高风速相干结构对通量输送影响的实验研究

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送.

Research on Relationships between High-Speed Coherent Structure and Turbulence Flux

The presence of coherent structures in turbulent shear flows is a great finding in turbulence research, which shows the orderly flows in turbulence which seems fully random. This article analyses the high-speed coherent structure in unstable PBL using the data measured at Pukou, Nanjing. These data include wind velocity measured by the sonic anemometer at the heights of 40 m and 2 m, and the vertical profiles of wind velocity measured with a Doppler wind-profile radar. The authors analyse the fluctuation velocity by employing wavelet transforming at the time scale of 400 s, and the high-speed coherent structure is recognized and located using the threshold value. By comparison with the corresponding wind profiles acquired by radar, these coherent structures satisfy our convention understanding which has long time scales (more than 5 min) and large vertical scales (about thickness of PBL). The nondimensional interval between the coherent structures for the three days is about 6, which means that at the distance of six thickness of PBL, one coherent structure can be found. The wavelet coefficient of vertical fluctuation velocity is correlated with the corresponding wavelet coefficient of horizontal fluctuation velocity, which shows that the downward vertical velocity apparently exists in the high-speed coherent structure, this result is similar to the “Gustiness” theory in earlier research. Sweep motion and ejection motion are two dominant types of turbulence motion, which transport minus momentum flux. The high-speed coherent structure influences significantly the momentum flux by promoting the sweep motion and restraining the ejection motion.