  We analyze the primitive variables of fluid flow and scalar fields through
fast Fourier transform (FFT) in the near and far wake of an elliptic cylinder.
Numerical simulation of flow and scalar fields behind an elliptic cylinder of
axis ratio 0.4 at a Reynolds number of 130 is performed. The semi-major axis of
the elliptic cylinder is kept perpendicular to the incoming flow, where the
fluid flow is two-dimensional and the Prandtl number is 0.71. The scalar is
injected into the flow field by means of heating the cylinder continuously. The
simulation is run for a long time to show that the secondary vortex street is a
time-dependent phenomenon. Three distinguishable flow and scalar regions are
observed in the wake of the cylinder. This study reveals the presence of
low-frequency structures besides the primary shedding structures in linear,
transition and saturation regions of temporal wake development. We show that
the spectral source of the primary frequency is the saturated state of the
temporal wake development, while its physical source is the periodic
arrangement of structures of primitive variables, which inhibits the
transmutation of their wavelength. On the other hand, the secondary low
frequency is embedded in the transitional developing stage of the wake and its
physical source is the chaotic behaviour of the transition process, which aids
in the transmutation of the wavelength of the structures. Our spectral analysis
also reveals that the scalar is predominately carried by the streamwise
velocity and the pressure throughout the wake.
