Effect of bed roughness on turbulent boundary layer and net sediment transport under asymmetric waves

This paper presents an investigation of the roughness effects in the turbulent boundary layer for asymmetric waves by using the baseline (BSL) k–ω model. This model is validated by a set of the experimental data with different wave non-linearity index, N_i (namely, N_i = 0.67, N_i = 0.60 and N_i = 0.58). It is further used to simulate asymmetric wave velocity flows with several values of the roughness parameter (a_m/k_s) which increase gradually, namely from a_m/k_s = 35 to a_m/k_s = 963. The effect of the roughness tends to increase the turbulent kinetic energy and to decrease the mean velocity distribution in the inner boundary layer, whereas in the outer boundary layer, the roughness alters the turbulent kinetic energy and the mean velocity distribution is relatively unaffected. A new simple calculation method of bottom shear stress based on incorporating velocity and acceleration terms is proposed and applied into the calculation of the rate of bed-load transport induced by asymmetric waves. And further, the effect of bed roughness on the bottom shear stress and bed-load sediment transport under asymmetric waves is examined with the turbulent model, the newly proposed method, and the existing calculation method. It is found that the higher roughness elements increase the magnitude of bottom shear stress along a wave cycle and consequently, the potential net sediment transport rate. Moreover, the wave non-linearity also shows a big impact on the bottom shear stress and the net sediment transport.     

Mass transport in a two-layer wave boundary layer

The transport of a chemical species under the pure action of surface progressive waves in the benthic boundary layer which is loaded with dense suspended sediments is studied theoretically. The flow structure of the boundary layer is approximated by that of a two-layer Stokes boundary layer with a sharp interface between clear water and a heavy fluid. The simplest model of constant eddy diffusivities is adopted and the exchange of matter with the bed is ignored. For a thin layer of heavy fluid, whose thickness is comparable to the surface wave amplitude and the Stokes boundary layer thickness, effective transport equations are deduced using an averaging technique based on the method of homogenization. The effective advection velocity is found to be equal to the depth-averaged mass transport velocity, while the dispersion coefficient can be shown to be positive definite. Explicit expressions for the transport coefficients are obtained as functions of fluid properties and flow kinematics. Physical discussions on their relations are also presented.     

三沙湾夏季底边界层动力过程及悬沙输运特征

基于2018年8月福建三沙湾湾内外共两个定点站位的船基和座底三脚架观测数据,研究了三沙湾底边界动力过程及悬沙输运特征。结果表明,三沙湾湾内湾外两个站位均表现出涨落潮历时相近但涨落潮流速明显不对称的现象,即湾内涨潮流速大于落潮流速,湾外则相反。湾内水体受淡水输入影响较大,表现出落潮期间显著的温盐层化,而涨潮期间水体混合良好;湾外水体受淡水影响不明显,表现为水体温度主导的层化。通过对底边界层动力过程的分析表明,湾内(距底0.75m)、湾外(距底0.50m)站位底边界层的平均摩阻流速分别是0.016m/s、0.013m/s,且两个站位拖曳系数基本相等(2.03×10^–3),表明在相同流速下湾内站位的底部切应力更大,近底沉积物再悬浮和搬运相对湾外站位更为显著。因此观测期间悬沙浓度最大值出现在湾内站位,为109mg/L,且悬沙在垂向上的分布可达上层水体;湾外站位悬沙浓度更低,并且底部悬浮泥沙仅能影响至距底5m的水体。悬沙通量机制分解结果表明,三沙湾夏季的潮周期单宽悬沙从湾外向湾内方向净输运,湾内站位向湾内方向净输运74.88 g/(m·s),平流输沙占主导作用,贡献率41....     

A multi-frequency Acoustic Concentration and Velocity Profiler (ACVP) for boundary layer measurements of fine-scale flow and sediment transport processes

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.     

Characteristics of turbulent boundary layers over a rough bed under saw-tooth waves and its application to sediment transport

A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, a_c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k–ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found.     

