Coherent structure and energy distribution of atmospheric surface layer turbulent flow over a barchan dune in Minqin, China

The coherent structure and energy distribution of turbulence are major concerns in the research of turbulence dynamics. The present work conducts field measurements of atmospheric surface layer (ASL) turbulent motions on the windward slope centerline of an individual barchan dune located in Minqin, China. On the basis of comparing basic turbulence statistics (including the mean wind velocity, streamline angle, turbulence intensity and Reynolds stress) on the dune windward slope with existing results to confirm the reliability of our experimental methods and observation data, the present work analyzes the variation patterns of turbulent coherent structures and energy distributions along the windward slope. The results reveal that low-frequency (large-scale) turbulent motions increasingly dominate the airflow as turbulence evolves along the windward slope toward the slope crest, as reflected in the gradual increase in structure scales (the coherence scale and the largest energycontaining structure scale) along the slope and the more significant energy contribution of low-frequency coherent structures along the slope. The variation in turbulent kinetic energy distribution between frequencies (scales) makes the self-organizing state of turbulence change significantly along the dune windward slope; although the second-order streamwise structure functions still satisfy the Kolmogorov's two-thirds law in the inertial subregion of turbulence, the reduced energy-containing region causes the natural logarithmic scaling law of second-order structure functions to gradually to become insignificant along the windward slope, implying that the effects of favorable pressure gradient and airflow acceleration diminish the suitability of the attached eddy hypothesis as the streamwise position approaches the slope crest. The present work is the first to analyze the coherent structures and energy distribution characteristics of ASL turbulence on the windward slope of a natural barchan dune, which may provide assistance in gaining comprehensive insights into turbulent motions over dune terrains.