Transient growth of finite three-dimensional perturbations in wall-bounded laminar shear
flows has been extensively studied theoretical, numerical and experimentally. With regard
to the experimental work, the vast majority of the investigations have been focused only
on the behaviour of the streamwise velocity component, u', despite the importance of
the spanwise gradient of the wall-normal velocity component, [Symbols appear here. To view, please open pdf attachment], as the forcing
agent. The present study provides a step forward in the experimental characterisation
of transient growth providing, by Laser Doppler Anemometry, simultaneous and highly
spatially resolved measurements of both streamwise and wall-normal velocity components
of three-dimensional disturbances embedded in a two-dimensional laminar boundary layer.
The disturbances are deterministic, as they are generated by a spanwise array of cylindrical
roughness elements. The readings are concentrated in both the wake and u'-energy growth
regions. Contrary to non-normal operator theory, which describes transient growth as
a linear mechanism, a spectral analysis in the present study demonstrates that there
exists an underlying non-linear mechanism. This non-linear behaviour is attributed to the
influence of the wake in the u'-energy growth region. Numerical simulations considering
non-linear terms confirm this conclusion. Both spectral and wavelet analyses have been
conducted, providing information regarding the spatial location of the components of
the perturbation as well as the relationship between the scales of motion. The numerical
simulations are also employed as a workbench to design actuators for open-loop control
of transient growth.