The influence of turbulence on drag

The use of the Reynolds number as the only correlating factor for drag force measurements may be inadequate in circumstances involving highly turbulent flows. The results of previous investigations relating to the effects of turbulence scale and intensity are examined. Of special interest is the possibility of a drag minimum, even at low Reynolds number, for a free-stream turbulence intensity of about 5%. This appears to be the result of interaction between the free stream and the boundary layer. As intensity increases beyond 5%, the minimum may be succeeded by an increase in drag to values exceeding the laminar flow values. Further elucidation of the subject is required, particularly because of its importance in various problems related to geophysical flows.     

Comparison of laminar model,RANS, LES and VLES for simulation of liquid sloshing

Liquid sloshing in storage tank is a fundamental problem of great engineering importance. Sloshing motion can be laminar or turbulent. However, the necessity for inclusion of turbulence in CFD simulation of sloshing flows has not yet been established. In this paper, three roll–induced sloshing cases are studied to assess the merits and shortcomings of the laminar model and three most–commonly used turbulence models (RANS k–ε, LES and Very LES). To overcome the deficiencies in the RANS and LES, the new Very LES (VLES) model, which combines the RANS k–ε and LES, is developed in this paper. The free surface profiles are reconstructed by a coupled Level–Set and Volume–of–Fluid (CLSVOF) method. To the authors’ knowledge, the comprehensive and systematical assessment of the effect of turbulence on sloshing simulation has not been reported in the literature. The numerical results are evaluated using experimental measurements from Delorme and Souto−Iglesias. The present study indicates that the inclusion of an appropriate turbulence model has a profound influence on the simulations of violent and non–violent sloshing flows. The VLES and LES models can provide accurate predictions of free surface profiles and impact pressures, whereas the laminar flow assumption and the RANS model cannot adequately capture the energy dissipation in the sloshing simulation and lead to the inaccurate flow predictions.     

Hybrid sediment gravity flow deposits – Classification, origin and significance

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.     

Centrifuge Study on the Submarine Mudflows

Submarine mudflow is one of the most serious geohazards. A new Mini-drum Centrifuge apparatus was developed for use. A series of experiments were performed to investigate the flow behaviour of submarine mud, based on the following aspects: (i) side view; (ii) change in basal pore pressure; (iii) change in water content and (iv) change in the Reynolds number. In this series of centrifuge tests in the subaqueous environment, the submarine mudflow transformed from a highly concentrated mudflow to a turbulent current at a water content of 113% to 121%. Accordingly, the Reynolds number Re, as the threshold of turbulent flow was about 1,150–2,250. Based on the water content measured before and after each test, the water entrainment during the flowing process was minimal for laminar flows, but large for turbulent flows. In addition, the evolution of basal pore pressure was found to be closely related to the regime of submarine mudflow: laminar or turbulent.     

长江口湍流剖面的观测与分析

小尺度湍流过程对河口物质输运与能量交换至关重要。受传统观测方法的限制, 河口浅水区域的剖面观测资料至今较为匮乏, 进而限制了湍流过程的研究。为此, 采用新型5波束声学多普勒流速剖面仪(Nortek Signature 1 000 kHz AD2CP)在长江口开展湍流剖面观测, 获取高频、低噪的高质量湍流剖面数据, 并与声学多普勒点式流速仪(acoustic doppler velocimeters, ADV)同步观测的数据进行对比。结果表明, 通过AD2CP与ADV获得的近底部边界层摩阻流速u_*、拖曳系数C_d、雷诺应力S_R等特征参数基本一致, 底摩擦与波浪能量为河口区域湍动能的主要输入源。湍流垂向结构存在显著的非局地平衡, 即温盐等斜压作用引起的浮力通量、对流项以及强波浪作用影响的脉动压力做功、黏性输运等因素可能为长江口湍流非局地平衡的主因。     

Turbulent mixing in stably stratified flows of the environment: The current state of the problem (Review)

