Ripple Crests Research Articles

On the Role of Ripple Secondary Crest on Nearbed Hydrodynamics

AbstractRipples are ubiquitous in sandy beds and their geometry plays an important role in determining seabed roughness and intensifying near‐bed turbulence. Ripple geometry in natural settings often deviates from equilibrium configurations. To understand how such nonequilibrium geometry and structures impact near‐bottom hydrodynamics and hydraulic roughness, we performed laboratory experiments examining the effect of two different types of ripple configurations. We employed two distinct fixed 3D‐printed ripple morphologies, uniform ripples and ripples with superimposed secondary crests, and replicated natural conditions by adhering sand grains, matching in size to the ripple scale, onto their surfaces. Our results show that the introduction of secondary crest disrupts the flow over not only the modified ripple but also over its neighbors. Secondary crests induce a thicker boundary layer than regular ripples. Velocities over the upstream side of secondary crest show substantial deviation from the regular ripple baseline case, while the downstream side experiences a lower effect. The shear velocity at the crest of ripples with a secondary feature is significantly higher, indicating an increased capacity for sediment transport and bedform evolution. The turbulent kinetic energy over ripples with secondary crests is twice as high as that over regular ripples. Our results further affirm that the hydraulic roughness is a function of not only the height and wavelength of the ripples, but also of specific structures and ripple geometry.

Insights into Martian bedform migration: Results from Gale, Jezero and Pasteur craters

AbstractAn attempt is made in this study to advance the understanding of the sand movement on Mars by studying the bedform migration at Gale, Jezero and Pasteur craters. The study on the grain size distribution at Gale Crater using Curiosity rover (MAHLI and APXS) observations reveals that the grains with smaller diameters (~50–150 μ) are more prone to migration and vice‐versa, which gives an idea of the necessary requirements that initiate bedform migration. The chemical analysis of the surface materials at the Gale crater revealed elevated concentrations of P2O5, SO3, Cl and Zn in soil compared to sand and active transportation processes for sand but not soil. The comprehensive chemical makeup of the Martian soil (inactive bedforms) and sand (active bedforms) is characterized by its basaltic nature, with enriched volatile elements such as sulphur, chlorine and zinc, and the presence of minerals like plagioclase, pyroxene and olivine due to the cohesive nature of inactive bedforms. Physical weathering and wind flow velocity play a pivotal role in the formation of different sedimentary bodies, impacting grain size distribution and mineralogy. The effect of dust‐lifting on surface features is studied by analysing Perseverance‐MEDA observations at the Jezero crater to understand the short‐term changes in the bedform. These events are found to involve the redistribution of only a small amount of materials and, thereby, changing surface features on Mars over a short period. To detect the bedform migration in the Pasteur crater, several HiRISE images acquired over different time intervals were used. The changes in the ripple crest (~0.29–1.18 m/Earth year) and dune slip face suggest new grain flow events. In the Pasteur crater, extensive changes in sand deposits near the dunes signify a widespread bedform migration. The stronger north‐westerly and north‐easterly winds dominate these changes. Thus, the bedform migration in the three tropical craters exhibits significant variability driven by localized aeolian processes. This variability is crucial for understanding Mars' geological history, current surface dynamics and eventually, helps in planning future missions.

Field observations on the characteristics of sand ripples on tidal flats

Field observations on the characteristics of sand ripples on tidal flats

Laboratory investigations of wave-induced transport of plastic debris over a rippled bottom

Laboratory experiments are conducted in a wave flume to investigate the effect of water waves on the transport of plastic pellets over a rippled bottom. The horizontal velocities of plastic debris are analyzed over the rippled bottom for different wave conditions and plastic elements with different properties. Laboratory investigations determined the characteristic transport patterns of wave-induced plastic debris with a density of ∼2.0g/cm3 moving along the rippled bottom. In the first, swing-type motion, the grains move only in the ripple trough with velocities lower than 0.10m/s. For sliding-type movement, the grains move along the entire rippled surface with velocities in the range of 0.10-0.13m/s. For higher velocities in the range of 0.15-0.20m/s, a saltation-type motion becomes dominant. The results show that plastic grains may move up to 2-3cm above the ripple crest depending on hydrodynamic conditions. The analysis shows that for velocity-skewed flows, sliding-type motion and onshore transport dominate. For acceleration-skewed flows, saltation-type motion and offshore transport dominate, which is attributed to higher boundary layer thickness and phase lag effects. The analysis of the relationship between the particle Reynolds number and the thickness of the turbulent boundary layer reveals that for values of Rep≥1000 and a boundary layer thickness mm saltation-type motion becomes dominant. The direction of transport is affected not only by the density of the sediment and the wave skewness coefficients but also by the dimensions of the bottom ripples. The laboratory investigations also provide insight into the hydrodynamic conditions affecting the transport of plastic debris along the bottom covered with ripples in oscillating nonlinear water flows.

