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Wavenumber Spiral FSAT: actuation mode simulation
Fig. 1 shows how the spiral FSAT geometry can be chosen so that the active wavenumber range corresponds to decoupled bands for the fundamental A0 and S0 Lamb modes, thus allowing for single-mode excitation. Tone burst excitation at different frequencies within one of these bands generates A0 or S0 radiation in different directions.	Fig. 1: Frequency-wavenumber dispersion relations for a 0.75 mm thick Al plate. The active wavenumber range associated to a typical FSAT geometry corresponds to decoupled bands for A0 and S0.
Illumination of an entire 2D region can be achieved by broadband excitation. For example, the active angular range associated with the FSAT can be sequentially scanned through a chirp actuation, as demonstrated by the simulation in Movie 1, or the area can be simultaneously illuminated in all directions with a pulse excitation. However, it is interesting to notice how the FSAT spatial filtering selects a different propagation wavelength for each direction, and hence different wave velocities due to dispersion.
Movie 1: Chirp excitation for sequential monitoring of a 2D area.
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