Specific features of the turbulent transfer of the momentum and heat in stably stratified geophysical flows, as well as possibilities for including them into RANS turbulence models, are analyzed. The momentum (but not heat) transfer by internal gravity waves under conditions of strong stability is, for example, one such feature. Laboratory data and measurements in the atmosphere fix a clear dropping trend of the inverse turbulent Prandtl number with an increasing gradient Richardson number, which must be reproduced by turbulence models. Ignoring this feature can cause a false diffusion of heat under conditions of strong stability and lead, in particular, to noticeable errors in calculations of the temperature in the atmospheric boundary layer. Therefore, models of turbulent transfer must include the effect of the action of buoyancy and internal gravity waves on turbulent flows of the momentum. Such a strategy of modeling the stratified turbulence is presented in the review by a concrete RANS model and original results obtained during the modeling of stratified flows in the environment. Semiempirical turbulence models used for calculations of complex turbulent flows in deep stratified bodies of water are also analyzed. This part of the review is based on the data of investigations within the framework of the large international scientific Comparative Analysis and Rationalization of Second-Moment Turbulence Models (CARTUM) project and other publications of leading specialists. The most economical and effective approach associated with modified two-parameter turbulence models is a real alternative to classical variants of these models. A class of test problems and laboratory and full-scale experiments used by the participants of the CARTUM project for the approbation of numerical models are considered.     

Bottom friction caused by boundary layer streaming beneath random waves for laminar and smooth turbulent flow

The effect of boundary layer streaming on the sea bed shear stresses, beneath random waves, is investigated for laminar flow as well as smooth turbulent flow. It is demonstrated how bottom friction formulas for regular waves can be used to obtain the bed shear stresses resulting from steady streaming under random waves. As a result, friction factors for steady streaming under random waves are provided, and the effect of streaming versus the effect of linear waves is discussed. For laminar flow the effect of second order Stokes waves is also included. Examples are included to illustrate the applicability of the present practical method, and results are obtained using data typical for field conditions.     

波浪破碎下湍流混合的实验研究

波浪破碎过程产生的湍流动量和能量垂向输运对于加快海洋上混合层中垂向混合具有显著效果.采用二维实验室水槽中对波浪破碎过程进行模拟.对采集的波浪振幅时间序列采用希尔伯特变换定位破碎波位置,波浪的破碎率随有效波高的增加而增大,波浪谱分析得到的波浪基本周期与有效周期结果相似.实验中采用粒子图像测速技术(particle ima...     

Numerical simulation of micro-bubble drag reduction using population balance model

The phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian-Eulerian two-fluid model. Multiple-size group (MUSIG) based on population balance models, which resolve a wide range of bubble sizes taking into account the bubble break-up and coalescence have been used for this purpose. The simulated results are compared against the experimental findings of Madavan et al. [1984. Reduction of turbulent skin friction by micro-bubbles. Physics of Fluids 27, 356-363] and also other numerical studies explaining the sophisticated phenomena of drag reduction. For the two Reynolds number cases considered, the buoyancy with the plate on the bottom configuration is investigated, as from the experiments it is seen that buoyancy seem to play a role in the drag reduction. Numerical model employed in the investigation comprises of a micro-bubble laden flow wherein two independent sets of Reynolds averaged Navier-Stokes (RANS) transport equations were used to describe both the phases of the flow. The shear stress transport (SST) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles. Change in the mean streamwise velocity profiles, void fraction, turbulence modification and other results are presented and discussed with corresponding change in the gas injection rates. The complex mechanism of drag reduction are scrutinised and explained in context to our numerical findings. Special attentions have been also devoted to divulge the effect of bubble coalescence and break-up caused by random collision and turbulent impact. Numerical results showed good agreement for the skin-friction coefficients against experimental data throughout various air injection rates. The MUSIG model was found to be one of the best candidates to resolve the bubble dynamics in micro-bubble-induced drag reduction problems.     

各向同性均匀湍流近壁结构及泥沙起动

采用振动格栅产生的各向同性均匀湍流研究了泥沙的起动。应用PTV技术和局部光学放大技术对湍流场进行了测量，发现近壁面湍流动能的指数分布规律。分析了泥沙在均匀同性湍流中的起动机制，并用概率统计理论对泥沙的起动进行了定量的描述。通过测定泥沙起动事件的概率，得到了同种泥沙在不同流动参数下起动的临界湍流动能。     

Calculation of wave-induced turbulent flows in estuaries

The k-ε turbulence model which relates the eddy viscosity to turbulent kinetic energy, k, and to the rate of its dissipation, ε, and determines the distribution of these two quantities from modeled transport equations, is employed in calculating the vertical structure of wave-induced turbulent flows in two-dimensional estuaries. The empirical constants in this model are given the standard values cited in the literature and used successfully for calculating a large variety of steady flows; they are not tuned to the oscillating flows. The free surface elevation and the eddy-viscosity distribution are also calculated as a part of the solution. First, calculations performed for oscillatory laminar flows are compared with analytic solutions to ensure the proper performance of the numerical scheme. The turbulent flow in a laboratory-estuary model with one end closed, and in the Humber Estuary is simulated with the numerical model, and the results are compared with the corresponding flume and field measurements. The influence of frequency and roughness is thereby investigated. The results indicate that the time and space variation of the velocity field and the phase lag between the surface slope and the horizontal velocity can be predicted satisfactorily in wave-induced turbulent flows.     

