Three-dimensional sand ripples as the product of vortex instability

Formation of sand ripples under a turbulent liquid flow

Sand ripples are commonly observed in both nature and industry. For example, they are found on riverbeds and in oil pipelines that transport sand. In both natural and industrial cases, ripples increase friction between the bed and fluid and are related to flooding, high pressure drops, and transients. Ripples appear when sediments are entrained as bed load (a mobile granular layer) and are usually considered to be the result of initial bedforms that eventually saturate. Given the small aspect ratio of the initial bedforms, linear analyses can be used to understand the formation of ripples. Thi…

The formation and migration of sand ripples in closed conduits: experiments with turbulent water flows

The transport of solid particles by a fluid flow is frequently found in nature and industry. Some examples are the transport of sand in rivers and hydrocarbon pipelines. When the shear stresses exerted by a fluid flow on a granular bed remain moderate, some grains are set in motion without fluidizing the bed; the moving grains form a layer, known as bed load, that moves while maintaining contact with the fixed part of the bed. Under bed load conditions, the granular bed may become unstable, generating ripples and dunes. Sand ripples are commonly observed in closed conduits and pipes such as in…

Nonlinearity in Oscillatory Flow over Sand Ripple

In this report, We investigated the nonlinear phenomena in the study of flow dynamics of velocity component. In our studies, we observed nonlinear term in the vertical component of velocity by vittori et al \cite{vit} The time series simulation of vertical component which increases with Reynold stress value. We developed direct numerical simulation under two dimensional grid system to study the flow dynamics and vorticity parameter. Flow pattern and flow dynamics near wavy boundary wall in the victinity of ripple bottom was readdressed under direct numerical simualtion(DNS) framework.

Concentration profiles for fine and coarse sediments suspended by waves over ripples: An analytical study with the 1-DV gradient diffusion model

Field and laboratory measurements of suspended sediments over wave ripples show, for time-averaged concentration profiles in semi-log plots, a contrast between upward convex profiles for fine sand and upward concave profiles for coarse sand. Careful examination of experimental data for coarse sand shows a near-bed upward convex profile beneath the main upward concave profile. Available models fail to predict these two profiles for coarse sediments. The 1-DV gradient diffusion model predicts the main upward concave profile for coarse sediments thanks to a suitable $β$(y)-function (where $β$ is …

地球上共同演化的气动沙纹与撞击沙纹

Coevolving aerodynamic and impact ripples on Earth

本研究通过风洞实验与数值模拟发现地球上存在共同演化的厘米级撞击沙纹与分米级气动沙纹，揭示两种中尺度不稳定性，修正了风沙响应速度的认知，并提出太阳系中尺度颗粒床形地貌的统一框架。

Linear and nonlinear instabilities of a granular bed: determination of the scales of ripples and dunes in rivers

Granular media are frequently found in nature and in industry and their transport by a fluid flow is of great importance to human activities. One case of particular interest is the transport of sand in open-channel and river flows. In many instances, the shear stresses exerted by the fluid flow are bounded to certain limits and some grains are entrained as bed-load: a mobile layer which stays in contact with the fixed part of the granular bed. Under these conditions, an initially flat granular bed may be unstable, generating ripples and dunes such as those observed on the bed of rivers. In fre…