Multi-scale Self-similarity Characteristics Of Coherent Structures In The Atmospheric Surface Layer

In recent years,the self-similarity characteristics of coherent structures at different scales have been gradually revealed and received more and more attention.However,due to the limitations of high Reynolds number numerical simulation and insufficient experimental methods,the multi-scale self-similarity under high Reynolds number conditions is rarely seen.The surface layer of the atmosphere is a typical wall turbulent motion with high Reynolds number.The observation and analysis of the surface layer flow in the atmosphere will help to understand the similar characteristics of the coherent structure.Three-dimensional velocity components and air temperature were obtained simultaneously by sonic anemometers to explore the scale-dependent self-similarity of coherent structures in the atmospheric surface layer(ASL)at high Reynolds numbers wall-bounded turbulent flow.First,through the statistical analysis of the non-neutral data obtained from the Qingtu Lake field observation array,it is found that the statistical characteristics of turbulence under the non-neutral stratification condition are consistent with the existing theoretical results,which confirms the stratification data can be used for the study of non-neutral stratification flow.This thesis carried out the study of the self-similarity characteristics of the coherent structure in the flow field with different stratification stability and different roughness under the atmospheric surface layer.The observed experiment suggests that the self-similar wall-attached structures indicate the Aspect Ratios between streamwise,wall-normal and spanwise directions as 14:1:1 under near-neutral and unstable conditions.It also permits us an opportunity to focus on the scale-dependent structure inclination angles under different stratification stability conditions.Smaller scale tends to be at a steeper angle when affected by the positive buoyancy.Secondly,this paper reveals the scale-dependence of the inclination angle of the coherent structure for the first time.It is found that the inclination angle of all structures increases by the effect of buoyancy,the increase in the inclination angle of small-scale structures is more significant.Based on this,this paper establishes a quantitative characterization model of structural inclination considering scale-dependence under different thermal stability conditions,which can be used to improve the existing near-wall model.This finding also means that it is not sufficient to judge the self-similarity between different scales under unstable stratification only based on the aspect ratio,because the structural inclination angles of different structures have also changed at the same time.Finally,this paper finds that the temperature field under unstable stratification in atmospheric flow also has large-scale structural characteristics similar to turbulent coherent structures.The scales of the temperature field based on the autocorrelation of temperature fluctuations decreases with the increase of thermal buoyancy,which is qualitatively contrary to the law that the turbulent structure in the flow field with the increase of thermal buoyancy.Moreover,the structures of the temperature field increase linearly with height.The coherent structure of temperature fluctuations also has the characteristics of self-similarity with different scales,and the aspect ratio of streamwise to wall-normal directions decreases with the increase of thermal buoyancy but increases with the increase of roughness.