# FEM or CFD

Is a calculation with the "Finite Element Method" FEM or a "Finite Element Analysis" FEA sufficient or do you need a flow calculation with "Computational Fluid Dynamics" CFD or even both?

As can be read in the glossary article on FEM simulation and CFD simulation, the prerequisites are relevant where or how a component or assembly is used.

While a table is essentially subjected to purely static forces, it is a different story for a bridge.

Bridges not only have to bear their own weight and the forces generated by vehicles and their manoeuvres, but are also exposed to environmental loads such as wind loads, snow loads and water loads.

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A prime example and perennial favourite for the effects that unconsidered influences can have is the Tacoma Narrows Bridge. This bridge was already built in 1940 in an "optimised" design. That is, it was the slimmest bridge of its time in relation to its size. Shortly after its completion, it was discovered that it showed unacceptably high vibrations under certain wind conditions.

Due to the low stiffness and thus low natural frequencies, a certain wind speed that remained constant for a long time caused the periodically detaching vortices to cause the bridge to vibrate more and more until it finally collapsed.

This type of vortex formation is called Kármán vortex street. If the frequency of the detaching vortices meets a natural frequency of a bridge, this form of oscillation builds up more and more until the deformations and stresses become so high that failure occurs and the bridge collapses. This is also referred to as resonance frequency.

This is a very clear example of when CFD simulations can be usefully applied to buildings. Here, the CFD simulation provides the resulting wind loads based on the pressure distribution, but also the dynamic, transient behaviour of the flow loads acting on the structure.

The FEM or FEA simulation can determine the natural frequencies of the structure, but also calculate the static and dynamic loads of the structure as deformation, stress and strain and compare them with permissible values.

Another example that we all still remember is the train accident in Eschede.

An ICE, one of the most modern trains in the world at the time, derailed because a wheel tyre broke.

We will spare ourselves the details of the entire accident here and only highlight the part that is relevant from an FEM and CFD point of view.

The flow simulation is not needed to explain the damage. This is a purely structural-mechanical problem that can be calculated using FEM or FEA (finite element analysis).

The ICE Wilhem Conrad Röntgen was travelling at about 200km/h from Munich to Hamburg when a wheel tyre broke and drilled through the floor of a carriage. Due to a chain of further circumstances, this resulted in the largest railway accident in German history.

Since 1992, a new type of wheel had been used in high-speed rail traffic in Germany. A ring, a so-called wheel tyre, was placed on the wheel. In between was a 20mm thick layer of hard rubber. The reason for developing this type of wheel was that the railway's customers had been complaining about loud humming and vibrations for some time. In addition, the wagons could also be affected by the increasing vibrations. The reason for this was again the so-called monobloc wheel. A wheel that was made of one piece and transmitted vibrations directly to the wagons due to increasing wear.

One of the complaints about the new wheel tyres was that they had not been tested sufficiently. Among other things, not to the final wear limit. (Source: Wikipedia)

Rubber is also a material whose properties are highly non-linear, vary greatly with temperature and are subject to an ageing process.

Both the effects of wear and the material behaviour can be investigated in advance using FEM calculations.

However, there are also topics that are purely CFD flow topics. For example, floods during heavy rainfall, such as the flood of the century in the Ahr valley in Rhineland-Palatinate and North Rhine-Westphalia with more than 180 deaths on 14 and 15 July 2021, can already be simulated in advance. Torrential rainfall with 150 litres of rain per square metre had led to flooding that devastated entire areas.

Unfortunately, it is always catastrophes that drive developments, because here either the need suddenly increases or attention is first drawn to a problem. Today, we are a big step ahead in the field of developing and forecasting the behaviour of bodies under stress, but also of tidal waves and tsunamis, and we are working to ensure that such disasters - at least from a technical background - are prevented as much as possible.