Mass transport at the bottom of propagating surface waves over a rippled bottom

The sea surface can be described by means of the superposition of many sinusoidal functions. However, quite often the amplitude of each component turns out to be much smaller than its wavelength, and any component evolves independently of the others. Hence, it is common to investigate the dynamics of a simple monochromatic surface wave. Hereinafter, the flow generated by a monochromatic surface wave within the bottom boundary layer over a rippled sea bed is determined by means of the numerical integration of vorticity and continuity equations. The forcing term that drives the fluid motion within the boundary layer is evaluated assuming that the steepness of the monochromatic surface wave is much smaller than one and considering the first term of the Stokes expansion. Even though the irrotational flow that forces the viscous rotational flow near the sea bottom is symmetric with respect to the ripple crests, Blondeaux and Vittori [“A route to chaos in an oscillatory flow: Feigenbaum scenario,” Phys. Fluids A 3(11), 2492–2495 (1991a)] showed that the symmetry of the flow field is broken when the Reynolds number becomes larger than a threshold value Rδ,t1 that depends on the geometrical characteristics of the ripples. The results of Blondeaux and Vittori [“A route to chaos in an oscillatory flow: Feigenbaum scenario,” Phys. Fluids A 3(11), 2492–2495 (1991a)] suggest that, when the Reynolds number is larger than Rδ,t1 but not too far from it, a steady current is also generated. Hereinafter, the steady velocity component is determined as a function of the ripple characteristics.

Particle-resolved direct numerical simulation of the oscillatory flow and sediment motion over a rippled bed

Particle-resolved direct numerical simulation of the oscillatory flow and sediment motion over a rippled bed

Sands on Meridiani Planum, Mars

AbstractThe analyses of 184 sediment targets and more than 70,000 individual grains revealed that along the Opportunity rover traverse there are four distinct fractions of deposits related to different geomorphological settings: (a) dust mixed with very fine sand is common behind topographical obstacles, (b) fine sands are deposited in depressions, (c) very coarse sands occur in coarse‐grained ripple fields, and (d) gravel dominates at the rims of craters and on bedrock as lag deposits. Medium size sands were not observed on the plains, but they can be trapped in relatively large craters, where they form dunes and are within coarse‐grained ripples and transverse aeolian ridges (TARs). The fine sands show no regional variations in chemical composition and granulometry, as these sands are easily transported by wind. The very coarse sands vary in composition and shape between the plains and the Endeavour crater rim as their sources are local and their transport distances are short. On the plains, the gravel and the coarse sands are enriched in iron and characterized by higher roundness than the grains from the Endeavour crater rim. The source of iron‐rich, rounded grains on the plains are hematite spherules that are eroded out of Burns formation rocks. The smallest, the best sorted, and the least rounded coarse sand samples are found on coarse‐grained ripple crests. They are mainly composed of spherule fragments and their low roundness indicates shorter transport path lengths than those of grains transported in the past when coarse‐grained ripples migrated.

Paleoecology and paleoenvironment of the Early Cretaceous theropod-dominated ichnoassemblage of the Los Corrales del Pelejón tracksite, Teruel Province, Spain

Paleoecology and paleoenvironment of the Early Cretaceous theropod-dominated ichnoassemblage of the Los Corrales del Pelejón tracksite, Teruel Province, Spain

Biotic vs abiotic origin of unusual features from Mesoproterozoic of Vindhyan Supergroup, India

Biotic vs abiotic origin of unusual features from Mesoproterozoic of Vindhyan Supergroup, India

The Physical Properties and Geochemistry of Grains on Aeolian Bedforms at Gale Crater, Mars

