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Simulating multicomponent systems or, ever faster, more accurate, more realistic

Simulation: one wrong steering maneuver at excessive speed causes a truck to tip over. In this case it's the forces between the tires and the roadway that play an important role. (Start the simulation by clicking on the image.)

In order to properly design a system like a car, train, wagon or airplane, multicomponent simulation is now frequently being utilized: the actual system is reproduced on the computer with separate, interconnected parts like components, springs or absorbers. Forces are exerted on the individual components, which causes them to move. In turn, these forces are affected by the location and movement of the various components with respect to each other. All these interactions taken together result in the net movement behavior of the entire system. The equations which describe this behavior are generated by the computer automatically from information about how the system is constructed. Solving them makes it possible to predict how the system as a whole will move.

Simulation: in an accident, advantageously-shaped car parts can reduce harm to a pedestrian. This calculation requires modeling the mechanics of contact during the crash. (Start the simulation by clicking on the image.)

The SIMPACK program developed at DLR is one of these multicomponent simulation programs. You can enter almost any configuration of systems and almost any kind of rules for forces, from very simple formulas for springs up to complicated interrelationships like those describing the contact between tires and street, or between a train wheel and its track. Since these programs basically do nothing but calculate forces from the movement of the individul components, and from these forces the movement of the entire system, one can calculate almost anything that moves, if the relationships between the movements and the forces are known.

In principle a useful simulation has to satisfy two requirements:

• It must be precise enough, i.e., what one calculates should match reality as closely as possible.
• It has to be fast.

Research in the field of vehicle system dynamics is concerned with perfecting multicomponent simulations which meet these criteria.

In order to make the simulation even more precise, new descriptions of physical effects are being developed, for example, aerodynamic forces arising from the kind of fluid flow that occurs with aircraft or high speed trains, or forces which are exerted on wheels which are not rolling over a paved street but on soft ground like a meadow, or components which can be permanently deformed, like train wheels subjected to overloading.

Simulation: high-speed trains are very sensitive to the aerodynamic forces which arise with crosswinds or during train encounters (red: excessively high pressure, yellow: high pressure, green: neutral, blue: low pressure or suction). In the worst cases the train can even derail. Coupling various simulations, for example, multicomponent and flow simulations, makes it possible to estimate such risks.

In order to make the simulation run faster, new calculation methods are being developed. The equations which have to be solved in the simulation program are frequently so complex that they can only be solved iteratively, by testing and successively improving the result. New methodologies and algorithms are being developed which can solve also complex mathematical problems rapidly but nevertheless accurately and reliably.

Simulation: the elasticity of wheel sets affects the running behavior of rail vehicles. Taking into account the structural elasticity of parts is the latest trend in multicomponent simulations, now possible with access to more powerful computers.

Improvements in the simulation yield more and more realistic predictions about how a machine or vehicle is going to behave. In this way it is possible to test machines and vehicles even before they are built for any weaknesses or design errors, making them even safer, more reliable, and more efficient.