A large amount of the aerodynamic drag is produced by turbulence phenomena occurring in the near-wall layer. Their detailed identification and characterisation is a challenging task, due to the fact that the most important interactions occur within the very thin viscosity-affected sublayer close to the wall. This paper examines the above interactions by analysing DNS data for a compressible boundary layer at Mach = 2.3. The specific aim is to characterize and predict the effects of large-scale outer structures in the log-law region on the near-wall layer. At each wall-normal level, the turbulence field is separated into large-scale and small-scale motions using a two-dimensional variant of the "Empirical Mode Decomposition (EMD)". The response of the near-wall conditions to the large-scale structures in the outer flow is then investigated by a statistical analysis involving spectra and joint PDFs constructed from conditionally sampled data for the small-scale motions within the large-scale footprints. Finally, it is shown that a phenomenological model, designed to predict the "universal" turbulence field, free from the influence of large-scale motions, and originally developed for channel flow by Agostini & Leschziner , also holds for the present compressible boundary layer.
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