The Modelling of Lightning Strikes

Lightning strikes most commonly occur in clouds: either inter- or intra-cloud or cloud-to-ground. The simulation of indirect lightning effects on structures with metallic shells can be simulated effectively using CST MICROWAVE STUDIO® (CST MWS). Surface current distributions can be calculated and the coupling into shielded structures predicted.

The fast rise time and slow decay of typical lightning signals is typically simulated by a double exponential type waveform [1]. This may easily be implemented in CST MWS using a user-defined excitation as the reference signal. The signal is applied between the boundary and the impact point on the structure, in this case the nose of an aircraft, as shown in Figure 1. The discharge channel is modelled by placing a 300 Ohm lumped element resistor between the tail of the airplane and the boundary, which is fully electric to provide a closed current path.

A time-domain H-field monitor was used to record the surface currents due to the lightning strike. The current density is shown in Figure 2. A rapid rise in current is followed by a slower decay. The current peak at the impact point on the nose is expected, but the high current density along the inner front wing edges, the tail fin and at the tail tip may be of concern for EMI mitigation.

The objective of this article was to demonstrate how a user defined signal can be employed to simulate a lightning strike in the time-domain using CST MWS. The actual indirect effects, i.e. the coupling of the resulting surface current into a shielded structure, are of great interest but are beyond the scope of this article. Another potentially interesting application is the analysis of heating of the structure due to dielectric or surface losses. This could be calculated in the same interface using the link to CST EM STUDIO™'s thermal solver.

Note that the aircraft model used is full scale: both its length and wingspan exceed 25 metres. Despite this size, the simulation using the transient solver with a hexahedral mesh (FPBA) took only about 15 minutes to run on a standard 3 GHz desktop PC with 2 GB RAM.

References:

[1]  Buccella, C,  S. Cristina and A. Orlandi, "Frequency analysis of the induced effects due to the lightning stroke radiated electromagnetic field," IEEE Trans. EMC, Vol. 34, pp. 338-344, 1992.