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