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Three-dimensional (3-D) quasi-instantaneous acoustic Doppler velocity profiles at the center of uniform, turbulent open-channel flow over smooth and rough beds have been analyzed for the dynamics of coherent structures. The qualitative aspects of simultaneously measured Eulerian velocity and shear stress signatures identify coherent structures in the water column. A cumulant discard method is applied to describe the statistical properties of the covariance terms u'w' along the mean flow and v'w' across the mean flow relative to their time means. Conditional statistics and conditional sampling are used to compare the theoretical and experimental relative covariance contributions from the four quadrants in the longitudinal and transverse planes. The results in the (u', w') plane show the dominance of ejections (quadrant 2; u'<0, w'>0) and sweeps (quadrant 4; u'>0, w'<0). In contrast, the distribution of fractional v'w' events in the transverse plane is quasi-uniform over the four quadrants. Based on these experimentally determined statistical properties of the covariance terms in different flow conditions, a simplified form of the vertical turbulent energy flux in the intermediate flow region is given and the concept of wall similarity in turbulent boundary layers is validated. Since the validity of the wall similarity concept over a wide range of bed roughness has been shown, it is proposed to determine the mean bed friction velocity from the evaluation of the vertical turbulent energy flux 相似文献
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The present experimental investigation focuses on the characteristics of near bed turbulence in a fully rough, uniform open-channel
flow over a gravel-type bed. Due to bed topography small scale heterogeneity, the flow is not uniform locally in the near
bed region and a double averaging methodology is applied over a length scale much larger than the gravel size. The double-averaged
Turbulent Kinetic Energy (TKE) budget derived in the context of the present flow over a gravel bed differs from the TKE budget
written for flow over a vegetation canopy. The non-constant shape of the roughness function measured in our gravel bed leads
to an additional bed-induced production term which is null for vertical roughness elements, such as simplified vegetation
elements.
The experimental estimation of the terms of the TKE budget reveals that the maximum turbulent activity takes place away from
the reference plane, near the roughness crests. However, within the interface sublayer the work of the bed induced velocity
fluctuations against the Reynolds stress is of the same magnitude as the main turbulence production term. Consequently, the
characteristics of the TKE budget have similarities with uniform flows over canopies and strongly differ from uniform flows
over smooth and transitionally rough flows over sedimentlike beds. 相似文献
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David Hurther Peter D. Thorne Mickaël Bricault Ulrich Lemmin Jean-Marc Barnoud 《Coastal Engineering》2011
The use of acoustics to measure sediment transport boundary layer processes has gained increasing acceptance over the past two decades. This has occurred through the development of increasingly sophisticated measuring systems and theoretical developments, which have enabled flow and suspended sediment parameters to be obtained from acoustic data with a high degree of accuracy. Until relatively recently, separate acoustic systems were used to measure flow and suspended sediment concentration. Over the past few years, however, the technology has become sufficiently advanced so that flow and sediment measurements can be integrated into a single system. This integration provides, quasi-instantaneous, non-intrusive, co-located, high temporal-spatial resolution measurements of benthic flow and sediment processes. Here the development of such an instrument, the Acoustic Concentration and Velocity Profiler (ACVP) is described. The theory underpinning its application is outlined, new approaches to velocity de-aliasing and suspended sediment inversion instabilities using multi-frequency capabilities are presented and the application of the system to sediment transport processes over a sandy ripple bed is illustrated. The observations clearly show the value of such instrumentation for studying the dynamical interaction between the bed, the flow and the sediments at and within the bottom boundary layer. 相似文献
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