A measuring method for three-dimensional turbulent velocities based on vector decomposition and synthesis

Study on the turbulence velocity and intensity distribution structure of a fluid flow containing sediment is essential original conductive research of natural river dynamics in a lab [1-2]. It mainly depends on development of measurement techniques and the authenticity and accuracy of historical turbulence records during experiments to understand the fluid flow turbulence velocity. Throughout the development of the current velocimeter (turbulence velocity measurement in sediment content flow), a silicon piezoresistance velocity meter and Laser Doppler Velocimeter (LDV) which could be used only in very low sediment content flow are widely used. The former adopts the intrusive contact measurement and could be used in hyper-sediment flow under conditions with a certain acceptable interference to the flow and needs to be a dedicatedly designed and manufactured shape and size to minimize error. Meanwhile, the transducer also must be calibrated by a specially designed calibration system to make its outputs strictly correspond to the flow in a layer flow situation, and the pipe pulse jet flow calibration must be set to calibrate its resonant frequency in order to ensure its dynamic performance [3-4]. LDV adopts a non-contact measurement method that could only be used in transparent water flow or very low sediment content flow, but not hyper sediment content fluid [5-6]. As is known, Acoustic Doppler Velocimeter (ADV) is another intrusive transducer used to measure flow velocity [7-8], but ultrasonic sensors could not precisely identify the flow or water movement due to the echoed sound intermittently coming from a complex flow unit, and regularly the size and shape disturbed the flow too much. PIV is often used for the validation of Reynolds-averaged Navier–Stokes (RANS) computations and the study of high Reynolds number flows where the direct numerical simulation (DNS) of the governing equations is not feasible [9-10]. It has been a bottleneck to measure three-dimensional turbulence velocity in flow with hyper-sediment content for many years since 1989 [11-12] due to the two features of the hyper sediment content fluid flow: (1) non-transparence, which hinders the use of LDV and the feature, and (2) concessional disturbance to the flow, which makes it difficult to find a structure as an intrusive transducer to sense the 3D flow velocity within an acceptable sensor size and shape. Until now it has been a puzzle to find a structure to design the transducer [12]. Meanwhile, the above measurement equipment are mostly used for one-dimensional and twodimensional turbulence velocity measurement of the flow. The reason why there is a limitation for measuring 3D turbulence flow is that the precision of the volume of the sensing unit and the sensitivity of the sensor are so strictly required. The sensing part is the soul of a transducer which converts the measured physical information to electronic signals. A well-known method to measure turbulence velocity is directly sensing the flow force, which had been practiced since the 1950’s [13-14] until the time the 2D sensor was designed and tested in the sediment content fluid flume in The State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University in 1995 [15-16]. But it has been difficult to measure threeABSTRACT