Observed characteristics of flow,water mass,and turbulent mixing in the Preparis Channel

Preparis Channel is the very important exchange path of energy and materials between the northern Bay of Bengal and Andaman Sea(AS). A set of hydrographic measurements, a microstructure profiler, and a deep mooring were used to determine the characteristics of water masses, turbulent mixing, and flows in the Preparis Channel. The unprecedented short-term mooring data reveal that a deep current in the deep narrow passage(below 400 m) of the Preparis Channel flows toward the Bay of Bengal(BoB) wit...     

Reynolds number variation in oscillatory boundary layers: Part I. Purely oscillatory motion

A numerical model based upon a low Reynolds number turbulence closure is proposed to study Reynolds number variation in reciprocating oscillatory boundary layers. The model is used to compute the boundary layer for flow regimes ranging from smooth laminar to rough turbulent. Criteria for fully developed turbulence are derived for walls of the smooth and rough types. In particular, a new criterion to identify the rough turbulent regime is determined based on the time-averaged turbulence intensity. The reliability of the present model is assessed through comparisons with detailed experimental data collected by other investigators. The model globally improves upon standard high Reynolds number closures. Variation through the wave cycle of the main flow variables (ensemble-averaged velocity, shear stress, turbulent kinetic energy) is remarkably well-predicted for smooth walls. Predictions are satisfactory for rough walls as well. Yet, the turbulence level in the rough turbulent regime is overpredicted in the vicinity of the bed.     

不同湍流模式下钱塘江涌潮水流三维模拟

钱塘江涌潮具有动力作用强和流速变化快等特点。涌潮水流紊动复杂,流速的垂向分布和紊动强度息息相关。通过涌潮水流实测资料的分析可以发现,涌潮作用下流速垂向分布在底部和上层存在差异。为研究涌潮作用下流速垂向分布的特征,应用基于非结构网格下有限体积法模型FVCOM对钱塘江涌潮河段水流运动进行三维数值模拟。考虑到涌潮紊动作用复杂且对流速的垂向分布起着重要影响,采用不同的湍流模式对涌潮传播过程中水流的运动特征开展研究。通过与涌潮河段实测资料的验证,复演涌潮到达前后水流运动特征,给出涌潮水流湍动能的变化过程。研究成果有助于深入认识涌潮水流紊动特征和流速的分布规律,为涌潮作用下物质输运的研究提供基础。     

大洋细结构的一种可能的机制Ⅲ.对混合过程动能耗散的估计

继第部分之后研究了惯性内波和近惯性内波由f～的作用所致的剪切不稳定引起的破碎机制。物理上,该机制很象存在由风应力所致薄表面涡旋漂流层时表面波的破碎与饱和过程。惯性内波和近惯性内波的破碎产物与小尺度湍流一起形成了混合块,它与Gregg等人(1986)的持久混合观测结果一致。依据Thorpe(1973)实验的结果作者提出了一个估计湍流动能耗散率和消衰时间的方法。结果表明,在剪切不稳定中近惯性内波在湍动耗散中起了关键作用,而惯性内波引起非常弱的湍动耗散。使用内波能量谱的标准总能量密度估计出的近惯性内波的耗散率和消衰时间与PATCHEX测量结果非常一致。文中还讨论了几个与此破碎机制有关的问题。     

Depositional processes and stratigraphic architecture within a coarse-grained rift-margin turbidite system: The Wollaston Forland Group,east Greenland

