Large- and Very Large-scale Structures - Transport and Coherent Structures in Wall Turbulence - page 422

two-dimensional spectra and cospectra - particularly those
observed in the cospectra of uv (see Figure 6.19) - do not, a
priori , always indicate the existence of elongated structures.
Out of a variety of plausible examples, we can point to the
stochastic process

() ( )

where is the Dirac s x = δ x − x

∑

δ

i

i

function and

x are the positions of the Poisson pulses with

arrival rate . The spectral density of this process is λ

2

()

[PAP 84]. It contains the constant at all

2 πλ δ k + λ λ

x

wavelengths.