基于新型高频声学多普勒流速剖面仪的河口近底部边界层观测初探

A preliminary study on in situ bottom boundary layer measurement near the seabed of estuaries by using a novel high-frequency Acoustic Doppler Current Profiler

河口物质输运、能量交换与底边界层内的水动力过程密切相关,底边界层参数(如切应力、拖曳系数)的确定至关重要。挪威Nortek公司生产的新型声学多普勒流速剖面仪AD2CP相比传统ADCP具有高频、低噪的优点,可用于高频(16Hz)流速剖面观测,而被广泛应用于底边界层观测的ADV只能测量单点高频流速。本文采用AD2CP在长江口南槽最大浑浊带区域进行座底式观测,并与同步近底部三脚架上ADV的观测结果进行对比。结果表明,使用AD2CP测得的近底部平均流速与ADV的测量结果吻合良好;使用惯性耗散法计算了底切应力,基于ADV的单点高频流速数据计算结果为2.16×10^-2~5.69×10^-1 N/m^2,基于AD2CP的结果为2.09×10^-2~4.26×10^-1 N/m^2,二者范围大致相当。在此基础上,基于AD2CP数据计算出摩阻流速为4.55×10^-3~2.06×10^-2 m/s、底拖曳系数范围为1.84×10^-4~2.49×10^-3,与ADV的计算结果基本一致。此外,由于AD2CP可以获得高频的流速剖面数据,优于单点ADV,具备观测近底部边界层参数和边界层内湍流剖面的潜力。

Since the material transport and energy exchange in estuary are closely related to the physical processes in the bottom boundary layer(BBL),it is necessary to accurately determine the BBL parameters(e.g.bottom shear stress and drag coefficient).The Nortek's new Acoustic Doppler Current Profiler(AD2CP)has the advantages of high frequency,broadband and low Doppler noise compared with the traditional ADCP,it can be used in measuring high-frequency(16 Hz)velocity profile.The acoustic Doppler velocimeters(i.e.ADV)is widely used in BBL measurements,however it can only measure high-frequency velocity at a single point.Therefore,we applied an AD2CP in the turbidity maximum zone in South Channel of the Changjiang(Yangtze)River estuary,comparing with an ADV near seabed,by deployment of bottom tripods.The results show that the near seabed along-channel velocity of AD2CP agrees well with that of ADV.Based on high-frequency velocity,we used inertial subrange method to calculate the bottom shear stress.Result calculated from AD2CP data is during 2.16×10^-2-5.69×10^-1 N/m^2,and that from AD2CPs data is during 2.09×10^-2-4.26×10^-1 N/m^2,they are relatively equivalent.Besides,according to the result calculated from the data measured by AD2CP,the friction velocity range is 4.55×10^-3-2.06×10^-2 m/s,the bottom drag coefficient range is 1.84×10^-4~2.49×10^-3,which are consistent with those measured by ADV.In addition,according to the AD2CP can be used in observing high-frequency velocity profile,which is better than ADV,has excellent potential for BBL observation and measuring turbulence profile.