AbstractThis study reports on the physical properties and geochemistry of aeolian bedform grains along the Curiosity rover traverse in Gale crater from Vera Rubin Ridge to the Sands of Forvie (Sols 1902–2995), and includes comparisons to results made earlier in the mission. Volumetrically, <150 μm grains dominate active aeolian bedforms in the study area, similar to previous findings elsewhere at Gale crater and at other locations on Mars. Coarser grains, up to 2.9 mm long, are present on larger active bedforms. The larger 1–3 mm active grains commonly are reddish or whitish in color and irregular in shape, suggesting erosion of local bedrock as sources. One inactive megaripple had a surface of dust‐covered 2–15 mm grains, with smaller <150 μm grains between and within the bedform interior. Geochemical measurements show element concentrations vary according to position on the bedform, sand activity, and grain contributions from local bedrock. A strong positive correlation between Mg and Ni is identified on active bedform surfaces, with the highest Ni always corresponding to ripple crests where the coarsest gray and clear grains were commonly found. There is also a correlation between Ti and Cr for the majority of active sands, with the finer active sands in ripple troughs and sand patches having the greatest number of red grains and highest Cr concentrations. These results show the smaller scale physical properties and geochemistry of several types of aeolian bedforms on Mars formed under current and ancient environments.

Morphodynamics of vortex ripple creation under constant and changing oscillatory flow conditions

The morphodynamics of orbital vortex ripples due to oscillatory flow and induced sediment transport was studied numerically. The methodology was based on large-eddy simulations of the full coupling of turbulent oscillatory fluid flow, bed and suspended sediment transport, and bed evolution. The Immersed Boundary method was implemented for the imposition of fluid and sediment boundary conditions on the mobile bed surface. Results are presented for ripple creation starting from flat or rippled beds, as well as results of ripples adapting to changing flow (wave) direction. The computed equilibrium ripple dimensions are in agreement with existing empirical predictions; however, the computed ripple profile is not as steep in the vicinity of the ripple crest as the analytical one in Sleath (1984). The time needed for the bed geometry to reach equilibrium, in cases where the initial bed is almost flat or it is formed by ripples that shorten towards equilibrium, is longer than in cases where the initial bed is formed by ripples that widen towards equilibrium. Moreover, the ripple crests are reoriented when the flow changes direction by 45°; the new equilibrium ripple crestline is perpendicular to the flow direction. Finally, the numerical results are compared with corresponding results of flow above fixed ripples. The bed load flux does not present substantial differences between the fixed and mobile bed cases. On the contrary, a strong increase of the suspended load flux is observed for the mobile bed case in comparison to the fixed bed case. • LES Coupling between oscillatory flow, sediment transport, and bed morphodynamics. • Same flow forcing results into same bed equilibrium regardless of initial bed form. • Evolution of ripples longer than equilibrium ones is slower than for shorter ones. • Ripples are reoriented perpendicular to the flow direction. • Suspended load over mobile rippled bed is larger than over fixed ripples.

Particle based Large Eddy Simulation of vortex ripple dynamics using an Euler–Lagrange approach

A volume-filtered Large Eddy Simulation (LES) of oscillatory flow over a mobile rippled bed is conducted using an Euler–Lagrange approach. The modeling is done to complement the experimental work, which shows quasi-steady state ripples in a sand bed under oscillatory flow, as observed in unsteady marine flows over sedimentary beds. Simulations approximate the experimental configuration with a sinusoidal pressure gradient driven flow and a sinusoidally shaped rippled bed of particles. The LES equations, which are volume-filtered to account for the effect of the particles, are solved on an Eulerian grid, and the particles are tracked in a Lagrangian framework. A Discrete Particle Method (DPM) is used, where the particle collisions are handled by a soft-sphere model, and the liquid and solid phases are coupled through volume fraction and momentum exchange terms. Comparison of the numerical results to the experimental data show that the LES-DPM captures the mesoscale features of the system. The simulations reproduced the large scale shedding of vortices from the ripple crests observed in experiments. Likewise, the simulations exhibited quantitative agreement between the wall-normal flow statistics, and qualitative agreement in ripple shape evolution and sand particle entrainment behavior above the fluid-bed interface. The numerical data provide insight into the formation of shear layers and eddies within the flow field above and through the ripple and their influence on the particle transport and the dilation of the ripple mobile layer thickness during an oscillatory period. The resolution of flow structures within the particle bed in the LES-DPM simulations make the framework a unique tool for investigating ripple driven sediment transport and distribution of momentum over the ripple.

