  This paper concerns the study of direct numerical simulation (DNS) data of a
wavepacket in laminar turbulent transition in a Blasius boundary layer. The
decomposition of this wavepacket into a set of "modes" (a basis that spans an
approximate solution space) can be achieved in a wide variety of ways. Two
well-known tools are the fast Fourier transform (FFT) and the proper orthogonal
decomposition (POD). To synergize the strengths of both methods, a hybrid
POD-FFT is pioneered, using the FFT as a tool for interpreting the POD modes.
The POD-FFT automatically identifies well-known fundamental, subharmonic and
Klebanoff modes in the flow, even though it is blind to the underlying physics.
Moreover, the POD-FFT further separates the subharmonic content of the
wavepacket into three fairly distinct parts: a positively detuned mode
resembling a Lambda-vortex, a Craik-type tuned mode and a Herbert-type
positive-negative detuned mode pair, in decreasing order of energy. This
distinction is less widely recognized, but it provides a possible explanation
for the slightly positively detuned subharmonic mode often observed in previous
experiments and simulations.
