Fluctuations in test sections of transonic wind tunnels have been investigated with the help of the hot-wire technique, developed by the authors. The data on the intensity and spectra of fluctuation modes (vorticity, entropy, acoustic) were obtained using the approach of the fluctuation diagrams. It is shown that acoustic waves generated by permeable walls make dominant contribution into fluctuations in blow-down wind-tunnels. The main part of disturbances in transonic wind tunnels with closed circuit results from the temperature non-uniformities due to the injection and evaporation of liquid nitrogen or due to the dissipation of mechanical energy from the fan.
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