Features And Characterizations Of Large-Scale Three-Dimensional Structures In The Atmospheric Surface Layer

The understanding of the large-scale structures is still limited because of lacking high Reynolds numbers turbulent facilities and high resolution instruments.Therefore,in order to obtain high Reynolds numbers experimental data,we performed synchronous multi-point measurements for the three components of wind velocity,temperature and dust concentration in the atmospheric surface layer?ASL?,which is located at the Qingtu Lake Observation Array?QLOA?site in western China.After applying the pre-processing and data selection criteria,high quality data which can be used in the study of high Reynolds [Re_t ⁴⁶?10⁶?] wall-bounded turbulent flows were acquired.Based on the selected high Reynolds numbers experimental data,the large-scale three-dimensional structures were investigated quantitatively,including constructing a volumetric view,scaling the feature quantity,and characterizing the three-dimensional shape of these structures.Furthermore,the thermal stability effects are also analyzed.Some new results are obtained from the quantitative investigation.Firstly,we find that the inclination angle is dominated by the vertical velocity gradient?i.e.,vertical wind shear?.A non-dimensional is proposed to characterize the variation of the structure inclination angle.Secondly,the findings reveal the Reynolds number scaling of the spatial length and the general law of the scale growth over three orders of magnitude change in Reynolds number [Re_t ⁴⁶?10³?-?46??10⁶?].The outer-scaled spatial length scales are found to have Reynolds number invariance when the scale separation is adequate?Re_t > 2000?.The growth of the spatial length scales with the wall-normal distance shows a good collapse on outer-scaled axes for all the available data.The streamwise length scale exhibits a gradually slowing increase across the log region,while the spanwise and the wall-normal length scales display a piecewise linear increase throughout the boundary layer.Then,a characterization model of large-scale three-dimensional structures is proposed based on the quantitative analysis of the topological equivalent transformation of the two-dimensional?2D?‘slice'.The calculated results agree well with the experimental results for both the current ASL and the laboratory boundary layer.Finally,it is found that the large-scale structures change drastically under different stability conditions in the surface layer.In the unstable surface layer,the positive buoyancy has a ‘lifting' effect that altering the or ientation of the large-scale structures,enhancing the turbulent transport and momentum transfer,thus resulting in the relatively large spatial length and inclination angle.In the stable surface layer,the turbulent motions are suppressed by the negative buoyancy,the cold and denser air near the surface restricts the turbulent transport and momentum transfer,and resulting in the relatively short spatial length and small inclination angle.Furthermore,the spatial length and inclination angle varying systematically with the Monin-Obukhov stability parameter,increasing with the decrease of stability and follows a log-linear increase in the unstable regime.The present work contributes to a better understanding of the large-scale three-dimensional structures.