Numerical investigation of sediment transport mechanism under breaking waves by DEM-MPS coupling scheme

Plunging waves generate vortices that suspend significant amounts of sediment in the key sediment transport mechanism in the surf and swash zones of beaches, causing major changes in the beach morphology over time. The intermittency and locality of vortices and sediment transport make it difficult to measure the flow fields and sediment concentrations in laboratory experiments or field investigations, making such studies of limited value for studying the sediment transport at beaches or as a basis for predicting changes in beach morphology. An attempt is made in this study to overcome these shortcomings through a particle-scale numerical investigation of sediment transport mechanisms under breaking waves. The numerical simulation was performed using three-dimensional coupling of the discrete element method (DEM) and moving particle semi-implicit (MPS) method, with the validity of the numerical model confirmed by comparison with experimental results obtained in a small wave flume. The numerical results revealed the intra-wave sediment motion and the contribution of the wave-generated pressure gradient to the sediment transport near the ripple crest. Sediment transport mechanisms was investigated at the spatial scale of the individual sediment grains by associating it with the vortices, turbulence, and exerted forces near the ripple trough. The results revealed that vortices generated at the water surface by plunging waves affected the bed material motions mainly through the action of a large pressure gradient force that arises near the bed surface owing to a local minimum of pressure, which can be associated with a vortex region.

Granule Ripples in the Kumtagh Desert, China: Morphological and Sedimentary Characteristics, and Development Processes

AbstractGranule ripples are widespread on Earth and are a fundamental component of dryland sedimentary systems, but the relationship between their wavelength and surface particle size is unclear; in addition, their sedimentary bedding and granule ripple development are incompletely understood. In this study, we examined the relationship between granule ripple development, morphology, and the surface grain‐size distribution. We measured the morphology and particle‐size distribution of granule ripples in China's Kumtagh Desert, and found an exponential relationship between ripple wavelength and average crest particle size, which contradicts previous results. We hypothesize the ripples resulted from creep of coarse particles caused by saltation impacts, accompanied by decreasing bed elevation, leading to crest migration and increased crest height. The size composition and wind regime determined the movement mode of particles, which controlled ripple development and size, but dry–wet and freeze–thaw cycles also contributed. We established a new conceptual model for the development of large granule ripples. Sand ripples and small granule ripples develop similarly, but large ripples did not form from merger of smaller ripples. We report for the first time that particles between 2.3 and 4.7 mm in diameter were missing in the ripple crests (confirmed in wind tunnel experiments). This gap affects the particle movement mode. Since the threshold wind speed for particle saltation at 4.7 mm approaches the regional maximum wind speed, quantifying the missing particles may permit prediction of regional maximum wind speeds, and has practical significance for identifying wind conditions elsewhere, including the surface of Mars.

High‐Resolution Large Eddy Simulations of Vortex Dynamics Over Ripple Defects Under Oscillatory Flow

AbstractWe carried out high‐resolution Large Eddy Simulations to study the three‐dimensional hydrodynamic characteristics over regular and irregular wave‐generated ripples. Two types of irregularities in ripple crestlines are considered, namely ripples with a termination defect and with a bifurcation defect. Over regular ripples, the flow and vortex structures vary in the spanwise direction, displaying evident three‐dimensional vortex structures. Compared to regular ripples, the presence of a bifurcation or a termination defect implies local variations in ripple height and spacing. As a result, the maximum velocity, vortex strength and the position of the vortex center over ripples neighboring a defect are all affected by the presence of the defect. The maximum velocity at the ripple crest neighboring a defect is linearly related to the position of the vortex center. The presence of defects induces an asymmetric distribution of shear stress and sediment flux that will ultimately affect bedform evolution.

