liquid water masses on an airfoil surface. The numerical ice shapes are compared to rime and glaze shapes obtained experimentally in wind tunnel for a NACA0012 wing profile. The new roughness computation method generates the complex ice shapes observed experimentally in wet and dry regimes and the results agree well with icing profiles obtained in wind tunnel experiments and in many cases are better than those predicted by the models available.

Aerodynamic Flow-off of Type II and Type IV Aircraft Ground Anti-icing Fluids

The certification process for aircraft ground anti-icing fluids involves flat plate wind tunnel aerodynamic flow-off tests. This test method was developed in 1990 from flight and wind tunnel tests results on full scale and model airfoils, and flat plates; the resulting lift losses were then correlated to the Boundary Layer Displacement Thickness (BLDT) on a flat plate. This correlation was made for Type II fluids existing at the time. Since the introduction of Type IV fluids in 1994, with significantly longer anti-icing endurance times, the same test procedure was applied. However, Type IV fluids are generally more viscous than Type II fluids of the same concentration. At the FAAs request, a study was undertaken to see if aerodynamic certification testing should be different for Type IV fluids as opposed to Type II. After a comparison of existing certification BLDT data which showed no significant differences between Type II and Type IV fluids, aerodynamic tests were performed on five commercial fluids, two typical Type II and three typical Type IV fluids. Tests with different initial thickness showed that the thickness had little effect on BLDT and fluid elimination data, with the exception of one fluid, near the temperature at which it is acceptable. Examination of energy data showed that more energy was required by the wind tunnel to move the Type II fluids as opposed the Type IV fluids. When these fluids were tested using an identical fixed fan speed profile at 0, -10 and -25°C, little difference was seen in the BLDTs as compared to those generated by a certification profile adjusted to obtain the same acceleration. These preliminary tests suggest that the aerodynamic certification method developed for Type II fluids is adequate for assessing Type IV fluids.

Endurance and Aerodynamic Performance Certification of Aircraft De/Anti-icing Fluids

Three performance certification tests are required for the assessment of aircraft de/anti-icing fluids. The two first, measuring anti-icing endurance, consists of the Water Spray (WSET) and the High Humidity Endurance Tests (HHET). The third, for aerodynamic performance, consists of the Flat Plate Elimination Test (FPET). The three performance tests, used for both deicing and anti-icing fluids, are described in the annexes of AMS1424 and AMS1428. Since February, 2003 they are covered by aerospace standards AS5900 for aerodynamic, and AS5901 for anti-icing endurance performance. The WSET anti-icing endurance test measures the time that a fluid protects a plate, inclined at a 10° angle, subjected to freezing precipitation, from a specified amount of icing. This WSET time then defines the fluid type: it must exceed 3 minutes for a Type I, 30 minutes for a Type II and 80 minutes for a Type IV fluid. The aerodynamic FPET performance test involves measuring the Boundary Layer Displacement Thickness (BLDT) during fluid elimination on a flat plate in a wind tunnel at temperatures from 0 °C down to a possible –45 °C. Two take-off acceleration profiles can be simulated, one for large transport type aircraft, and the other for commuter-type aircraft. A fluid is considered acceptable at a temperature interval if its BLDT is below an acceptance criteria defined by BLDT measurements of a reference fluid tested simultaneously and the fluid elimination is below a given percentage depending on fluid type.