Direct mode–mode coupling observation in the fluctuations of nonstationary transparent fluid

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Abstract
An experimental method by which a given time and space fluctuating medium can be qualified as equilibrium (laminar), or turbulent, is presented. It employs an optical observation to obtain directly the complex amplitude of the space Fourier transform of mass density distribution in a transparent gas. This observation is provided by coherent Rayleigh scattering and heterodyne detection. Several Fourier modes with different wavenumbers and frequencies are observed simultaneously. Their nonlinear quadratic mode–mode coupling is analyzed by using third‐order moment statistics and the full space‐time bispectrum. An experiment using this double (optical and numerical) technique on the air density fluctuations in a round free‐jet is reported. Significant nonlinear coupling is observed in the mixing layer and in the fully developed turbulence zone.