Open-channel flow over evolving subaqueous ripples

We have numerically investigated the turbulent flow and sediment grain motion in an open-channel flow configuration over a subaqueous sediment bed featuring two-dimensional transverse ripples at moderate Reynolds number and super-critical Shields number values. The simulation data, which were generated by means of particle-resolved direct numerical simulation, are the same as in our previous work (Kidanemariam & Uhlmann,J. Fluid Mech., vol. 818, 2017, pp. 716–743). By carefully choosing the computational box sizes, we were able to accommodate single ripple units which form over an initially flat sediment bed at a wavelength equal to the domain length. The ripples then evolve into their asymmetric shape relatively quickly and eventually migrate downstream steadily while maintaining their shape and size. In the present study, using a ripple-conditioned phase-averaging procedure, we are able to obtain novel insights into the evolution of the turbulent flow and particle motion over the bedforms, in particular the spatial structure of the basal shear stress and its relation to the particle flow rate. Our analysis confirms that the boundary shear-stress maximum is located upstream of the ripple crest, while the particle flow rate is essentially in phase with the ripple topology, with an average phase difference between the two in the range of 18–19 particle diameters for the considered parameter values. We were further able to confirm the link between the sediment flux relaxation behaviour and the observed shear-stress/geometry lag, by direct evaluation of the saturation length scale.

Collinear Interaction of Waves and Current in the Presence of Ripple in the U-Tube

Ripple formation and evolution as well as vortex separation along the bedform profile strongly influence surface waves and sediment transport. These features were investigated in a U-Tube at the Hydraulics Laboratory of the University of Messina. During the experimental campaign, tests in the presence of wave only, current only, and collinear wave plus current in wave dominated regime were carried out. The experiments involved both live bed and fixed bed conditions. It was observed that, when the current superimposes to the wave, a longer time is required for the bedforms to stabilize; the vortex separating at the ripple crest reduces with respect to the wave only case. Accordingly, in the fixed rippled bed case, velocity measured in current only condition is larger than that in the wave plus current flow. As vortex shedding influences the way sediments are transported close to the bed, the obtained results may improve the present knowledge on wave current interaction in the presence of bedforms with repercussions in turn on sediment dynamics.

RANSE Modeling of the Oscillatory Flow Over Two‐Dimensional Rigid Ripples

AbstractThe oscillatory turbulent flow over two‐dimensional ripples is numerically simulated. Both the profile of the ripples and the values of the parameters used in the present study are typical of vortex ripples at the bottom of sea waves. The flow is computed numerically by using a RANSE approach. The results show that, as the ratio between the fluid excursion and the ripple wavelength is increased, the vortex shed on one side of the ripple crest is no longer the mirror image of the vortex shed on the other side and the dynamics of the vortex structures becomes quite complex. Moreover, the formation of steady recirculating cells, which might distort the profile of ripples in an erodible bed, is observed. A horizontal steady streaming is also generated, whose positive/negative direction depends on the values of the parameters.

Nucleation and detachment of polystyrene nanoparticles from plowing-induced surface wrinkling

We report the formation of spherical particles (up to 250 nm in diameter) from polystyrene surfaces repeatedly scratched by atomic force microscopy nanotips (nominal radius < 10 nm) along a series of parallel lines. The particles nucleate from the crests of the ripple profiles formed in the beginning of the scratch process. They are subsequently detached and progressively displaced by the tip across the ripples till the edge of the scanned area, where they pile up without coalescing. The detachment occurs smoothly without static friction peaks, suggesting that the particles are torn off as a result of a crazing mechanism induced by the tip when it is pushed against the ripple crests. Considering the negative impact of nanoplastics on the environment and human health, and the absence of established methodologies for a quantitative analysis of these processes at the level of single particles, our results will help to promote systematic characterization of plowing wear on different polymeric materials and different environmental conditions.

The Occurrences and Formation of Ladderback Ripples on Barchans Dunes of Najaf Dunes Field, Iraq, in Aeolian Environments

This paper concerns the study of ripples that occur on the windward of Barchan dunes from the dunes field of Najaf governorate, Iraq. These dunes consist mainly of sand sediments with variable sizes, including medium, fine, and very fine sands. Quartz represents the major light mineral in the Najaf Dunes sand. The prevailing wind direction in the study area is NW-SE. The major ripple crest series of every pattern are oriented perpendicular to the NW-SE wind direction, whereas imbricated ripple groups within the troughs of the preexisting ripples are created by the WSW-ENE wind trend. These ripples tend to be formed by shortened ripples that occupy the troughs of the prolonged series. All crests of the ladderback ripples are oriented at right angles to asymmetry ripples. The ladderback ripples were noticed from fine to very fine- grained sediments, which consist mainly of quartz. The wavelength of the ladderback ripples ranges from 2 – 4 cm, while they are 0.1 – 0.2 cm in height. The occurrence of ladderback ripples within an aeolian environment indicates a variety of wind directions, which influenced the arrangements of the crest ripples.