The Wollaston Forland Basin, NE Greenland, is a half-graben with a Middle Jurassic to Lower Cretaceous basin-fill. In this outcrop study we investigate the facies, architectural elements, depositional environments and sediment delivery systems of the deep marine syn-rift succession. Coarse-grained sand and gravel, as well as large boulders, were emplaced by rock-falls, debris flows and turbulent flows sourced from the immediate footwall. The bulk of these sediments were point-sourced and accumulated in a system of coalescing fans that formed a clastic wedge along the boundary fault system. In addition, this clastic wedge was supplied by a sand-rich turbidite system that is interpreted to have entered the basin axially, possibly via a prominent relay ramp within the main fault system. The proximal part of the clastic wedge consists of a steeply dipping, conformable succession of thick-bedded deposits from gravity flows that transformed down-slope from laminar to turbulent flow behaviour. Pervasive scour-and-fill features are observed at the base of the depositional slope of the clastic wedge, c. 5 km into the basin. These scour-fills are interpreted to have formed from high-density turbulent flows that were forced to decelerate and likely became subject to a hydraulic jump, forming plunge pools at the base of slope. The distal part of the wedge represents a basin plain environment and is characterised by a series of crude fining upward successions that are interpreted to reflect changes in the rate of accommodation generation and sediment supply, following from periodic increases in fault activity. This study demonstrates how rift basin physiography directly influences the behaviour of gravity flows. Conceptual models for the stratigraphic response to periodic fault activity, and the transformation and deposition of coarse-grained gravity flows in a deep water basin with strong contrasts in slope gradients, are presented and discussed.     

Simulation of vortex-induced vibrations of a cylinder using ANSYS CFX

In this paper, vortex-induced vibrations of a cylinder are simulated by use of ANSYS CFX simulation code. The cylinder is treated as a rigid body and transverse displacements are obtained by use of a one degree of freedom spring damper system. 2-D as well as 3-D analysis is performed using air as the fluid. Reynolds number is varied from 40 to 16000 approx., covering the laminar and turbulent regimes of flow. The experimental results of （Khalak and Williamson, 1997） and other researchers are used for validation purposes. The results obtained are comparable.     

Transport properties of entrained topographic waves during their interaction with turbulence

In the Boussinesq approximation, topographic waves entrained by a sloping bottom are considered with allowance for turbulent viscosity and diffusion. The mean flows induced by a wave through nonlinearity are determined. The turbulent exchange coefficients are expressed in terms of the density of turbulent energy by using the relations of the semiempirical theory of turbulence. The equation for boundary-layer wave solutions and the equation of turbulent energy balance are solved jointly, which makes it possible to determine the vertical distribution of the density of turbulent energy in the area of the given wave. In the diffusion approximation, the vertical distribution of the concentration of wave-suspended sediments is obtained for the case when the tangential bottom stress exceeds the critical values corresponding to the start of sediment motion.     

江苏近海潮流湍流切应力研究

潮流的流速分布和湍流切应力的分布密切相关。为了研究潮流湍流切应力的特征和变化规律,本文从潮流运动方程推导出随潮流呈周期性变化的湍流切应力随深度的分布表达式。从江苏近海现场实测流速剖面中计算得到一个潮周期内的湍流切应力的变化过程。通过将实测值与本文理论解的比较发现,计算值能很好地反映出实测值的变化特征,尤其是能正确地反映潮流在加速和减速过程中,湍流切应力偏离线性分布呈现上凹和下凹的现象,以及只有在加减速转换的时候切应力才呈现线性分布的特点。最后,通过分析湍流切应力振幅及相位沿水深的变化情况,认为反映潮流周期、涡粘性和水深之间关系的参数Ri是决定湍流切应力弯曲程度及分布形态的重要参数。对于近海潮流,水深是影响湍流切应力偏离线性程度的主要因素。     

基于精细温度观测的南海东北部陆坡-深海盆底层湍流混合*

南海是存在强湍流混合的边缘海之一, 但前人对南海湍流混合的研究更多关注的是中上层, 对底层则鲜有关注。本文基于高分辨率温度传感器于2019年5月在南海东北部22个站位海底上方0.5m处持续观测4.4d的温度数据, 分析了2216~3200m深度范围内底层海水温度的时间变化特征, 并探讨了地形粗糙度和内潮对底层湍流混合的影响。分析结果表明, 南海东北部各站位底层海水的温度变化量级约为10^-4~10^-3℃; 温度变化趋势与正压潮变化趋势不同, 温度能谱显示多数站位在全日和半日频带区间出现谱峰, 温度变化更多地受斜压潮影响, 全日、半日内潮起主要调制作用。陆坡-深海盆过渡区及深海盆底层的湍动能耗散率量级为10^-10~10^-9m²∙s^-3, 涡扩散系数量级为10^-4~10^-3m²∙s^-1。观测数据未能显示底层湍流混合与地形粗糙度存在明显的相关性。底层湍流混合的空间分布与过去观测到的南海北部深海盆内潮的南北不对称性分布一致。