Numerical simulation of breaking wave generated sediment suspension and transport process based on CLSVOF algorithm

The sediment suspension and transport process under complex breaking wave situation is investigated using large eddy simulation （abbreviated as LES hereafter） method. The coupled level set （LS） and volume of fluid （VOF） method is used to accurately capture the evolution of air？water interface. The wall effect at the bottom is modeled based on the wave friction term while the complicate bottom boundary condition for sediment is tackled using Chou and Fringer’s sediment erosion and deposition flux method. A simulation is carried out to study the sediment suspension and transport process under periodic plunging breaking waves. The comparison between the results by CLSVOF method and those obtained by the LS method is given. It shows that the latter performs as well as the CLSVOF method in the pre-breaking weak-surface deformation situation. However, a serious mass conservation problem in the later stages of wave breaking makes it inappropriate for this study by use of the LS method and thus the CLSVOF method is suggested. The flow field and the distribution of suspended sediment concentration are then analyzed in detail. At the early stage of breaking, the sediment is mainly concentrated near the bottom area. During the wave breaking process, when the entrapped large-scale air bubble travels downward to approach the bottom, strong shear is induced and the sediment is highly entrained.     

Field measurements of fairweather bottom boundary layer processes and sediment suspension on the Louisiana inner continental shelf

Field measurements of bottom boundary layer and sediment-transport processes were made on the Louisiana inner continental shelf in spring 1992 at a depth of 15.5 m, and in spring and summer 1993 at a depth of 20.5 m. Two different wave–current boundary layer/sediment-transport models were applied to the measured near-bed flows. In addition, the log-profile method was applied to estimate hydraulic roughness and bed stress. Consistent with the results of others, our measurements show that near-bed flows were very weak under non-storm conditions. Bed stresses were typically too low to resuspend bed sediments. However, the advection of high-turbidity layers or plumes past the instrumentation apparently caused a sustained period of high suspended sediment concentration throughout the log layer in spring 1993. In the absence of wave activity or high suspended sediment concentrations, boundary layer profiles showed the bed to have been hydraulically very smooth with

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cm. However, wave agitation, combined with increased suspended sediment concentration caused hydraulically rough conditions with

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cm.     

A model of oscillatory rough turbulent boundary layer flow

Approximate analytical solutions of the boundary layer equation are obtained in closed form for oscillatory rough turbulent flow. The solutions are based on a time-varying eddy viscosity, and the aim of the study is to assess the effects of these time variations on the properties of the wave boundary layer. The flow and the eddy viscosity are made interdependent by a closure assumption which relates the peak value of viscosity in the wave cycle to the peak value of shear velocity. Instantaneous vertical profiles of horizontal velocity and shear stress, and time series of the bed shear stress, are presented for a typical case study. In addition, the wave drag coefficient, the boundary layer thickness and the phase lead of peak bed shear stress over peak free-stream velocity, are determined as functions of both the relative roughness and the parameter governing the magnitude of the time variations in viscosity. Reasonable agreement is demonstrated with previous experimental and theoretical results.     

Three simple models of wave sediment transport

The process of sediment transport by nonbreaking waves over horizontal, rippled beds is presented by three different models which are evaluated through comparison with wave flume data. The conclusion from this testing is “the simpler the better”. It is shown that the process in question can in fact be modelled adequately without quantitative consideration of the suspended sediment distribution. Only a reference average concentration at the bed is needed. On the other hand, classical diffusion models severely underpredict the transport of coarse sand in suspension because the process is much more organized than diffusion. The possibility of adapting the models to conditions involving sheet flow and/or breaking waves is discussed.     

Observations of boundary layer parameters and suspended sediment transport over the intertidal flats of northern Jiangsu, China

A current-turbidity monitoring system (CTMS) was deployed on the intertidal flat at Wanggang, northern Jiangsu during October 16-17, 2000, to measure the tidal current speeds and seawater turbidities at 5 levels above the seabed. Based upon the logarithmic-prof‘de equation, the boundary layer parameters, i.e., u*, Z0 and C60, were obtained for 247 tidal flow velocity prof‘des. Around 90% of the profiles were logarithmic according to the critical correlation coefficient. Internal consistency analysis shows that these parameters derived by different methods are consistent with each other. In addition, the height of the bedforms observed is close to the seabed roughness lengths calculated from the velocity prof‘des, indicating that the boundary layer parameters obtained can reveal the conditions at the sedimentwater interface on the intertidal flats. Suspended sediment concentrations were obtained from the 5 CTMS turbidity meters using laboratory and in-situ calibrations. The results show that the in-situ calibrated SSCs have a much higher accuracy than the laboratory calibrated ones. Calculation of suspended sediment fluxes on the intertidal flats, with a magnitude of 104 kg/m per spring tidal cycle, indicates that suspended sediment moves towards the northwest, which is reversal to the transport pattern controlled by the southward Northern Jiangsu Coastal Current in the sub-tidal zone and adjacent shallow waters.     

