Researchers are relying more and more on a combination of flight testing, wind tunnel testing and computational analysis to see how aircraft designs and ice-protection systems will react in icing conditions. Computer simulation has seen the most advances in recent years. Faster computers, better algorithms, and advanced software let engineers and scientists more accurately model and understand ice buildup, evaluate changes in aircraft performance, and analyze ice-protection systems. There are four basic steps to the simulation. Engineers first calculate the flow field surrounding the aircraft. With respect to the rather loose accuracy requirements for flow calculations used to simulate ice, potential flow codes can be used to evaluate airflow, which do not need a grid for the field surrounding the body which had to be updated after each iteration. These data are used to then determine the trajectories of water droplets in the flow field and where those drops hit the aircraft. The next step is calculating how much water along the body surface has turned to ice. To do this, engineers divide the surface of the aircraft into small surface elements and perform a mass and energy balance on each element. This result is used in the final step which calculates the shape of the ice, thus generating the ice surface for the following time step. This four-step process is repeated until it replicates the entire time the surface is exposed to icing conditions. Engineers then use the shapes generated from such simulations to evaluate changes in aircraft performance using flight test or wind-tunnel testing. Ice shapes are simulated by dummies attached to a wing or a wing model to be checked out in wind tunnels. Engineers also simulate the effects of ice on performance CFD tools. However, unlike the rather loose requirements on accuracy for flow calculations used to simulate ice growth, determining the changes in aerodynamic parameters such as lift and drag requires a higher degree of fidelity. Details of the grid surrounding the highly irregular geometry, including flow over rough surfaces and flow separation at low angles of attack in some regions are to be accounted for as well as unsteady, completely separated flow at or near maximum lift conditions. These requirements push the limits of most CFD tools and are the subject of current research. Computational methods are also used to simulate ice-protection systems, but they currently cover only thermal systems. The most common type of thermal ice protection is the hot-air bleed system, where heat from the engine is piped to the area behind the wing's leading edge or engine cowling, prime spots for ice formation. The heat prevents ice from forming and water can be heated to the point that it does not freeze on aft portions of the aircraft. Mechanical deicing systems use various schemes (e.g. inflatable rubber boots on the leading edge of the wings) to mechanically debond ice from the wing and let aerodynamic forces sweep it away. Chemical systems apply freezing-point depressant fluids to the wings, thus preventing ice from forming if temperatures aren't too low. The simulation methods described here require verification (determining whether the equations are properly represented in the computer code) and validation (evaluating whether the equations properly simulate the physics). At NASA, the development of validation databases has been undertaken using the Icing Research Tunnel (IRT) and an Icing Research Aircraft (IRA). Engineers have used the IRT to validate data for water-droplet trajectories and ice shapes, and to evaluate iced aircraft performance and various ice-protection systems. Built at the end of World War II and since then updated with computer controls, a larger fan motor, better fan blades, and electronic data collection, this closed-loop subsonic wind tunnel generates airspeeds from 50 to about 400 mph and temperatures as low as -30 deg F. The IRA is a specially equipped DeHaviland DHC-6 Twin Otter. It has been used for many purposes including characterizing clouds that tend to create ice, evaluating cloud instrumentation and ice-protection systems, gathering data on ice shapes, and cataloging performance changes due to ice buildup. Cloud instrumentation carried on the IRA measures water-droplet size and quantity, ice particle size and quantity, as well as total water content of the cloud.


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    Title :

    Simulating ice


    Additional title:

    Numerische Simulation der Eisbildung an Tragflächen


    Contributors:

    Published in:

    Machine Design ; 77 , 16 ; 64-67


    Publication date :

    2005


    Size :

    4 Seiten, 7 Bilder



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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