Full-range equation for wave boundary layer thickness

Full-range equation covering all the flow regimes in a wave boundary layer is proposed for the boundary layer thickness. The results are compared with the available experimental data and good agreement has been found. In case of wave boundary layers, there are three definitions of boundary layer thickness in use. Therefore, the full-range equation is derived for three of the definitions. The findings of this study may be useful in calculating suspended sediment transport in coastal environments and studying wave–current combined motion.     

An experimental study of turbulent boundary layer with slit suction

Experiments are performed on a flat plate with a transverse suction slit in the Reynolds number range 5 × 10⁵ < R_e < 1.1 × 10⁶. Mean velocity profiles, RMS values are measured with hot wire anemometry. Friction velocity is numerically calculated. The experiments showed that a classical boundary layer parameter α is related to the suction coefficient S_c with an equation of the form: .The value of A seems to depend strongly on the relative location with respect to suction slit and possibly weakly on Reynolds number.     

Numerical simulation of sediment transport in harbors

Numerical simulation of sediment transport is a coupled problem based on computation of profiles of water velocity and suspended sediment concentration. Effects of the water velocity profile and turbulent pulsation on vertical motion of suspended particles are emphasized in this study. Explanation of amplified sediment deposit in zones of separated flow near both natural and manmade bottom irregularities (like walls of shipping channels) is given. A semi-empirical approach to determination of long-term sediment deposit/erosion is suggested.     

On a theoretical model of rough turbulent wave boundary layers

An analytical theory which describes the motion in a turbulent wave boundary layer near a rough sea bottom by using a two-layer time invariant eddy viscosity model is presented. The eddy viscosity in the inner layer increases quadratically with the height above the sea bottom. In the outer layer the eddy viscosity is taken as a constant. The mean velocity and shear stress profiles, the bottom shear stress and the bottom friction coefficient are presented, and comparisons are made with experimental results.     

非对称歪斜波引起的片流输沙数值研究

本文用了一个可考虑相位差作用和波浪边界层非对称性的瞬态理论模型和一个两相紊流模型共同研究非对称歪斜波引起的片流输沙现象。为了解速度偏度和加速度偏度对输沙通量和输沙率的贡献,两相流模型为理论模型提供了必要的相位超前、瞬时侵蚀深度和边界层的发展过程。理论模型研究显示了由速度偏度和加速度偏度引起的向岸阶段和离岸阶段的泥沙运动非对称性,解释了净输沙的产生原因。在以往的非对称歪斜波片流输沙研究中,净输沙的产生主要被归结于相位差作用。本文的研究则表明了非对称的边界层发展所产生的净流量和动床面效应在净输沙产生过程中的比相位差作用更为重要。     

Mellor-Yamada模型在波浪边界层中的运用

利用Mellor-Yamada 2.5阶封闭格式建立一维垂线波浪边界层数学模型,利用有限元方法求解,模拟线性波作用下的层流边界层和乔治沙洲南侧海岸一侧点的波浪边界层流速分布,并同理论或其他的计算方法进行比较,结果显示该模型具有较高的精度,能广泛运用于波浪边界层的研究中。     

Theoretical models of the height of the atmospheric boundary layer and turbulent entrainment at its upper boundary

The planetary boundary layer (PBL), which directly interacts with the underlying surface, differs significantly in its nature from the low-turbulent stably stratified free atmosphere. Fluctuations of the Earth’s surface heat balance immediately affect the PBL and assimilate there owing to the effective mechanism of turbulent heat transfer. In this case the upper boundary of the PBL plays the role of a cover, preventing the direct penetration of thermal effects and contaminants into an overlying atmospheric layer. In view of this, air pollution is especially dangerous when associated with shallow PBL. In addition, local peculiarities of climate change are mainly determined by the PBL height due to the high sensitivity of thin stably stratified PBLs to the thermal effects. Deep convective PBLs are not very sensitive to weak thermal effects, but they significantly affect the formation of convective cloudiness and the climate system as a whole by means of the turbulent entrainment of the thermal energy, humidity, aerosols, and other admixtures through the upper boundary. The PBL height and turbulent entrainment must be calculated when simulating and forecasting air pollution, abnormal frosts and heat, and other hazardous phenomena. In this paper we discuss the state-of-the-art knowledge in the area of PBL height simulation and suggest a new model of turbulent entrainment for convective PBLs.